Waterproof breathable layers – A review

Advances in Colloid and Interface Science 268 (2019) 114–135

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Historical perspective

Waterproof breathable layers – A review Ali Reza Tehrani-Bagha ⁎ Department of Chemical and Petroleum Engineering, American University of Beirut, PO Box 11-236, Beirut 1107-2020, Lebanon

a r t i c l e

i n f o

Article history: 25 January 2019 Available online 13 March 2019 Keywords: Waterproof Breathable layer Filter Membrane Characterization Application

a b s t r a c t Waterproof breathable layers (WPBLs) can be classified into two large groups of hydrophilic nonporous and hydrophobic porous layers. These layers (e.g., fabrics, films, membranes, and meshes) can be produced by various continuous and non-continuous processes such as coating, laminating, film stretching, casting, etc. The most common methods for production, characterization, and testing of WPBLs are presented and discussed in light of recent publications. The materials with high level of waterproofness and breathability are often used in outerwear for winter sports, sailing apparel, raincoats, military/police jackets, backpacks, tents, cargo raps, footwear and etc. WPBLs can also be used for other specialized applications such as membrane distillation, oil-water filtration, and wound dressing. These applications are discussed by presenting several good examples. The main challenge in the production of these layers is to compromise between waterproofness and breathability with opposing nature. The related research gaps, challenges, and future outlook are highlighted to shed more light on the topic. © 2019 Elsevier B.V. All rights reserved.

Contents 1.

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3.

Introduction . . . . . . . . . . . . . . . . . . . . . 1.1. Theoretical background . . . . . . . . . . . . . 1.2. Production . . . . . . . . . . . . . . . . . . . Characterization of WPBLs . . . . . . . . . . . . . . . 2.1. Thickness, areal density and mechanical properties 2.2. Surface properties . . . . . . . . . . . . . . . 2.3. Pore size distribution . . . . . . . . . . . . . . 2.4. Gas permeability . . . . . . . . . . . . . . . . 2.5. Water vapor permeability . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . 3.1. Water proof breathable fabrics (WPBFs) . . . . . 3.2. Gas diffusion layer . . . . . . . . . . . . . . . 3.3. Skin wound healing and vascular grafts . . . . . . 3.4. Membrane distillation (MD) . . . . . . . . . . . 3.5. Oil-water separation . . . . . . . . . . . . . .

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Abbreviations: ABS, Acrylonitrile-butadiene-styrene copolymer; AFM, Atomic force microscope; AGMD, Air gap membrane distillation; ASTM, American society for testing and materials; CMC, Carboxymethyle chitosan; CPL, Caprolactam; DCMD, Direct contact membrane distillation; DMF, Dimethylformamide; FPU, Fluorinated polyurethane; GO, Graphene oxide; HPH, Hydrostatic pressure Head; JIS, Japanese international system; MD, Membrane distillation; MFC, Membrane fuel cells; PCL, Poly(e-caprolactone); PDMS-b-P4VP, Poly (dimethylsiloxane)-block-poly(4-vinylpyridine); PET, Polyethylene terephthalate; PMMA, Polymethylmethacrylate; PMMA-b-PNIPAAm, Copolymer poly(methyl methacrylate)-blockpoly(N-isopropylacrylamide); PMMA-co-PDEAEMA, Poly(N,N-dimethylaminoethyl methacrylate); PNIPAm, Poly(N-isopropylacrylamide); PS, Polystyrene; PSMA, Poly(stearyl methacrylate); PTFE, Polytetrafluoroethylene; PU, Polyurethane; PVB, Polyvinyl butyral; PVDF, Polyvinylidene fluoride; PVDF-HFP, Polyvinylidene fluoride-co-hexafluoropropylene; RO, Reverse osmosis; SDS, Sodium dodecyl sulfate; SEM, Scanning electron microscope; SGMD, Sweep gas membrane distillation; SPM, Scanning probe microscope; THF, Tetrahydrofuran; TPC, Temperature polarization coefficient; VMD, Vacuum membrane distillation; WPBF, Waterproof breathable fabric; WPBL, Waterproof breathable layer; WPBM, Waterproof breathable membrane; WV, Water vapor. ⁎ Corresponding author. E-mail addresses: [email protected], [email protected].

https://doi.org/10.1016/j.cis.2019.03.006 0001-8686/© 2019 Elsevier B.V. All rights reserved.

A.R. Tehrani-Bagha / Advances in Colloid and Interface Science 268 (2019) 114–135

4. Summary and future outlook 5. Conclusion remarks . . . . . Acknowledgement . . . . . . . . References . . . . . . . . . . .

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1. Introduction Waterproof breathable layers (WPBLs) are impermeable to water and permeable to water vapor and air. The waterproofness and breathability, with contrasting nature, should be tuned based on the end-use requirements. WPBLs in the form of membranes, fabrics, films, or meshes are normally classified based on the level of waterproofness which varies from one application to another [1,2]. The waterproofness is defined as the maximum hydrostatic pressure head (HPH), normally expressed in cm H2O or kPa, that the WPBL can tolerate before leaking. The breathability is also evaluated based on the water vapor transmission rate (WVTR) expressed in Kg/m2/24 h. WPBLs can be classified into two large groups of hydrophilic nonporous, and hydrophobic porous layers. One example from each of these classes along with their advantages and shortcomings are provided in Table 1. Current WPBLs are claimed to have very high HPH (above 196 kPa) and WVTR (above 20 Kg/m2/24 h) [3]. The performance and durability of WPBLs are affected mainly by: (a) the type and nature of polymers (e.g., hydrophilicity/hydrophobicity, chemical stability, and mechanical properties), (b) the production techniques (e.g., casting, coating, laminating, thermal stretching, …), and (c) the mechanical stability of their support layers (e.g., porous fabric, nonwoven mesh, and etc.).

1.1. Theoretical background In hydrophilic non-porous/dense membranes, the hydrophilic segments (e.g., oxyethylene groups) or side groups (e.g., -OH, -COOH, -NH2) are responsible for transfer of water vapor (WV). The WV transfer through the amorphous regions of the layer is mainly by the solutiondiffusion mechanism. The water vapor pressure and humidity are directly proportional; and the gradient of WV pressure or humidity on both sides of the layer, is the driving force for WV transmission through the membrane. The nature of the hydrophilic segments, their density, and temperature affect the diffusion rate of water molecules through the nonporous section of WPBLs [4–6]. Based on the solution-diffusion mechanism, the WV diffusive molar permeate flux through a hydrophilic non-porous/dense membrane can be described by Fick's first law (Eq. (1)) [7]. The WV molecules move from a region of high concentration to a region of low concentration through the membrane. The concentration of WV on both sides of the membrane can be calculated by (Eq. (2)) knowing the solubility

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115

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coefficient of the WV molecules in the membranes and the corresponding vapor pressure. The total WV mass flux can be then calculated using Eq. (3) Jsolution−diffusion ¼ −Dg

dC dx

ð1Þ

Ci ¼ Sg  pi Jsolution−diffusion ¼ Dg  Sg 

ð2Þ ðp2 −p1 Þ t

Reported values in 2007 [4]

Reported values in 2017 [3]

Hydrophilic non-porous layer

Hydrophilic PU non-porous film HPH = 140 kPa WVTR = 2 Kg/m2/24 h

HPH N 196 kPa WVTR N 20 Kg/m2/24 h

Hydrophobic porous layer

PTFE Porous membrane HPH = 110 kPa WVTR = 6.3 Kg/m2/24 h

HPH N 276 kPa WVTR N 20 Kg/m2/24 h

ð3Þ

Where Dg and Sg are the diffusion and solubility coefficients of the gas/WV, respectively, Ci is the concentration of the gas/WV in the membrane, p1 and p2 are the vapor pressures of the gas/WV on both sides of the membrane surface, and t is the membrane thickness. It should be noted that the term Dg × Sg is the membrane permeability. In porous hydrophobic membranes, the gas/WV transmission is achieved mainly through the pores; but, the solution-diffusion mechanism (Eq. (3)) cannot be neglected completely. The mean free path of gas/WV molecules through pores can be calculated using the following equation: kB T λW ¼ pffiffiffi 2π pm ðσ W Þ2

ð4Þ

Where kB is the Boltzmann constant, pm is the mean pressure within the membrane pores, T is the absolute temperature, and σW is the collision diameter of water molecules (σW=2.641 × 10−10 m) [7]. If the pores are relatively small (r b 0.05λW), the diffusive or Knudsen flow (Eq. (8)) is mainly responsible for the mass transfer through the membrane pore (i.e., the WV molecules colloid more frequently with the pore wall than to each other) [8,9]. But, if the pores are relatively large (r N 50λW), these WV molecules mainly transfer by viscose or Poiseuille flow (Eq. (10)). For the WV transfer through the pores (0.05λW b r b 50λW), a combination of these two equations can be used (Eq. (9)). Fig. 1 shows the WV transport through a porous WPBL via the aforementioned mechanisms. J ¼ gas permeanace  total number of pores per unit area  ΔP GK ¼

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   32π 9MRT

Table 1 Classification of WPBLs [3,4]. Type of WPBLs

129 132 133 133

Advantages and shortcomings - Very promising HPH - Very low level of WVTR which can be enhanced by swelling and increased relative humidity (i.e, selective permeability) - Relatively cheap due to simpler manufacturing process - Promising HPH which can be improved by decreasing the pore sizes and lamination - The HPH deceases by repeated washing cycles and in the presence of contaminations and surfactants - Non-selective permeability - Relatively expensive

ð5Þ ð6Þ

116

GP ¼

A.R. Tehrani-Bagha / Advances in Colloid and Interface Science 268 (2019) 114–135

1 16μRT

JKnudsen ¼

NK  GK  r3  ΔP when r b 0:05λW τt

JTransition ¼

JPoiseuille ¼

 NKP  GK  r3 þ GP  r4  p τt  ΔP when 0:05λW b r b 50λW NP  GP  r4  p  ΔP when r N 50λW τt

ð7Þ ð8Þ

ð9Þ ð10Þ

Where ΔP is the vapor pressure of the gas/WV on both sides of the layer (p2 − p1), r is the capillary pore radius, M is the molecular weight of the gas/WV, R is the gas constant, T is the gas absolute temperature, μ is the gas/WV viscosity, and NK, NKP, and NP are the number of pores per unit area that are (r b 0.05λW), (0.05λW b r b 50λW), and (r N 50λW), respectively. t is the membrane thickness, and τ is the pore tortuosity [10]. A general equation for estimating the total gas/WV flow based on the aforementioned mechanisms has been derived for porous WPBLs [7]. The surface porosity of the WPBLs (εS) can also be calculated from the following equation: εS ¼

n X Apores ¼ Nπ f j r 2j AWPBL j¼1

The polymer film extrusion can be used for making hydrophilic PU films and hydrophobic porous PTFE membranes. The biaxial stretching of a PTFE film results in a microporous structure with many interconnected pores (Fig. 3) [13]. These WPBLs are not very strong and they should be protected against mechanical damage by lamination. Densely woven or non-woven fabrics can be used for supporting the layer [1,11,12]. Coating a support layer with a polymeric solution or foam is also common in making WPBLs. The fabric support layer can be coated with a polymer solution (e.g., PU or PVDF in DMF) by various methods (e.g. impregnation, spraying, casting, etc.). In order to make a porous layer, the coated fabric should be passed through a conditioning chamber followed by a coagulation bath. The organic solvent diffuses from the coating to the coagulation bath and leaves behind some pores. The polymeric solution concentration, coagulation bath formulation, temperature, and fixing conditions should be properly adjusted for achieving the best results [12]. For producing WPBLs on a laboratory scale, many other techniques can be employed individually or together such as solution casting, coating, interfacial polymerization, layer by layer deposition, solution wet/dry spinning, electrospinning, ion/electron tracketching, wet coagulation, 3D printing, laser, UV-irradiation, plasmainduced polymerization, etc. [11,12,14–22]. 2. Characterization of WPBLs

ð11Þ

Where Apores and AWPBL are the surface area of pores and the total WPBL surface area, respectively. N is the number of pores per unit area, and fj is the fraction of pores with radius rj. 1.2. Production Different support materials and preparation techniques have been employed for making WPBLs. The most common processes are shown in Fig. 2. The phase inversion technique is a very common practice for making WPBLs. The selected polymer is dissolved in a good solvent and casted into a film with a thickness in the range of 20–250 μm on a proper support. The casted film is immersed in a coagulation bath (non-solvent). A large number of pores with various structures and diameters are obtained due to the solvent exchange. The most important effective parameters are the nature and molecular weight of the polymer, the type and concentration of additives, the type of solvent and non-solvent, temperature, drying time, etc. [11–15]

There are several analytical methods for characterization of WPBLs and some of the most important ones will be presented briefly here: 2.1. Thickness, areal density and mechanical properties A precision thickness gauge is conventionally used for measuring the thickness (t) of a layer. The force applied on the surface during the measurement may affect the result especially if the layer is bulky and porous. Therefore, a standard test method should be employed for this purpose. By knowing the dimension of a sample, its volume can be measured based on the measured thickness. The areal density of the layer (ρM) can then be calculated by dividing the weight of each sample by its volume. The porosity (i.e., air volume fraction) of the layer can be calculated from the following equation assuming that the layer is composed of a known polymer with the density (ρP) and neglecting the density of air: ρM ¼ ρAir  V Air þ ρP  ð1−V Air Þ

ð12Þ

Fig. 1. Water vapor transport through a WPBL via four different mechanisms: 1-solution-diffusion for nonporous areas of the layer (Eq. (3)), 2-Knudsen flow (Eq. (8)) for the pores with (r b 0.05λW), 3- Transition flow (Eq. (9)) for the pores (0.05λW b r b 50λW), and 4- Poiseuille flow (Eq. (10)) for the pores (r N 50λW), The WV transport through a nonporous membrane can only happen via the first mechanism [8,10].

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117

Fig. 2. Common processes for production of WPBLs: (a) film extrusion and biaxial stretching, (b) Dip coating with or without wet-/thermo- coagulation, (c) laminating, and (d) spraying [11–15].

Porosity ð%Þ ¼ V Air

  ρ  100 ¼ 1− M  100 ρP

where VAir is the air volume fraction of the layer, and ρM and ρP are the density of the layer and the polymer, respectively. If the layer is

composed of more than one polymer, the average densities of the mixture should be used in Eq. (12). Mechanical properties of WPBLs are generally very important for various applications and affect their performance and durability. Some of these properties along with their corresponding ASTM standard test methods have been summarized in Table 2 [23]. The most important

Fig. 3. PTFE hydrophobic porous membrane is produced by biaxial stretching of PTFE film at elevated temperature. The SEM micrographs show the effect of uniaxial and biaxial stretching on the structure of PTFE films: (a) lengthwise stretching of the membrane (300%) followed by (b) widthwise stretching of the membrane (~214%). Adapted with permission from [13].

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Table 2 Typical ASTM standard test methods for assessing mechanical properties of textile fabrics and WPBLs [23]. Mechanical properties

Instrument

ASTM

Tensile strength, strain, and modulus

electromechanical tensile tester machine

Bursting strength/Diaphragm bursting strength Tearing strength

Inflated diaphragm bursting tester or a tensile tester with a ring clam

D5034, D5035 D2261, D4851 D3786

Low temperature bend test Abrasion resistance

Falling-pendulum (Elmendorf-Type) tester a bending fixture inside a low temperature chamber A uniform abrasion tester

D1424 D2136 D4158

mechanical properties that should be normally measured and reported are the tensile strength (σM∗), the tensile strain (ϵM∗), and the tensile modulus (EM). The bursting pressure of the layers can also be measured using the same instrument. A circular membrane is cut and fixed in a round horizontal sample holder and a steel ball is pressed on it with a controlled speed to measure the maximum pressure that the layer can tolerate before failure [24]. WPBLs are generally thin and should be normally supported by strong layers of woven or non-woven fabrics. Thus, the tensile strength of the composite layers can be improved by wet/dry coating or laminating the support layer [11,25]. In case of using WPBLs in clothing (e.g. sportswear, raincoats), they are seamed and sealed by waterproof sealing tape to prevent the fabric from water leaking through the stitch holes [26]. In one of the studies, twenty-two commercially available WPBFs, made of PU direct wet/dry coating or PTFE lamination, seamed and sealed under constant conditions. The rupture force of sewn seams was measured using the grab test procedure according to

ASTM D 1682 standard test method. The seam strength and elongation were influenced by sewing conditions (e.g., seam type, seam allowance, stitch type, stitch density, needle size, sewing thread). A waterproof sealing tape was used for sealing the seamed fabrics. As a result, the tensile strength and elongation of the seamed and sealed fabric improved [27]. 2.2. Surface properties The surface properties of WPBLs can be investigated by a number of techniques. Scanning electron microscopy (SEM) is commonly used for studying the surface morphology and cross-sectional area of the WPBLs (Fig. 4) [28,29]. The SEM micrographs can be used for measuring the size of the pores at the surface, thickness and fiber diameter of the layers. The surface wettability and the surface free energy of WPBLs can be evaluated by contact angle measurements using an optical tensiometer/ goniometer. A small droplet of liquid with known surface tension is added on the surface of the layer and the contact angle is measured (Eq. 13 and Fig. 5). In case the liquid droplet is water, the contact angle reveals the hydrophilicity (θ b 90°) or hydrophobicity (θ N 90°) of the layer. The surface free energy of WPBLs can also be calculated by measuring the contact angle of two different liquids on the surface of the same membrane (Owens-Wendt method) using the following equations [30,31]. γSG ¼ γSL þ γLG :Cos θ

ð13Þ

γLG ¼ γdLG þ γ pLG

ð14Þ

γSG ¼ γdSG þ γ pSG

ð15Þ

Fig. 4. SEM images of two different WPBLs (a,b) fabricated free-standing PVDF membranes, [28] and (c,d) nanofibrous FPU/PU with 0.75% CNTs [29]. The PVDF membrane has been prepared from 60 g/l PVDF in DMF with 1 mmol citric acid as a precursor through a special procedure of tape-casting, drying, immersing and skimming. The FPU/PU nanofibrous membrane has been prepared by electrospinning of 1.5 wt% FPU1:8PU +0.75% CNTs in a mixture of DMF1:1THF. Adapted with permission from [28,29]. Copyright (2015) Elsevier and American Chemical Society.

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119

Fig. 5. Schemes showing a droplet of liquid on a flat solid surface: (a) with contact angle lower than 90°, and (b) with contact angle larger than 90°, and a droplet deposited on a surface with holes or spikes: (c) Wenzel state, and (d) Cassie-Baxter state.

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi γLG :ð1 þ Cos θÞ ¼ 2 γ dSG :γ dLG þ 2 γ pSG :γ pLG

ð16Þ

Where γLG, γSG, and γSL are the surface tension of the liquid, the surface free energy of the solid surface, and the surface interfacial tension between the liquid and surface, respectively. γSGd, γSGp, γLGd, and γLGp are the dispersion and polar components of the surface free energy of the solid surface, and the dispersion and polar components of the surface tension of the corresponding liquid, respectively. γLG, γLGd, and γLGp have been measured and reported for various liquids and can be used for the calculation of γSG using Eqs. (15) and (16) [31]. Using Eq. (16) entails measuring the contact angle of at least two different liquids (typically, water and methylene iodide) at the surface of the membrane and solving two equations simultaneously to find γSGd, and γSGp. It should be noted that there are some other methods for the estimation of γSG that have been reported in more details elsewhere [32,33]. Nevertheless, the aforementioned method remains as a simple and reliable method for estimating the surface free energy of solid surfaces. Eqs. (13)–(16) are valid for flat solid surfaces. However, most of WPBLs may have rough and textured surfaces at submicron or nano scales. The liquid droplet may penetrate into the texture and stick to the surface with high contact angle hysteresis (i.e., Wenzel state). The relation between the measured contact angle (θm) and that of the Young's contact angle (θ) in Eq. (13) has been proposed by Wenzel (Eq. (17)). In Cassie-Baxter state, entrapped air prevents the droplet from penetrating into the texture. The Cassie-Baxter droplet can easily roll on the surface by small perturbation [34]. The surface roughness or presence of macroscopic holes or spikes geometry of WPBLs can affect the contact angle measurement according to the following equations [35–37]: Cos θm ¼ r  Cos θ ðWenzel stateÞ

r ¼ roughness factor ¼

2.3. Pore size distribution Researchers have used various techniques for measuring the pore size of the membranes such as (a) Gas permeation test [44–46] (b) Capillary flow porometry [47], (c) Mercury intrusion porosimetry that is based on the intrusion of mercury into a porous structure under stringently controlled pressures [48,49], (d) AFM surface analysis [50] and (e) SEM image analysis when the through pores are relatively large and uniform [51,52]. In SEM analysis, the sample should be dried and coated with a layer of conductive metal under vacuum which can affect the texture and pore size of the soft and hydrated samples [53]. In gas permeation method, one can obtain the average pore size of a porous WPBL from the linear plot of the volume permeation flux of gas (J) at different capillary pressure drop (ΔP). The average pore size (dg) can be calculated using the following equation [45]:

ð17Þ ð18Þ

ð19Þ

Where θ and θm are the Young's and measured contact angles, respectively, r is the surface roughness, and ∅ is the fraction of solid/liquid interface where the droplet is in contact with the surface [35,36,38]. When the droplet is completely in contact with the surface (i.e., ∅ = 1), Eqs. (17) and (18) are the same. As can be seen from Eqs. (17) to (19), when both r and ∅ are equal to one, the measured contact angle is reliable for measuring the surface free energy (i.e., θm = θ). This can be the case for nonporous hydrophilic WPBLs [39]. For measuring the real contact with porous hydrophobic WPBLs, one should also measure the r and ∅ values accurately. Researchers have used various techniques for measuring the roughness (r) such as atomic force microscopy [40], profilometry [41], laser scanning microscopy [42], or contact mode SPM analysis [43].

dg ¼ Cos θm ¼ r  ∅  Cos θ þ ∅−1 ðCassie−Baxter stateÞ

actual surface geometric surface

  rffiffiffiffiffiffiffiffiffi 16 S 8RT   μ 3 I πM

ð20Þ

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Where S and I are the slope and intercept of the linear plot of Q vs. ΔP, respectively, R is the gas constant, T is the absolute temperature, M is the molecular weight of the gas, and μ is the gas viscosity [45]. The limitation of this method is that the pore size distribution cannot be obtained. In order to improve the average pore size estimation, a more generalized model for porous WPBL should be used to account for porosity, tortuosity, noncircular cross-sectional area, wetting, and their interactions [54]. The capillary pressure (ΔP) is directly proportional to the through pore size (D) according to the following equation: ΔP ¼

−4:γ:Cos θ β D

ð21Þ

Where γ is the surface tension of the wetting liquid, θ is the contact angle of the liquid with the surface, and β is the pore geometrical correction factor (0bβ≤1). The concept of determination of the through pore size distribution has been shown schematically in Fig. 6 [55]. The WPBL is tested before and after wetting with a good wetting agent. The air flow (Q) as a function of pressure drop (ΔP) is obtained for both wet and dry tests. The pore size at any specific ΔP is calculated from Eq. (21). By using the dry and wet curves, the pore size distribution can be calculated from the following differential equation: pore size distribution ¼ d

  Q wet  100 =dD Q dry

ð22Þ

Where Qwet and Qdry are the flow rates through the wet and dry sample, respectively. So, the ratio of (Qwet/Qdry) at two different diameters (D1 and D2) can be subtracted from each other to calculate the percentage of flow passing through the pores within the specified range (D1-D2) [55]. To account for irregular pore structures, the correction factor β has been introduced to Eq. (21). β is 1, 0.914, and 0.843 for circular, square, and equilateral triangle cross sections, respectively [56]. β can be estimated for pores with elliptical cross section as can be seen in Fig. 7 [55]: 2.4. Gas permeability Gas permeability of WPBLs is defined as the gas flow divided by the cross sectional area of the layer at a certain pressure [57]. This is a very

important property in various applications (e.g., air filtration, wind proof coats, and tents) that should be optimized for the best performance. For increasing the flow of air through a WPBL with low air permeability, a high pressure difference should be applied on its both sides (Eq. (21)). The gas permeability can be measured according to the standard test methods ASTM D737-96 or ISO 4638. The average pore size of the WPBL can also be calculated using (Eq. (20)) as it was discussed in the previous section. 2.5. Water vapor permeability Performing the water vapor permeability test is crucial for characterizating of WPBLs. Fig. 8 shows four of the most common standard test methods for measuring the WVTR through WPBLs schematically. The WVTR is a function of water vapor pressure gradient which determines the driving force of water vapor diffusion and the mass transfer per unit time [58–60]. The theoretical models related to WVTR through a WPBL have been already discussed in Section 1.1. WPBLs are normally cut and placed on test cups that are filled with either 100 ml of distilled water (Fig. 8 a and b) or saturated desiccant solution (Fig. 8 c). The latter can be made by dissolving 300 g of potassium acetate in 100 ml of water, which generates a relative humidity of about 23% at 20 °C. The cups are then covered with tight gaskets and put inside an environmental chamber. The description of the standard test methods can be found elsewhere in more details [58–60]. The WVTR of WPBLs can be evaluated after a certain period of time using the following equation: WVTR ¼

ðm f −mi Þ Δt  A

ð23Þ

Where mf and mi are the final and initial masses of the test body/cup, respectively, Δt is the test period of time, and A is the effective surface area of the exposed WPBL. The other common test method known as dynamic moisture permeation cell (Fig. 8 d) entails using an instrument that can regulate the relative humidity on both sides of the layer accurately. Dry nitrogen gas is typically employed for this purpose and its humidity is set and controlled by using a set of bubblers filled with distilled water. According to ASTM F 2298 standard test method, the initial gas humidity is set at 95% and 5% on both sides of the layer, respectively, without applying any pressure gradient. The gas flow rate and the chamber temperature

Fig. 6. Capillary flow porometry for measuring the through pore distribution of a porous WPBL. (a) the WPBL is fixed in the holder and sandwiched between two plates with a fixed aperture size. The WPBL is wetted with a good wetting agent (normally perfluoropolyether with low surface tension) and all of the pores are filled spontaneously. The flow of a nonreacting gas (normally dry air) is gradually increased on top of the membrane, and as a result the pressure builds up. (b) By increasing the pressure above Pmin. (also known as the bubble pressure), the through pores are opened from the largest to the smallest according to Eq. (21). (c) The gas flow rate as a function of the measured pressure is plotted for the wet and dry sample. The pore size distribution graph is then obtained accordingly to Eq. (22) [55].

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Fig. 7. Shape factor (β) for pores with elliptical cross section [55].

are controlled at around 2 L/min and 20 °C, respectively. The humidity of the gas flow outlets is continuously measured and monitored until the steady-state condition is reached. The WVTR of the tested layer can then be calculated using the following equation: WVTR ¼

ðC f −Ci Þ  Q A

ð24Þ

Where Cf and Ci are the bottom outgoing and incoming water vapor transmission concentrations (mass/volume), respectively, Q is the volumetric gas flow rate (volume/time), and A is the real surface area of the layer. It should be noted that this method depends directly on the gradient of initial gas humidity and temperature. The water vapor diffusion rate can then be estimated by knowing the thickness of the layer [58]. Table 3 shows the breathability characteristic of 26 WPBFs using 4 different standard test methods [58]. This table serves as a reliable and valuable source for comparing the WVTR of commercially available WPBFs. The lowest and highest WVTR values for the same sample can

be obtained using ASTM 96 B upright cup method and JIS L 1099 desiccant inverted cup method, respectively. The reported values by the latter is approximately 16 times larger than those obtained by the former. Thus, the same fabric or membrane can be ranked differently by these methods. The results clearly show that there are generally very poor correlations between these standard test methods (Table 4). The only exception is the relatively higher correlation (R* = 0.97) between the upright cup method (ASTM E 96 B) and the dynamic moisture permeation cell method (ASTM F 2298). The poor correlations should be related to the WVTR test conditions and the nature of the test materials [4,6,61]. The discrepancy among the WVTR standard test methods entails the development of new and more reliable standard test methods for this purpose. The reader is directed to review the following papers to get more information about the new proposed standard method [60,62]. The other suggestion is that a commercially available porous membrane with known pore size distribution should be tested in addition to the other samples under the same standard conditions for benchmarking and comparison.

Fig. 8. Scheme showing four different methods for measuring the WVTR: (a) Upright cup (ASTM E 96 B), (b) Inverted cup (ASTM E 96 BW), (c) Desiccant inverted cup (JIS L 1099), and (d) dynamic moisture permeation cell (ASTM F 2298).

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Table 3 The WVTR of 26 different commercially available WPBFs. Adapted with permission from [58]. Standard test methods →

(a) ASTM E 96 B T = 23 °C R.H. = 50% Air flow = 2.8 m/s

(b) ASTM E 96 BW T = 23 °C R.H. = 50% Air flow = 2.8 m/s

(c) JIS L 1099 Tchamber = 30 °C Twater = 23 °C

(d) ASTM F 2298 R.H.1 = 95% R.H.2 = 5% Gas flow = 2 L/s

Sample size→

Circular, D = 74 mm

Circular, D = 74 mm

Square, 20 × 20 cm2

Rectangle, 5 × 6.5 cm2

↓ WPB fabrics ↓

g/24 h/m

With durable water repellent finish Clima F.I.T.® Epic™ Hyper D-WR With microporous coatings/laminate Entrant G2™–XT (type C) eVent™ (Nylon fabric) eVent™ (Polyester fabric) Helly-Tech® Extreme Omni-Tech Dry™ Omni-Tech Mini-Faille™ Proof Ace® (type M) Triple Point Ceramic® With monolithic coating/laminate Dermizax® Diaplex (Rip stop weave) Diaplex (Plain weave) Gelanots® (Rip stop weave) Gelanots® (Plain weave) Marmot Membrain® Pertextion Sympatex® Xalt™ With biocomponent treatments Eclipse Twin Sensor™ (Rip stop weave) Eclipse Twin Sensor™ (Plain weave) Gore-Tex® XCR Gore-Tex® Marmot Dry Touch Storm F.I.T.®

2

g/24 h/m

RTln

P sat

g/24 h/m

2

g/24 h/m2

892.4 800.8 801.6

4788.0 3113.6 3302.4

13,420.8 6852.0 6824.8

4775.1 3238.5 3743.6

926.0 984.8 942.8 785.2 913.6 742.4 690.8 776.8

5084.8 7265.6 6201.6 3056.8 5317.2 4360.0 3012.8 2972.0

21,272.8 27,825.6 20,716.0 6696.0 16,728.8 7788.0 6050.4 5305.6

5742.0 6162.5 6039.2 3353.5 5098.5 2499.4 2199.0 3094.2

700.0 742.4 715.2 624.4 724.4 618.8 446.4 783.2 566.4

6608.4 6180.4 7285.6 5801.2 7634.4 4368.0 4510.0 5876.0 5992.8

12,357.6 14,508.0 12,052.8 11,676.8 12,707.2 8728.8 6672.8 11,669.6 8220.8

2245.5 2654.2 2441.8 2052.4 2424.5 1962.2 1174.5 2960.1 1692.1

811.6 782.0 864.4 758.8 875.6 804.8

5441.6 4243.2 7513.2 5674.8 4537.6 7604.4

14,998.4 10,361.6 21,193.6 16,612.8 12,616.8 15,360.8

3840.7 3163.1 3193.3 2865.6 3769.5 3053.5

The presence of small pores in WPBLs improves the WVTR and prevents large water droplets from entering the layer. The capillary condensation of water vapor molecules into the pores depends mainly on the geometry and size of the pores. The condensation can occur due to the large pressure needed for water to pass through according to Eq. (25) (the Simplified Kelvin Equation) [8,63,64].

P Ksat:

2

3. Applications WPBLs can be used in a wide range of applications as can be seen in Fig. 9 [65]. These applications will be discussed in more details in this section to highlight the new advancement in the field. 3.1. Water proof breathable fabrics (WPBFs)

! ¼ −V m 

4:γ:Cos θ D

ð25Þ

Where Psat.K and Psat are the saturation vapor pressures in equilibrium with a curved liquid meniscus and next to a flat interface, respectively, γ is the surface tension, Vm is the molar volume, θ is the contact angle, and D is the pore diameter. In reality, the thickness of the adsorbed layer, that appears before condensation and remains after vaporization, should be deducted from D [64].

WPBFs are commonly used in outerwear for winter sports, sailing apparel, raincoats, military/police jackets, backpacks, tents, cargo raps, footwear and etc. The market for WPBFs was worth $1.43 billion in 2015 and expected to grow at an approximate rate of 6%/year to reach $2.3 billion by 2024 [66]. Layers that are impermeable to both water and water vapor can be made form non-porous hydrophobic films (e.g., PU, PET, etc.), or coating conventional fabrics with a continuous flexible layer of hydrophobic materials (e.g., wax, oil, etc.). Based on the quality and durability of the finishing treatment/film, temporary or permanent property can be

Table 4 Correlation between WVTR results reported using 4 different standard test methods [58]. Correlation between (a) ASTM E 96 B (a) ASTM E 96 B (a) ASTM E 96 B (b) ASTM E 96 BW (b) ASTM E 96 BW (c) JIS L 1099

& (b) ASTM E 96 BW & (c) JIS L 1099 & (d) ASTM F 2298 & (c) JIS L 1099 & (d) ASTM F 2298 & (d) ASTM F 2298

Pearson correlation R

Least square correlation (R2)

Spearman rank order R*

0.12 0.64 0.91 0.56 0.08 0.69

0.02 0.41 0.83 0.31 0.01 0.47

0.1 0.63 0.97 0.7 0.05 0.51

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Fig. 9. Most important applications of WPBLs.

achieved [6,12]. Wearing a non-permeable cloth may cause adverse effects on the body and increase the risk of heat stress (i.e., hyperthermia) due to increment of air insulation inside the clothing [12]. There is also a high risk of suffocation for sleeping in a tent made of non-permeable layers. It is worth mentioning that the rate of body perspiration for thermoregulation, due to evaporation cooling, increases corresponding to the heat energy produced by various human activities. This rate varies from a minimum of about 2–3 Kg/m2/24 h for sleeping and sitting to about 14–22 Kg/m2/24 h for running at room temperature [2,67]. These values drop sharply at reduced surrounding temperatures, but, the wearer may experience heat stress if the WVTR of a layer is below these values. The hydrostatic pressure head of 100 cm H2O (~9.8 kPa) and the water vapor transmission rate (WVTR) of 5 Kg/m2/24 h (according to ASTM E96-CaCl2 standards desiccant method at 37 °C) are the minimum accepted levels for sportswear and raincoats [1,2]. The WVTR of a survival suit should be around 10 Kg/m2/24 h and a high

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pressure difference is needed on its both sides to achieve such a rate in cold weather. This value of WVTR can be seen as an optimistic target to prevent the wearer from hypothermia [4]. The WVTR of several commercial WPBFs has been reported in Table 3. Expanded PTFE membranes and nonporous hydrophilic PU or PET films are the most popular WPBFs in the market. To protect these layers against physical damage and mechanical wear and tear, they are normally sandwiched between several layers of fabrics [4]. But, their performance and waterproofness diminishes if they get punctured. One of the advances in the field of WPBFs was to generate self-sealing and healable layers. This was achieved by coating a nonporous poly(ether ester) multi-block copolymer (Sympatex®) with (2-hydroxyethyl acrylate) cross-linked with hydrophobic poly(dimethylsiloxane). The resulting composite fabric was able to heal a large through hole by swelling mechanism in contact with water [68]. There are some other self-healing polymers that have the potential to be used for making self-sealing WPBLs in response to various stimuli (e.g., heat, light, pH, etc.) [69–75]. This interesting research domain needs more attention from both academia and industry. Using self-sealing WPBLs can also solve one of the important challenges in the field of WPB clothing to seal the holes after seaming the coat, tent, etc. which has made the process difficult and relatively expensive [11,12,76,77]. Lightweight and durable WPBFs with multifunctional properties (e.g., anti-microbial, anti-dirt, …) are anticipated to dominate the market in the near future [66]. One of the relatively new and promising technologies for the in production of WPBFs is electrospinning which can be used for covering fabrics and other substrates [17]. The recent advances in the field of electrospun fabrication techniques have been reviewed and a needle-less electrospinning setup is presented here as an example (Fig. 10) [78]. Despite electrospinning's simple setup, there are many parameters involved that should be optimized to achieve suitable fiber diameter, porosity, and pore size distribution. The type of polymer and solvent, the polymer concentration, the high voltage, the distance between the spinneret and the collector, temperature, humidity, feed and collecting rates are among the most important effective parameters [79–81].

Fig. 10. Scheme showing a needleless electrospinning machine which can continuously cover a fabric support layer. High voltage is required to overcome the viscosity and surface tension of the polymeric solution. The polymeric solution is stretched towards the grounded collector and turns into several solid micro/nanofibers [78]. Electrospinning produces a porous structure and if a hydrophobic polymer is used, a WPBL can be made [81]. Active materials such as nanoparticles, drugs, and perfumes can also be encapsulated within the fibers.

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The topic of WPBFs has been well covered in a number of book chapters [11,12,76,82,83] and review papers [4–6,61,65,84].

diffusion layer on the performance of the systems has been investigated in a number of studies [95–97].

3.2. Gas diffusion layer

3.3. Skin wound healing and vascular grafts

WPBLs can be used as gas diffusion layer in membrane fuel cells (MFC). Two good examples are provided here for highlighting the importance of WPBLs: (a) proton exchange MFCs that are very promising candidates for green energy conversion with high energy density and zero CO2 emission. They can be used in portable power generation systems such as vehicles. In proton exchange MFCs, hydrogen (H2) feed at the anode is oxidized to release protons (H+) and electrons (e−). As can be seen in Fig. 11(a), protons pass through a proton exchange membrane to recombine with electrons and oxygen to produce water. Therefore, there is a need for a WPBL as a gas diffusion layer to bring in oxygen and remove water vapor from the cathode side of the MFC [85–89], and (b) Microbial MFCs that are electrochemical devices that use active microorganisms as the anode catalyst to oxidize the waste biomass and generate electricity. This is a promising and attractive technology for the treatment of wastewater and generating clean bioenergy. As can be seen in Fig. 11(b), organic compounds are oxidized at the surface of anode and as a result, smaller oxidized organic compounds are produced along with proton and carbon dioxide. Oxygen and protons are combined at the surface of the cathode to generate water [90]. In order to reduce the high cost of bioenergy production by MFCs, researchers have omitted the relatively expensive proton exchange membrane between the anode and cathode and used air as a cheap and sustainable source of oxygen in a so called single-chamber air-cathode MFC. This is emerging as a promising and practical alternative with higher power density [91,92]. Therefore, the single-chamber aircathode MFC needs a WPBL as a gas diffusion layer that is permeable to air and prevents the system from water flooding [92]. The thickness and pore size distribution of the WPBL affect the power output and coulombic efficiency of MFCs [93]. The performance of activated carbon-air-cathodes MFCs was studied in the presence of several different WPBLs. The results showed that these layers can significantly increase the coulombic efficiency and power density of the MFC while preventing the water loss through the cathode [94]. The proton exchange membrane can also be made from WPBLs. The role of this diffusion layer is to facilitate the passage of protons while preventing water and oxygen from entering the Anode side as much as possible. The effect of PTFE content and distribution in the gas

The WVTR plays an important role in wound healing as it can control the moisture content of the wound for the proliferation of epidermal cells and fibroblasts. One way to create such a desirable microenvironment around a wound is by wound dressing using WPBLs. In one of the studies, five WPB PU membranes with various level of WVTRs were synthesized and used for wound dressing. The fastest wound healing process was achieved using the porous PU membrane with a WVTR of about 2028 ± 237 g/m2/24 h. This was confirmed by both in vitro and in vivo studies. The PU membranes with higher or lower WVTRs showed noticeably slower wound healing rates (Fig. 12) [98]. The infection of wounds is a severe problem for many patients who have diabetic skin ulcers and the treatment is relatively expensive. Therefore, these wounds should be treated effectively and at an early stage to prevent infection [99]. WPBLs can prevent the wound infection by controlled breathability (i.e., WVTR) in contrast to the traditional wound dressing with bandages. In one of the published papers, the authors investigated a double-layered wound dressing using a breathable composite liquid dressing (i.e., Carboxymethyl chitosan film) and a Polyvinyl butyral (PVB) WPB film. Carboxymethyl chitosan (CMC) solution was first sprayed on the wound and then PVB in a sterile liquid formulation (a mixture of ethyl acetate 1:8 ethanol 75%) was brushed on it. After solvent evaporation, a WPBL remained on the wound and protected it from external water and bacteria while keeping the wound beds moist without risking dehydration or exudate accumulation (Fig. 13) [100]. The results showed that the WPB wound dressing reduced inflammation and improved the wound healing process in the presence and absence of a water soluble drug. The gradual blockage of arteries with fat, cholesterol, etc. known as atherosclerosis, is a common health problem that can lead to heart attack and even death. There are different medical treatments to prevent, delay, or open the blocked blood vessel. In case the blocked/damaged artery needs to be replaced/bypassed by surgery, the preferred treatment option is to use a healthy blood vessel from the same patient (i.e., autologous vascular graft). This needs a second surgery which is painful, risky, and not always possible due to the patient's health condition, previously failed usage, trauma, etc. To overcome these problems,

Fig. 11. Scheme diagram of (a) proton exchange MFC and (b) air cathode microbial MFC for production of electricity and pure water.

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Fig. 12. The WVTRs of the PU porous membranes as a function of porosity (left) and the 3D diagram of wound healing (%) as a function of WVTR of the membranes (right). The wound healing is accelerated when the wound is covered with a PU membrane with a medium porosity and the WVTR of about 2028 g/24 h/m2. Adapted with permission from [98].

synthetic vascular grafts should be developed to replace similarly to autologous grafts. The mechanical properties of natural animal autologous grafts have been provided in Table 5 from one of the studies. An ideal tissue graft should mimic the mechanical properties of natural autologous graft and only include a patient's own cells and extracellular matrix components [101]. These grafts should be able to carry oxygenrich blood under pressure without any leakage; and therefore, WPBLs are good candidates for this purpose [102]. PTFE porous tubular membranes (Gore-Tex®) are now commercially available in different diameters (N6 mm) as vascular grafts [103]. Fig. 14 shows a bilayered electrospun vascular graft made of PCL/collagen. The internal layer has been designed to be less porous to prevent the leakage of blood under pressure and the outer layer is relatively more porous to enhance the vascular smooth muscle cells infiltration, proliferation and attach. In general, the small pores of electrospun scaffolds have limited cellular infiltration. By using PCL as a biocompatible and resorbable polymer, the scaffold slowly degrades and cells remodel and replace the scaffold with their natural extracellular matrix proteins. The preclinical large animal study of this vascular graft has been very

promising with a high degree of graft patency without showing an inflammatory response [106]. 3.4. Membrane distillation (MD) MD technology is a combination of membrane separation and thermal distillation. The driving force of separation in MD is a partial vapor pressure gradient as a result of the temperature difference between liquid feed and permeate sides. A hydrophobic porous membrane (WPBM) is normally used for this purpose which separates the liquid feed from the permeate. The four commonly used configurations in the MD setup are (a) direct contact membrane distillation (DCMD), (b) air gap membrane distillation (AGMD), (c) sweep gas membrane distillation (SGMD), (d) vacuum membrane distillation (VMD). Fig. 15 shows these four configurations schematically. In DCMD, both the hot feed and cold permeate aqueous solutions are circulated in direct contact with the WPBM. The temperature difference at both sides of the membrane induces a vapor pressure difference; thus, volatile molecules evaporate at the hot liquid/vapor interface, pass through the pores, and

Fig. 13. Scheme showing the double-layered wound dressing composed of a CMC film and PVB WPBL on top of it. As reported, the film adheres effectively to the skin and has a non-sticky surface. Adapted with permission from [100].

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Table 5 The mechanical properties of natural autologous grafts of adult female mongrel dogs [101], native carotid artery of dogs [104], and human internal mammary arteries [105]. Autologous grafts ↓

Tensile strength (MPa)

Elongation point at break (%)

Bursting strength (kPa)

Suture tolerance strength (N)

Fresh autogenous living tissue biological tubes of dogs Natural femoral arteries of dogs Native carotid artery of dogs Human internal mammary arteries

4.7 ± 2.3 9.3 ± 3.2 3.3 ± 0.43 0.22 ± 0.023

34 ± 8 91 ± 27 99 ± 11 –

146.6 ± 25 304 ± 42 – –

2.5 ± 0.3 3.2 ± 0.4 – 0.56 ± 0.12

condense in the cold liquid/vapor interface inside the membrane module. Most of the MD studies (N60) are focused on the DCMD configuration due to its simple design and operation. In AGMD configuration, there is an air gap between the membrane and cold plate inside the module. The transferred vapor is condensed in this air gap into liquid. When the transferred vapor is carried out of the module by sweeping gas and vacuum, the configuration is called SGMD and VMD, respectively. These configurations have different performance and energy consumptions. As a good example to highlight this matter, the permeate flux can be enhanced by applying vacuum (i.e., the VMD configuration) which consumes relatively more energy than the other MD configurations. The membrane needed in the VMD should also have relatively smaller pore sizes to prevent pore wetting [10,107–110]. The order of permeate flux for various MD configurations is roughly in the order of: VMD N DCMD N SGMD N AGMD. The advantages and disadvantages of these configurations have been summarized in Table 6 [111–113]. The MD technology in general has received lots of attention from academia and industry during the past years due to its potential for desalination. In contrast to the high pressure needed by the reverse osmosis (RO) setup for sea water desalination, the MD can be performed at atmospheric pressure [114]. The amount of thermal energy needed for the MD process is also relatively lower than that of conventional thermal technologies [115]. In addition, the MD technology can be used for concentrating of juice, removing volatile compounds from water, treating wastewater, and removing of ammonia from water [112,116,117].

A large number of WPBLs made of polymers (e.g. PP, PS, PU, PVDFHFP, PTFE, …) and ceramics (e.g. zirconia, titania, …) have already been fabricated and tested for the DCMD process. However, PTFE and PVDF-HFP membranes have shown better results as they have lower surface free energy, higher thermal conductivity, better thermal stability, and good chemical stability (Table 7) [10,108]. Some of the general conclusions from these works are listed below [118–126]: - The permeate flux increases by increasing the temperature difference between the hot and cold sides roughly 1 Kg/m2/h for each 1 °C (See Fig. 16) and the membrane should have a very good thermal stability. - The permeate flux increases by increasing the flow rates of the feed and permeate sides. However, above a certain flow rate, the membrane starts leaking and the salt rejection rate deteriorates. - The type of membrane and gemometrical design of the module have noticable effects on the permeation flux [10]. Benchmarking the system with a commercially available WPBL is missing from the majority of the papers. - The condensation of water vapor within the pores of the membrane is a serious problem and can negatively affect the performance of the MD system. The chance of pore wetting will increase by increasing the membrane thickness and pore size. - For increasing the permeate flux, the pore size should be increased according to (Eqs. (5)–(10)). But, this can reduce the liquid entry pressure (Eq. (21)) and salt rejection rates. Therefore, the optimum

Fig. 14. (A) A picture of bilayered electrospun PCL/collagen vascular scaffold (4.75 mm of inner diameter and 0.4 mm of wall thickness). SEM micrographs of (B) the scaffold, (C) its outer layer, (D) its cross-sectional interface, and (E) inner layer of the bilayered vascular scaffold. Adapted with permission from [106].

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(a) DCMD : The WPBL is in direct contact with the hot feed side (e.g., sea water) and the cold permeate side (e.g., pure water)

(b) AGMD : an air gap between the WPBL and the cold side acts as an condenser to turn vapor into liquid.

(c) SGMD : a cold inert gas sweeps the transferred vapor out of the MD module. A condenser separates the vapor from the sweeping gas and turns it into liquid (product).

(d) VMD : the vapor is vacuumed out of the air gap. A condenser turns the vapor into liquid (product).

Fig. 15. Four different configurations of Membrane Distillation (MD) [10,113].

pore size of a membrane for this application should be determined. The average pore size of the membranes used for this application is typically 1 μm. The incorporation of various nanomaterials such as calcium carbonate [122], graphene oxide (GO) [123], carboxylated carbon nanotubes [126] at the surface of WPBMs has shown to be an effective way to enhance the permeate flux. The immobilization of graphene oxide (GO) on a PTFE membrane has enhanced the water desalination performance (i.e. permeate flux) in the DCMD for minimum 20% (Fig. 16) [123]. It has been proposed that the presence of polar groups (–OH and – COOH groups) at the surface of the WPBMs can enhance their interaction with water vapor molecules and facilitate their diffusion and permeation through the membrane (Fig. 17). Graphene has a very high in-plane thermal conductivity (upto 5300 W/m/K) [127]. The horizontally oriented GO platelets at the surface of the composite membrane are expected to improve the “in-plane” heat conduction and therefore increase the temperature polarization coeficient (TPC) of the DCMD process (Eq. (26)) while the total heat loss through the membrane remains unchanged due to the large surface−surface contact resistance between GO platelets [128].

TPC ¼

actual driving force ðinterfacialÞ Tmf −Tmp ¼ theoretical driving force ðbulkÞ Tbf −Tbp

ð26Þ

Where Tmf is the feed/membrane interface temperature, Tmp is the permeate/membrane interface temperature, and Tbf and Tbp are the bulk feed (hot side) and permeate (cold side) temperatures, respectively. The DCMD process is limited by heat and mass transfer depending on the TPC value. When the heat losses are high, the membrane/interface

Table 6 MD configurations and their advantages and disadvantages [111–113]. MD configuration

Advantages

VMD

▪ High flux ▪ Improved mass transfer ▪ Negligible conductive heat loss

DCMD

▪ Relatively high flux ▪ Simple design

SGMD

▪ Better mass transfer than AGMD ▪ Less heat loss

AGMD

▪ Flux close to that of DCMD ▪ Internal heat recovery ▪ Less heat loss due to conduction

Disadvantages ▪ Higher risk of membrane wetting ▪ Limited heat recovery ▪ High energy consumption ▪ High sensitivity to foulants ▪ High conduction heat loss ▪ Cold feed cannot be used as coolant ▪ Additional cost for sweeping gas ▪ External condenser with large volume ▪ Low sensitivity to foulants ▪ Air gap is limiting the mass transfer

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Table 7 Comparison of water desalination results from 35 g/l NaCl feed solution using a laboratory-scale DCMD setup. Type of WPBL Electrospun PS

Pore size (μm)

Thot−Tcold = ΔT (°C)

Permeate flux Kg/m2/h

Salt rejection (%)

Ref.

D ¼0.19 Dmax=0.44

70-17 = 53

19.4

99.9

[118]

80-17 = 63 50

31 51

99.9

[119] [120]

Electrospun PS

D ¼1.15 Dmax=2.08

Electrospun PVDF-HFP

D ¼1 Dmax=2.5

60-20 = 40

20

N98.5

Commercial PTFE

D ¼0.22 Not provided

60-20 = 40

20

99.9

60-17 = 43 70-17 = 53 80-17 = 63 63-20 = 43

40.9 60.7 85.6 22.8

99.9

[121]

99

[122]

73-20 = 53 83-20 = 63 60-20 = 40 70-20 = 50 80-20 = 60 60-20 = 40 70-20 = 50 80-20 = 60

33.5 49.4 49 66 69 74 86 94

Nonsolvent-induced phase separation PVDF 15% + ε-CPL 85%

Dope casting on support and wet coagulation of PVDF+CaCO3 NP

D ¼0.19 Dmax=0.33

Commercial PTFE

0.2

GO immobilized on PTFE

0.2

temperatures are different from the bulk temperatures and the TPC value is low [129]. For a well-designed DCMD system, this value will be close to unity and the system is mainly controlled by the mass transfer through the membrane. In order to achieve this, the heat transfer of both the feed and the permeate layers should be very close to the bulk temperatures. To minimize the heat loss by conduction through the membrane, one can also place an air gap between the membrane and the cold condensing side (i.e., the AGMD configuration). This reduces the heat loss by conduction and temperature polarization (i.e., TPC close to one). However, the air gap will be a barrier for the permeate flux and should be minimized inorder to optimize the system [10]. 3.5. Oil-water separation Separation is among the promising technologies for oil spill cleanup, oil purification, wastewater treatment, and etc. Therefore, there is a high demand towards the development of cheap, environmentally friendly, and recyclable/reusable filters that can separate oil from water in a large scale efficiently [130–134]. The filters should be able to separate water with high surface tension (above 70 mN/m) and organic solvents and oils with surface tensions typically below 35 mN/m (Table 8). Table 9 shows the results of oil separation using WPBLs. Some of the general conclusions from these studies can be listed below [25,28,138–145]: - Very high oil separation fluxes and efficiencies can be obtained by WPBLs in general even for the membranes with small pores. The oil separation fluxes can be enhanced by increasing the applied pressure and/or the pore size according to Eq. (21). In principle, the surface property and pore structure of WPBLs influence the directional movement of fluid through the capillary channels. The breakthrough pressure for an interwoven WPBL with predominant cylindrical texture can be calculated based on Eq. (27) [150–153]. When the HPH is above this pressure, the water will also permeate through the WPBL.

Pbreakthrough ¼

Pref: ¼

R:l ð1−Cos θÞ  cap     Pref: R Sin θ D2 1 þ 2 D

2γ ; lcap ¼ lcap

rffiffiffiffiffiffiffi γ ρg:

ð27Þ

99 99.9

[123]

99.9

Where θ is the contact angle between the liquid and the WPBL, R is the cylinder radius and 2D is the inter-cylinder spacing, γ is the surface tension of liquid, Pref. is the minimum possible differential pressure across a millimeter sized liquid droplet, lcap is the capillary length of liquid, ρ is the density of liquid, g is the acceleration due to gravity. - The Hagen–Poiseuille equation (Eq. (10)) can also be used for modeling the oil flux through a porous WPBL. The oil separation flux reduces with viscosity and increases with pore size of the WPBL [142,146]. - The separation of oil from oil-in-water emulsions stabilized with surfactants is quite chanllenging and relatively slower than that without a surfactant as emulsifier [25,142]. Surfactants reduce the surface tension of water and the hydrophobic pores get wet. As a result, the water can also pass through the hydrophobic layer (Eq. (27)). - Many of the reported oil-water separation results are based on a simple, laboratory-scale filtration unit with low Pbreakthrough (Eq. (27)). Such WPBLs cannot be used in industrial applications under high pressures [145,146,148,154–158]. - The incorporation of some additives (e.g., hydrophobic nano SiO2, graphene, etc.) in the structure of WPBLs can improve the oil filtration flux [25,141]. However, there are some papers showing that modifying the membrane can reduce the oil separation flux while increasing its separation efficiency [140]. If these additives improve the number and size of the pores during the production of WPBLs, they can improve the oil filtration flux. The additives may also alter the surface roughness and hydrophobicity which in return affect the separation efficiency and selectivity. - The WPBLs should be chemically resistant and mechanically strong to withstand any minor damage [131,159]. - The design of separation/filtration units has an important role on the reported oil fluxes [139,140,143]. A reliable benchmarking with commercially available WPBLs is missing from the majority of the relevant oil-water separation papers. Researchers may use some commercially available porous WPBLs with known characteristics for the test under the same experimental conditions. - The textile fabrics have been hydrophobized with various chemical treatments and used for oil-water separation. The stability of the treatment has been studied by measuring the WCA of the fabrics after repeated laundry cycles, adhesive tape peeling or immersing in various solvents [154–156,158,160]. Some of these chemical treatments can reduce the tensile strength of the fabrics [161].

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Fig. 16. Comparison between two WPBLs (PTFE and PTFE modified with graphene oxide) in the DCMD process. The effects of (a) the feed temperature and (b) the feed flow rate on water desalination permeate flux. Adapted with permission from [123].

- The WCA of the WPBLs decreases by repeated abrasion cycles [154,155,158,161]. - The oil-water separation efficiency of the membranes can be enhanced by polymer blending [162]. There is also demand for the development of WPBLs that are stretchable, healable, or responsive at various conditions [149,154,157,163]. Switchable oil-water separation layers have been developed that can filter oil from water or vice versa in response to change in temperature [142,148,164], pH [144,165–175], ammonia [149], electricity [172,176],

magnetic field [177], photo-thermal irradiation [178] or surrounding gas atmosphere (Fig. 18) [143]. Three examples of such switchable layers are provided in Table 10. The readers are directed to the following review papers to get more information on this topic [179,180].

4. Summary and future outlook Several standard test methods have been developed for the characterization of WPBLs. Some of these methods have their own limitations and shortcomings due to the high porosity and compressibility of the

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A.R. Tehrani-Bagha / Advances in Colloid and Interface Science 268 (2019) 114–135 Table 8 Surface tension of various solvents and oils at 24–25 °C [135–137].

Fig. 17. Scheme showing how the immobilization of graphene oxide on a WPBM can enhance the permeate flux or WVTR. Adapted with permission from [123].

layers. As an example, the thickness of WPBLs can be measured with different thickness gauges: micro screw, laser, ultrasonic, etc. It is important to note that a conventional thickness measurement such as micro screw gauge may introduce a systematic error and underestimate the thickness value. SEM is a useful instrument for visualizing the surface topography of the layers or measuring the fiber diameters. However, it has its own limitations for measuring the through pore size of the layers when they are not uniform in size and shape. Other techniques such as capillary flow porometry or mercury intrusion porosimetry can be satisfactorily used for measuring the pore size distribution of the layers by plotting Q vs. ΔP as it was discussed in Section 2. However, if the measured values of Q and ΔP are inaccurate, or the correction factor (β) in Eq. (21) is not wisely set, the calculated pore size values may not be trusted. One way to deal with this issue is to test a commercially available membrane with known pore size distribution before measuring the pore size of the main samples. The auto-calibration features should be also added to these techniques to make them more reliable. There are poor correlations between the main standard test methods for measuring the WVTR of WPBLs. The suppliers of WPBFs often claim very high WVTR for their products without mentioning the standard test method used for the measurement. Thus, there is a need for the development of a reliable and meaningful standard test method for measuring the WVTR of WPBLs. WPBLs are generally tested in unstretched planar state. In reality, these layers should have very high HPH and WVTR when they are stretched or bended or covered with water under very hot or cold conditions. Characterization of the WPBLs after repeated washing cycles, repeated loading/ unloading cycles, abrasion test, sweating test, etc. can provide more information about the durability and performance of the layers.

Materials

Surface tension (mN/m)

Water Butyl acetate (ester) Corn oil Decane (aliphatic hydrocarbon) Diesel fuel Heavy crude oil Isopropanol Light crude oil Liquid paraffin Liquid petroleum Mineral oil Olive oil o-xylene (aromatic hydrocarbon) Soybean oil

72 25 33 24 25 25–35 23 32 26–28 33 30 33 30 32

The main and most important application of WPBLs is in textile clothing (e.g., rain coats, skiwear, footwear, tents, etc.). Three common WPBFs in the market are based on PTFE (~34%), PU (33%), and polyetsr (21%). The WPBF market is estimated to reach above $2 billion by 2024. The general trend is to make these layers lighter, durable, cheaper, healable, and self-sealable [66]. Researchers are continuously developing new WPBLs for other applications (i.e., MFC, MD, oil-water separation, biomedical engineering and wound healing). One of the new applications of WPBLs is in air-toair heat exchangers for regulating the humidity of incoming cold air. These heat exchangers are recommended in cold climates to transfer heat and humidity between the exhaust and supply air to provide acceptable indoor thermal comfort. The common problem in such systems is the frosting of water vapor on the surface of the WPBL when the temperature is below the freezing point of the surrounding humid air. WPBLs with anti-frosting properties are demanded for this application [186–189]. Inspired by icephobicity of Penguins' feathers, researchers have developed a WPBL made of polyimide nanofibers using high-pressure electrostatic spinning that resists against ice formation. Although penguins live in extremely cold places, frost and ice are seldom found on their feathers [190]. This interesting research area needs further investigation and development. The addition of other functionalities and producing responsive so called “smart” WPBLs is another interesting research area. Stomatex® made of Neoprene insulating foam, developed initially by AkzoNobel, is one of those smart WPBLs that has been inspired by nature. Inspired by stomatal closure feature of plant leaves, the domes of Stomatex® can open or close depending on the relative humidity and regulate the WVTR (Fig. 19). It has been claimed that these layers have a higher WVTR at a higher level of activity [65,83,191–193]. Although there are several reports on stimuli-responsive membranes whose filtration performance changes in response to pH, temperature, light, salt, or magnetic field [180], research is very limited on stomatainspired WPBLs that can selectively control the transfer of humidity and temperature to achieve indoor climate regulation [20,21]. A reversible self-actuated thermo-responsive PTFE porous membrane with pores filled with poly(N-isopropylacrylamide) (PNIPAm) is one of these research works. The pores of this membrane can open and close completely and reversibly in response to temperature (Fig. 19c) [20]. We should expect to witness WPBLs with smart pores/gates that can open to increase the level of WVTR on demand or in response to internal/external stimuli. In the future, military, police, and firefighter coats will probably have electronic sensors, gates, and fans to regulate the temperature and breathability. The environmental concerns regarding perfluorinated and halogenated compounds used in the production of WPBLs are expected to

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131

Table 9 Comparison of oil-water separation results using different WPBLs. Type of WPBLs Electrospun PVDF

Pore size (μm)

Oil-Water system

Oil flux L/m2/h Efficiency (%)

Ref.

D ¼1 Dmax=1.8

Hexane-W

~2400

~60

[138]

10 ml:10 ml Hexane-W

~3080

~80

30

79–318

91 Operating P = .1 MPa 87 Operating P = .1 MPa N99.6

[28]

20–65

N90

[141]

60–240

N99.9

[142]

1637-2982

99

[25]

Electrospun PU

D ¼0.6 Dmax=2.25 Commercial Nylon microfilter dip-coated in PTFE D≤0.45 dispersion Comemrcial PTFE D ¼0.45 a

Porous PVDF membranes by tape-casting, drying, N.P. b1 by SEM immersing and skimming PET Tubular braid reinforced with PVDF and graphene by a dry−wet spinning process thermo-responsive (PVDF)/(PSMA) composite membranes by casting

D ¼0.16 Dmax=0.19 0.5–1

(ECTFE)-SiO2 Hybrid porous membrane by thermally induced phase separation with hydrophobic SiO2 as the additive Electrospun PMMA-co-PDEAEMA

0.24

Electrospun PDMS-b-P4VP

N.P.a

Filter paper with average pore size 17–25 μm spray-coated with ABS copolymer Cu mesh covered with polydopamin and ODA

N.P.a b2 by SEM

Nickle foam treated with dopamoin and ODA

PMMA-b-PNIPAAm electrospun/casted on a stainless steel mesh Cotton fabric dip-coated in a suspension of hydrophobic nickle stearate particles

a

N.P.a

10 ml:10 ml 1%W/O emulsion Inkam-1 coolant fluid 1%W/O emulsion Oil:I-20 industrial oil, Emulsifier: SDS and starch 1%W/O emulsions Droplets 0.1–1 μm Oils: hexane, cyclohexane, isooctane, chloroform Kerosene:W 100 ml:100 ml Oil:W 114:1 Oils: Toluene, chloroform Emulsifier: Span 80 Oil:water 114:1 Oils: kerosene, chloroform and toluene emulsifier: none Emulsifier: Span 80 O:W 40 ml:40 ml Oils: Hexan Petroleum ether Heptane O:W 100 ml:100 ml Oil:hexane Span 80-stabilized water-in-oil emulsions

Square ~180 × 180 μm2 Hexane Disel Olive oil 200 μm Lubricating oil Motor oil Silicone oil N.P.a O:W 100 ml:100 ml Oils: petroleum ether, heptane, gasoline N.P.a Oil:W 100:1 Oils: Toluene Emulsifier: Span 80 2 g/l

2760

[139] [140]

338–417 95 14,000–17,000 99.9

[143]

9000

N98

[144]

4000–13,000

99.9

[145]

6382 4958 754 2823 2337 1844 ~4300 ES

N99

[146]

N.P.a

[147]

N98

[148]

~1600 casted 486

N98 N98

[149]

N.P.: Not provided.

shift the market/research towards greener products. Thus, the research should be focused on the development of more sustainable and ecofriendly chemicals and processes. This entails finding good replacements for PTFE based WPBLs that have over 30% of the market share.

Some companies have already developed eco-friendly WPBLs based on recycled polyethylene terephthalate bottles and it is forecasted that this trend will continue in to rise the future to make WPBLs from sustainable starting materials and processes [66].

Fig. 18. Stimuli responsive or switchable layers for oil-water separation. These layers can become either super-hydrophobic & oleophilic or super-hydrophilic and oleophobic in response to temperature, pH, gas purging, electricity, light, etc. [179,181–185].

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Table 10 Oil-water separation flux by smart switchable WPBLs under various conditions. Responsive separation layers

Oil (Hexane) flux L/m2/h

Water flux L/m2/h

Reference

Temperature-responsive electrospun PMMA-b-PNIPAAm pH-Responsive electrospun (PDMS-b-P4VP) CO2 Responsive electrospun PMMA-co-PDEAEMA

4200 at high temperature 9000 at water pH 4 17,000 In air/N2 bubbling

9400 at room temperature 27,000 at water pH 4 9554 Under CO2 purging

[148] [144] [143]

Fig. 19. Smart membranes: (a) A picture showing the cross section of a leaf and its transpiration through stomata guard cells; (b) The respirational pore structure of a leaf (stomata) that can open and close in response to light, temperature and humidity for gas exchange; (c) A scheme showing a stomata-inspired membrane with reversible self-actuated thermo-responsive pores; (d) Top and cross sectional views of a synthetic pore structure that can open and close in response to temperature. A smart WPBL can be fabricated with this concept.

5. Conclusion remarks The applications of WPBLs are quite diverse and span from waterproof breathable clothing (e.g., raincoat, tent, etc.) to advanced biomedical applications and tissue engineering. Researchers have also used them in MFC, MD, and for oil-water separation. The pore size distribution has a very important effect on the performance of hydrophobic porous layers and affects both their WVTR (Eqs. (5)–(10)) and HPH (Eq. (21)). These parameters are in contrast with each other; therefore, the pore size of the layer should be carefully engineered to achieve the best performance for various applications. There are poor correlations between the standard test methods for measuring the WVTR of WPBLs. The obtained values are sometimes an order of magnitude different (Tables 2 and 3). Thus, there is a need for more reliable and meaningful standard test methods. The reported HPH values of WPBLs are also typically lower than the predicted values

by (Eq. (21)) and a correction factor accounting for the shape factor and tortuosity of the pores should be considered. Hydrophobic porous membranes with an the average pore size b1 μm are commonly used for the MD process. To enhance the permeation flux, WPBMs can be coated with micro/nanoparticles (e.g., graphene oxide, carboxylated CNTs, etc.). This is a promising domain for the development of new and advanced composite membranes with enhanced performance. WPBLs with very large pore sizes have been used successfully for oilwater separation in laboratory-scale. When the HPH is above Pbreakthrough (Eq. (27)), the water can also pass through the WPBL. This is an important consideration for scaling up the oil-water separation process. The research trend is towards the development of durable and long-lasting layers with a high separation flux. The challenges remain in the development of layers capable of efficiently separating viscose oils from water in the presence of surfactants.

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There is a need for developing stomata-inspired humidity-responsive WPBLs for various applications such as WPB clothing, air-to-air heat exchanger, filtration, etc. Such WPBLs can regulate the temperature and humidity smarter as a function of humidity or temperature.

Acknowledgement The author is grateful for the grants received from the Masri Institute and URB-AUB. There is no conflict of interest. References [1] Fung W. Coated and laminated textiles in sportswear. In: Shishoo R, editor. Textiles in sport. Woodhead Publishing Ltd; 2002. [2] Keighley JH. Breathable fabrics and comfort in clothing. J Coated Fabrics 1985;15: 89–104. [3] Gugel M. All about waterproof fabrics. http://www.paddypallin.com.au/blog/allabout-waterproof-fabrics/; 2017. [4] Lomax GR. Breathable polyurethane membranes for textile and related industries. J Mater Chem 2007;17:2775–84. [5] Painter CJ. Waterproof, breathable fabric laminates: a perspective from film to market place. J Coated Fabrics 1996;26:107–30. [6] Lomax GR. Hydrophilic polyurethane coatings. J Coated Fabrics 1990;20:88–107. [7] Khayet M, Matsuura T. Pervaporation and vacuum membrane distillation processes: modeling and experiments. Aiche J 2004;50:1697–712. [8] Mulder M. Basic principles of membrane technology. Dordrecht: Kluwer; 1991. [9] Bui N, Meshot ER, Kim S, Pena J, Gibson PW, Wu KJ, et al. Ultrabreathable and protective membranes with Sub-5 nm carbon nanotube pores. Adv Mater 2016;28: 5871–7. [10] Khayet-Souhaimi M, Matsuura T. Membrane distillation: principles and applications. Elsevier; 2011. [11] Mukhopadhyay A, Midha VK. Waterproof breathable fabrics. In: Horrocks AR, Anand SC, editors. Handbook of technical textiles. Woodhead Publishing Ltd; 2016. p. 27–55. [12] Holmes DA. Waterproof breathable fabrics. In: Horrocks AR, Anand SC, editors. Handbook of technical textiles. Woodhead Publishing Ltd; 2000. p. 282–315. [13] Kurumada K, Kitamura T, Fukumoto N, Oshima M, Tanigaki M, Kanazawa S. Structure generation in PTFE porous membranes induced by the uniaxial and biaxial stretching operations. J Membrane Sci 1998;149:51–7. [14] Lalia BS, Kochkodan V, Hashaikeh R, Hilal N. A review on membrane fabrication: structure, properties and performance relationship. Desalination 2013;326:77–95. [15] Singha K. A review on coating & lamination in textiles: processes and applications. Am J Polym Sci 2012;2:39–49. [16] Lee A, Elam JW, Darling SB. Membrane materials for water purification: design, development, and application. Environ Sci Water Res 2016;2:17–42. [17] Maksoud FJ, Lameh M, Fayyad S, Ismail N, Tehrani-Bagha AR, Ghaddar N, et al. Electrospun waterproof breathable membrane with a high level of aerosol filtration. J Appl Polym Sci 2018;135:45660. [18] Kim SY, Kanamori T, Shinbo T. Preparation of thermal-responsive poly(propylene) membranes grafted with N-isopropylacrylamide by plasma-induced polymerization and their water permeation. J Appl Polym Sci 2002;84:1168–77. [19] Xie R, Chu LY, Chen WM, Xiao W, Wang HD, Qu JB. Characterization of microstructure of poly (N-isopropylacrylamide)-grafted polycarbonate track-etched membranes prepared by plasma-graft pore-filling polymerization. J Membrane Sci 2005;258:157–66. [20] Park Y, Gutierrez MP, Lee LP. Reversible self-actuated thermo-responsive pore membrane. Sci Rep UK 2016;6. [21] Kim H, Lee SJ. Fabrication of triple-parted stomata-inspired membrane with stimulus-responsive functions. Sci Rep-UK 2016;6. [22] Lindsey J. New fabric tech could be outerwear's biggest advance in 40 years. https://www.wired.com/2015/02/voormi-core-technology/; 2015. [23] ASTM. Annual book of ASTM standards. https://www.astm.org/; 2018. [24] Czichos H, Saito T, Smith LE. Springer handbook of materials measurement methods. Berlin Heidelberg: Springer; 2007. [25] Pan J, Xiao CF, Huang QL, Liu HL, Zhang T. ECTFE hybrid porous membrane with hierarchical micro/nano-structural surface for efficient oil/water separation. J Membrane Sci 2017;524:623–30. [26] Jeong WY, An SK. Mechanical properties of breathable waterproof fabrics with seaming and sealing processes. Fiber Polym 2004;5:316–20. [27] Jeong WY, An SK. Seam characteristics of breathable waterproof fabrics with various finishing methods. Fiber Polym 2003;4:71–6. [28] Chen LW, Si YF, Zhu H, Jiang T, Guo ZG. A study on the fabrication of porous PVDF membranes by in-situ elimination and their applications in separating oil/water mixtures and nano-emulsions. J Membrane Sci 2016;520:760–8. [29] Li Y, Zhu ZG, Yu JY, Ding B. Carbon nanotubes enhanced fluorinated polyurethane macroporous membranes for waterproof and breathable application. ACS Appl Mater Inter 2015;7:13538–46. [30] Fowkes FM. Attractive forces at interfaces. Indus Eng Chem 1964;56:40–52. [31] Owens DK, Wendt RC. Estimation of the surface free energy of polymers. J Appl Polym Sci 1969;13:1741–7. [32] Van-Oss CJ, Chaudhury MK, Good RJ. Interfacial Lifshitz-van der Waals and polar interactions in macroscopic systems. Chem Rev 1988;88:927–41.

133

[33] Schuster JM, Schvezov CE, Rosenberger MR. Analysis of the results of surface free energy measurement of Ti6Al4V by different methods. Procedia Mater Sci 2015;8: 732–41. [34] Ishino C, Okumura K. Wetting transitions on textured hydrophilic surfaces. Eur Phys J E Soft Matter 2008;25:415–24. [35] Wenzel RN. Resistance of solid surfaces to wetting by water. Indus Eng Chem 1936; 28:988–94. [36] Bico J, Thiele U, Quéré D. Wetting of textured surfaces. Colloids Surf A Physicochem Eng Asp 2002;206:41–6. [37] Michielsen S, Lee HJ. Design of a superhydrophobic surface using woven structures. Langmuir 2007;23:6004–10. [38] Chu ZL, Feng YJ, Seeger S. Oil/water separation with selective superantiwetting/ superwetting surface materials. Angew Chem Int Ed 2015;54:2328–38. [39] Yuan Y, Lee TR, Bracco G, Holst B. Contact angle and wetting properties. Surface science techniques; 2013. p. 3–34. [40] Zhao YY, Wang XM, Yang HW, Xie YFF. Effects of organic fouling and cleaning on the retention of pharmaceutically active compounds by ceramic nanofiltration membranes. J Membrane Sci 2018;563:734–42. [41] Wang YX, Al Shakhshir S, Li XG. Development and impact of sandwich wettability structure for gas distribution media on PEM fuel cell performance. Appl Energ 2011;88:2168–75. [42] Shahabadi SMS, Brant JA. Bio-inspired superhydrophobic and superoleophilic nanofibrous membranes for non-aqueous solvent and oil separation from water. Sep Purif Technol 2019;210:587–99. [43] Lakhotia SR, Mukhopadhyay M, Kumari P. Cerium oxide nanoparticles embedded thin-film nanocomposite nanofiltration membrane for water treatment. Sci Rep UK 2018;8. [44] Nakao S. Determination of pore-size and pore-size distribution 3. Filtration membranes. J Membrane Sci 1994;96:131–65. [45] Khayet M, Matsuura T. Determination of surface and bulk pore sizes of flat-sheet and hollow-fiber membranes by atomic force microscopy, gas permeation and solute transport methods. Desalination 2003;158:57–64. [46] Vermisoglou EC, Pilatos G, Romanos GE, Karanikolos GN, Boukos N, Mertis K, et al. Synthesis and characterisation of carbon nanotube modified anodised alumina membranes. Micropor Mesopor Mater 2008;110:25–36. [47] Ahmad NA, Leo CP, Ahmad AL, Nur Izwanne M. Swelling reduction of polyvinylidenefluoride hollow fiber membrane incorporated with silicoaluminophosphate-34 zeotype filler for membrane gas absorption. Sep Purif Technol 2019;212:941–51. [48] Lu C-L, Chang C-P, Guo Y-H, Yeh T-K, Su Y-C, Wang P-C, et al. High-performance and low-leakage phosphoric acid fuel cell with synergic composite membrane stacking of micro glass microfiber and nano PTFE. Renew Energy 2019;134:982–8. [49] Meng T, Xie R, Chen YC, Cheng CJ, Li PF, Ju XJ, et al. A thermo-responsive affinity membrane with nano-structured pores and grafted poly(Nisopropylacrylamide) surface layer for hydrophobic adsorption. J Membrane Sci 2010;349:258–67. [50] Khayet M. The effects of air gap length on the internal and external morphology of hollow fiber membranes. Chem Eng Sci 2003;58:3091–104. [51] Huang B, Vyas C, Roberts I, Poutrel Q-A, Chiang W-H, Blaker JJ, et al. Fabrication and characterisation of 3D printed MWCNT composite porous scaffolds for bone regeneration. Mater Sci Eng C 2019;98:266–78. [52] Xu LY, Wu GG, Zhao M, Yin B, Yang MB. Effect of external field on the lamellar crystalline structure and properties of poly(4-methyl-1-pentene) casting film. J Appl Polym Sci 2019;136. [53] Tomlins P, Grant P, Mikhalovsky S, James S, Mikhalovska L. Measurement of pore size and porosity of tissue scaffolds. Am Soc Test Mater 2004;1452:3–11. [54] Cai JC, Perfect E, Cheng CL, Hu XY. Generalized modeling of spontaneous imbibition based on Hagen-Poiseuille flow in tortuous capillaries with variably shaped apertures. Langmuir 2014;30:5142–51. [55] Jena A, Gupta K. Liquid extrusion techniques for pore structure evaluation of nonwovens. Int Nonwovens J 2003;12:45–53. [56] Washburn EW. The dynamics of capillary flow. Phys Rev 1921;17:273–83. [57] Abuzade RA, Zadhoush A, Gharehaghaji AA. Air permeability of electrospun polyacrylonitrile nanoweb. J Appl Polym Sci 2012;126:232–43. [58] McCullough EA, Kwon M, Shim H. A comparison of standard methods for measuring water vapour permeability of fabrics. Meas Sci Technol 2003;14:1402–8. [59] Huang JH, Qian XM. Comparison of test methods for measuring water vapor permeability of fabrics. Text Res J 2008;78:342–52. [60] Huang M. A device for characterizing water vapor permeability of polymer membranes. Int J Polym Anal Ch 2008;13:37–48. [61] Lomax R. Coating and laminating fabrics for cold weather apparel. Woodhead Publ Text 2009;93:56–83. [62] Huang JH. A new test method for determining water vapor transport properties of polymer membranes. Polym Test 2007;26:685–91. [63] Adamson AW, Gast AP. Physical chemistry of surfaces. . 6th ed.Wiley; 1997. [64] Calvo JI, Pradanos P, Hernandez A, Bowen WR, Hilal N, Lovitt RW, et al. Bulk and surface characterization of composite UF membranes atomic force microscopy, gas adsorption-desorption and liquid displacement techniques. J Membrane Sci 1997;128:7–21. [65] Mukhopadhyay A, Midha VK. A review on designing the waterproof breathable fabrics part I: fundamental principles and designing aspects of breathable fabrics. J Indus Text 2008;37:225–62. [66] Grand-View-Research. Waterproof Breathable Textiles (WBT) market analysis. http://www.grandviewresearch.com/press-release/global-waterproof-breathabletextiles-industry; 2016. [Report ID: 978-971-68038-68316-68034].

134

A.R. Tehrani-Bagha / Advances in Colloid and Interface Science 268 (2019) 114–135

[67] Mack GW, Nadel ER. Body fluid balance during heat stress in humans. In: Fregly MJ, Blatteis CM, editors. Handbook of physiology. Section 4: environmental physiology. New York: Oxford University Press; 1996. p. 187–214. [68] Rother M, Barmettler J, Reichmuth A, Araujo JV, Rytka C, Glaied O, et al. Self-sealing and puncture resistant breathable membranes for water-evaporation applications. Adv Mater 2015;27 [6620-+]. [69] Amaral AJR, Pasparakis G. Stimuli responsive self-healing polymers: gels, elastomers and membranes. Polym Chem UK 2017;8:6464–84. [70] Nehache S, Tyagi P, Semsarilar M, Deratani A, Phan TNT, Gigmes D, et al. Translocation across a self-healing block copolymer membrane. Soft Matter 2017;13:6689–93. [71] Huang CH, Liu YL. Self-healing polymeric materials for membrane separation: an example of a polybenzimidazole-based membrane for pervaporation dehydration on isopropanol aqueous solution. RSC Adv 2017;7:38360–6. [72] Getachew BA, Kim SR, Kim JH. Self-healing hydrogel pore-filled water filtration membranes. Environ Sci Technol 2017;51:905–13. [73] Kim SR, Getachew BA, Park SJ, Kwon OS, Ryu WH, Taylor AD, et al. Toward microcapsule-embedded self-healing membranes. Environ Sci Tech Lett 2016;3:216–21. [74] Chen L, Si LP, Wu F, Chan SY, Yu PY, Fei B. Electrical and mechanical self-healing membrane using gold nanoparticles as localized “nano-heaters”. J Mater Chem C 2016;4:10018–25. [75] Tyagi P, Deratani A, Bouyer D, Cot D, Gence V, Barboiu M, et al. Dynamic interactive membranes with pressure-driven tunable porosity and self-healing ability. Angew Chem Int Ed 2012;51:7166–70. [76] Hunter L, Fan J. In: Fan J, Hunter L, editors. Waterproofing and breathability of fabrics and garments. Woodhead Publ Text, Woodhead Publishing Ltd; 2009. p. 283–308. [77] Jassal M, Agrawal AK. Intelligent breathable coatings and laminates for textile applications. Smart textile coatings and laminates; 2010. p. 189–221. [78] Nayak R, Padhye R, Kyratzis I, Truong YB, Arnold L. Recent advances in nanofibre fabrication techniques. Text Res J 2012;82:129–47. [79] Ismail N, Maksoud FJ, Ghaddar N, Ghali K, Tehrani-Bagha AR. Simplified modeling of the electrospinning process from the stable jet region to the unstable region for predicting the final nanofiber diameter. J Appl Polym Sci 2016;133:44112. [80] Ismail N, Maksoud FJ, Ghaddar N, Ghali K, Tehrani-Bagha AT. Effect of processing parameters on the performance of nanofiber filtration efficiency and morphology. J Aerosol Sci 2017;5 Revised manuscript submitted March. [81] Maksoud FJ, Lameh M, Fayyad S, Ismail N, Tehrani-Bagha AR, Ghaddar N, et al. Electrospun waterproof breathable membrane with a high level of aerosol filtration. J Appl Polym Sci 2018;135. [82] Holmes DA. Performance characteristics of waterproof breathable fabrics. J Indus Text 2000;29:306–16. [83] Midha VK, Mukhopadhyay A. Smart breathable coatings for textiles. In: Hu J, editor. Active coatings for smart textiles. Woodhead Publishing Ltd; 2016. p. 81–111. [84] Mukhopadhyay A, Midha VK. A review on designing the waterproof breathable fabrics part II: construction and suitability of breathable fabrics for different uses. J Indus Text 2008;38:17–41. [85] Majlan EH, Rohendi D, Daud WRW, Husaini T, Haque MA. Electrode for proton exchange membrane fuel cells: a review. Renew Sustain Energy Rev 2018;89:117–34. [86] Truong VM, Wang CL, Yang MK, Yang H. Effect of tunable hydrophobic level in the gas diffusion substrate and microporous layer on anion exchange membrane fuel cells. J Power Sources 2018;402:301–10. [87] Shrestha P, Banerjee R, Lee J, Ge N, Muirhead D, Liu H, et al. Hydrophilic microporous layer coatings for polymer electrolyte membrane fuel cells operating without anode humidification. J Power Sources 2018;402:468–82. [88] Shakerinejad E, Kayhani MH, Nazari M, Tamayol A. Increasing the performance of gas diffusion layer by insertion of small hydrophilic layer in proton-exchange membrane fuel cells. Int J Hydrogen Energ 2018;43:2410–28. [89] Hunsom M, Kaewsai D, Kannan AM. Recent developments in bifunctional air electrodes for unitized regenerative proton exchange membrane fuel cells - a review. Int J Hydrogen Energ 2018;43:21478–501. [90] Santoro C, Arbizzani C, Erable B, Ieropoulos I. Microbial fuel cells: from fundamentals to applications. A review. J Power Sources 2017;356:225–44. [91] Xing DF, Tang Y, Mei XX, Liu BF. Electricity generation of microbial fuel cell with waterproof breathable membrane cathode. J Power Sources 2015;300:491–5. [92] Fu Z, Li K, Pu L, Ge B, Chen Z. Waterproof breathable membrane used as gas diffusion layer in activated carbon air cathode microbial fuel cells. Fuel Cells 2016;16: 839–44. [93] Cheng S, Liu H, Logan BE. Increased performance of single-chamber microbial fuel cells using an improved cathode structure. Electrochem Commun 2006;8:489–94. [94] Zhang XY, He WH, Yang WL, Liu J, Wang QY, Liang P, et al. Diffusion layer characteristics for increasing the performance of activated carbon air cathodes in microbial fuel cells. Environ Sci Water Res 2016;2:266–73. [95] Chen T, Liu SH, Zhang JW, Tang MN. Study on the characteristics of GDL with different PTFE content and its effect on the performance of PEMFC. Int J Heat Mass Traner 2019;128:1168–74. [96] Kakaee AH, Molaeimanesh GR, Garmaroudi MHE. Impact of PTFE distribution across the GDL on the water droplet removal from a PEM fuel cell electrode containing binder. Int J Hydrogen Energ 2018;43:15481–91. [97] Moosavi SM, Niffeler M, Gostick J, Haussener S. Transport characteristics of saturated gas diffusion layers treated with hydrophobic coatings. Chem Eng Sci 2018; 176:503–14. [98] Xu R, Xia HS, He WF, Li ZC, Zhao J, Liu B, et al. Controlled water vapor transmission rate promotes wound-healing via wound re-epithelialization and contraction enhancement. Sci Rep UK 2016;6. [99] Silver FH, Wang MC. A review of the etiology and treatment of skin ulcers with wound dressings - comparison of the effects of occlusive and nonocclusive dressings. J Long-Term Eff Med 1992;2:267–88.

[100] Xia DL, Chen YP, Wang YF, Li XD, Bao N, He H, et al. Fabrication of waterproof, breathable composite liquid dressing and its application in diabetic skin ulcer repair. Adv Skin Wound Care 2016;29:499–508. [101] Chen XS, Ou TW, Zhang J, Li JX, Chen B, Yu HX, et al. Histological and mechanical properties of autologous living tissue biotubes. Exp Ther Med 2013;5:1613–8. [102] Pashneh-Tala S, MacNeil S, Claeyssens F. The tissue-engineered vascular graft-past, present, and future. Tissue Eng B Res 2016;22:68–100. [103] Bonardelli S, Nodari F. Hybrid Gore vascular graft: A new tool for the hybrid treatment of Thoraco-abdominal aneurysms. J Vasc Med Surg 2015;03. [104] Zhou M, Qiao W, Liu Z, Shang T, Qiao T, Mao C, et al. Development and in vivo evaluation of small-diameter vascular grafts engineered by outgrowth endothelial cells and electrospun chitosan/poly(epsilon-caprolactone) nanofibrous scaffolds. Tissue Eng A 2014;20:79–91. [105] Kajbafzadeh AM, Khorramirouz R, Kameli SM, Hashemi J, Bagheri A. Decellularization of human internal mammary artery: biomechanical properties and histopathological evaluation. Biores Open Acc 2017;6:74–84. [106] Ju YM, Ahn H, Arenas-Herrera J, Kim C, Abolbashari M, Atala A, et al. Electrospun vascular scaffold for cellularized small diameter blood vessels: a preclinical large animal study. Acta Biomater 2017;59:58–67. [107] Eykens L, De Sitter K, Dotremont C, Pinoy L, Van der Bruggen B. Membrane synthesis for membrane distillation: a review. Sep Purif Technol 2017;182:36–51. [108] Ashoor BB, Mansour S, Giwa A, Dufour V, Hasan SW. Principles and applications of direct contact membrane distillation (DCMD): a comprehensive review. Desalination 2016;398:222–46. [109] Abu-Zeid MA, Zhang YQ, Dong H, Zhang L, Chen HL, Hou L. A comprehensive review of vacuum membrane distillation technique. Desalination 2015;356:1–14. [110] Alkhudhiri A, Darwish N, Hilal N. Membrane distillation: a comprehensive review. Desalination 2012;287:2–18. [111] Chen HY, Wu CR, Jia Y, Wang X, Lu XL. Comparison of three membrane distillation configurations and seawater desalination by vacuum membrane distillation. Desalin Water Treat 2011;28:321–7. [112] Ding ZW, Liu LY, Li ZM, Ma RY, Yang ZR. Experimental study of ammonia removal from water by membrane distillation (MD): the comparison of three configurations. J Membrane Sci 2006;286:93–103. [113] Eykens L, Reyns T, De Sitter K, Dotremont C, Pinoy L, Van der Bruggen B. How to select a membrane distillation configuration? Process conditions and membrane influence unraveled. Desalination 2016;399:105–15. [114] Bottino A, Comite A, Ferrari F, Jezowska A, Voena A, Capannelli G. Comparison between reverse osmosis and membrane distillation for bilge water treatment. Proc Eng 2012;44:1700–2. [115] Plattner J, Naidu G, Wintgens T, Vigneswaran S, Kazner C. Fluoride removal from groundwater using direct contact membrane distillation (DCMD) and vacuum enhanced DCMD (VEDCMD). Sep Purif Technol 2017;180:125–32. [116] Mehta GD. Comparison of membrane processes with distillation for alcohol water separation. J Membrane Sci 1982;12:1–26. [117] Carnevale MC, Gnisci E, Hilal J, Criscuoli A. Direct contact and vacuum membrane distillation application for the olive mill wastewater treatment. Sep Purif Technol 2016;169:121–7. [118] Ke H, Feldman E, Guzman P, Cole J, Wei QF, Chu B, et al. Electrospun polystyrene nanofibrous membranes for direct contact membrane distillation. J Membrane Sci 2016;515:86–97. [119] Li X, Wang C, Yang Y, Wang XF, Zhu MF, Hsiao BS. Dual-biomimetic Superhydrophobic electrospun polystyrene nanofibrous membranes for membrane distillation. ACS Appl Mater Inter 2014;6:2431–8. [120] Tijing LD, Woo YC, Johir MA, Choi JS, Shon HK. A novel dual-layer bicomponent electrospun nanofibrous membrane for desalination by direct contact membrane distillation. Chem Eng J 2014;256:155–9. [121] Xiao TH, Wang P, Yang X, Cai XH, Lu J. Fabrication and characterization of novel asymmetric polyvinylidene fluoride (PVDF) membranes by the nonsolvent thermally induced phase separation (NTIPS) method for membrane distillation applications. J Membrane Sci 2015;489:160–74. [122] Hou DY, Dai GH, Fan H, Wang JN, Zhao CW, Huang HJ. Effects of calcium carbonate nano-particles on the properties of PVDF/nonwoven fabric flat-sheet composite membranes for direct contact membrane distillation. Desalination 2014;347: 25–33. [123] Bhadra M, Roy S, Mitra S. Desalination across a graphene oxide membrane via direct contact membrane distillation. Desalination 2016;378:37–43. [124] Shirazi MMA, Kargari A, Tabatabaei M. Evaluation of commercial PTFE membranes in desalination by direct contact membrane distillation. Chem Eng Process 2014; 76:16–25. [125] Ahmad HM, Khalifa AE, Antar MA. Water desalination using direct contact membrane distillation system. Proceedings of the ASME international mechanical engineering congress and exposition, 2015, vol. 6a. ; 2016. [126] Bhadra M, Roy S, Mitra S. Flux enhancement in direct contact membrane distillation by implementing carbon nanotube immobilized PTFE membrane. Sep Purif Technol 2016;161:136–43. [127] Balandin AA, Ghosh S, Bao WZ, Calizo I, Teweldebrhan D, Miao F, et al. Superior thermal conductivity of single-layer graphene. Nano Lett 2008;8:902–7. [128] Jung H, Yu S, Bae NS, Cho SM, Kim RH, Cho SH, et al. High through-plane thermal conduction of graphene nanoflake filled polymer composites melt-processed in an L-shape kinked tube. ACS Appl Mater Inter 2015;7:15256–62. [129] Qtaishat M, Matsuura T, Kruczek B, Khayet M. Heat and mass transfer analysis in direct contact membrane distillation. Desalination 2008;219:272–92. [130] Padaki M, Murali RS, Abdullah MS, Misdan N, Moslehyani A, Kassim MA, et al. Membrane technology enhancement in oil-water separation. A review. Desalination 2015;357:197–207.

A.R. Tehrani-Bagha / Advances in Colloid and Interface Science 268 (2019) 114–135 [131] Gupta RK, Dunderdale GJ, England MW, Hozumi A. Oil/water separation techniques: a review of recent progresses and future directions. J Mater Chem A 2017;5:16025–58. [132] Ma LN, Dong XF, Chen ML, Zhu L, Wang CX, Yang FL, et al. Fabrication and water treatment application of carbon nanotubes (CNTs)-based composite membranes: a review. Membranes 2017;7. [133] Dickhout JM, Moreno Y, Biesheuvel PM, Boels L, Lanunertink RGH, de Vos WM. Produced water treatment by membranes: a review from a colloidal perspective. J Colloid Interf Sci 2017;487:523–34. [134] Otitoju TA, Ahmad AL, Ooi BS. Polyvinylidene fluoride (PVDF) membrane for oil rejection from oily wastewater: a performance review. J Water Process Eng 2016;14: 41–59. [135] Dorshow RB. The measurement of interfacial-tension and viscosity in crude-oil Systems at elevated-temperatures and pressures using surface laser-light scattering. Coll Col Ser 1992;50:97–105. [136] Halpern A. The surface tension of oils. J Phys Colloid Chem 1949;53:895–7. [137] Harvey EH. The surface tension of crude oils. Ind Eng Chem 1925;17:85. [138] Gu J, Gu H, Zhang Q, Zhao Y, Li N, Xiong J. Sandwich-structured composite fibrous membranes with tunable porous structure for waterproof, breathable, and oilwater separation applications. J Colloid Interface Sci 2017;514:386–95. [139] Fazullin DD, Mavrin GV, Nasyrov IA, Shaikhiev IG. Dynamic membranes of nylonPtfe for separation of water-oil emulsions. J Fundam Appl Sci 2017;9:1441–9. [140] Fazullin DD, Mavrin GV, Shaikhiev IG. Modified PTFE-PANI membranes for the recovery of oil products from aqueous oil emulsions. Petrol Chem+ 2017;57:165–71. [141] Hao JQ, Xiao CF, Zhang T, Zhao J, Fan ZW, Chen L. Preparation and performance of PET-braid-reinforced poly(vinylidene fluoride)/graphene hollow-Fiber membranes. Ind Eng Chem Res 2016;55:2174–82. [142] Yuan XY, Li W, Zhu ZG, Han N, Zhang XX. Thermo-responsive PVDF/PSMA composite membranes with micro/nanoscale hierarchical structures for oil/water emulsion separation. Colloid Surf A 2017;516:305–16. [143] Che H, Huo M, Peng L, Fang T, Liu N, Feng L, et al. CO2 -responsive nanofibrous membranes with switchable oil/water wettability. Angew Chem Int Ed Engl 2015;54:8934–8. [144] Li JJ, Zhou YN, Jiang ZD, Luo ZH. Electrospun fibrous mat with pH-switchable Superwettability that can separate layered oil/water mixtures. Langmuir 2016; 32:13358–66. [145] Deng WS, Long MY, Zhou QN, Wen N, Deng WL. One-step preparation of superhydrophobic acrylonitrile-butadiene-styrene copolymer coating for ultrafast separation of water-in-oil emulsions. J Colloid Interf Sci 2018;511:21–6. [146] Han N, Lim YT, Jang W, Koo HY, Choi WS. Polydopamine-mediated all-in-one device with superhydrophilicity and superhydrophobicity for one-step oil/water separation and pollutant purification. Polymer 2016;107:1–11. [147] Wang EQ, Wang HY, Liu ZJ, Yuan RX, Zhu YJ. One-step fabrication of a nickel foambased superhydrophobic and superoleophilic box for continuous oil-water separation. J Mater Sci 2015;50:4707–16. [148] Li JJ, Zhu LT, Luo ZH. Electrospun fibrous membrane with enhanced swithchable oil/water wettability for oily water separation. Chem Eng J 2016;287:474–81. [149] Chen X, He Y, Fan Y, Yang QB, Yang X, Zeng GY. Facile preparation of a smart membrane with ammonia-responsive wettability transition for controllable oil/water separation. J Mater Sci 2018;53:516–27. [150] Gupta P, Kandasubramanian B. Directional fluid gating by Janus membranes with heterogeneous wetting Properties for selective oil-water separation. Acs Appl Mater Inter 2017;9:19102–13. [151] Tuteja A, Choi W, Mabry JM, McKinley GH, Cohen RE. Robust omniphobic surfaces. Proc Natl Acad Sci USA 2008;105:18200–5. [152] Tian XL, Li J, Wang X. Anisotropic liquid penetration arising from a cross-sectional wettability gradient. Soft Matter 2012;8:2633–7. [153] Choi W, Tuteja A, Chhatre S, Mabry JM, Cohen RE, McKinley GH. Fabrics with tunable oleophobicity. Adv Mater 2009;21 [2190-+]. [154] Xue CH, Bai X, Jia ST. Robust, self-healing superhydrophobic fabrics prepared by one-step coating of PDMS and octadecylamine. Sci Rep UK 2016;6. [155] Zhou XY, Zhang ZZ, Xu XH, Guo F, Zhu XT, Men XH, et al. Robust and durable Superhydrophobic cotton fabrics for oil/water separation. Acs Appl Mater Inter 2013;5:7208–14. [156] Singh AK, Singh JK. Fabrication of zirconia based durable superhydrophobicsuperoleophilic fabrics using non fluorinated materials for oil-water separation and water purification. RSC Adv 2016;6:103632–40. [157] Rather AM, Manna U. Stretchable and durable superhydrophobicity that acts both in air and under oil. J Mater Chem A 2017;5:15208–16. [158] Zhou CL, Chen ZD, Yang H, Hou K, Zeng XJ, Zheng YF, et al. Nature-inspired strategy toward superhydrophobic fabrics for versatile oil/water separation. Acs Appl Mater Inter 2017;9:9184–94. [159] Piao C, Winandy JE, Shupe TF. From hydrophilicity to hydrophobicity: a critical review: part I. Wettability and surface behavior. Wood Fiber Sci 2010;42:490–510. [160] Gu SJ, Yang L, Huang W, Bu YM, Chen DZ, Huang JJ, et al. Fabrication of hydrophobic cotton fabrics inspired by polyphenol chemistry. Cellulose 2017;24:2635–46. [161] Gao SW, Dong XL, Huang JY, Li SH, Li YW, Chen Z, et al. Rational construction of highly transparent superhydrophobic coatings based on a non-particle, fluorinefree and water-rich system for versatile oil-water separation. Chem Eng J 2018; 333:621–9. [162] Jurdi R, Zaidouny L, Abou-Daher M, Tehrani-Bagha AR, Ghaddar N, Ghali K. Electrospun polymer blend with tunable structure for oil-water separation. J Appl Polym Sci 2018;135:46890.

135

[163] Wu MC, An N, Li Y, Sun JQ. Layer-by-layer assembly of fluorine-free polyelectrolyte surfactant complexes for the fabrication of self-healing Superhydrophobic films. Langmuir 2016;32:12361–9. [164] Ou RW, Wei J, Jiang L, Simon GP, Wang HT. Robust thermoresponsive polymer composite membrane with switchable superhydrophilicity and superhydrophobicity for efficient oil-water separation. Environ Sci Technol 2016; 50:906–14. [165] Li JJ, Zhou YN, Luo ZH. Smart fiber membrane for pH-induced oil/water separation. ACS Appl Mater Inter 2015;7:19643–50. [166] Xu ZG, Zhao Y, Wang HX, Zhou H, Qin CX, Wang XG, et al. Fluorine-free superhydrophobic coatings with pH-induced wettability transition for controllable oil-water separation. ACS Appl Mater Inter 2016;8:5661–7. [167] Cheng ZJ, Wang JW, Lai H, Du Y, Hou R, Li C, et al. pH-controllable on-demand oil/ water separation on the switchable superhydrophobic/superhydrophilic and underwater low-adhesive superoleophobic copper mesh film. Langmuir 2015;31: 1393–9. [168] Zeng XJ, Yang KQ, Huang CY, Yang K, Xu SP, Wang L, Pi PH, Wen XF. Novel pH-responsive smart fabric: From switchable wettability to controllable on-demand oil/ water separation. Acs Sustain Chem Eng 2019;7:368–76. [169] Cheng BW, Li ZJ, Li QX, Ju JG, Kang WM, Naebe M. Development of smart poly(vinylidene fluoride)-graft-poly(acrylic acid) tree-like nanofiber membrane for pHresponsive oil/water separation. J Membrane Sci 2017;534:1–8. [170] Cheng ZJ, Li C, Lai H, Du Y, Liu HW, Liu M, et al. A pH-responsive superwetting nanostructured copper mesh film for separating both water-in-oil and oil-inwater emulsions. RSC Adv 2016;6:72317–25. [171] Fu YC, Jin BY, Zhang QH, Zhan XL, Chen FQ. pH-induced switchable superwettability of efficient antibacterial fabrics for durable selective oil/water separation. ACS Appl Mater Inter 2017;9:30161–70. [172] Kung CH, Zahiri B, Sow PK, Merida W. On-demand oil-water separation via lowvoltage wettability switching of core-shell structures on copper substrates. Appl Surf Sci 2018;444:15–27. [173] Lei ZW, Zhang GZ, Deng YH, Wang CY. Surface modification of melamine sponges for pH-responsive oil absorption and desorption. Appl Surf Sci 2017;416:798–804. [174] Li XY, Zhang QD, Zhang WF, Qu RX, Wei Y, Feng L. Smart nylon membranes with pH-responsive wettability: high-efficiency separation on demand for various oil/ water mixtures and surfactant-stabilized emulsions. Adv Mater Interf 2018;5. [175] Liu MM, Hou YY, Li J, Tie L, Guo ZG. pH-responsive superwetting fabric for on-demand oil-water separation. Chem Lett 2018;47:923–6. [176] Du L, Quan X, Fan XF, Chen S, Yu HT. Electro-responsive carbon membranes with reversible superhydrophobicity/superhydrophilicity switch for efficient oil/water separation. Sep Purif Technol 2019;210:891–9. [177] Fu SD, Zhou H, Wang HX, Ding J, Liu S, Zhao Y, et al. Magnet-responsive, superhydrophobic fabrics from waterborne, fluoride-free coatings. RSC Adv 2018; 8:717–23. [178] Hu L, Gao S, Ding X, Wang D, Jiang J, Jin J, et al. Photothermal-responsive singlewalled carbon nanotube-based ultrathin membranes for on/off switchable separation of oil-in-water nanoemulsions. ACS Nano 2015;9:4835–42. [179] Li JJ, Zhou YN, Luo ZH. Polymeric materials with switchable superwettability for controllable oil/water separation: a comprehensive review. Prog Polym Sci 2018; 87:1–33. [180] Chang J, Zhang LB, Wang P. Intelligent environmental nanomaterials. Environ Sci Nano 2018;5:811–36. [181] Dutta K, De S. Smart responsive materials for water purification: an overview. J Mater Chem A 2017;5:22095–112. [182] Wang HQ, Hu XY, Ke ZW, Du CZ, Zheng LJ, Wang CY, et al. Review: porous metal filters and membranes for oil-water separation. Nanoscale Res Lett 2018;13. [183] Wang YF, Ma KK, Xin JH. Stimuli-responsive bioinspired materials for controllable liquid manipulation: principles, fabrication, and applications. Adv Funct Mater 2018;28. [184] Zheng X, Zhou YF, Chen SY, Liu XY, Zha LS. Stimuli-responsive electrospun nanofibers. Prog Chem 2018;30:958–75. [185] Chen X, He Y, Shi CC, Fu WG, Bi SY, Wang ZY, et al. Temperature- and pHresponsive membranes based on poly (vinylidene fluoride) functionalized with microgels. J Membrane Sci 2014;469:447–57. [186] Niroomand S, Fauchoux MT, Simonson CJ. Effect of moisture transfer through a semipermeable membrane on condensation/frosting limit. J Heat Trans T ASME 2018;140. [187] Navid P, Niroomand S, Simonson CJ. A new approach to delay or prevent frost formation in membranes. J Heat Trans T ASME 2019;141. [188] Niroomand S, Fauchoux MT, Simonson CJ. Evaluation of the frost properties on a semipermeable membrane. Int J Heat Mass Tranfer 2019;133:435–44. [189] Alonso MJ, Liu P, Mathisen HM, Ge GM, Simonson C. Review of heat/energy recovery exchangers for use in ZEBs in cold climate countries. Build Environ 2015;84: 228–37. [190] Wang SY, Yang ZJ, Gong GM, Wang JM, Wu JT, Yang SK, et al. Icephobicity of penguins spheniscus humboldti and an artificial replica of penguin feather with air-infused hierarchical rough structures. J Phys Chem C 2016;120:15923–9. [191] Save NS, Jassal M, Agrawal AK. Smart breathable fabric. J Indus Text 2005;34: 139–55. [192] Doles PI. Smart barrier membranes for protective clothing. In: Chapman R, editor. Smart textiles for protection. Elsevier; 2013. [193] Al-Anzi BS, Siang OC. Recent developments of carbon based nanomaterials and membranes for oily wastewater treatment. RSC Adv 2017;7:20981–94.