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liquid interfaces and their implications for anti-fouling surfaces
Materials and Design 106 (2016) 226–234
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microscopic mechanisms of fluid flow at solid/liquid interfaces and their implications for anti-fouling surfaces Bing Yin a, Zhuhui Qiao a,⁎, Chaohong Liu b,⁎ a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China Haier Group, Qingdao 266103, China
a r t i c l e
i n f o
Article history: Received 8 March 2016 Received in revised form 26 May 2016 Accepted 30 May 2016 Available online 31 May 2016 Keywords: Fouling process Solid/liquid interfaces Planktonic microorganisms Molecular dynamics Velocity gradient
a b s t r a c t Although understanding the correlation between the microscopic mechanism of fluid flow and the fouling process at solid/liquid interfaces is of great theoretical and practical importance, this topic remains poorly studied. We investigated the microscopic mechanisms of flow at solid/liquid interfaces using molecular dynamics simulations and experiments with specially designed surfaces to understand the implications for anti-fouling processes. The interfacial layers absorbed close to the solid surfaces were closely related to surface fouling. Based on the analysis of the microscopic simulations and macroscopic experiments, weaker interfacial adsorption (c: 2–0.2) or lower surface roughness (L: 7.85–1.96 Å) or higher flow velocity (v: 10–80 m/s) leads to lower density and shear viscosity in the interfacial layer. Furthermore, a larger velocity gradient generates a higher shear stress. These changes in properties greatly reduce aggregation in the interfacial layer along with adhesion of planktonic microorganisms and suspended solids to the solid surface, which are conducive to anti-fouling. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Fouling has disastrous consequences for marine materials and facilities; [1–3] thus, studies on the mechanisms of fouling and the development of environmentally friendly anti-fouling strategies are necessary and important [4]. The use of fouling release coatings is a non-toxic and non-polluting approach to protect surfaces. The release behavior of fouling release coatings is caused by the high shear condition of the surface, resulting in the self-cleaning of the surface and reduction in surface resistance. Liquid environments are hotbeds for biological fouling. When an interface is examined at the nanometer level, a series of phenomena different from macroscopic phenomena appear, including liquid phase change, interface slip, and interlayer slip. Thus, studying these interfacial phenomena from only the macroscopic perspective using continuous media cannot produce an in-depth understanding of the fouling mechanism. Even microscopic studies are not sufficient; instead, molecular-level investigations of the origin and mechanism of liquid slip are required to understand the nature of shear and to realize superlubricity. Recently, liquid slip has been observed in micro- and nanoscale experimental flow devices, and the researchers reported that proper solid– fluid interaction depends both on the characteristic length scale of the fluid and the chemical and physical properties of the solid surface [5]. However, the study of liquid slip on solid/liquid interfaces still presents many challenges [6]. Furthermore, if the fluid scale shrinks to the ⁎ Corresponding authors. E-mail addresses: [email protected] (Z. Qiao), [email protected] (C. Liu).
http://dx.doi.org/10.1016/j.matdes.2016.05.116 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
molecular level, the continuum model can no longer be used to investigate its flow. In such a case, the fluid should be considered as an ensemble of individual molecules. Molecular dynamics (MD) simulations provide a powerful tool to study fluids at the molecular level. MD simulations can be used to complement experiments and provide insights into shear rate, vibrational frequency, and fluid velocity gradient close to the solid/liquid interface. Some simulations have already been employed to determine the dependence of slip on shear rate, chemical bond strength, and surface roughness [7,8]. However, the correlation between the microscopic mechanism of fluid flow and the fouling process at the solid/liquid interface remains unclear. This study aimed to examine the effects of interfacial interaction, flow velocity, surface roughness, and shear stress using MD simulations in order to provide guidance for research on anti-fouling surfaces and coatings. In addition, the simulation of fluid behavior at the solid/liquid interface and the comparison of molecular-level fluid behavior with macroscale (continuous fluid) behavior are important and useful for designing future anti-fouling coatings and establishing interface fluid theory. 2. Experimental details 2.1. MD simulation MD simulations were performed to investigate the flow boundary conditions and microscopic mechanisms at solid/liquid interfaces. All simulations were conducted in the NVT (constant number of particles,
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Fig. 1. Fabrication process of the multilayer films with different surface energy.
volume and temperature) ensemble with the COMPASS force field [9] and a 15 Å cutoff distance. The Coulomb and van der Waals longrange, non-bond interactions were handled using the standard Ewald and atom-based summation methods. A fixed time step size of 1 fs was used. The Andersen thermostat method was employed to control the system at a temperature of 298 K [10]. The initial velocity of each water molecule of the system was random. The systems were first integrated for 100 ps in equilibration runs followed by 120-ps production runs during which data were collected. The simulation system was comprised fluid molecules confined between two parallel planar walls. The properties of the planar solid wall could affect the liquid slip at the solid/liquid interface; thus, we chose a flexible wall model (solid atoms are allowed to vibrate) wherein the solid wall had a face-centered-cubic (fcc) lattice structure with the (1 1 1) surface in contact with the liquid. The solid walls were parallel to the XY plane and arranged as fcc unit cells. The walls were constructed of 960 platinum atoms tethered to lattice sites with a stiff linear spring. We chose water as the model fluid and achieved Couette flow by translating the two parallel planar (the upper and lower walls) walls with constant but opposite velocities in the X-direction. The initial
density of water was 1 g cm−3, and the translating velocities could be altered. 2.2. Special surfaces Layer-by-layer self-assembly is a handy and versatile surface modification method. Special surfaces were prepared using this method in the work. Multilayered films were constructed by first immersing fresh platinum substrates in a polycation solution (1 mg mL−1 PEI, pH 7.0) for 15 min followed by rinsing via three 1-min immersions in water. The substrates were then immersed in aqueous polyanion (3 mg mL−1 PAA, pH 5.0) solutions for 15 min followed by an identical rinsing procedure. This process was repeated until 14 layers were obtained. The naturally formed micro- and nanostructures of the multilayered films were chemically modified by PEI (S3) and fluorine silane molecules (S2, S1) to build coatings with different surface energies (Fig. 1). The coverage of silane molecules determines the surface energy. Controlling the modification time is to render the surfaces with lower surface energy [11]. Multilayer film samples S1 (130 °C for 3 h) and S2 (130 °C for 1 h) were modified with different amount of FAS
Fig. 2. (a) Schematic diagram of the apparatus for testing the fouling on sample surfaces, (b) forces acting on larva L in flow [12], u-flow velocity, D-drag; A-acceleration, R-resultant, (c) the tray of the samples.
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Fig. 3. SEM images of sample S3, S2, and S1 surfaces. The inset shows the contact angle for corresponding surface.
The samples were immersed in a suspension of SRB in the stationary growth phase, and the anti-fouling properties of S1 and S2 were determined based on their ability to prevent bacterial adhesion and biofilm formation on their surfaces. The rotating cylinder method is designed to test fouling as a function of velocity (Fig. 2). The system utilizes a rotating cylinder for specimen testing and is based on the rotating drum standard [13,14]. The sample surfaces were analyzed with scanning electron microscopy (SEM, JEOL JSM-7500F). The in-situ microscope images of aggregated bacteria were obtained using a fluorescence microscope (NIKON/Ti-E).
studies have been conducted in a previous work [15]. As the polyelectrolytes were assembled, the surface topography changed; the microand nanostructures increased in size with an increasing layer number, and the distance between them increased. The sizes of the worm-like structures on the multilayered films with 15 layers (S3) were on the order of 60 ± 5 nm, and the contact angle (CA) on the S3 surface was approximately 47°; this CA can be further increased to generate a superhydrophobic surface. Fluorine silane molecules covered the surfaces of the multilayered films, resulting in low surface energies. As shown in Fig. 3, the different films exhibited large differences in coverage of silane molecules. The distinguishable hierarchical micro- and nanostructures increased surface roughness, lowered surface energy, and resulted in a desirable superhydrophobic surface [16]. The hydrophobicity usually can be quantified by the CA of a drop of water on the surface. Because of their different surface properties, the CAs on the S3, S2, and S1 surfaces were measured to be 47 ± 2°, 107 ± 2° and 153 ± 2° (Fig. 3).
3. Results and discussion
3.2. Interfacial interaction and anti-fouling
Biomimetic research allows scientists to realize special wettability on functional surfaces, which may be applied in various applications in the future. To investigate the flow behaviors at solid/liquid interfaces, a series of simulations and anti-fouling tests under different experimental conditions were performed.
Compared with the liquid in the bulk region, the liquid in the solid/ liquid interface region exhibits different physical properties. Water molecules in the interface region manifest certain steady-state structural characteristics (e.g., density, viscosity, and velocity profile). These characteristics may be related to anti-fouling. The liquid molecules in the interface region are more ordered and stratified than those in the bulk liquid region because they are either adsorbed on solid walls with crystalline or semi-solid structures or are mobile with intrinsic slip velocity Vs at the wall surfaces. Adsorption requires a strong interaction between solid and liquid atoms. Once adsorption occurs, the liquid density profile along the Z-direction near the solid surface will have a much sharper peak. Furthermore, the velocity of the adsorbed layer should be almost
((tridecafluoroctyl)-triethoxysilane, CF3(CF2)5(CH2)2Si(OCH2CH3)3, DYNASYLAN F8261, Degussa) and then placed in an oven at 180 °C for 2.5 h to remove the unreacted FAS molecules. 2.3. Anti-fouling tests
3.1. Special surfaces Via the diffusion mechanism of “in” and “out”, molecular crosslinking took place between layers, and polyelectrolyte molecules were distorted accordingly, eventually causing the assembled film to grow exponentially and the surface to become uneven (Fig. 3a). Similar
Fig. 4. (a) The microscopic structure of the interfacial region after 100 ps, (b) density profile of water molecules as a function of their Z position.
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Fig. 5. Density profiles of water molecules as a function of their Z position for the simulation system (a) c = 0.2, (b) c = 0.8, (c) c = 1.4, (d) c = 2.0.
equal to that of the solid wall, causing interlayer slip between the adsorbed layer and the liquid next to it. As expected, the simulation results show that the microscopic structure of the interfacial region was different from that of the bulk region in the ways mentioned above (Fig. 4a). Couette flow was achieved as a result of the two parallel planar walls, which moved with constant but opposite velocities in the X-direction. First, the molecular density profiles of water were analyzed as a function of Z position and used to determine the interfacial distributions of water molecules. Fig. 4b shows the molecular density profile of water as a function of Z position in the simulation system. The reference z = 0 is the center atomic position of the bottom layer of the lower planar wall surface. As in all simulations, the temperature was maintained at 298 K throughout the simulation. Adsorption, which is an important factor affecting the distribution of interfacial water molecules, is generally related to the distance from the solid surface and the surface properties. The simulated density profiles of water indicate that density is maximized near the surfaces and becomes homogeneous moving away from each surface. Moving further from the solid surface, the effect of interfacial adsorption decreases, and the characteristics of the fluid change accordingly.
The Lennard-Jones (LJ) potential describes the interaction between neutral atoms or molecules (van der Waals forces) in MD computer simulations. In this study, the LJ potential was expressed as ϕab ¼ 4cε ab
σ 12 ab
r
σ 6 − ab r
ð1Þ
where ε is the depth of the potential well, c is a coefficient that adjusts the infiltration between the fluid and the solid wall, σ is the finite distance at which the interparticle potential is zero, and r is the distance between the particles. The greater the potential energy functions between fluid molecules and surface atoms increase the strength of the force between them. In MD simulations, the infiltration between the fluid and the solid wall is closely related to the interaction between them: [17] cosθLJ ¼ 2
cεab −1 ε
ð2Þ
pffiffiffiffiffiffiffiffiffiffi where εab ¼ εa εb, the εab and ε are constants in a simulation model, θLJ is the CA determined by the potential energy function. From the
Fig. 6. Fluorescence microscope images of aggregated bacteria on the (a) S1, (b) S2, and (c) S3 surfaces after immersion in a simulated SRB bacterial suspension fluid for 2 days.
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Fig. 7. SEM images of SRB adhered to the (a) S1, (b) S2, and (c) S3 surfaces after immersion in a simulated SRB bacterial suspension fluid for 2 days.
above formula, a greater coefficient c will result in a smaller contact angle. The affected region of the LJ potential energy function is a few atomic diameters in size; however, it is important when determining the nanoscale fluid properties on a solid surface. The potential energy function of the surface first affects the distribution of fluid molecules in the vicinity of the surface. Furthermore, past studies have indicated that the flow of a fluid is closely related to its microscopic distribution [18,19]. The configurations and density distributions of the fluid in simulation system are shown in Fig. 5. The statistical distribution of fluid molecules is uneven, and spatial fluctuations are observed near the solid surface. The size of the affected area is related to the potential energy between the fluid and surface. For c = 2, the interfacial interaction between fluid and solid surface is stronger, and the effect on the distribution near the surface is more noticeable. The molecules form a stable sticking layer at the interface [17], wherein the relative velocity between the molecule and solid wall is almost zero. In general, the width of the interfacial region (c = 2) is on the order of a few molecular diameters (multi-layer fluid molecules stick on the wall), which is larger than those of the other relative coefficients. However, owing to weak interfacial interaction at c = 0.2, the interfacial region is only one molecular diameter in width, corresponding to the first layer of fluid intensive distribution (Fig. 5a). Therefore, because of the effect of potential energy from the solid surface, the fluid molecules near the solid surface are layered and regular and fluctuate in space. For hydrophilic surfaces, the fluid molecules near the wall show solid-like properties, and the fluid density and molecular regularity increase. In contrast, for hydrophobic surfaces, the distribution of fluid molecules near the wall decrease and the fluid molecules near the wall form a relatively lower-density layer. The microscopic structure (Fig. 4) and density profile (Fig. 5) indicate the aggregation of water molecules near the atomically smooth surface, similar to that in previous reports [20]. The solid-like phase
grew in the interfacial region. The interfacial layer is approximately several monolayers thick, and this thickness may change with surface conditions. In general, the width of the interfacial region is expected to be on the order of a few molecular diameters. Interfacial adsorption is an important parameter that affects the interfacial layers and increases the chance that fluid molecules, microorganisms, and suspended solids will make contact with the solid surface. As the interfacial adsorption between the solid surface and fluid molecules weakens, the interfacial layers will become thinner, and the anti-fouling effect of the solid surface will change correspondingly. The in-situ microscope images show the interfacial structure. For the anti-fouling experiments, the fluorescence microscope images of attached bacteria on surfaces with different surface energies show the state of aggregation (Fig. 6); the aggregation of microorganisms at the solid/liquid interface decreased with decreasing surface energy, in agreement with the results of the MD simulations (Fig. 5). Biological fouling is a gradual process including conditional film formation followed by microbial adhesion, growth, reproduction, and fall off. Eventually, fouling results in the formation of a biofilm on a solid surface. Often, suspended solids and microorganisms are first present in the local environment of biofilm formation. Controlling the conditional film formation and microbial adhesion is important in anti-fouling operations. To clarify the relationship between surface properties and anti-fouling ability, the important aspect that should be looked at is the biofouling resistance for the simulated surface, which is with different interfacial interaction. Fig. 7 shows SEM images of the surfaces after immersion in a simulated SRB bacterial suspension fluid for 2 days. Compared with other two samples, sample S1 exhibited a much lower amount of adsorbed SRB, and the amount of adsorbed SRB was lower on S2 than on S3. These results are attributed to the effect of interfacial shear on interfacial interaction, which finally affects the microbial adhesion. Weaker interface adsorption and stronger interface
Fig. 8. Density profile as a function of Z position for a system with different velocities.
Fig. 9. Model of shear stress at a solid/liquid interface.
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Fig. 10. (a) The velocity distribution of the fluid; (b) MSD of the fluid in X direction; (c) MSD of the fluid in Y direction; (d) MSD of the fluid in Z direction; (e) The shear viscosity of fluid.
shearing help reduce adsorption and are also beneficial for fouling release. Interfacial interactions determine the distributions of liquid molecules and suspended solids in liquids in the Z-direction, which are closely related to the fouling of the solid/liquid interface. The interfacial interaction is an important factor for the velocity slip, microbial
adhesion, fouling release, and so on. While interfacial interaction is usually related to the surface energy of solid surface. Solid surfaces with different surface energies were used in the anti-fouling experiments under the same fluid conditions. A solid surface with a low surface energy is helpful in reducing the adsorption of suspended solids and microorganisms near the solid/liquid interface. In this case, the influence to
Fig. 11. SEM images of SRB adhered to the S1 surfaces after immersion in a simulated SRB bacterial suspension fluid for 3 days with the velocity of (a) 0.1 m/s, (b) 0.01 m/s, and (c) 0 m/s.
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Fig. 12. SEM images of SRB adhered to the S2 surfaces after immersion in a simulated flow of suspension of SRB for 3 days with the velocity of (a) 0.1 m/s, (b) 0.01 m/s, and (c) 0 m/s, respectively.
microbial adhesion activity by the shear stress caused by flow field will increase greatly, and fouling release will be easier (Figs. 6 and 7). The results of this study indicate that microbial adhesion, fouling release, and biofilm formation are strongly affected by interfacial interactions at the solid/liquid interface. 3.3. Velocity and anti-fouling In this study, the walls moved at a constant velocity, and the interfacial fluid between the parallel planar walls flowed along the neighboring wall owing to the strong adsorption. Changes in flow velocity did not affect the distribution of liquid molecules in the interfacial region (Fig. 8). It will generate a velocity profile along the direction perpendicular to the wall at a certain velocity and stable flow condition. An obvious correlation between the interfacial velocity profile and flow velocity was observed. In continuum mechanics, the fluid has a continuous linear velocity profile. Once subjected to the differences of force field, fluid velocity slip will occur at the solid/liquid interface and in the fluid inside [21]. Figs. 9 and 10a show that owing to strong adsorption from the nearby wall, the molecular layer moves at a velocity close to that of the wall.
Interlayer slip occurs between the adsorption layer and the free layer of the fluid molecular; the velocity distribution of the fluid is linear in the free layer, whereas the relative fluid velocity declines sharply near the adsorption layer. This phenomenon becomes more obvious moving nearer to the adsorption layer. This effect can be explained by Newton's formula, which is used when the velocity does not vary linearly in the Zdirection: τ¼η
du dz
ð3Þ
where τ is the shear force, du/dz is the local shear velocity, and η is the viscosity, which is related to the fluid properties. The above formula assumes that the fluid flows along parallel lines, and that the Z-axis is perpendicular to the fluid, pointing in the direction of maximum shear velocity. The shear stress is related to the velocity gradient du/dz of the fluid. As shown by the velocity distribution in Fig. 10a, moving closer to the surface, the velocity gradient du/dz decreases, and the shear stress and velocity slip decrease accordingly, enhancing the adhesion of the interfacial layer. Therefore, the shear stress hardly affects the microbial
Fig. 13. (a) Topology of the rough surface, with different asperities; (b) density profiles of water molecules as a function of the Z position for the system; (c) MSD of the fluid in X direction.
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movement patterns of planktonic microorganisms in the fluid, including the attachment of suspended particles, microbial adhesion, fouling release, and biofilm formation. The velocity gradient du/dz of the interface layer close to the solid surface is nearly zero; thus, fouling adhesion will be easy and stable. Increasing the shear stress and preventing fouling materials from reaching the anti-fouling surface are effective anti-fouling strategies. For the same solid/liquid interface system, increasing the velocity of the fluid will cause the slope of the velocity distribution line to steepen, and the shear stress will be greater in the same position. The mean square displacement (MSD) was also studied for fluids at solid/liquid interfaces. The MSD can express the mobility of fluid molecules, and the MSD result of different direction shows the molecular mobility in that direction. The slope of the MSD line represents the molecular mobility of the corresponding liquid. A large slope of the MSD line indicates a strong ability to migrate (mobility); that is, the molecules can migrate more easily in the simulation system. Fig. 10 shows that the velocity of a fluid affects the slope of the MSD line; when fluid velocity increases, the molecules tend to migrate more easily especially in the X and Y directions. As the simulation system getting balance, the mobility of fluid molecules is gradually reduced in Z direction. Due to the interface adsorption, the fluid molecules near the wall show solidlike properties, and the fluid density and molecular regularity increase, this also was further validated by MSD in Z direction. The shear viscosity of a system measures is resistance to flow. Fig. 10e shows that the shear viscosity also decreases as velocity increase. Thus, in theory, the velocity of the fluid near the solid/liquid interface is an important parameter affecting mobility and shear viscosity, and it is the key factor influencing surface anti-fouling. When other experimental conditions are controlled, if the velocity of the fluid decreases, the shear stress and mobility will decrease, the shear viscosity will increase, and the anti-fouling performance will suffer. To verify this result, additional anti-fouling experiments were conducted. As shown in Fig. 11, fewer cells were observed on the S1 (a) and S1 (b) surfaces, but the solid particles and a few of bacteria coverage on the S1 (c) sample. At the same time, the surface in S1 (b) is cleaner than the surface in S1 (c). It gradually increased as velocity was reduced during the experiment, indicating that the bacteria began to gradually adsorb on some parts of the surface in (c). The experimental results are consistent with the above MD results. The anti-fouling processes are also clearly affected by the interface shearing action and fluid viscosity at the solid/liquid interface. As flow velocity increased, the shearing action increased, and viscosity decreased, which could eventually improve the anti-fouling effect. For the S2 surfaces in similar experiments, the results validated the conclusion again (Fig. 12). 3.4. Surface roughness and anti-fouling The solid surface may exhibit certain degree of roughness introduced by manufacturing techniques or by adhesion of biological particles from the liquids [22,23], which is interesting to ask about the effects of surface roughness on solid/liquid antifouling. Three surface
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topologies, a flat (R0) and two rough surfaces (R1, R2), were studied in this work (Fig. 14a). The roughness was modeled by the introduction of asperities with truncated cone geometry (the height h and distance L). R0: h = 0.00 Å, L = 1.96 Å; R1: h = 3.92 Å, L = 5.89 Å; R2: h = 5.89 Å, L = 7.85 Å. For the rough solid surface, lamellar liquid molecules will also stick on the surface. While the liquid density profile along the Z-direction near the surface will have some irregular sharp peaks, which are affected by the microscopic structures on surface. For the concave valleys in a certain size range, the water molecules have invaded and been trapped in them (Fig. 13). There are mainly two aspects about the effect of the surface roughness to interfacial fluid. It changes the distribution of the fluid near surface microscopic structures. And rough structure on the surface disturbs the flow field of the solid/liquid interface, increasing the flow resistance and reducing the migration. Adjust the layers of self-assembled polyelectrolyte on substrates to control the surface roughness [15]. Fig. 14 shows SEM images of the surfaces with different surface roughness (Ra b Rb b Rc) after immersion in a simulated SRB bacterial suspension fluid for 1 day. Compared with other two samples, sample with 6 layers exhibited a much lower amount of adsorbed SRB, and the amount of adsorbed SRB got higher as the increasing of the roughness. These results are attributed to the effect of surface microscopic structures on surface roughness, which finally affect the microbial adhesion. The experimental anti-fouling results are consistent with the above MD results. The anti-fouling processes are also clearly affected by the roughness of solid surface. Within a certain range, higher surface roughness hinders the releasing, and is more likely to provide favorable conditions for microorganisms. 4. Conclusions At present, most techniques cannot directly observe the microscopic mechanisms of fluid and microbial movement patterns because of the difficulty in setting up accurate and controllable flow fields while visualizing and quantifying the microbial response. Investigating fluid flow at solid/liquid interfaces is important for understanding anti-fouling from the microcosmic perspective. MD simulations were performed to investigate fluid behaviors at the solid/liquid interface. MD simulations allow the direct visualization of how molecules are affected by flow and solid surfaces, providing an effective way to understand the relationship between the microscopic mechanism of fluid flow and microbial movement patterns at solid/liquid interfaces. The simulation results show that an interfacial layer is absorbed close to the solid surface. Interfacial interactions are an important factor affecting anti-fouling. Decreasing the distance between the adsorbate and the solid/liquid interface or increasing the surface energy strengthens the interfacial adsorption. This occurs because of increases
Fig. 14. SEM images of SRB adhered to surfaces after immersion in a simulated SRB bacterial suspension fluid for 1 day (a) Ra with 6 layers, (b) Rb with 10 layers, and (c) Rc with 14 layers.
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in the density and viscosity of the fluid in the interfacial layer, which increases the probability of fluid molecules, microorganisms, and suspended solids coming into contact with the solid surface. The fluid velocity is another important factor affecting anti-fouling. When the fluid velocity increases, the shear stress increases, whereas shear viscosity decreases. The effect of surface roughness also has been studied. In a certain range, higher surface roughness hinders the releasing, and is more likely to provide favorable conditions for microorganisms. These factors affect the microbial movement patterns of planktonic microorganisms in the fluid, which could explain the adhesion of suspended solids and microorganisms to the solid surface, resulting in biofilm formation. The simulations and experimental results provided micro- and macroscale insights, respectively. Reducing the interfacial interactions or increasing the flow velocity near the solid surface is not only helpful for reducing the adsorption of suspended particles and microbes but also for promoting fouling release. Consequently, the surface will maintain the original properties, facilitating anti-fouling. Acknowledgments This work was supported by The National Natural Science Foundation of China (51303188). References [1] A.M. Al Amer, T. Laoui, A. Abbas, N. Al-Aqeeli, F. Patel, M. Khraisheh, M. Ali Atieh, N. Hilal, Fabrication and antifouling behaviour of a carbon nanotube membrane, Mater. Des. 89 (2016) 549–558. [2] L. Wu, J. Sun, An improved process for polyvinylidene fluoride membrane preparation by using a water soluble diluent via thermally induced phase separation technique, Mater. Des. 86 (2015) 204–214. [3] H. Rajabi, N. Ghaemi, S.S. Madaeni, P. Daraei, B. Astinchap, S. Zinadini, S.H. Razavizadeh, Nano-zno embedded mixed matrix polyethersulfone (pes) membrane: influence of nanofiller shape on characterization and fouling resistance, Appl. Surf. Sci. 349 (2015) 66–77. [4] A. Javid, M. Kumar, L. Wen, S. Yoon, S.B. Jin, J.H. Lee, J.G. Han, Surface energy and wettability control in bio-inspired PEG like thin films, Mater. Des. 92 (2016) 405–413. [5] E. Luaga, H.A. Stone, Effective slip in pressure driven stokes flow, J. Fluid Mech. 489 (2003) 55.
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Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microscopic mechanisms of fluid flow at solid/liquid interfaces and their implications for anti-fouling surfaces Bing Yin a, Zhuhui Qiao a,⁎, Chaohong Liu b,⁎ a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China Haier Group, Qingdao 266103, China
a r t i c l e
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Article history: Received 8 March 2016 Received in revised form 26 May 2016 Accepted 30 May 2016 Available online 31 May 2016 Keywords: Fouling process Solid/liquid interfaces Planktonic microorganisms Molecular dynamics Velocity gradient
a b s t r a c t Although understanding the correlation between the microscopic mechanism of fluid flow and the fouling process at solid/liquid interfaces is of great theoretical and practical importance, this topic remains poorly studied. We investigated the microscopic mechanisms of flow at solid/liquid interfaces using molecular dynamics simulations and experiments with specially designed surfaces to understand the implications for anti-fouling processes. The interfacial layers absorbed close to the solid surfaces were closely related to surface fouling. Based on the analysis of the microscopic simulations and macroscopic experiments, weaker interfacial adsorption (c: 2–0.2) or lower surface roughness (L: 7.85–1.96 Å) or higher flow velocity (v: 10–80 m/s) leads to lower density and shear viscosity in the interfacial layer. Furthermore, a larger velocity gradient generates a higher shear stress. These changes in properties greatly reduce aggregation in the interfacial layer along with adhesion of planktonic microorganisms and suspended solids to the solid surface, which are conducive to anti-fouling. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Fouling has disastrous consequences for marine materials and facilities; [1–3] thus, studies on the mechanisms of fouling and the development of environmentally friendly anti-fouling strategies are necessary and important [4]. The use of fouling release coatings is a non-toxic and non-polluting approach to protect surfaces. The release behavior of fouling release coatings is caused by the high shear condition of the surface, resulting in the self-cleaning of the surface and reduction in surface resistance. Liquid environments are hotbeds for biological fouling. When an interface is examined at the nanometer level, a series of phenomena different from macroscopic phenomena appear, including liquid phase change, interface slip, and interlayer slip. Thus, studying these interfacial phenomena from only the macroscopic perspective using continuous media cannot produce an in-depth understanding of the fouling mechanism. Even microscopic studies are not sufficient; instead, molecular-level investigations of the origin and mechanism of liquid slip are required to understand the nature of shear and to realize superlubricity. Recently, liquid slip has been observed in micro- and nanoscale experimental flow devices, and the researchers reported that proper solid– fluid interaction depends both on the characteristic length scale of the fluid and the chemical and physical properties of the solid surface [5]. However, the study of liquid slip on solid/liquid interfaces still presents many challenges [6]. Furthermore, if the fluid scale shrinks to the ⁎ Corresponding authors. E-mail addresses: [email protected] (Z. Qiao), [email protected] (C. Liu).
http://dx.doi.org/10.1016/j.matdes.2016.05.116 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
molecular level, the continuum model can no longer be used to investigate its flow. In such a case, the fluid should be considered as an ensemble of individual molecules. Molecular dynamics (MD) simulations provide a powerful tool to study fluids at the molecular level. MD simulations can be used to complement experiments and provide insights into shear rate, vibrational frequency, and fluid velocity gradient close to the solid/liquid interface. Some simulations have already been employed to determine the dependence of slip on shear rate, chemical bond strength, and surface roughness [7,8]. However, the correlation between the microscopic mechanism of fluid flow and the fouling process at the solid/liquid interface remains unclear. This study aimed to examine the effects of interfacial interaction, flow velocity, surface roughness, and shear stress using MD simulations in order to provide guidance for research on anti-fouling surfaces and coatings. In addition, the simulation of fluid behavior at the solid/liquid interface and the comparison of molecular-level fluid behavior with macroscale (continuous fluid) behavior are important and useful for designing future anti-fouling coatings and establishing interface fluid theory. 2. Experimental details 2.1. MD simulation MD simulations were performed to investigate the flow boundary conditions and microscopic mechanisms at solid/liquid interfaces. All simulations were conducted in the NVT (constant number of particles,
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Fig. 1. Fabrication process of the multilayer films with different surface energy.
volume and temperature) ensemble with the COMPASS force field [9] and a 15 Å cutoff distance. The Coulomb and van der Waals longrange, non-bond interactions were handled using the standard Ewald and atom-based summation methods. A fixed time step size of 1 fs was used. The Andersen thermostat method was employed to control the system at a temperature of 298 K [10]. The initial velocity of each water molecule of the system was random. The systems were first integrated for 100 ps in equilibration runs followed by 120-ps production runs during which data were collected. The simulation system was comprised fluid molecules confined between two parallel planar walls. The properties of the planar solid wall could affect the liquid slip at the solid/liquid interface; thus, we chose a flexible wall model (solid atoms are allowed to vibrate) wherein the solid wall had a face-centered-cubic (fcc) lattice structure with the (1 1 1) surface in contact with the liquid. The solid walls were parallel to the XY plane and arranged as fcc unit cells. The walls were constructed of 960 platinum atoms tethered to lattice sites with a stiff linear spring. We chose water as the model fluid and achieved Couette flow by translating the two parallel planar (the upper and lower walls) walls with constant but opposite velocities in the X-direction. The initial
density of water was 1 g cm−3, and the translating velocities could be altered. 2.2. Special surfaces Layer-by-layer self-assembly is a handy and versatile surface modification method. Special surfaces were prepared using this method in the work. Multilayered films were constructed by first immersing fresh platinum substrates in a polycation solution (1 mg mL−1 PEI, pH 7.0) for 15 min followed by rinsing via three 1-min immersions in water. The substrates were then immersed in aqueous polyanion (3 mg mL−1 PAA, pH 5.0) solutions for 15 min followed by an identical rinsing procedure. This process was repeated until 14 layers were obtained. The naturally formed micro- and nanostructures of the multilayered films were chemically modified by PEI (S3) and fluorine silane molecules (S2, S1) to build coatings with different surface energies (Fig. 1). The coverage of silane molecules determines the surface energy. Controlling the modification time is to render the surfaces with lower surface energy [11]. Multilayer film samples S1 (130 °C for 3 h) and S2 (130 °C for 1 h) were modified with different amount of FAS
Fig. 2. (a) Schematic diagram of the apparatus for testing the fouling on sample surfaces, (b) forces acting on larva L in flow [12], u-flow velocity, D-drag; A-acceleration, R-resultant, (c) the tray of the samples.
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Fig. 3. SEM images of sample S3, S2, and S1 surfaces. The inset shows the contact angle for corresponding surface.
The samples were immersed in a suspension of SRB in the stationary growth phase, and the anti-fouling properties of S1 and S2 were determined based on their ability to prevent bacterial adhesion and biofilm formation on their surfaces. The rotating cylinder method is designed to test fouling as a function of velocity (Fig. 2). The system utilizes a rotating cylinder for specimen testing and is based on the rotating drum standard [13,14]. The sample surfaces were analyzed with scanning electron microscopy (SEM, JEOL JSM-7500F). The in-situ microscope images of aggregated bacteria were obtained using a fluorescence microscope (NIKON/Ti-E).
studies have been conducted in a previous work [15]. As the polyelectrolytes were assembled, the surface topography changed; the microand nanostructures increased in size with an increasing layer number, and the distance between them increased. The sizes of the worm-like structures on the multilayered films with 15 layers (S3) were on the order of 60 ± 5 nm, and the contact angle (CA) on the S3 surface was approximately 47°; this CA can be further increased to generate a superhydrophobic surface. Fluorine silane molecules covered the surfaces of the multilayered films, resulting in low surface energies. As shown in Fig. 3, the different films exhibited large differences in coverage of silane molecules. The distinguishable hierarchical micro- and nanostructures increased surface roughness, lowered surface energy, and resulted in a desirable superhydrophobic surface [16]. The hydrophobicity usually can be quantified by the CA of a drop of water on the surface. Because of their different surface properties, the CAs on the S3, S2, and S1 surfaces were measured to be 47 ± 2°, 107 ± 2° and 153 ± 2° (Fig. 3).
3. Results and discussion
3.2. Interfacial interaction and anti-fouling
Biomimetic research allows scientists to realize special wettability on functional surfaces, which may be applied in various applications in the future. To investigate the flow behaviors at solid/liquid interfaces, a series of simulations and anti-fouling tests under different experimental conditions were performed.
Compared with the liquid in the bulk region, the liquid in the solid/ liquid interface region exhibits different physical properties. Water molecules in the interface region manifest certain steady-state structural characteristics (e.g., density, viscosity, and velocity profile). These characteristics may be related to anti-fouling. The liquid molecules in the interface region are more ordered and stratified than those in the bulk liquid region because they are either adsorbed on solid walls with crystalline or semi-solid structures or are mobile with intrinsic slip velocity Vs at the wall surfaces. Adsorption requires a strong interaction between solid and liquid atoms. Once adsorption occurs, the liquid density profile along the Z-direction near the solid surface will have a much sharper peak. Furthermore, the velocity of the adsorbed layer should be almost
((tridecafluoroctyl)-triethoxysilane, CF3(CF2)5(CH2)2Si(OCH2CH3)3, DYNASYLAN F8261, Degussa) and then placed in an oven at 180 °C for 2.5 h to remove the unreacted FAS molecules. 2.3. Anti-fouling tests
3.1. Special surfaces Via the diffusion mechanism of “in” and “out”, molecular crosslinking took place between layers, and polyelectrolyte molecules were distorted accordingly, eventually causing the assembled film to grow exponentially and the surface to become uneven (Fig. 3a). Similar
Fig. 4. (a) The microscopic structure of the interfacial region after 100 ps, (b) density profile of water molecules as a function of their Z position.
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Fig. 5. Density profiles of water molecules as a function of their Z position for the simulation system (a) c = 0.2, (b) c = 0.8, (c) c = 1.4, (d) c = 2.0.
equal to that of the solid wall, causing interlayer slip between the adsorbed layer and the liquid next to it. As expected, the simulation results show that the microscopic structure of the interfacial region was different from that of the bulk region in the ways mentioned above (Fig. 4a). Couette flow was achieved as a result of the two parallel planar walls, which moved with constant but opposite velocities in the X-direction. First, the molecular density profiles of water were analyzed as a function of Z position and used to determine the interfacial distributions of water molecules. Fig. 4b shows the molecular density profile of water as a function of Z position in the simulation system. The reference z = 0 is the center atomic position of the bottom layer of the lower planar wall surface. As in all simulations, the temperature was maintained at 298 K throughout the simulation. Adsorption, which is an important factor affecting the distribution of interfacial water molecules, is generally related to the distance from the solid surface and the surface properties. The simulated density profiles of water indicate that density is maximized near the surfaces and becomes homogeneous moving away from each surface. Moving further from the solid surface, the effect of interfacial adsorption decreases, and the characteristics of the fluid change accordingly.
The Lennard-Jones (LJ) potential describes the interaction between neutral atoms or molecules (van der Waals forces) in MD computer simulations. In this study, the LJ potential was expressed as ϕab ¼ 4cε ab
σ 12 ab
r
σ 6 − ab r
ð1Þ
where ε is the depth of the potential well, c is a coefficient that adjusts the infiltration between the fluid and the solid wall, σ is the finite distance at which the interparticle potential is zero, and r is the distance between the particles. The greater the potential energy functions between fluid molecules and surface atoms increase the strength of the force between them. In MD simulations, the infiltration between the fluid and the solid wall is closely related to the interaction between them: [17] cosθLJ ¼ 2
cεab −1 ε
ð2Þ
pffiffiffiffiffiffiffiffiffiffi where εab ¼ εa εb, the εab and ε are constants in a simulation model, θLJ is the CA determined by the potential energy function. From the
Fig. 6. Fluorescence microscope images of aggregated bacteria on the (a) S1, (b) S2, and (c) S3 surfaces after immersion in a simulated SRB bacterial suspension fluid for 2 days.
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Fig. 7. SEM images of SRB adhered to the (a) S1, (b) S2, and (c) S3 surfaces after immersion in a simulated SRB bacterial suspension fluid for 2 days.
above formula, a greater coefficient c will result in a smaller contact angle. The affected region of the LJ potential energy function is a few atomic diameters in size; however, it is important when determining the nanoscale fluid properties on a solid surface. The potential energy function of the surface first affects the distribution of fluid molecules in the vicinity of the surface. Furthermore, past studies have indicated that the flow of a fluid is closely related to its microscopic distribution [18,19]. The configurations and density distributions of the fluid in simulation system are shown in Fig. 5. The statistical distribution of fluid molecules is uneven, and spatial fluctuations are observed near the solid surface. The size of the affected area is related to the potential energy between the fluid and surface. For c = 2, the interfacial interaction between fluid and solid surface is stronger, and the effect on the distribution near the surface is more noticeable. The molecules form a stable sticking layer at the interface [17], wherein the relative velocity between the molecule and solid wall is almost zero. In general, the width of the interfacial region (c = 2) is on the order of a few molecular diameters (multi-layer fluid molecules stick on the wall), which is larger than those of the other relative coefficients. However, owing to weak interfacial interaction at c = 0.2, the interfacial region is only one molecular diameter in width, corresponding to the first layer of fluid intensive distribution (Fig. 5a). Therefore, because of the effect of potential energy from the solid surface, the fluid molecules near the solid surface are layered and regular and fluctuate in space. For hydrophilic surfaces, the fluid molecules near the wall show solid-like properties, and the fluid density and molecular regularity increase. In contrast, for hydrophobic surfaces, the distribution of fluid molecules near the wall decrease and the fluid molecules near the wall form a relatively lower-density layer. The microscopic structure (Fig. 4) and density profile (Fig. 5) indicate the aggregation of water molecules near the atomically smooth surface, similar to that in previous reports [20]. The solid-like phase
grew in the interfacial region. The interfacial layer is approximately several monolayers thick, and this thickness may change with surface conditions. In general, the width of the interfacial region is expected to be on the order of a few molecular diameters. Interfacial adsorption is an important parameter that affects the interfacial layers and increases the chance that fluid molecules, microorganisms, and suspended solids will make contact with the solid surface. As the interfacial adsorption between the solid surface and fluid molecules weakens, the interfacial layers will become thinner, and the anti-fouling effect of the solid surface will change correspondingly. The in-situ microscope images show the interfacial structure. For the anti-fouling experiments, the fluorescence microscope images of attached bacteria on surfaces with different surface energies show the state of aggregation (Fig. 6); the aggregation of microorganisms at the solid/liquid interface decreased with decreasing surface energy, in agreement with the results of the MD simulations (Fig. 5). Biological fouling is a gradual process including conditional film formation followed by microbial adhesion, growth, reproduction, and fall off. Eventually, fouling results in the formation of a biofilm on a solid surface. Often, suspended solids and microorganisms are first present in the local environment of biofilm formation. Controlling the conditional film formation and microbial adhesion is important in anti-fouling operations. To clarify the relationship between surface properties and anti-fouling ability, the important aspect that should be looked at is the biofouling resistance for the simulated surface, which is with different interfacial interaction. Fig. 7 shows SEM images of the surfaces after immersion in a simulated SRB bacterial suspension fluid for 2 days. Compared with other two samples, sample S1 exhibited a much lower amount of adsorbed SRB, and the amount of adsorbed SRB was lower on S2 than on S3. These results are attributed to the effect of interfacial shear on interfacial interaction, which finally affects the microbial adhesion. Weaker interface adsorption and stronger interface
Fig. 8. Density profile as a function of Z position for a system with different velocities.
Fig. 9. Model of shear stress at a solid/liquid interface.
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Fig. 10. (a) The velocity distribution of the fluid; (b) MSD of the fluid in X direction; (c) MSD of the fluid in Y direction; (d) MSD of the fluid in Z direction; (e) The shear viscosity of fluid.
shearing help reduce adsorption and are also beneficial for fouling release. Interfacial interactions determine the distributions of liquid molecules and suspended solids in liquids in the Z-direction, which are closely related to the fouling of the solid/liquid interface. The interfacial interaction is an important factor for the velocity slip, microbial
adhesion, fouling release, and so on. While interfacial interaction is usually related to the surface energy of solid surface. Solid surfaces with different surface energies were used in the anti-fouling experiments under the same fluid conditions. A solid surface with a low surface energy is helpful in reducing the adsorption of suspended solids and microorganisms near the solid/liquid interface. In this case, the influence to
Fig. 11. SEM images of SRB adhered to the S1 surfaces after immersion in a simulated SRB bacterial suspension fluid for 3 days with the velocity of (a) 0.1 m/s, (b) 0.01 m/s, and (c) 0 m/s.
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Fig. 12. SEM images of SRB adhered to the S2 surfaces after immersion in a simulated flow of suspension of SRB for 3 days with the velocity of (a) 0.1 m/s, (b) 0.01 m/s, and (c) 0 m/s, respectively.
microbial adhesion activity by the shear stress caused by flow field will increase greatly, and fouling release will be easier (Figs. 6 and 7). The results of this study indicate that microbial adhesion, fouling release, and biofilm formation are strongly affected by interfacial interactions at the solid/liquid interface. 3.3. Velocity and anti-fouling In this study, the walls moved at a constant velocity, and the interfacial fluid between the parallel planar walls flowed along the neighboring wall owing to the strong adsorption. Changes in flow velocity did not affect the distribution of liquid molecules in the interfacial region (Fig. 8). It will generate a velocity profile along the direction perpendicular to the wall at a certain velocity and stable flow condition. An obvious correlation between the interfacial velocity profile and flow velocity was observed. In continuum mechanics, the fluid has a continuous linear velocity profile. Once subjected to the differences of force field, fluid velocity slip will occur at the solid/liquid interface and in the fluid inside [21]. Figs. 9 and 10a show that owing to strong adsorption from the nearby wall, the molecular layer moves at a velocity close to that of the wall.
Interlayer slip occurs between the adsorption layer and the free layer of the fluid molecular; the velocity distribution of the fluid is linear in the free layer, whereas the relative fluid velocity declines sharply near the adsorption layer. This phenomenon becomes more obvious moving nearer to the adsorption layer. This effect can be explained by Newton's formula, which is used when the velocity does not vary linearly in the Zdirection: τ¼η
du dz
ð3Þ
where τ is the shear force, du/dz is the local shear velocity, and η is the viscosity, which is related to the fluid properties. The above formula assumes that the fluid flows along parallel lines, and that the Z-axis is perpendicular to the fluid, pointing in the direction of maximum shear velocity. The shear stress is related to the velocity gradient du/dz of the fluid. As shown by the velocity distribution in Fig. 10a, moving closer to the surface, the velocity gradient du/dz decreases, and the shear stress and velocity slip decrease accordingly, enhancing the adhesion of the interfacial layer. Therefore, the shear stress hardly affects the microbial
Fig. 13. (a) Topology of the rough surface, with different asperities; (b) density profiles of water molecules as a function of the Z position for the system; (c) MSD of the fluid in X direction.
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movement patterns of planktonic microorganisms in the fluid, including the attachment of suspended particles, microbial adhesion, fouling release, and biofilm formation. The velocity gradient du/dz of the interface layer close to the solid surface is nearly zero; thus, fouling adhesion will be easy and stable. Increasing the shear stress and preventing fouling materials from reaching the anti-fouling surface are effective anti-fouling strategies. For the same solid/liquid interface system, increasing the velocity of the fluid will cause the slope of the velocity distribution line to steepen, and the shear stress will be greater in the same position. The mean square displacement (MSD) was also studied for fluids at solid/liquid interfaces. The MSD can express the mobility of fluid molecules, and the MSD result of different direction shows the molecular mobility in that direction. The slope of the MSD line represents the molecular mobility of the corresponding liquid. A large slope of the MSD line indicates a strong ability to migrate (mobility); that is, the molecules can migrate more easily in the simulation system. Fig. 10 shows that the velocity of a fluid affects the slope of the MSD line; when fluid velocity increases, the molecules tend to migrate more easily especially in the X and Y directions. As the simulation system getting balance, the mobility of fluid molecules is gradually reduced in Z direction. Due to the interface adsorption, the fluid molecules near the wall show solidlike properties, and the fluid density and molecular regularity increase, this also was further validated by MSD in Z direction. The shear viscosity of a system measures is resistance to flow. Fig. 10e shows that the shear viscosity also decreases as velocity increase. Thus, in theory, the velocity of the fluid near the solid/liquid interface is an important parameter affecting mobility and shear viscosity, and it is the key factor influencing surface anti-fouling. When other experimental conditions are controlled, if the velocity of the fluid decreases, the shear stress and mobility will decrease, the shear viscosity will increase, and the anti-fouling performance will suffer. To verify this result, additional anti-fouling experiments were conducted. As shown in Fig. 11, fewer cells were observed on the S1 (a) and S1 (b) surfaces, but the solid particles and a few of bacteria coverage on the S1 (c) sample. At the same time, the surface in S1 (b) is cleaner than the surface in S1 (c). It gradually increased as velocity was reduced during the experiment, indicating that the bacteria began to gradually adsorb on some parts of the surface in (c). The experimental results are consistent with the above MD results. The anti-fouling processes are also clearly affected by the interface shearing action and fluid viscosity at the solid/liquid interface. As flow velocity increased, the shearing action increased, and viscosity decreased, which could eventually improve the anti-fouling effect. For the S2 surfaces in similar experiments, the results validated the conclusion again (Fig. 12). 3.4. Surface roughness and anti-fouling The solid surface may exhibit certain degree of roughness introduced by manufacturing techniques or by adhesion of biological particles from the liquids [22,23], which is interesting to ask about the effects of surface roughness on solid/liquid antifouling. Three surface
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topologies, a flat (R0) and two rough surfaces (R1, R2), were studied in this work (Fig. 14a). The roughness was modeled by the introduction of asperities with truncated cone geometry (the height h and distance L). R0: h = 0.00 Å, L = 1.96 Å; R1: h = 3.92 Å, L = 5.89 Å; R2: h = 5.89 Å, L = 7.85 Å. For the rough solid surface, lamellar liquid molecules will also stick on the surface. While the liquid density profile along the Z-direction near the surface will have some irregular sharp peaks, which are affected by the microscopic structures on surface. For the concave valleys in a certain size range, the water molecules have invaded and been trapped in them (Fig. 13). There are mainly two aspects about the effect of the surface roughness to interfacial fluid. It changes the distribution of the fluid near surface microscopic structures. And rough structure on the surface disturbs the flow field of the solid/liquid interface, increasing the flow resistance and reducing the migration. Adjust the layers of self-assembled polyelectrolyte on substrates to control the surface roughness [15]. Fig. 14 shows SEM images of the surfaces with different surface roughness (Ra b Rb b Rc) after immersion in a simulated SRB bacterial suspension fluid for 1 day. Compared with other two samples, sample with 6 layers exhibited a much lower amount of adsorbed SRB, and the amount of adsorbed SRB got higher as the increasing of the roughness. These results are attributed to the effect of surface microscopic structures on surface roughness, which finally affect the microbial adhesion. The experimental anti-fouling results are consistent with the above MD results. The anti-fouling processes are also clearly affected by the roughness of solid surface. Within a certain range, higher surface roughness hinders the releasing, and is more likely to provide favorable conditions for microorganisms. 4. Conclusions At present, most techniques cannot directly observe the microscopic mechanisms of fluid and microbial movement patterns because of the difficulty in setting up accurate and controllable flow fields while visualizing and quantifying the microbial response. Investigating fluid flow at solid/liquid interfaces is important for understanding anti-fouling from the microcosmic perspective. MD simulations were performed to investigate fluid behaviors at the solid/liquid interface. MD simulations allow the direct visualization of how molecules are affected by flow and solid surfaces, providing an effective way to understand the relationship between the microscopic mechanism of fluid flow and microbial movement patterns at solid/liquid interfaces. The simulation results show that an interfacial layer is absorbed close to the solid surface. Interfacial interactions are an important factor affecting anti-fouling. Decreasing the distance between the adsorbate and the solid/liquid interface or increasing the surface energy strengthens the interfacial adsorption. This occurs because of increases
Fig. 14. SEM images of SRB adhered to surfaces after immersion in a simulated SRB bacterial suspension fluid for 1 day (a) Ra with 6 layers, (b) Rb with 10 layers, and (c) Rc with 14 layers.
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in the density and viscosity of the fluid in the interfacial layer, which increases the probability of fluid molecules, microorganisms, and suspended solids coming into contact with the solid surface. The fluid velocity is another important factor affecting anti-fouling. When the fluid velocity increases, the shear stress increases, whereas shear viscosity decreases. The effect of surface roughness also has been studied. In a certain range, higher surface roughness hinders the releasing, and is more likely to provide favorable conditions for microorganisms. These factors affect the microbial movement patterns of planktonic microorganisms in the fluid, which could explain the adhesion of suspended solids and microorganisms to the solid surface, resulting in biofilm formation. The simulations and experimental results provided micro- and macroscale insights, respectively. Reducing the interfacial interactions or increasing the flow velocity near the solid surface is not only helpful for reducing the adsorption of suspended particles and microbes but also for promoting fouling release. Consequently, the surface will maintain the original properties, facilitating anti-fouling. Acknowledgments This work was supported by The National Natural Science Foundation of China (51303188). References [1] A.M. Al Amer, T. Laoui, A. Abbas, N. Al-Aqeeli, F. Patel, M. Khraisheh, M. Ali Atieh, N. Hilal, Fabrication and antifouling behaviour of a carbon nanotube membrane, Mater. Des. 89 (2016) 549–558. [2] L. Wu, J. Sun, An improved process for polyvinylidene fluoride membrane preparation by using a water soluble diluent via thermally induced phase separation technique, Mater. Des. 86 (2015) 204–214. [3] H. Rajabi, N. Ghaemi, S.S. Madaeni, P. Daraei, B. Astinchap, S. Zinadini, S.H. Razavizadeh, Nano-zno embedded mixed matrix polyethersulfone (pes) membrane: influence of nanofiller shape on characterization and fouling resistance, Appl. Surf. Sci. 349 (2015) 66–77. [4] A. Javid, M. Kumar, L. Wen, S. Yoon, S.B. Jin, J.H. Lee, J.G. Han, Surface energy and wettability control in bio-inspired PEG like thin films, Mater. Des. 92 (2016) 405–413. [5] E. Luaga, H.A. Stone, Effective slip in pressure driven stokes flow, J. Fluid Mech. 489 (2003) 55.
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