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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)
Membrane Journal Vol.31 No.3 pp.170-183
DOI : https://doi.org/10.14579/MEMBRANE_JOURNAL.2021.31.3.170

Recent Progress in Patterned Membranes for Membrane-Based Separation Process

Hein Htet Aung, Rajkumar Patel†
†Corresponding author(e-mail: rajkumar@yonsei.ac.kr; http://orcid.org/0000-0002-3820-141X)
May 15, 2021 ; June 27, 2021 ; June 28, 2021

Abstract


Fouling has continued to be a problem that hinders the effectiveness of membrane properties. To solve this problem of reducing fouling effects on membrane surface properties, different and innovative types of membrane patterning has been proposed. This article reviews on the progress of patterned membranes and their separation process concerning the fouling effects of membranes. The types of separation processes that utilize the maximum effectiveness of the patterned membranes include nanofiltration (NF), reverse osmosis (RO), microfiltration (MF), ultrafiltration (UF), and pervaporation (PV). Using these separation processes have shown and prove to have a major effect on reducing fouling effects, and in addition, they also add beneficial properties to the patterned membranes. Each patterned membrane and their separation processes gave notable results in threshold towards flux, salt rejections, hydrophilicity and much more, but there are also some unsolved cases to be pointed out. In this review, the effects of patterned membrane for separation processes will be discussed.


fouling , membrane , thin film composite membrane

분리공정을 위한 패턴화 멤브레인 최근 연구 동향

헤인 탯 엉, 라즈쿠마 파텔†
연세대학교 언더우드국제대학 융합과학공학부 에너지환경과학공학

초록


파울링은 멤브레인 성질의 효과를 저해한다는 점에서 큰 문제다. 멤브레인의 표면적인 특성에 파울링의 영향이 감소하는 문제를 해결하기 위해 다양하고 기발한 멤브레인의 패턴화가 제시되었다. 패턴화된 멤브레인의 진행 과정과 멤브레 인의 파울링 효과의 분리 과정을 이 글을 통해 짚어보려 한다. 패턴화된 멤브레인의 효과를 최대한 활용하는 다양한 분리 과 정을 분석해야 하는데 바로 나노여과, 역삼투, 미세여과, 한외여과, 투과증발이다. 이러한 분리 과정들을 사용하면 파울링 효 과를 줄이는데 큰 효과가 있다는 점이 증명되었다. 또한, 패턴화된 멤브레인에 이로운 요소도 더해진다. 각각의 패턴화된 멤 브레인과 분리 과정들은 플럭스, 염 제거율, 친수성 등에 대한 임계값을 구하는데 주목할 만한 결과가 나왔지만, 아직 파악되 지 않은 부분을 확인할 필요도 있다. 본 총설에서는 분리공정을 위한 패턴화된 멤브레인의 효과에 대해 논의할 것이다.


    1. Introduction

    Even as the world progresses in many ways, water treatment continues to be a major issue that must be focused on. One of the main challenges that is still recognizable, despite many advancements on membrane development, is fouling effects on membranes. Many researches have been ongoing in order to control and reduce fouling[1-8]. These researches have resulted in the use of patterned membranes in separation processes that allow scientists, not only to find more efficient solutions towards fouling control, but also obtaining unexpected results that are beneficial in the modified membrane.

    There are a variety of approaches towards decreasing the fouling rate on membrane surfaces. One method is membrane surface modification, which can be classified as chemical or physical modifications. Chemical modifications have been focused on making the membrane surface more hydrophilic via grafting, adsorption, or coating[9-14]. Physical modifications draw from utilization of surface topography to prevent fouling, and recent studies results of surface topography can reduce foulant-surface interactions and thus mitigate fouling. It is from the latter that membrane scientists have been inspired to apply patterned surfaces on membrane separation processes. There have been ongoing efforts such as reports on direct nano-patterning of a polyamide (PA) thin-film composite (TFC) NF membrane combined with surface chemistry to reduce alginate fouling. It has shown to be a success and opened up possibilities for patterned membranes[15-17].

    Various patterned surfaces would be applied on membranes to test on anti-fouling effects. These membranes would show that there are lots of advantageous properties and potential to improve fouling resistance. However, there are some risks involved in their own field. In this review, patterned membranes with applications in NF, RO, MF, UF and PV are discussed. Fig. 1 represents the schematic diagram of a pattern membrane and Table 1 represents the summarized table of articles discussed in this review.

    2. Separation Process

    2.1. Nanofiltration

    Malakian et al. worked on the effects of colloidal fouling resistance in NF membranes by observing the effects of patterns and fouland chemistry on threshold flux[18]. To work on this, PA NF membranes were applied with a line-and-groove pattern through nanoimprint lithography (NIL). The characterization for the modified membrane includes atomic force microscopy (AFM) for membrane morphology. The modified membrane and as-received membrane will perform NF using silica nanoparticles (SiNPs) that possess four different surface chemistries. The results of this test will show that the modified membranes were superior to the as-received membranes. In the pure SiNP feed solution, the modified membrane showed a flux of 210 l m-2 h-1 and the as-received membrane showed a flux of 150 l m-2 h-1. In addition to this, all modified membranes showed about 20~25% increase in threshold flux for the foulant types. Foulant chemistry also played a role in increasing the threshold flux, which could be seen between the foulants and the modified membrane. The main contribution is the differences in Coulombic interaction and hydrophilicity which impacted the fouling rates. However, it remains that the patterning affecting the threshold flux was more notable.

    Shang et al. reported on the impacts of millimeterscale patterns for desalinations with a focus on antifouling properties[19]. To obtain this membrane, polyethylene (PE) support, which was made from melt casting and stretching, has been used for NF, giving the membrane a micrometer-scale surface. The PENF membrane would then be imprinted with a permeate spacer, which would give it a millimeter-scale pattern. This modified membrane would be compared with other NF membranes such as DF304i and DF304. Fig. 2 represents the field-emission scanning electron micro scopy (FE-SEM) images of DF304i, DF304, and PENF, and their performances of zeta potential, contact angle, and transport properties. Fig. 3 represents the illustrations of scaling and dead-end and crossflow conditions of DF304i, DF304, and PENF over periods of time. For the characterization of the membrane, FE-SEM, a SurPASS electrokinetic analyzer, and X-ray photoelectron spectroscopy (XPS) characterize the surface morphology, charge, and elemental compositions of the membranes, respectively. Fig. 4 represents the SEM images, scaling properties, and permeation performances on PENF nonwoven, spacer, and reverse spacer. DF304i, DF304, and PENF showed smooth surfaces, turing structures, and rough surfaces, respectively. Fig. 5 represents the average shear stress and streamline profiles of PENF spacer and reverse spacer. While results showed that the high water permeabilities were 14~22 l m-2 h-1 and a high rejection of greater than 95%, there are indications of PENF and PENF spacer having a slightly milder decline in flux compared to DF304i and DF304. This is the result of DF304i and DF304 being more hydrophobic. However, the PENF and PENF spacer exhibited stronger anti-scaling properties. The PENF micrometer-level pattern membrane exhibited properties that could control the growth of salt crystals, which contributed to reduced scaling rates. Meanwhile, the PENF spacer millimeter-level patterns could affect hydrodynamic impacts on the crystal growth of the membrane surface.

    In this study, Wang et al. reported on the study of a NF membrane for its antifouling effects[20]. The membrane was made with a poly(piperazine-amide) active layer atop a PE support layer, which was fabricated via an interfacial polymerization (IP) technique. For characterization, an attenuated total reflection fourier- transform infrared (ATR-FTIR) analyzed the surface chemistry and an AFM determined the surface roughness. The modified membrane, PENF-P, was compared to the original PENF and other NF membranes. The PENF-P showed superior fouling resistance towards scaling, organic, and inorganic fouling, to the point of even combined fouling due to its surface patterns. The PENF-P also exhibited a glucose rejection of over 80% when applied under pressure and had a water permeability of greater than 11 l m-2 h-1 bar-1. Not only did it exhibit such traits, but the membrane could also run for over 3000 hours while maintaining high performance in its anti-fouling traits.

    In this article, Weinman et al. reported on what factors could control the patternability of thin-film composite (TFC) NF and RO membranes[21]. Here, the report developed a set of heuristics to use NIL to nanopattern any TFC membranes. For characterization, SEM was used for membrane top and cross-section morphology. AFM was used for the surface morphology. The results showed that the multiple TFC NF and RO membranes with different chemistries did not have much correlation between each other, aside from certain membranes reaching a higher average peak height. The notable feature in this study was the presence of a humectant. The experiment showed that when changing the original humectant for a 15% glycerol humectant, the membrane average peak heights became more uniform. This shows that humectants can affect patterning, however more research will be required.

    Weinman et al. worked on the fouling resistance of a NF membrane that has physical and chemical modifications[ 22]. The membrane was created by having a nanoscale line-and-groove pattern be combined with a PA TFC membrane through thermal embossing. Afterwards, poly(ethylene glycol) diglycidyl ether (PEGDE) was applied onto the membrane as a chemical coating. For characterization, ATR-FTIR was used for the surface chemistry of as-received and modified membranes. The modified membrane showed a slight decline in flux compared to the as-received membranes depending on the amount of PEGDE applied. However, the modified membrane showed superior flux decline of about 0-8% reduction compared to the as-received membrane at an average of 22% over periods of time. The modified membrane was shown to sustain a flux of 120 l m-2 h-1 during a 2 hour test while maintaining its fouling resistance properties of a consistent 95% salt rejection.

    2.2. Reverse Osmosis

    Choi et al. studied the synthesis of RO membranes and their anti-fouling effects[23]. The preparation of the membrane was made in processes. First, RO membrane was created with a molecular layer-by-layer (mLbL) technique. Secondly, The membrane was applied with submicron titanium oxide (TiO2) pillar patterns through a sol-gel based nanoimprinting technique. AFM and SEM characterized the membrane morphology. X-ray diffraction (XRD) characterized the structural properties of the pattern. Fig. 6 represents the illustration of the preparation of the TiO2-patterned membrane, and AFM and SEM images from different angles. Fig. 7 represents the water flux and sodium chloride (NaCl) rejection of the neat mLbL and TiO2-patterned membranes as well as AFM, SEM, and AFM height profiles. Fig. 8 represents the schematics and graphs of the water flux of the neat mLbL, TiO2-flat, and TiO2-patterned membranes at different functions. Fig. 9 represents the contour of the shear stress, local wall shear stress values, and vertical positions of TiO2-patterned and TiO2-flat membrane surfaces. The results compared the neat mLbL, flat TiO2, and patterned TiO2 membranes. The modified membrane showed a lower water flux of 10.8 ± 1.9 l m-2 h-1 and higher NaCl rejection of 98.3 ± 0.8%. Additionally, the modified membrane also showed a lower flux reduction of around 8% while unpatterned and neat membrane showed a flux decline of around 20% and 30%, respectively. This was also the same case for the BSA rejection. The patterned membrane showed a flux decline at 15% while the unpatterned and neat membrane showed a decline at 40% and 90%, respectively. The patterned membrane also showed better antifouling properties for BSA, bacteria, and organic foulants due to its improved surface hydrophilicity.

    Choi et al. worked on a study for a series of Sharklet patterns combined with RO membranes and anti-biofouling effects[24]. The Sharklet-patterned RO membranes were synthesized with phase separation-induced micromolding followed by a layered IP. For characterization, FTIR was used for the chemical structures of the support and membrane samples. A total of four dimensional spacings were studied, which were 1.5, 2, 3, and 6 micrometer spacing. The Sharklet-patterned mem branes observed a similar water flux and high NaCl rejection. However, the Sharklet-patterned membrane with 2 micrometer spacings proved to be the most effective membrane. It performed the best dynamic biofouling trend and the second best static biofouling trend. Additionally, it also had the slowest flux decline compared to other dimension spacings, which allowed it to be the most effective membrane in the study.

    Zhou et al. presented on multiple models of RO membranes that were patterned with different geometries to understand their hydrodynamics[25]. The geometries with various patterns were created with SolidWorks. The computation fluid dynamics (CFD) were used to identify many different performances given by the membranes. Overall, the patterned membranes showed superior traits compared to flat membranes. The results did show that the flat membrane has superior permeate flux to the patterned membranes, but only in the actual area. The projected area, on the other hand, resulted in patterned membranes being superior. One patterned membrane, which consisted of a rectangular pillar pattern, showed a 40% higher projected- area flux and 41% greater surface area compared to the flat membrane. Another membrane, which had a long line-and-groove circle pattern, showed a 24% higher flux and 25% higher projected area than the flat membrane. Projected-area calculation showed that the high water flux is correlated to the increased surface area caused by the patterns. It was conclusive that the patterned membrane was superior to the flat membrane in the simulations, as projected-area flux is more relevant in full-scale systems.

    2.3. Microfiltration

    Gencal et al. reported on a patterned MF membrane [26]. The patterned MF membrane was made with a phase separation microfabrication (PSMF), making use of vapor-induced phase separation. To obtain the membranes, 5 minutes of vapor exposure was required for the symmetric unpatterned membrane while it took 5 and 10 minutes for the asymmetric patterned membranes. For characterization, SEM was used for membrane morphologies. The patterned membranes showed superior pure water permeance and pure water permeability when compared to the flat membrane. Additionally, the patterned membrane gained an increased surface area of 103% and 52% for 5 minutes and 10 minutes of vapor exposure, respectively.

    2.4. Ultrafiltration

    Ilyas et al. worked on a study that focuses on the prime solution casted on poluvinylidene fluoride (PVDF) and the effects of it[27]. To prepare this membrane, a flat PVDF membrane was fabricated with spray-modified non-solvent induced phase separation (s-NIPS). However, this suffers from a slow phase separation and thus a casting solution of 20% PVDF, 6.7% poly (vinylpyrrolidone) (PVP), and 1% H2O was applied and resulted in patterned PVDF membrane that will be investigated. For characterization SEM will be used for the membranes’ morphologies and dimensions. The modified membrane showed a higher performance of pure water permeance by 140%, but suffered a 20% decrease in BSA reduction. The modified membrane also showed much superior higher final permeance after 80 minutes of filtration time. It had a 12 times higher permeance after the filtration time compared to the unpatterned membrane.

    In this article, Maruf et al. described the effects of topography and IP on TFC membranes[28]. Patterned TFC membranes were synthesized with IP using concentrations such as m-phenylenediamine (MPD) or piperazine (PIP). Aromatic and semi-aromatic patterned layers would form on top of the UF membranes. For characterization, FE-SEM and ATF-FTIR were used for the cross-section and surface of the membranes. The modified membranes showed slightly better flux and salt rejection depending on the percent of the concentration. The modified membranes, however, also showed some non-conformal growth on the surfaces of the UF membranes. Fortunately, this could be prevented by adjusting the concentration of the amines used in the UF membranes.

    Maruf et al. studies on the fouling control of UF membranes by measuring the critical flux[29]. The UF membranes with submicron surface patterns were synthesized with NIL and compared to the commercial UF membranes. For characterization, AFM was used for the surface topography of the membranes. During the tests, it would be seen that the membranes would have high critical flux in correlation to the particle size, crossflow velocity and water contact angle. This modified membrane would also prove to be more superior when compared to the unpatterned membrane. The anti- fouling behaviour was also studied and could be attributed to the enhanced shear-induced diffusion.

    In accordance with Mazinani et al. reports on the 3D printed composite membranes and its outcome[30]. The membranes were fabricated through CFD and 3D printed technology. For characterization, SEM was used for the membrane support, selective layer, and 3D composite membranes. AFM was used for the surface roughness membranes. Fig. 10 represents preparation of wavy 3D composite membranes. Fig. 11 represents geometry and mesh discretizations of wavy and flat membranes. Fig. 12 represents the SEM micrograph and pore size distribution of the wavy support. Fig. 13 represents different magnifications of the wavy composite membrane. Results showed that the modified membranes had a 10% higher pure water permeance to the flat membrane. In addition, they also had a superior permeance recovery ratio of 87% to 54% from the flat membrane. The modified membrane also could retain around 87% of its initial pure water permeance and fouling resistant properties after 10 complete filtration cycles.

    2.5. Pervaporation

    In this article, He et al. studied the properties of bio-ethanol recovery[31]. For the preparation of the membrane, micro patterned polydimethylsiloxane (PDMS) was synthesized with phase separation micro-molding and modified immersion precipitation techniques. Then, the PDMS layer was cross linked with three crosslink agents, which were tetraethyl orthosilicate (TEOS), triethoxyvinylsilane (VTES), and p-tolyltriethoxysilane (p-TTES). For characterization, FTIR was used for the chemical structure of the composite membrane. The results of the study shows that the membrane crosslinked with TEOS was the most effective. The TEOS membrane had the highest flux of 977 g m-2 h-1, which was 2.11 times as high as the unpatterned membrane. The reason as to why TEOS membrane was superior is attributed to the patterns of the membranes. The TEOS membrane had a greater pattern height, larger surface area, and more roughness.

    Kharraz et al. reported on the performances of a PVDF membrane with the addition of both physical and chemical modifications[32]. The two-layer patterned superhydrophobic PVDF membrane was synthesized with the combination of NIPS and vapor induced phase separation (VIPS). SEM was used for morphology of the membrane. The modified membrane had many positive attributes. It had a higher surface area and achieved an 18% higher flux compared to the as-received membrane. It also had a slower flux decline of 15.2% over 97 hours of operation against 100% flux reduction of the as-received membrane.

    3. Others

    In this article, Ilyas et al. reported on a PVDF micro- patterned membrane and its anti-fouling effects[33]. The micro-patterned PVDF was prepared with s-NIPS. Then, the casting knives of rectangular and triangular patterning were fabricated into the membrane. For characterization, SEM was used to identify the morphologies and dimensions. The patterned membrane showed a lot of qualities. The results indicate that the patterned membranes had an overall higher pure water permeance ranging from 54-144% compared to the flat membrane. However, it did have a lower BSA rejection of 5-30%. Unlike the flat membrane, the patterned membranes were able to hold 3 hours of continuous filtration time and higher final permeances. The patterned membranes also showed higher surface area, porosity, and anti-fouling effects.

    Koupaei et al. worked on the control of the surface topography[34]. The creation of the membrane was used with electrohydrodynamic (EHD) patterning processes to be adopted on the preparation and outcome of patterning on polyethersulfone (PES) membranes. The patterning, growth of pillars, and membrane formation was visualized with a confocal scanning microscopy. Fig. 14 represents the schematic of the setup. Fig. 15 represents optical images of the EHD surface-patterned membrane. Fig. 16 represents SEM images of the EHD surface-patterned membrane produced. Fig. 17 represents SEM image and top view of the EHD surface- patterned membrane. The results indicate that the optimal size of these pillars ranged between 98 and 150 μm for width and height, respectively. The results showed that these dimensions would be the most effective anti-fouling effects.

    Zhao et al. reported on a patterned membrane and studied its membrane effects towards microalgae harvesting[ 35]. The negatively charged membrane was prepared from polysulfone/sulfonated polysulfone (PSf/sPSf) using a spray-modified phase inversion technique. For characterization, ATR-FTIR was used for the surface chemical property. TGA was used for thermal stability. The membrane exhibited the highest clean water permeance of 2420 L m-2 h-1 bar-1, and the highest critical flux at 55 l m-2 h-1.

    4. Conclusion

    Various methods and applications to prevent any membrane fouling have been addressed in this review article. In the section of patterned NFmembranes, PA NF with line and groove patterns, or PENF were used to test fouling resistance. There were results of increased threshold flux, hydrophilicity, increased water permeability and salt rejection. In the experiments with RO, patterned membranes known as submicron TiO2 pillar patterns, or Sharklet-patterned membranes were used that would utilize surface topography to prevent fouling accumulation, which maximized anti-fouling performances. In the case of MF, patterned MF membranes were fabricated through a PSMF, and results yielded that patterned membranes show to have less fouling rate than non-patterned membranes due to increased surface area. In the results of UF, patterned TFC membranes, or patterned, flat-sheet PVDF membranes synthesized via the s-NIPS method were used. These showed huge increase in performances for pure water permeances, but showed a decrease in BSA rejection. With PV, surface patterned nonporous PDMS composite membranes with enhanced ethanol recovery efficiency, or a two-layer patterned superhydrophobic PVDF membrane were fabricated, and these gave the results of a higher total flux, salt rejection and achieving enhanced fouling effect. The performance and results of the patterned membranes have shown to be beneficial, but there is still room for improvement. Considering the global issue regarding fouling, progress on patterned membranes for the separation process should be researched continuously.

    Figures

    MEMBRANE_JOURNAL-31-3-170_F1.gif

    Schematic diagram of patterned membrane.

    MEMBRANE_JOURNAL-31-3-170_F2.gif

    (a)–(c) FESEM images of (a) DF304i, (b) DF304, and (c) PENF; (d) ζ potential on membrane surfaces; (e) contact angle of membranes; and (f) transport properties of membranes under 2 bar. (Reproduced with permission from Shang et al., 19, Copyright 2020, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F3.gif

    (a) Illustrations of dead-end (no stirring) and crossflow nanofiltration; (b, c) normalized flux decline of three membranes in (b) dead-end scaling and (c) crossflow scaling; (d–f) SEM images of crystal formation on the PENF membrane under (d) dead-end conditions after 15 min, (e) dead-end conditions after 1 h, (f) crossflow conditions after 1 h, and (e) crossflow conditions after 2 h; illustrations of scaling on (h) microcrumpled PENF membrane (direct spatial effect), (i) microcrumpled PENF membrane (hydrodynamic effect), (j) smooth DF304i membrane, and (k) nanopatterned DF304 membrane. The scale bar shows the shear stress due to feed flow simulated by CFD (Reproduced with permission from Shang et al., 19, Copyright 2020, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F4.gif

    SEM images of the surfaces of the (a) spacer and the (b) reverse side of the spacer; photos of the surfaces of (c) spacer-imprinted PENF membrane and (d) reverse spacer-imprinted PENF membrane; SEM images of the cross-section of (e) spacer-imprinted PENF membrane and (f) reverse spacer-imprinted PENF membrane; (g) mechanical properties of PE, PES, PSF, and nonwoven; and normalized flux of (h) dead-end scaling, (i) crossflow scaling, and (j) alginate fouling tests (Reproduced with permission from Shang et al., 19, Copyright 2020, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F5.gif

    (a) Average shear stress on a smooth surface and PENF on spacers of different configurations and (b, c) streamline profiles on PENF imprinted by (b) spacer and (c) reverse spacer (Reproduced with permission from Shang et al., 19, Copyright 2020, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F6.gif

    (a) Schematic illustration of the fabrication of the submicron TiO2-patterned membrane. (b) AFM image of the molecular layer-by-layer (mLbL)-assembled membrane. (c, d) Top-down and cross-sectional SEM images of the mLbL-assembled membrane. (e) AFM image of the TiO2-patterned membrane. (f, g) Top-down and cross-sectional SEM images of the TiO2-patterned membrane. Scale bar = 500 nm for all SEM images (Reproduced with permission from Choi et al., 22, Copyright 2016, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F7.gif

    (a) Water flux and NaCl rejection of the neat mLbL and TiO2-patterned membranes. (b–d) Top-down SEM, AFM, and AFM height profiles of the TiO2-patterned membrane before permeation test. (e–g) Top-down SEM, AFM, and AFM height profiles of the TiO2-patterned membrane after permeation test. Scale bar = 500 nm for SEM images (Reproduced with permission from Choi et al., 22, Copyright 2016, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F8.gif

    Fouling test of the three membrane materials. (a) Normalized water flux of the neat mLbL, TiO2-flat, and TiO2-patterned membranes as a function of filtration time with the addition of BSA. The SEM images show cross sections of three membranes after the fouling test. Scale bar = 500 nm. (b) Normalized water flux of the neat mLbL, TiO2-flat, and TiO2-patterned membranes as a function of filtration time with the addition of P. aeruginosa. The SEM images show cross sections of three membranes after the fouling test. (c) Schematic representation of the accumulation of the foulant layer for the three membrane surfaces with filtration time (Reproduced with permission from Choi et al., 22, Copyright 2016, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F9.gif

    (a, b) Contour of the shear stress in the vicinity of the TiO2-patterned and TiO2-flat membrane surfaces. (c) Local wall shear stress values as a function of (d) the vertical position of the pillar pattern. The inset SEM micrograph shows the cross section of the pillar after fouling test (Scale bar = 100 nm). The dashed line denotes the value of averaged overall wall shear stress of the TiO2- flat membrane (Reproduced with permission from Choi et al., 22, Copyright 2016, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F10.gif

    Preparation of wavy 3D composite membranes: (a) 3D printing of the wavy support, (b) casting of the PES-selective layer, and (c) vacuum filtration is used to ensure adhesion of the selective layer onto the wavy support (Reproduced with permission from Mazinani et al., 29, Copyright 2019, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F11.gif

    Geometry for (a) wavy and (b) flat membranes along with mesh discretizations for (c) wavy and (d) flat used in the simulations, with dimensions matching the cross section at the midpoint of the filtration cell. A denser mesh was applied to the membrane surface and upper wall region where the velocity gradients were expected to vary more rapidly (Reproduced with permission from Mazinani et al., 29, Copyright 2019, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F12.gif

    (a) SEM micrograph of the wavy support with nominal pore size and interpore spacing of 200 μm. More than 100 pores from randomly selected locations on the support (e.g., five locations were highlighted) were analyzed to determine the openness and size of the pores by using ImageJ; (b) resulting pore size distribution of more than 100 randomly selected pores with nominal pore size and interpore spacing of 200 μm. Average uncertainty of the pore size is ±3 μm (Reproduced with permission from Mazinani et al., 29, Copyright 2019, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F13.gif

    (a) Optical and (b) digital micrographs of 3D printed wavy support. The color map represents the morphology with red, indicating the peaks and blue indicating the valleys; (c) the SEM micrograph of the 3D wavy support (top view); SEM micrographs of (d) top view and cross section of the PES-selective layer at (e) ×10 000 and (f) ×1000 magnifications; SEM micrographs (g) top view and (h) cross section, and (i) optical micrograph of the wavy composite membrane. The color map represents the morphology with red indicating the peaks and blue indicating the valleys (Reproduced with permission from Mazinani et al., 29, Copyright 2019, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F14.gif

    Schematic of the (a) experimental cell and (b) confocal microscopy setup used in this study (note: components are not scaled) (Reproduced with permission from Kpupaei et al., 33, Copyright 2019, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F15.gif

    Optical images of the EHD surface-patterned membrane produced after voltage application. Initial liquid film composition: PES 13%, PVP 2% in NMP solvent. Electrode distance: 10 μm, applied voltage: 2,100 dc. (A–E) In situ visualization of the growing features with time—scale bar: 200 μm. (F) Demonstration of a growing pillar before touching the surface of the upper electrode—scale bar: 50 μm. (G) Surface area spreading rate with time. (H) Top view showing the collection of grown pillars—scale bar: 100 μm. (Reproduced with permission from Kpupaei et al., 33, Copyright 2019, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F16.gif

    SEM images of the EHD surface-patterned membrane produced after phase inversion. Initial liquid film composition: PES 13%, PVP 2% in NMP solvent. Electrode distance: 25 μm, applied voltage: 2100 dc. (A) Top view showing a collection of pillars—scale bar: 200 μm. (B) Top view showing a single pillar—scale bar: 20 μm. (C) Top view showing the porous structure of the surface of a single pillar—scale bar: 400 nm. (D) Side view featuring a cylindrical concave shape of pillars—scale bar: 20 μm. (E) Side view of a single pillar—scale bar: 20 μm. (F) Side view featuring the porous structure of a single pillar—scale bar: 1 μm (Reproduced with permission from Kpupaei et al., 33, Copyright 2019, American Chemical Society).

    MEMBRANE_JOURNAL-31-3-170_F17.gif

    SEM image of the EHD surface-patterned membrane produced after phase separation by water. Top view showing the base of the membrane being porous—scale bar: 400 nm (Reproduced with permission from Kpupaei et al., 33, Copyright 2019, American Chemical Society).

    Tables

    Summary of Patterned Membranes

    References

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