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sea water separation
Materials Today Communications 21 (2019) 100618
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Facile preparation of antifouling hydrogel architectures for drag reduction and oil/sea water separation
T
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Zitong Wanga,b, Shuanhong Maa, Haiquan Zhongc, Wufang Yanga, , Xiaowei Peia, Bo Yua, Kai Sua, Feng Zhoua a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China Qiushi Honors College, Tianjin University, Tianjin, 300350, China c Dongfang Electric Machinery Co., Ltd., Deyang, 6180000, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous hydrogel Antifouling Drag reduction Oil/sea water separation
Materials and technologies which can resist the adhesion of microorganisms, purify the wastewater and transport fluid with lower drag have been explored avidly, but a number of challenges still remain. A hydrogel coating anchored on the structured surface was fabricated through a facile surface self-catalyzed polymerization in situ. The necessary physicochemical properties of as-prepared hydrogel substrate were characterized, and the experimental results indicated that a controllable fluid drag was achieved through regulating the weak interaction between the mobile oil phase and hydrogel surface, meanwhile, the corresponding maximum of drag reduction efficiency was up to 64%. Besides, driven solely by gravity, the porous hydrogel-based iron net prototype was capable of generating purified seawater at a high rate of permeation flux 42,000 L m−2 h-1, and its longer-time application for either oil/water separation or fluid transportation could be preserved with the help of high efficiency antifouling behavior and good abrasion resistance capability. As a consequence, because of the remarkable drag reduction behavior and high-efficiency oil/sea water separation ability, hydrogel coated but structured substrate fabricated using a facile, cost-effective, and scalable method is expected to open up new avenues for large-scale sea water purification, oil transportation application and some other relevant fields.
1. Introduction Water pollution always occur inevitably during the rapid development of modern industry and has been more serious in recent years [1]. Especially for the frequent oil leakages induced water pollution which generally occurs during the marine transport, different oil collecting techniques have been proposed to cope with this daunting problem. While, their corresponding filtration efficiencies dramatically decline due to the settlement of protein, microorganisms and algae [2]. Meanwhile, considering the enormous economic and technological impact of the strategies with excellent drag reduction ability, effective oil/water separation and self-cleaning characters, multifunction combined strategies involved in oil medium transport therefore are critically necessary for avoiding either oil spill or collecting leaking oil, and also have drawn worldwide attention recently [3]. For the oil/water mixture separation, many scientific efforts have been made which mainly relied on the optimized surface morphology as well as surface chemistry [4–6], such as surface functionalization or coating [7], and integration of inorganic nano-materials [8]. However,
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there still exist some critical issues with obviously limitations, such as the low flux induced by the ready fouling because of the adhesion of marine algae and the high energy consumption [9,10]. Accordingly, several vital methods have been developed to improve the antifouling performance of these oil/water separation surface, such as the surface modification techniques [11,12], click chemistry [13] and RAFT [14]. Meanwhile, engineered functional materials like neutral and charged hydrophilic polymers [15], dendritic polymers [16], quaternary ammonium polymers [17], amphiphilic polymers [18], thermo-responsive polymers [19], zwitterionic polymers [20], biomimetic adhesive agent [21], and fluoropolymers [22] have been employed to improve the antifouling performance of membrane through grafting and/or coating approaches. Nevertheless, long-term antifouling techniques without sacrificing the water flux for oil/water separation membranes are still a big challenge [23]. On the other hand, resistant force, a fluid imposes on an object in either open channel (external flow) or closed channel (internal flow) conditions, has been becoming increasingly attractive over the past decades because the easily available sources of energy are becoming
Corresponding author. E-mail address: [email protected] (W. Yang).
https://doi.org/10.1016/j.mtcomm.2019.100618 Received 22 June 2019; Received in revised form 22 August 2019; Accepted 22 August 2019 Available online 23 August 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.
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depleted. Another driver of interest in decreasing fluid resistance comes from the frictional resistance since it also accounts for oil spill at some extent through the increased interface interaction of petroleum with transmission pipeline. At the present, variety of drag reduction approaches have been established for fluid transportation, including the polymer additives absorbed on the internal surface of pipes [24], microbubbles technique [25], biomimetic riblet surfaces [26], melting ice surface [27], superhydrophobic/hydrophobic surfaces [28] and the combination of surface roughness and hydrophobicity [29]. Particularly, drag-reducing surfactant has become a common practice for the pipeline transportation [30], even though the surfactant can’t be discharged in seawater without any harmful effects on marine organisms, it is highly desirable that the surfactant should have been qualified with not only biodegradable aerobically/anaerobically but also eco-friendly. To our knowledge, although different drag reduction technologies have been developed, stable interface drag reduction methods for the oil medium flow are still lacking. Meanwhile, the intelligent surface with self-cleaning behavior, sea water purification ability, antifouling performance, drag reduction property and some other relevant merits has become a hot research field. Inspired by these ideas to design new engineering surfaces, one might strive to ‘grease’ the flow of liquids past solid surface by altering the boundary condition. For this purpose, hydrogel coated porous iron net was designed and prepared using a facile surface self-catalyzed polymerization in situ in this paper. The controllable drag reduction behavior of oil medium flowing over the underwater oleophobic hydrogel was achieved. Besides, the oil/sea water separation performance, antifouling behavior and durability of the structured hydrogel coated iron net were studied as well. Experimental investigation has shown that the hydrogel wall designed in this study could decrease the fluid shear stress, suppress biofouling and purify wastewater, which meant that the developed surface construction technology might have its application potential in the pipeline transport and even relevant micro-fluid devices.
deionized water to remove the redundant AAc/AAm monomer. Finally, the AAc/AAm hydrogel coated iron substrate was obtained. Interestingly, the surface chemistry of as-prepared hydrogel could be tailored by the weak electrostatic interactions of the surfactant hexadecyl trimethyl ammonium bromide (CTAB) with acid sodium. Here, hydrogel sample was immerged into 0.01 mol/L NaOH solution to tailor the carboxylic acid into acid sodium, and then 10 mmol CTAB was applied to absorb CTAB chain on the surface of AAc/AAm hydrogel. In order to control the adsorption amount of CTAB alkyl chains, hydrogel substrates with four different AAc/AAm molar ratios were prepared as follows, AAc0.2-AAm1, AAc1-AAm1, AAc1-AAm0.2 and AAc1-AAm0.1 (index words represent the relative molar ratio of AAc and AAm component, respectively). 2.3. Characterization The static underwater contact angles of diiodomethane, dichloroethane, raw petroleum and PFPE were measured using a DSA-100 optical contact angle meter (Kruss Company, Ltd, Germany) at ambient temperature. In the test process, 5 μL oil droplets were used. Three times static contact angle measurements were conducted and the corresponding mean values were regarded as the underwater apparent contact angles. While, for the characterization of rolling off angles, the hydrogel substrate holding the oil droplet was tilted gradually until the oil droplets started to move, the instantaneous tilt angles were regarded as the rolling off angles of the measured oil droplets. The micro-morphology of the hydrogel modified iron net was observed by optical microscopy (Olympus BX-51) and JSM-6701 F field emission scanning electron microscope (FE-SEM) at 5–10 kV. In order to obtain a deeper understanding of its original structure, all of the hydrogel samples were freeze-desiccation for 24 h prior to characterization. The underwater adhesive forces of various oil droplets on the hydrogel interface and CTAB treated interfaces were measured through a high-sensitivity micro-electromechanical balance system (Dataphysics TCAT11, Germany). Oleophilic metal cap with 5 μL oil suspended was connected to a micro-balance detector, the hydrogel surface was fixed on a movable stage which could drive the substrate upwards or downwards at a constant speed. Firstly, the stage moved upwards until the hydrogel substrate contacted sufficiently with the suspended oil droplet, and then it started to move away from the droplet surface. The instantaneous interaction force during the whole process was recorded. Accordingly, the adhesive force increased firstly and then decreased after reaching the maximum during the test process. The peak data recorded in the force-distance curve was considered as the adhesive break point. Drag-reduction characteristic of the as-prepared hydrogel substrate were conducted according to the previous report [33]. In this paper, in order to make a comparison and determine the adjacent shear stress of hydrogel/oil interface, rheometer (HAAKE, RS6000, Germany) with a coaxial two parallel plate model was employed to construct Couette flow and measure the instantaneous shear stress. The fresh untreated iron net was considered as control (nonslip plate) sample. While, for the slippery condition, hydrogel slides crapped on the lower surface of the platen was immerged into aquatic environment and the gap H (here, H = 1 mm) between two parallel plates was filled up with 1 mL oil medium. After that, the shear stress was detected when a monotonically increasing shear rate was applied under a stress-constant mode. The corresponding drag reduction behavior of the hydrogel wall was expressed in terms of the drag-reduction efficiency (Δτ/τ0) and was expressed as follows,
2. Materials and methods 2.1. Materials Acrylic acid (AAc, 99.5%, stabilized with 200 ppm MEHQ), acrylamide (AAm, 99%), potassium persulfate (K2S2O8, KPS, 99%), hexadecyl trimethyl ammonium bromide (CTAB, 99%) and N,N'-methylenebis (acrylamide) (MBA, 98%) were used as received from J&K Chemical Ltd. Sodium hydroxide (NaOH, ACS grade) was used to tailor the carboxylic acid into acid sodium. Both dichloroethane (C2H4Cl2, AR grade) and diiodomethane (CH2I2, AR grade) were obtained from Sinopharm Chemical Reagent Co. Ltd., China, and raw petroleum obtained from the Northern China Oil Field was used to characterize underwater contact angle. Perfluoropolyethers (PFPE) purchased from Shanghai ICAN Chemical S＆T Co., Ltd., China, was used as oil medium to study the drag-reduction performance of hydrogel with underwater oleophobicity. Iron net from local market was applied to prepare the hydrogel substrate. Deionized water and seawater were used for all experimental processes. 2.2. Preparation of hydrogel substrate composed of AAc and AAm The typical experimental process is as follows according to our previous references [31,32], 4.26 g (0.06 mol) AAm and 0.432 g (0.006 mol) AAc were added into 50 mL deionized water, then 0.027 g (0.1 mmol) KPS initiator and 0.00,618 g (0.04 mmol) chemical crosslinker MBAA were mixed with the solution. After the solution was deaired for 10 min with N2, ultrasonic treated iron net using dehydrated alcohol was immerged into the solution at room temperature. The surface self-catalyzed polymerization was conducted to construct the hydrogel coating in situ. After polymerization for a certain time (general for 3 min), the sample was taken out and washed with a large amount of
DR =
τno − slip − τslip τno − slip
× 100%
in which τno-slip and τslip were the shear stresses when the oil medium flows over the control iron net and hydrogel wall respectively. 2
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Scheme 1. Schematic illustration of the reaction mechanism of surface self-catalyzed polymerization and preparation process of hydrogel substrate composed of AAc and AAm.
a consequence, chain propagation reaction occurred, and hydrogel layer composed of AAc/AAm and physically absorbed onto iron substrate was fabricated [34]. More importantly, since the oxidation-reduction reaction of Fe2+ mainly happened close to the iron net interface, the enhanced mechanical properties induced by the coordination interaction between Fe3+ and carboxylate radical, therefore, could be expected, especially for the internal section. Besides, considering the surface self-catalyst polymerization occurred on both sides of substrate which was encapsulated tightly by the hydrogel, as a consequence, the durability of the constructed hydrogel mainly subjected to the stability of itself, and the contribution of interface adhesive strength of the hydrogel to their relevant application could be ignored. Finally, the corresponding surface chemistry of hydrogel could be easily regulated through the weak interaction of the cationic CTAB with acid sodium, in which the acid sodium was produced by immersing the hydrogel modified iron net into the 0.1 M sodium hydroxide solution an hour. More importantly, post-functionalization method, such as the presented strategy in this paper, could regulate the density of tethered alkyl chain by changing the surface chemistry components and provide a more efficient but simple route for fabricating controllable interface physicochemical performance. The successful preparation of AAc/AAm hydrogel and other samples were revealed with FT-IR characterization since the hydrogel samples were still loaded with acrylate groups and other unique components. For the FT-IR absorption spectra (Fig. 1), the obvious adsorption peaks − at 3337 cm−1 and 3190 cm 1 were attributed to the stretching vibration of NeH bonding, combined with the carbonyl stretching vibration near −1 and 1400 cm−1 1650 cm−1, 1600 cm , these typical adsorption peaks unambiguously affirmed the existence of acrylate/acrylamide groups in the synthesized polymer networks. Here, the reduced adsorption wavenumber was mainly induced by the p-π conjugation of O] CN and O = CO eeegroups. While, compared with the AAc-AAm hydrogel spectrum (red curve), a tremendous change taken place in C–H − stretching vibrations at 2914 cm−1 and 2845 cm 1 (green curve), and the −1 C–N stretching vibration around 1117 cm after the post-functionalization of the hydrogel with CTAB surfactant confirmed the successful integration of desired chemistry through electrostatic interaction. Either interface physicochemical properties or surface morphologies acted essential effect on the interface behaviors, such as wettability, adhesive property and even water flux capability. The micro-morphology of the as-prepared hydrogel was characterized using optical microscopy and FE-SEM, consequently. For the dehydrated hydrogel sample with varying AAc/AAm components, a necessary freeze-drying process was applied in order to reserve its original network as much as possible. Fig. 2 and Fig. S1 show the typical macro/micro topography of
Fig. 1. FT-IR spectra of AAc-AAm hydrogel layer before (red) and after post functionalization with CTAB cationic surfactant (green), where the peaks near − 1650 cm−1 and 1400 cm 1 denoted the carbonyl stretching vibration adsorption, and the C–O–C stretching vibration could be observed at 1200 cm−1, respectively (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
Besides, the anti-settlement behavior of the as-prepared hydrogel to marine algae was investigated. The red algae Porphyridium sp. and diatom Navicular sp. (supported by Freshwater Algae Culture Collection at the Institute of Hydrobiology, Chinese Academy of Sciences) were allowed to attach on the hydrogel substrate 24 h. After releasing the swimming algae through artificial seawater, the number of settled algae was visualized by auto-fluorescence image analysis (Olympus, BX-51). Thirty images were recorded and the number of adhered algae was calculated via Cell Profiler software, the average value was regarded as the typical density of the adhered algae (/mm2). 3. Results and discussion Contrasted to the traditional free-radical solution polymerization, lower active energy was required for the surface self-catalyzed polymerization because the chain initiation could be easily motivated under room temperature. As illustrated in the Scheme 1, polymerization process was initiated by the presence of acid medium (AAc) which leaded to the production of ferrous iron (Fe2+). After that, Fe2+ triggered the existence of both active free radical and Fe3+ through oxidation-reduction reaction with initiator potassium persulfate (KPS). As 3
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Fig. 2. FE-SEM images showing the typical surface and cross-section morphology of hydrogel substrates. The surface morphologies of dehydrated hydrogel samples of AAc0.2-AAm1 (a) and AAc1-AAm1 (e), surface appearance of CTAB decorated hydrogel samples of AAc0.2-AAm1 (b) and AAc1-AAm1 (f), cross-section morphological characterization of the dehydrated hydrogel samples of AAc0.2-AAm1 (c) and AAc1-AAm1 (g) and the cross-section FE-SEM of CTAB decorated hydrogel samples of AAc0.2-AAm1 (d) and AAc1-AAm1 (h). The scale bars were 10 μm in these images.
To be sure, this interesting phenomenon also could be illustrated by measuring the adhesive force of the oil droplets on CTAB modified hydrogel substrate with different AAc molar contents. For the AAc/ AAm hydrogel modified iron net (Fig. 3c), it almost could not catch any visible adhesive interaction during the whole test process. While induced by the hydrophobic interactions between the CTAB alkyl chains and oil droplets, an obviously interface interaction of CTAB modified hydrogel with oil droplet could be observed, even a small amount of oil component remained on the CTAB modified surface (Fig. 3d), for example, the adhesive force of PFPE droplet on the 10 mM CTAB treated AAc1-AAm0.1 hydrogel reached up to 50 μN. What’s more, as it has been analyzed statistically in the Fig. 3b, because of the electrostatic interaction between the AAc components and CTAB molecular chains, the average adhesive forces for the three kinds of selected oil medium, therefore, were proportional to the molar content of AAc and an underwater oleophobicity surface with switchable oil-adhesion behavior could be achieved as well. Similar with the actual fish scales and shark skin in their native underwater environment, such oleophobicity coupled with low adhesive force also could provide self-cleaning property reasonably. Furthermore, the hydrogel layer not only aided in the quick spread of water film at hydrogel-oil interface, but also retained the water barrier layer by microstructures of AAc/AAm hydrogel. As a result, this novel surface could create a slip effect for the adjacent fluid layer, and then effectively lower drag and increase the flow rate, especially for the transport of oil medium. Meanwhile, according to the adhesive force measurement, their corresponding drag reduction performance could be tailored by changing the hydrogel recipes. To demonstrate the drag reduction performance of oil medium transportation under partial or full-slip boundary condition, the influence of hydrogel and its surface chemistry on fluid drag regulation were investigated deliberately. Fig. 4a shows the constructed Couette Flow between two plates with a constant gap h (h = 1 mm). The upper plate was moving with varying shear rate and the lower one was stationary, velocity gradient alone the vertical direction formed because of the fluid internal friction. As shown in the Fig. 4a, in general, the boundary frictional drag impeding the relative motion of fluid was obvious enough, which meant that the velocity gradient was high. Besides, it was also assumed that the adjacent boundary velocity approached to zero. In this condition, the lower plate can be regarded as no-slippery and a higher shear stress was generally induced. While a slip boundary condition would be created when the lower plate was replaced by the hydrogel coated iron net (established model), and a lower interface
the as-prepared hydrogel samples. Firstly, uniform microporous structure (Fig. S1a), less than 200 μm, can be observed for the fresh iron net. After surface self-catalyzed polymerization, both sides of the grid framework were ideally encapsulated by the swollen hydrogel component. While, highly branched three-dimensional porous framework could be retained, and water penetration but oil repulsion phenomenon through the hydrogel coated microporous substrate might be endowed (Fig. 2a, c, e and g, Fig. S1c and d). On the contrary, there was no three-dimensional reticulate structure and multi-scale characters could be observed but relative compact, sag and fold surface structure for the CTAB treated hydrogel samples (Fig. 2b, d, f and h). On the other hand, the thickness of the as-prepared hydrogel coating was micron to millimeter scales (Fig. 2c, d, g and h). Meanwhile, the hydrogel layer became denser after CTAB surfactant modification, which might be attributed to the dehydration effect induced by the interface electrostatic interaction (Fig. 2d and h). These results indicated that the iron net acted as mainly skeleton and guaranteed the three-dimensional spatial structure of hydrogel. More importantly, as it has been generally accepted that large amount of free or combined water molecules could be retained in the hydrogel by hydrogen bonding interactions and also could penetrate fully into the hydrogel microstructures at molecular scale, a stable hydration layer was acquired by hydrogel coated iron network as a consequence. Considering the physicochemical properties of hydrogel coated iron net, this unique interface with tailorable surface chemistry and uniform microporous structure was expected to be applied in the field of fluid drag reduction, oil/water separation, antibiofouling and some other relevant aspects. Firstly, underwater oil wettability of hydrogel coated iron net was investigated. It can be observed from the Fig. 3a, all of the measured oil droplets were spherical., and underwater oleophobicity/ superoleophobicity was ensured because of the stable hydration effect of hydrogel substrate. Meanwhile, the corresponding rolling off angles for the different oil droplets were less than 10°, these experimental results were mainly attributed to the hydrogel component because the hydrogel could greatly whittle the adhesive interaction between oil droplet and iron net. Although the underwater oil contact angles were almost similar for both the hydrogel and CTAB decorated hydrogel samples, we saw in particular that the rolling off angle could be dramatically affected by changing the surface chemistry. Higher rolling off angles (about 20-30°) were obtained on the cationic surfactant CTAB modified surface, which was mainly attributed to the increased surface interaction between the alkyl chain and oil droplets. 4
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Fig. 3. Comparison of the underwater contact angels and roll off angles of organic solvent and oil drops with different surface tensions, here the contact angles (left ordinate) and rolling off angles (right ordinate) were plotted against various oil mediums (a), corresponding adhesion forces on the as-prepared hydrogels with varying AAc/AAm compositions after decorating with 10 mM CTAB solution (b), adhesive force measuring curves of droplet on the hydrogel surface (c) and on the CTAB decorated hydrogel substrate (d), respectively.
Fig. 4. Schematic diagram of velocity profiles of steady-state Couette flow geometry and their corresponding drag reduction mechanism (a), shear stress characterization of oil medium PFPE flowing over AAc0.2-AAm1 hydrogel modified substrate (b), and the drag reduction efficient characterization for PTFE in which different CTAB modified hydrogel wall were applied (c), plots of drag reduction efficiency are depicted as a function of shear rate, (A) 10 mM CTAB modified AAc1AAm0.1 hydrogel sample, (B) 10 mM CTAB modified AAc1-AAm1 hydrogel sample, (C) 10 mM CTAB modified AAc0.2-AAm1 hydrogel sample and (D) 10 mM CTAB modified AAc0.1-AAm1 hydrogel sample, respectively. 5
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though the flux for the sea water/liquid paraffin mixture was only about 3213 ± 80 L m-2 h-1. It was easy to understand that the lower flux was attributed to the high viscosity of liquid paraffin. To be sure, these seawater efficiencies were lower, however they were still competitive as these fluxes were higher than that of the common filtration membrane, which were mainly attributed to the porous character at micron scale for the hydrogel tethered iron net (Fig. S1). As a consequence, given that the oil/water separation process was conducted with higher fluxes but without external driving force, such hydrogel coated iron nets were therefore highly attractive in terms of sea water treatment. The last aspect to notice was that the long-term application for oil medium drag reduction and oil/water separation was always hampered by biofouling and performance attenuation. Therefore, antifouling behavior of the hydrogel coated iron net was investigated by the adhesion of diatom Navicular sp. and red algae Porphyridium sp. As depicted in the Fig. 6, there were more than 2500 and 1600 /mm2 red algae and diatom fouled on the control iron net, respectively. Because of the stable hydration barrier composed of AAc/AAm copolymer [35], the results, however, showed lower adhesion amounts of either red algae or diatom for the hydrogel coated iron net. Compared with the control substrate, the corresponding adhesion amounts decreased sharply with more than up to 97% and 93% for the red algae and diatom. Consistent with the underwater oil contact angle characterization, these results suggested high efficiency, broad-spectrum fouling resistance capability and self-cleaning property of the hydrogel coated iron net for their longterm application, and also synergistically confirmed the stability while oil/water separation and fluid drag reduction. Besides its excellent antifouling performance, in order to have a comprehensive understanding concerning the long-term application of the prepared coating, studying the mechanical stability and its acid/ alkaline abrasive resistance were also necessary. In this paper, the friction characterization was conducted to prove the wear performance of hydrogel as depicted in the Fig. 7. The friction coefficient versus time curves clearly revealed their superior anti-wear behavior under different loads (Fig. 7a), while the friction coefficient increased slightly from 10−3 to 10-2 over the 20 min period (Fig. 7b). Notably, because the as-prepared robust hydrogel could maintain its original structure and prevent the water molecule being squeezed out, lubrication layer could be retained under larger normal loads and give a lower friction capability during the test procedure. Finally, the hydrogel samples were immerged in acid and alkali solution for a month (Fig. S3). As expected, there was no change significantly happened for themselves, and only the alkaline solution became more yellow after immersion. These performance evaluations indicated that the synthesized hydrogel coated substrate was capable of offering inherently stability even in the face of long-term mechanical wear and acid/alkaline solutions. For example, their excellent oil/sea water separation capability was still reserved after acid/alkali solution immersion (Fig. S4).
shear stress could be produced because of the weaker interaction between the hydrogel and working fluid. In this case, the slope of the velocity gradient became slighter when compared with the no-slip condition. Besides, because of the soft characteristic of hydrogel, selfadaptation of hydrogel surface structure regarding the distribution of the boundary fluid field can be expected, which meant not only the momentum transfer but the mass transfer could be relieved [24]. Accordingly, lower shear stress was produced near the adjacent interface and an obvious fluid drag reduction phenomenon would be exhibited for the oil flow over the hydrogel coated surface. To quantify the influence of surface chemistry on fluid movement, Fig. 4b-c show the quantitative drag reduction behaviors of oil flowing (PFPE) over the hydrogel coated surface. Different from the control surface, the corresponding shear stress for the hydrogel coated substrate decreased greatly during the whole test procedure, which might indicate that the working fluid flowed with reduced interface sticking, and decreased weight loss or cross-contaminant. While, for the surfactant CTAB modified hydrogel surface, the corresponding shear stress was larger than hydrogel coated surface but smaller than that of the control surface obviously, which was mainly attributed to the influence of the adsorbed alkyl chain on the flowing performance of oil medium [29]. Besides, the equivalent representation of this data in terms of drag-reduction efficiency are included in the Fig. 4c. It is easy to note that the drag reduction efficiencies increased with decreasing the molar content of AAc to AAm for the CTAB modified hydrogel samples, and the effect of CTAB on the drag reduction efficiency was becoming more ignorable when the AAc content was very low. For example, the average drag-reduction efficiency for the CTAB modified AAc1-AAm1 was about 30%. While for the AAc0.1-AAm1 hydrogel sample, the drag reduction efficiency was not affected by the CTAB modification and almost kept constant before and after CTAB modification (64%). These experimental results indicated that enough amount of CTAB alkyl chains were exhibited on the hydrogel surface and sufficient to generate no-slip effect, especially for the CTAB modified AAc1-AAm0.1 sample, even an obvious resistance increase phenomenon was obtained. It was, therefore, assumed that the CTAB coated hydrogel surface rendered them fully or partially wetted by PFPE. Besides, the rate-dependent drag-reduction performance suggested that fluid was pinned to the CTAB modified hydrogel wall and endured a critical shear stress. Beyond this point, the moving fluid was detached from the surface in some sense, and increased drag reduction efficiencies were observed with increasing the shear rate, accordingly. Over all, it can be concluded that drag reduction efficiency was related to the fluid organization near the substrate. Because of the attractive interfacial interaction for the CTAB modified surface, the critical stress to detach the adsorbing oil medium from the substrate, therefore, appeared when a higher shear stress was employed. However, the interfacial interaction of the flowing medium with hydrogel substrate without CTAB modification was repulsive, the wall shear stress therefore decreased and finally resulted in macroscopic drag reduction behavior. As we have observed previously, the porous structure could act as the water channel in hydrogel membrane. Therefore, an only gravitydriven oil/sea water separation experiment was performed as shown in Fig. 5. Firstly, the hydrogel coated iron net was pre-wetted by water, and oil/sea water mixture of red-dyed oil and colorless sea water was poured into the tube. Once the mixture contacted the membrane, the sea water component permeated rapidly, while the oil phase was intercepted due to oleophobicity. All the fluxes were recorded after conducting the separation process till stabilization. Five kinds of oil/sea water mixtures with different oil density and surface tense were used to evaluate the separation behaviors as shown in Fig. 5b. Driven by gravity, the calculated fluxes for the sea water/diesel, sea water/dichloroethane, sea water/toluene, and sea water/n-hexadecane mixtures were 42,711 ± 910, 26,397 ± 1164, 38,690 ± 991, and 31,260 ± 980 L m−2 h-1, respectively. Meanwhile, the water separation efficiencies were over 97% for these measured mixtures, even
4. Conclusion In this paper, an environmentally friendly AAc/AAm hydrogel coated iron net was developed through a facile surface self-catalyzed polymerization. By regulating the weak interactions between the alkyl chain with oil medium, rheological experiments for the oil medium flowing against CTAB surfactant modified hydrogel substrate verified the existence of controllable drag reduction effect through and the corresponding maximum of drag reduction efficiency was up to 64%. Based on porous characters and underwater oleophobicity of the asprepared hydrogel, the oil/sea water separation also has been achieved with higher fluxes about 42,000 L m−2 h-1. Moreover, such structured hydrogel coating was environmentally friendly with excellent antifouling performance, good abrasion resistance, and therefore showed great attraction, especially for oil medium transport and the involved long-term oil/sea water mixture treatment. Overall, our finding 6
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Fig. 5. Schematic diagram of oil/sea water separation test where the dichloroethane labeled by red is retained and sea water passed through hydrogel substrate. The insets are digital images of the oil/sea water mixture before and after separation (a), separation efficiencies and water fluxes of the prepared hydrogel membranes for different oil/sea water mixture (b) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
Fig. 6. Typical fluorescence images of attached red algae Porphyridium sp. (a,b) and diatom Navicular sp. (c,d) after exposure to control iron net (a,d) and hydrogel coated substrates (b,e), the corresponding relative amounts of red algae Porphyridium sp. and diatom Navicular sp. attached on the samples (c and f). The fresh iron net without hydrogel coating was included for comparison. The scale bar was 50 μm (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
Fig. 7. Durability tests of the hydrogel shrouded porous network through friction characterization, different payloads as depicted in the (a) were employed under constant frequency (1 Hz) through ball on disc rubbing test, and a long-term friction measurement reflected the robust adhesive force on the substrate as well as the coating durability, (b) the corresponding friction coefficient statistically calculated from the friction curves. 7
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demonstrated how altering chemical components and wettability can be used to affect surface fluid drag and thus control the drag-reduction efficiency. This novel surface construction technology capable of a considerable boundary slippage effect might be more practice for pipeline transport of flue, and even suggest a possibility of helping fluid transportation of micro-fluid devices.
[15]
[16]
Acknowledgements
[17]
The authors gratefully acknowledge the funding support from the NSFC (51905519, 51573198, 51403220, and21773274), Ministry of Science and Technology (2016YFC1100401) and the financial supports provided by the Dongfang Electric Machinery Co., ltd. (No. 2017GZ0053).
[18]
[19]
Appendix A. Supplementary data [20]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mtcomm.2019. 100618.
[21]
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Facile preparation of antifouling hydrogel architectures for drag reduction and oil/sea water separation
T
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Zitong Wanga,b, Shuanhong Maa, Haiquan Zhongc, Wufang Yanga, , Xiaowei Peia, Bo Yua, Kai Sua, Feng Zhoua a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China Qiushi Honors College, Tianjin University, Tianjin, 300350, China c Dongfang Electric Machinery Co., Ltd., Deyang, 6180000, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous hydrogel Antifouling Drag reduction Oil/sea water separation
Materials and technologies which can resist the adhesion of microorganisms, purify the wastewater and transport fluid with lower drag have been explored avidly, but a number of challenges still remain. A hydrogel coating anchored on the structured surface was fabricated through a facile surface self-catalyzed polymerization in situ. The necessary physicochemical properties of as-prepared hydrogel substrate were characterized, and the experimental results indicated that a controllable fluid drag was achieved through regulating the weak interaction between the mobile oil phase and hydrogel surface, meanwhile, the corresponding maximum of drag reduction efficiency was up to 64%. Besides, driven solely by gravity, the porous hydrogel-based iron net prototype was capable of generating purified seawater at a high rate of permeation flux 42,000 L m−2 h-1, and its longer-time application for either oil/water separation or fluid transportation could be preserved with the help of high efficiency antifouling behavior and good abrasion resistance capability. As a consequence, because of the remarkable drag reduction behavior and high-efficiency oil/sea water separation ability, hydrogel coated but structured substrate fabricated using a facile, cost-effective, and scalable method is expected to open up new avenues for large-scale sea water purification, oil transportation application and some other relevant fields.
1. Introduction Water pollution always occur inevitably during the rapid development of modern industry and has been more serious in recent years [1]. Especially for the frequent oil leakages induced water pollution which generally occurs during the marine transport, different oil collecting techniques have been proposed to cope with this daunting problem. While, their corresponding filtration efficiencies dramatically decline due to the settlement of protein, microorganisms and algae [2]. Meanwhile, considering the enormous economic and technological impact of the strategies with excellent drag reduction ability, effective oil/water separation and self-cleaning characters, multifunction combined strategies involved in oil medium transport therefore are critically necessary for avoiding either oil spill or collecting leaking oil, and also have drawn worldwide attention recently [3]. For the oil/water mixture separation, many scientific efforts have been made which mainly relied on the optimized surface morphology as well as surface chemistry [4–6], such as surface functionalization or coating [7], and integration of inorganic nano-materials [8]. However,
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there still exist some critical issues with obviously limitations, such as the low flux induced by the ready fouling because of the adhesion of marine algae and the high energy consumption [9,10]. Accordingly, several vital methods have been developed to improve the antifouling performance of these oil/water separation surface, such as the surface modification techniques [11,12], click chemistry [13] and RAFT [14]. Meanwhile, engineered functional materials like neutral and charged hydrophilic polymers [15], dendritic polymers [16], quaternary ammonium polymers [17], amphiphilic polymers [18], thermo-responsive polymers [19], zwitterionic polymers [20], biomimetic adhesive agent [21], and fluoropolymers [22] have been employed to improve the antifouling performance of membrane through grafting and/or coating approaches. Nevertheless, long-term antifouling techniques without sacrificing the water flux for oil/water separation membranes are still a big challenge [23]. On the other hand, resistant force, a fluid imposes on an object in either open channel (external flow) or closed channel (internal flow) conditions, has been becoming increasingly attractive over the past decades because the easily available sources of energy are becoming
Corresponding author. E-mail address: [email protected] (W. Yang).
https://doi.org/10.1016/j.mtcomm.2019.100618 Received 22 June 2019; Received in revised form 22 August 2019; Accepted 22 August 2019 Available online 23 August 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.
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depleted. Another driver of interest in decreasing fluid resistance comes from the frictional resistance since it also accounts for oil spill at some extent through the increased interface interaction of petroleum with transmission pipeline. At the present, variety of drag reduction approaches have been established for fluid transportation, including the polymer additives absorbed on the internal surface of pipes [24], microbubbles technique [25], biomimetic riblet surfaces [26], melting ice surface [27], superhydrophobic/hydrophobic surfaces [28] and the combination of surface roughness and hydrophobicity [29]. Particularly, drag-reducing surfactant has become a common practice for the pipeline transportation [30], even though the surfactant can’t be discharged in seawater without any harmful effects on marine organisms, it is highly desirable that the surfactant should have been qualified with not only biodegradable aerobically/anaerobically but also eco-friendly. To our knowledge, although different drag reduction technologies have been developed, stable interface drag reduction methods for the oil medium flow are still lacking. Meanwhile, the intelligent surface with self-cleaning behavior, sea water purification ability, antifouling performance, drag reduction property and some other relevant merits has become a hot research field. Inspired by these ideas to design new engineering surfaces, one might strive to ‘grease’ the flow of liquids past solid surface by altering the boundary condition. For this purpose, hydrogel coated porous iron net was designed and prepared using a facile surface self-catalyzed polymerization in situ in this paper. The controllable drag reduction behavior of oil medium flowing over the underwater oleophobic hydrogel was achieved. Besides, the oil/sea water separation performance, antifouling behavior and durability of the structured hydrogel coated iron net were studied as well. Experimental investigation has shown that the hydrogel wall designed in this study could decrease the fluid shear stress, suppress biofouling and purify wastewater, which meant that the developed surface construction technology might have its application potential in the pipeline transport and even relevant micro-fluid devices.
deionized water to remove the redundant AAc/AAm monomer. Finally, the AAc/AAm hydrogel coated iron substrate was obtained. Interestingly, the surface chemistry of as-prepared hydrogel could be tailored by the weak electrostatic interactions of the surfactant hexadecyl trimethyl ammonium bromide (CTAB) with acid sodium. Here, hydrogel sample was immerged into 0.01 mol/L NaOH solution to tailor the carboxylic acid into acid sodium, and then 10 mmol CTAB was applied to absorb CTAB chain on the surface of AAc/AAm hydrogel. In order to control the adsorption amount of CTAB alkyl chains, hydrogel substrates with four different AAc/AAm molar ratios were prepared as follows, AAc0.2-AAm1, AAc1-AAm1, AAc1-AAm0.2 and AAc1-AAm0.1 (index words represent the relative molar ratio of AAc and AAm component, respectively). 2.3. Characterization The static underwater contact angles of diiodomethane, dichloroethane, raw petroleum and PFPE were measured using a DSA-100 optical contact angle meter (Kruss Company, Ltd, Germany) at ambient temperature. In the test process, 5 μL oil droplets were used. Three times static contact angle measurements were conducted and the corresponding mean values were regarded as the underwater apparent contact angles. While, for the characterization of rolling off angles, the hydrogel substrate holding the oil droplet was tilted gradually until the oil droplets started to move, the instantaneous tilt angles were regarded as the rolling off angles of the measured oil droplets. The micro-morphology of the hydrogel modified iron net was observed by optical microscopy (Olympus BX-51) and JSM-6701 F field emission scanning electron microscope (FE-SEM) at 5–10 kV. In order to obtain a deeper understanding of its original structure, all of the hydrogel samples were freeze-desiccation for 24 h prior to characterization. The underwater adhesive forces of various oil droplets on the hydrogel interface and CTAB treated interfaces were measured through a high-sensitivity micro-electromechanical balance system (Dataphysics TCAT11, Germany). Oleophilic metal cap with 5 μL oil suspended was connected to a micro-balance detector, the hydrogel surface was fixed on a movable stage which could drive the substrate upwards or downwards at a constant speed. Firstly, the stage moved upwards until the hydrogel substrate contacted sufficiently with the suspended oil droplet, and then it started to move away from the droplet surface. The instantaneous interaction force during the whole process was recorded. Accordingly, the adhesive force increased firstly and then decreased after reaching the maximum during the test process. The peak data recorded in the force-distance curve was considered as the adhesive break point. Drag-reduction characteristic of the as-prepared hydrogel substrate were conducted according to the previous report [33]. In this paper, in order to make a comparison and determine the adjacent shear stress of hydrogel/oil interface, rheometer (HAAKE, RS6000, Germany) with a coaxial two parallel plate model was employed to construct Couette flow and measure the instantaneous shear stress. The fresh untreated iron net was considered as control (nonslip plate) sample. While, for the slippery condition, hydrogel slides crapped on the lower surface of the platen was immerged into aquatic environment and the gap H (here, H = 1 mm) between two parallel plates was filled up with 1 mL oil medium. After that, the shear stress was detected when a monotonically increasing shear rate was applied under a stress-constant mode. The corresponding drag reduction behavior of the hydrogel wall was expressed in terms of the drag-reduction efficiency (Δτ/τ0) and was expressed as follows,
2. Materials and methods 2.1. Materials Acrylic acid (AAc, 99.5%, stabilized with 200 ppm MEHQ), acrylamide (AAm, 99%), potassium persulfate (K2S2O8, KPS, 99%), hexadecyl trimethyl ammonium bromide (CTAB, 99%) and N,N'-methylenebis (acrylamide) (MBA, 98%) were used as received from J&K Chemical Ltd. Sodium hydroxide (NaOH, ACS grade) was used to tailor the carboxylic acid into acid sodium. Both dichloroethane (C2H4Cl2, AR grade) and diiodomethane (CH2I2, AR grade) were obtained from Sinopharm Chemical Reagent Co. Ltd., China, and raw petroleum obtained from the Northern China Oil Field was used to characterize underwater contact angle. Perfluoropolyethers (PFPE) purchased from Shanghai ICAN Chemical S＆T Co., Ltd., China, was used as oil medium to study the drag-reduction performance of hydrogel with underwater oleophobicity. Iron net from local market was applied to prepare the hydrogel substrate. Deionized water and seawater were used for all experimental processes. 2.2. Preparation of hydrogel substrate composed of AAc and AAm The typical experimental process is as follows according to our previous references [31,32], 4.26 g (0.06 mol) AAm and 0.432 g (0.006 mol) AAc were added into 50 mL deionized water, then 0.027 g (0.1 mmol) KPS initiator and 0.00,618 g (0.04 mmol) chemical crosslinker MBAA were mixed with the solution. After the solution was deaired for 10 min with N2, ultrasonic treated iron net using dehydrated alcohol was immerged into the solution at room temperature. The surface self-catalyzed polymerization was conducted to construct the hydrogel coating in situ. After polymerization for a certain time (general for 3 min), the sample was taken out and washed with a large amount of
DR =
τno − slip − τslip τno − slip
× 100%
in which τno-slip and τslip were the shear stresses when the oil medium flows over the control iron net and hydrogel wall respectively. 2
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Scheme 1. Schematic illustration of the reaction mechanism of surface self-catalyzed polymerization and preparation process of hydrogel substrate composed of AAc and AAm.
a consequence, chain propagation reaction occurred, and hydrogel layer composed of AAc/AAm and physically absorbed onto iron substrate was fabricated [34]. More importantly, since the oxidation-reduction reaction of Fe2+ mainly happened close to the iron net interface, the enhanced mechanical properties induced by the coordination interaction between Fe3+ and carboxylate radical, therefore, could be expected, especially for the internal section. Besides, considering the surface self-catalyst polymerization occurred on both sides of substrate which was encapsulated tightly by the hydrogel, as a consequence, the durability of the constructed hydrogel mainly subjected to the stability of itself, and the contribution of interface adhesive strength of the hydrogel to their relevant application could be ignored. Finally, the corresponding surface chemistry of hydrogel could be easily regulated through the weak interaction of the cationic CTAB with acid sodium, in which the acid sodium was produced by immersing the hydrogel modified iron net into the 0.1 M sodium hydroxide solution an hour. More importantly, post-functionalization method, such as the presented strategy in this paper, could regulate the density of tethered alkyl chain by changing the surface chemistry components and provide a more efficient but simple route for fabricating controllable interface physicochemical performance. The successful preparation of AAc/AAm hydrogel and other samples were revealed with FT-IR characterization since the hydrogel samples were still loaded with acrylate groups and other unique components. For the FT-IR absorption spectra (Fig. 1), the obvious adsorption peaks − at 3337 cm−1 and 3190 cm 1 were attributed to the stretching vibration of NeH bonding, combined with the carbonyl stretching vibration near −1 and 1400 cm−1 1650 cm−1, 1600 cm , these typical adsorption peaks unambiguously affirmed the existence of acrylate/acrylamide groups in the synthesized polymer networks. Here, the reduced adsorption wavenumber was mainly induced by the p-π conjugation of O] CN and O = CO eeegroups. While, compared with the AAc-AAm hydrogel spectrum (red curve), a tremendous change taken place in C–H − stretching vibrations at 2914 cm−1 and 2845 cm 1 (green curve), and the −1 C–N stretching vibration around 1117 cm after the post-functionalization of the hydrogel with CTAB surfactant confirmed the successful integration of desired chemistry through electrostatic interaction. Either interface physicochemical properties or surface morphologies acted essential effect on the interface behaviors, such as wettability, adhesive property and even water flux capability. The micro-morphology of the as-prepared hydrogel was characterized using optical microscopy and FE-SEM, consequently. For the dehydrated hydrogel sample with varying AAc/AAm components, a necessary freeze-drying process was applied in order to reserve its original network as much as possible. Fig. 2 and Fig. S1 show the typical macro/micro topography of
Fig. 1. FT-IR spectra of AAc-AAm hydrogel layer before (red) and after post functionalization with CTAB cationic surfactant (green), where the peaks near − 1650 cm−1 and 1400 cm 1 denoted the carbonyl stretching vibration adsorption, and the C–O–C stretching vibration could be observed at 1200 cm−1, respectively (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
Besides, the anti-settlement behavior of the as-prepared hydrogel to marine algae was investigated. The red algae Porphyridium sp. and diatom Navicular sp. (supported by Freshwater Algae Culture Collection at the Institute of Hydrobiology, Chinese Academy of Sciences) were allowed to attach on the hydrogel substrate 24 h. After releasing the swimming algae through artificial seawater, the number of settled algae was visualized by auto-fluorescence image analysis (Olympus, BX-51). Thirty images were recorded and the number of adhered algae was calculated via Cell Profiler software, the average value was regarded as the typical density of the adhered algae (/mm2). 3. Results and discussion Contrasted to the traditional free-radical solution polymerization, lower active energy was required for the surface self-catalyzed polymerization because the chain initiation could be easily motivated under room temperature. As illustrated in the Scheme 1, polymerization process was initiated by the presence of acid medium (AAc) which leaded to the production of ferrous iron (Fe2+). After that, Fe2+ triggered the existence of both active free radical and Fe3+ through oxidation-reduction reaction with initiator potassium persulfate (KPS). As 3
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Fig. 2. FE-SEM images showing the typical surface and cross-section morphology of hydrogel substrates. The surface morphologies of dehydrated hydrogel samples of AAc0.2-AAm1 (a) and AAc1-AAm1 (e), surface appearance of CTAB decorated hydrogel samples of AAc0.2-AAm1 (b) and AAc1-AAm1 (f), cross-section morphological characterization of the dehydrated hydrogel samples of AAc0.2-AAm1 (c) and AAc1-AAm1 (g) and the cross-section FE-SEM of CTAB decorated hydrogel samples of AAc0.2-AAm1 (d) and AAc1-AAm1 (h). The scale bars were 10 μm in these images.
To be sure, this interesting phenomenon also could be illustrated by measuring the adhesive force of the oil droplets on CTAB modified hydrogel substrate with different AAc molar contents. For the AAc/ AAm hydrogel modified iron net (Fig. 3c), it almost could not catch any visible adhesive interaction during the whole test process. While induced by the hydrophobic interactions between the CTAB alkyl chains and oil droplets, an obviously interface interaction of CTAB modified hydrogel with oil droplet could be observed, even a small amount of oil component remained on the CTAB modified surface (Fig. 3d), for example, the adhesive force of PFPE droplet on the 10 mM CTAB treated AAc1-AAm0.1 hydrogel reached up to 50 μN. What’s more, as it has been analyzed statistically in the Fig. 3b, because of the electrostatic interaction between the AAc components and CTAB molecular chains, the average adhesive forces for the three kinds of selected oil medium, therefore, were proportional to the molar content of AAc and an underwater oleophobicity surface with switchable oil-adhesion behavior could be achieved as well. Similar with the actual fish scales and shark skin in their native underwater environment, such oleophobicity coupled with low adhesive force also could provide self-cleaning property reasonably. Furthermore, the hydrogel layer not only aided in the quick spread of water film at hydrogel-oil interface, but also retained the water barrier layer by microstructures of AAc/AAm hydrogel. As a result, this novel surface could create a slip effect for the adjacent fluid layer, and then effectively lower drag and increase the flow rate, especially for the transport of oil medium. Meanwhile, according to the adhesive force measurement, their corresponding drag reduction performance could be tailored by changing the hydrogel recipes. To demonstrate the drag reduction performance of oil medium transportation under partial or full-slip boundary condition, the influence of hydrogel and its surface chemistry on fluid drag regulation were investigated deliberately. Fig. 4a shows the constructed Couette Flow between two plates with a constant gap h (h = 1 mm). The upper plate was moving with varying shear rate and the lower one was stationary, velocity gradient alone the vertical direction formed because of the fluid internal friction. As shown in the Fig. 4a, in general, the boundary frictional drag impeding the relative motion of fluid was obvious enough, which meant that the velocity gradient was high. Besides, it was also assumed that the adjacent boundary velocity approached to zero. In this condition, the lower plate can be regarded as no-slippery and a higher shear stress was generally induced. While a slip boundary condition would be created when the lower plate was replaced by the hydrogel coated iron net (established model), and a lower interface
the as-prepared hydrogel samples. Firstly, uniform microporous structure (Fig. S1a), less than 200 μm, can be observed for the fresh iron net. After surface self-catalyzed polymerization, both sides of the grid framework were ideally encapsulated by the swollen hydrogel component. While, highly branched three-dimensional porous framework could be retained, and water penetration but oil repulsion phenomenon through the hydrogel coated microporous substrate might be endowed (Fig. 2a, c, e and g, Fig. S1c and d). On the contrary, there was no three-dimensional reticulate structure and multi-scale characters could be observed but relative compact, sag and fold surface structure for the CTAB treated hydrogel samples (Fig. 2b, d, f and h). On the other hand, the thickness of the as-prepared hydrogel coating was micron to millimeter scales (Fig. 2c, d, g and h). Meanwhile, the hydrogel layer became denser after CTAB surfactant modification, which might be attributed to the dehydration effect induced by the interface electrostatic interaction (Fig. 2d and h). These results indicated that the iron net acted as mainly skeleton and guaranteed the three-dimensional spatial structure of hydrogel. More importantly, as it has been generally accepted that large amount of free or combined water molecules could be retained in the hydrogel by hydrogen bonding interactions and also could penetrate fully into the hydrogel microstructures at molecular scale, a stable hydration layer was acquired by hydrogel coated iron network as a consequence. Considering the physicochemical properties of hydrogel coated iron net, this unique interface with tailorable surface chemistry and uniform microporous structure was expected to be applied in the field of fluid drag reduction, oil/water separation, antibiofouling and some other relevant aspects. Firstly, underwater oil wettability of hydrogel coated iron net was investigated. It can be observed from the Fig. 3a, all of the measured oil droplets were spherical., and underwater oleophobicity/ superoleophobicity was ensured because of the stable hydration effect of hydrogel substrate. Meanwhile, the corresponding rolling off angles for the different oil droplets were less than 10°, these experimental results were mainly attributed to the hydrogel component because the hydrogel could greatly whittle the adhesive interaction between oil droplet and iron net. Although the underwater oil contact angles were almost similar for both the hydrogel and CTAB decorated hydrogel samples, we saw in particular that the rolling off angle could be dramatically affected by changing the surface chemistry. Higher rolling off angles (about 20-30°) were obtained on the cationic surfactant CTAB modified surface, which was mainly attributed to the increased surface interaction between the alkyl chain and oil droplets. 4
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Fig. 3. Comparison of the underwater contact angels and roll off angles of organic solvent and oil drops with different surface tensions, here the contact angles (left ordinate) and rolling off angles (right ordinate) were plotted against various oil mediums (a), corresponding adhesion forces on the as-prepared hydrogels with varying AAc/AAm compositions after decorating with 10 mM CTAB solution (b), adhesive force measuring curves of droplet on the hydrogel surface (c) and on the CTAB decorated hydrogel substrate (d), respectively.
Fig. 4. Schematic diagram of velocity profiles of steady-state Couette flow geometry and their corresponding drag reduction mechanism (a), shear stress characterization of oil medium PFPE flowing over AAc0.2-AAm1 hydrogel modified substrate (b), and the drag reduction efficient characterization for PTFE in which different CTAB modified hydrogel wall were applied (c), plots of drag reduction efficiency are depicted as a function of shear rate, (A) 10 mM CTAB modified AAc1AAm0.1 hydrogel sample, (B) 10 mM CTAB modified AAc1-AAm1 hydrogel sample, (C) 10 mM CTAB modified AAc0.2-AAm1 hydrogel sample and (D) 10 mM CTAB modified AAc0.1-AAm1 hydrogel sample, respectively. 5
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though the flux for the sea water/liquid paraffin mixture was only about 3213 ± 80 L m-2 h-1. It was easy to understand that the lower flux was attributed to the high viscosity of liquid paraffin. To be sure, these seawater efficiencies were lower, however they were still competitive as these fluxes were higher than that of the common filtration membrane, which were mainly attributed to the porous character at micron scale for the hydrogel tethered iron net (Fig. S1). As a consequence, given that the oil/water separation process was conducted with higher fluxes but without external driving force, such hydrogel coated iron nets were therefore highly attractive in terms of sea water treatment. The last aspect to notice was that the long-term application for oil medium drag reduction and oil/water separation was always hampered by biofouling and performance attenuation. Therefore, antifouling behavior of the hydrogel coated iron net was investigated by the adhesion of diatom Navicular sp. and red algae Porphyridium sp. As depicted in the Fig. 6, there were more than 2500 and 1600 /mm2 red algae and diatom fouled on the control iron net, respectively. Because of the stable hydration barrier composed of AAc/AAm copolymer [35], the results, however, showed lower adhesion amounts of either red algae or diatom for the hydrogel coated iron net. Compared with the control substrate, the corresponding adhesion amounts decreased sharply with more than up to 97% and 93% for the red algae and diatom. Consistent with the underwater oil contact angle characterization, these results suggested high efficiency, broad-spectrum fouling resistance capability and self-cleaning property of the hydrogel coated iron net for their longterm application, and also synergistically confirmed the stability while oil/water separation and fluid drag reduction. Besides its excellent antifouling performance, in order to have a comprehensive understanding concerning the long-term application of the prepared coating, studying the mechanical stability and its acid/ alkaline abrasive resistance were also necessary. In this paper, the friction characterization was conducted to prove the wear performance of hydrogel as depicted in the Fig. 7. The friction coefficient versus time curves clearly revealed their superior anti-wear behavior under different loads (Fig. 7a), while the friction coefficient increased slightly from 10−3 to 10-2 over the 20 min period (Fig. 7b). Notably, because the as-prepared robust hydrogel could maintain its original structure and prevent the water molecule being squeezed out, lubrication layer could be retained under larger normal loads and give a lower friction capability during the test procedure. Finally, the hydrogel samples were immerged in acid and alkali solution for a month (Fig. S3). As expected, there was no change significantly happened for themselves, and only the alkaline solution became more yellow after immersion. These performance evaluations indicated that the synthesized hydrogel coated substrate was capable of offering inherently stability even in the face of long-term mechanical wear and acid/alkaline solutions. For example, their excellent oil/sea water separation capability was still reserved after acid/alkali solution immersion (Fig. S4).
shear stress could be produced because of the weaker interaction between the hydrogel and working fluid. In this case, the slope of the velocity gradient became slighter when compared with the no-slip condition. Besides, because of the soft characteristic of hydrogel, selfadaptation of hydrogel surface structure regarding the distribution of the boundary fluid field can be expected, which meant not only the momentum transfer but the mass transfer could be relieved [24]. Accordingly, lower shear stress was produced near the adjacent interface and an obvious fluid drag reduction phenomenon would be exhibited for the oil flow over the hydrogel coated surface. To quantify the influence of surface chemistry on fluid movement, Fig. 4b-c show the quantitative drag reduction behaviors of oil flowing (PFPE) over the hydrogel coated surface. Different from the control surface, the corresponding shear stress for the hydrogel coated substrate decreased greatly during the whole test procedure, which might indicate that the working fluid flowed with reduced interface sticking, and decreased weight loss or cross-contaminant. While, for the surfactant CTAB modified hydrogel surface, the corresponding shear stress was larger than hydrogel coated surface but smaller than that of the control surface obviously, which was mainly attributed to the influence of the adsorbed alkyl chain on the flowing performance of oil medium [29]. Besides, the equivalent representation of this data in terms of drag-reduction efficiency are included in the Fig. 4c. It is easy to note that the drag reduction efficiencies increased with decreasing the molar content of AAc to AAm for the CTAB modified hydrogel samples, and the effect of CTAB on the drag reduction efficiency was becoming more ignorable when the AAc content was very low. For example, the average drag-reduction efficiency for the CTAB modified AAc1-AAm1 was about 30%. While for the AAc0.1-AAm1 hydrogel sample, the drag reduction efficiency was not affected by the CTAB modification and almost kept constant before and after CTAB modification (64%). These experimental results indicated that enough amount of CTAB alkyl chains were exhibited on the hydrogel surface and sufficient to generate no-slip effect, especially for the CTAB modified AAc1-AAm0.1 sample, even an obvious resistance increase phenomenon was obtained. It was, therefore, assumed that the CTAB coated hydrogel surface rendered them fully or partially wetted by PFPE. Besides, the rate-dependent drag-reduction performance suggested that fluid was pinned to the CTAB modified hydrogel wall and endured a critical shear stress. Beyond this point, the moving fluid was detached from the surface in some sense, and increased drag reduction efficiencies were observed with increasing the shear rate, accordingly. Over all, it can be concluded that drag reduction efficiency was related to the fluid organization near the substrate. Because of the attractive interfacial interaction for the CTAB modified surface, the critical stress to detach the adsorbing oil medium from the substrate, therefore, appeared when a higher shear stress was employed. However, the interfacial interaction of the flowing medium with hydrogel substrate without CTAB modification was repulsive, the wall shear stress therefore decreased and finally resulted in macroscopic drag reduction behavior. As we have observed previously, the porous structure could act as the water channel in hydrogel membrane. Therefore, an only gravitydriven oil/sea water separation experiment was performed as shown in Fig. 5. Firstly, the hydrogel coated iron net was pre-wetted by water, and oil/sea water mixture of red-dyed oil and colorless sea water was poured into the tube. Once the mixture contacted the membrane, the sea water component permeated rapidly, while the oil phase was intercepted due to oleophobicity. All the fluxes were recorded after conducting the separation process till stabilization. Five kinds of oil/sea water mixtures with different oil density and surface tense were used to evaluate the separation behaviors as shown in Fig. 5b. Driven by gravity, the calculated fluxes for the sea water/diesel, sea water/dichloroethane, sea water/toluene, and sea water/n-hexadecane mixtures were 42,711 ± 910, 26,397 ± 1164, 38,690 ± 991, and 31,260 ± 980 L m−2 h-1, respectively. Meanwhile, the water separation efficiencies were over 97% for these measured mixtures, even
4. Conclusion In this paper, an environmentally friendly AAc/AAm hydrogel coated iron net was developed through a facile surface self-catalyzed polymerization. By regulating the weak interactions between the alkyl chain with oil medium, rheological experiments for the oil medium flowing against CTAB surfactant modified hydrogel substrate verified the existence of controllable drag reduction effect through and the corresponding maximum of drag reduction efficiency was up to 64%. Based on porous characters and underwater oleophobicity of the asprepared hydrogel, the oil/sea water separation also has been achieved with higher fluxes about 42,000 L m−2 h-1. Moreover, such structured hydrogel coating was environmentally friendly with excellent antifouling performance, good abrasion resistance, and therefore showed great attraction, especially for oil medium transport and the involved long-term oil/sea water mixture treatment. Overall, our finding 6
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Fig. 5. Schematic diagram of oil/sea water separation test where the dichloroethane labeled by red is retained and sea water passed through hydrogel substrate. The insets are digital images of the oil/sea water mixture before and after separation (a), separation efficiencies and water fluxes of the prepared hydrogel membranes for different oil/sea water mixture (b) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
Fig. 6. Typical fluorescence images of attached red algae Porphyridium sp. (a,b) and diatom Navicular sp. (c,d) after exposure to control iron net (a,d) and hydrogel coated substrates (b,e), the corresponding relative amounts of red algae Porphyridium sp. and diatom Navicular sp. attached on the samples (c and f). The fresh iron net without hydrogel coating was included for comparison. The scale bar was 50 μm (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
Fig. 7. Durability tests of the hydrogel shrouded porous network through friction characterization, different payloads as depicted in the (a) were employed under constant frequency (1 Hz) through ball on disc rubbing test, and a long-term friction measurement reflected the robust adhesive force on the substrate as well as the coating durability, (b) the corresponding friction coefficient statistically calculated from the friction curves. 7
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demonstrated how altering chemical components and wettability can be used to affect surface fluid drag and thus control the drag-reduction efficiency. This novel surface construction technology capable of a considerable boundary slippage effect might be more practice for pipeline transport of flue, and even suggest a possibility of helping fluid transportation of micro-fluid devices.
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Acknowledgements
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The authors gratefully acknowledge the funding support from the NSFC (51905519, 51573198, 51403220, and21773274), Ministry of Science and Technology (2016YFC1100401) and the financial supports provided by the Dongfang Electric Machinery Co., ltd. (No. 2017GZ0053).
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Appendix A. Supplementary data [20]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mtcomm.2019. 100618.
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