Directional Fluid Gating by Janus Membranes with Heterogeneous

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Directional Fluid Gating By Janus Membranes With Heterogeneous Wetting Properties For Selective Oil-Water Separation Prashant Gupta, and Balasubramanian Kandasubramanian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 20, 2017

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ACS Applied Materials & Interfaces

Directional Fluid Gating By Janus Membranes With Heterogeneous Wetting Properties For Selective Oil-Water Separation Prashant Guptaa, Balasubramanian K.*,a a

Nano Surface Texturing Lab, Department of Materials Engineering, DIAT(DU), Ministry of

Defence, Girinagar, Pune-411025, India

KEYWORDS: Janus Membrane, Superhydrophobicity, Superoleophilicity, Directional Fluid Diode, Oil/Water Separation.

ABSTRACT:

The rising oil seepage accidents evolved into a global issue necessitating

immediate counter measure to abridge its catastrophic repercussions on sensitive marine ecosystem urging innovative techniques for effective oil/water separation. Here, we report surface tailored wettability modified superhydrophobic/superoleophilic Janus membrane by impregnating non-ionic surfactant stabilized nano-sized polytetrafluoroethylene (PTFE) dispersion polymerized via non-fluorinated processing aid onto cotton substrate using Meyer rod coating technique, exhibiting excellent separation efficiency up to 98% with various petroleum

ACS Paragon Plus Environment

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products and retaining its intrinsic properties for at least 30 recurrences. Morphological analysis revealed the generation of closely spaced irregularly patterned nanospindles on the microfibral cotton

surface

devising

hierarchical

dual-scale

surface

architecture,

followed

by

superhydrophobicization (WCA 168o ± 3o) and low ice-adhesion, illustrating deviation from conventional Wenzel and Cassie-Baxter wetting theories. The developed membrane exhibited flame retardancy, anti-icing characteristic and retained its superhydrophobic/superoleophillic characteristic of Janus membrane in hyper-saline solution, UV-irradiation of wavelenghth 254 nm, high temperature of 150oC and sub-zero temperature of -20oC. Furthermore, we hypothesized the developed membranes as a directional fluid diode, allowing lower surface tension liquids (oil) to permeate while barring higher surface tension liquids (water) from penetrating and the breakthrough pressure of 0.65 kPa for water permeation was also mathematically calculated.

This study systematically exemplifies the reported fabric as a

potentially competent alternative for cleaning massive marine oil seepages.

INTRODUCTION Globally, the rising oil-seepage accidents in marine and oceanic ecosystem via natural cataclysm or unannounced leakage is an appalling issue,1 which compels expeditious attention and deterrent measures. The mammoth quantity of oil discharge (~1.7 - 8.8 million tons) recorded each year2, 3 having a devastating impact on the environment and ecological system of aquariums4,

5

depicts the urgency for developing novel technologies to achieve effective oil-

water separation.6

- 9

Conventional techniques such as skimming, floatation, ultrasonic

separation, gravity separation, membrane filtration and coagulation-flocculation suffer limited


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applicability due to their high energy-cost, low separation efficiency, complex separation devices and secondary pollution.10

- 13

This precarious situation demands the development of eco-

friendly, cost-effective, efficient and reusable novel functional materials for oil-water separation, especially self-driven separation membranes. Recently, special wettable materials exhibiting distinct opposite affinities towards oil and water have attracted increased worldwide attention for oil-water separation due to their high separation efficiency and selectivity.14 - 16 These special wettable materials manifest directional fluid gating effect with integrated selectivity of either water or oil controlled by the surface property of the membrane.17 Special wettability of a membrane with oil and water can be achieved by altering either surface chemistry or surface architecture of the membrane surface.1820

The former involves chemically regulating the surface tension of membrane surface between

water (72.8 mN/m) and oil (30 mN/m) allowing the membrane to permeate oil and prevent water from penetrating.21 While the latter involves the introduction of multi-scale roughness on the surface which makes a hydrophillic surface more hydrophobic or even superhydrophobic due to the repulsive cushion of air pockets entrapped beneath the water droplet and an oleophillic surface becomes superoleophillic due to capillary effect.22,

23

Thus, a membrane surface

simultaneously exhibiting superhydrophobic and superoleophillic nature can be obtained by incorporating low surface tension and appropriate rough topography. This perception of dual functionality has been derived from Janus particles, in which half of the surface possesses hydrophobicity while the other half behaves as hydrophillic.24, 25 The membrane acquiring two or more discrete functional characteristics (in our case superhydrophobic and superoleophillic), can be termed as Janus membrane.21 Various techniques have been employed to develop superhydrophobic Janus membranes such as self assembly26,

27

, chemical etching28,


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electrospinning29, hydrothermal methods30, Wet-chemistry coating31 and others32

-

34

.

Nevertheless, lots of restraint still prevails for the scalable fabrication of such membranes due to costly, complicated and tedious procedures involved in these techniques, however, wetchemistry coating technique comprises of easy and cost-effective operations demonstrating a potential for large scale fabrication. In this context, we have delineated the detailed comparison of various membranes with Janus membranes in accord with their avails and implications in table 1. Table 1. Comparison of Janus membranes with other membranes for oil/water spills Membrane Ceramic Membranes

Advantages •

•

Disadvantages

High Selectivity towards

•

High cost35

oil and water35

•

Chemical

Chemical

and

required

thermal

stability36 •

treatment for

washing

fouled membrane36

Rugged

structural

•

Heavy in process35

stability36 Polymeric

•

High Efficiency37

•

Fouling39

Membranes

•

Low Energy Requirement4

•

Low Selectivity39

•

Inexpensive38

•

High

•

inhomogeneous

Polymeric

Mixed

Matrix Membranes •

separation

Efficiency40

dispersion

High selectivity41

nanoparticles41,42

of


4

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•

Low

Energy

•

Requirement40

un-controlled

pore

size41,42 •

Leaching

of

nanoparticles41,42 Surface

Modified

•

Janus Membrane

Selectivity

and

•

Separation Efficiency43

Polymeric Membranes

High

•

Enhanced Anti-fouling44

•

Controlled Pore Size45

•

Excellent Selectivity and

Complex

Fabrication

Technique43 •

High cost43

•

Low

stability

and

Separation Efficiency.22

flexibility

•

Self-driven separation46

incorporation of additives

•

Better Anti-fouling22

like TiO247,48

on

Feng et al., reported a superhydrophobic and superoleophillic Polytetrafluoroethylene (PTFE) spray-coated stainless steel mesh for oil-water separation having a water contact angle of 156.2o ± 2.8o and an oil contact angle of 0o.49 Due to the high electronegativity of fluorine, PTFE demonstrates mitigated london dispersion forces, as a result, it is intrinsically hydrophobic and oleophillic, making it a suitable candidate for altering the surface chemistry and surface architecture of the membrane substrate.50 PTFE is also highly non-reactive due to the aggregated effect of strong carbon-fluorine bonds, which can enhance the chemical durability of the coated membranes.51 Various solid substrate materials have been used for oil-water separation membranes such as metallic meshes49, fabrics/textiles52, 53 and sponges54. The inherent porous structure, abundance of hydrophillic hydroxyl groups in the cellulosic chain and biocompatibility


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coupled with chemical and thermal resistance encompasses cotton fabric as a suitable substrate for separation membranes.55-58 Herein, we report a fabrication of nano-engineered Janus membrane possessing special wettability by incorporating stable nano-sized PTFE dispersion polymerized via non-fluorinated processing aid and stabilized by a non-ionic surfactant onto a commercial cotton fabric using the Meyer rod coating technique.59 Wettability of the developed membranes towards oil and water demonstrating deviation from conventional wetting theories along with the absorption capacity, separation efficiency and recyclability were investigated with various petroleum products. Intrinsic hydrophobic property of PTFE, interconnected woven network of cotton fabric and uniform surface porosity of the developed membrane plays an important role in the heterogeneous wettability and anti-icing property of the Janus membranes. The reported membrane demonstrated flame retardancy while retaining its intrinsic properties and retained the innate properties of Janus membrane under extreme environments of hyper-saline solution, UVexposure, high temperature and sub-zero temperature. The engineered membranes were hypothesized as directional fluid diode exhibiting fluid gating on the basis of surface tension of the surface and the breakthrough pressure for water penetration was also calculated. This study contemplates the developed Janus membrane as a versatile, proficient and scalable alternative for environmental remediation of marine oil seepage accidents.

MATERIALS AND METHODS Materials: Raw woven cotton fabric (plain weave, 250 g/m2, thickness = 240µm) was procured from Ahmedabad Textile Industry's Research Association, India. PTFE dispersion was obtained


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from the Chemours company (Teflon™ PTFE DISP 30LX, PTFE content - 60%, Density - 1.51 g/cm3, particle size ~ 0.23µm, non-ionic surfactant stabilized and polymerized via nonfluorinated processing aid). Ethanol was procured from Sigma Aldrich Pvt. Ltd., India. Deionised water was acquired using Barnstead Nanopure Water System (cole-parmer, India) with purity 18.2 MΩ-cm. For oil absorption study, engine oil (Castrol ACTIV 4T 20W-40) was purchased from local store, Petrol and diesel were procured from Hindustan Petroleum Corporation Ltd., India. All chemical reagents and solvents were used as received without any purification.

Fabrication of Janus membrane: The heterogeneous wettable Janus membrane was fabricated by incorporating stable PTFE dispersion onto cotton fabric via Meyer rod coating technique as embellished in Figure 1. Cotton fabrics of dimension 12 X 12 cm2 were cleansed with acetone for 20 min and de-ionised water for 15 min in an ultrasonic bath at 20 KHz frequency to remove possible impurities. PTFE dispersion was impregnated into the cleansed cotton fabric by a manually controlled wire-wound metering rod (Meyer rod 10#), i.e., pushing the dispersion on top of the fabric with a Meyer rod. After drying the as-coated fabric in air at ambient temperature,

the

fabric

exhibited

hydrophillicity

and

the

transformation

from

superhydrophillicity to superhydrophobicity was realized by heat treating the coated fabric at 120oC for 6 hours to completely remove water as per the PTFE dispersion data sheet.60 The developed Janus membrane with uniform compact coating thickness of 70 µm (measured using digital thickness gauge) was then further characterized for morphology, wettability, anti-icing and oil-water separation.


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Figure 1. Schematic of the fabrication of Janus membrane

Characterization Techniques: The surface morphology of Janus membrane was analyzed via field emission scanning electron microscope (FE-SEM, Carl Zeiss AG, JSM-6700F, Germany) with 3 KV accelerating voltage. The static contact angle was measured on the contact angle goniometer (Krüss DSA100, Germany) using deionised water at ambient temperature. The Volume of individual droplet was kept 8 µl, contact angle was measured 2 sec after the attachment of droplet to the membrane and an average of five measurements was made to analyze the wettability of the membrane. The anti-icing of Janus membrane was analyzed by sprinkling 20µl droplets of de-ionised water (separately cooled at -5oC) via micropipette (Tarsons 2-20 µl (microlitre) T20 Accupipette) on the pre-cooled surface of Janus membrane placed in a NEWTRONIC deep freezer maintained at -20oC, to accrue condensed droplets on the membrane surface.

After the surface was sprinkled (~ 300 s), pressurized airstream was

progressively supplied at a rate of approximately 10 kPa/s via compressed gas tank placed outside of the freezer to the membrane surface until the accreted condensed droplets were fractured and detached from the surface. Fourier transform infrared (FT-IR) analysis of the


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cotton fabric and Janus membrane was done on PerkinElmer Spectrum BX FTIR system (PerkinElmer Inc., USA) at room temperature with KBr pellets in the range 4000 - 600 cm-1.

RESULTS AND DISCUSSION Morphological Analysis: The FE-SEM micrograph of pristine cotton fabric depicted in Figure 2(a) represents the uniformly conformed three-dimensional microfibers of cotton having smooth surface texture with an average diameter of 10 µm (calculated via ImageJ software).53 After the incorporation of PTFE dispersion onto the cotton fabric, closely spaced irregularly patterned PTFE nanospindles of diameter 220 nm (calculated via ImageJ software) were generated on the innate microfibral surface of fabric as depicted in Figure 2(b) and (c), which suggested the accomplishment of hierarchical micro- and nanoscale surface architecture as embellished in Figure 2(d). The generation of closely spaced irregularly patterned nanospindles on the microfibers of fabric was further confirmed by analyzing the FE-SEM micrographs with a scanning probe image processor (SPIP, Image metrology) program as displayed in Figure 2(e) and (f), which proffers the simulated three-dimensional view of FE-SEM micrograph. This three dimensional micrograph suggests the slightly concave tip of generated nanospindles, which imparts the adhesive properties to the surface due to the negative pressures produced by trapped air of small volume, a phenomenon reported by cheng et al.61 The nanospindle morphology yielded roughness at nanoscale complementing the inherent microscale roughness of the fabric weave. This hierarchical dual scale surface texture along with the inherent hydrophobic and oleophillic nature of PTFE facilitates the superhydrophobic and the superoleophillic nature of developed


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heterogeneous wettable Janus membrane,62 imparting directional fluid gating ability by the virtue of which it allows oils to permeate through it and inhibits permeation of water. The hierarchichal surface architechture is deliberated to aid the anti-icing ability of the membrane surface because of the high proportion of air trapped beneath the condensed droplets.63


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Figure 2. (a) FE-SEM micrograph of pristine cotton fabric, (b) low, (c) high magnification FESEM micrographs of Janus Membranes, (d) generation of PTFE nanospindles on cotton microfiber, (e) SPIP Micrograph and (f) 3D projection of SEM micrograph.

Fourier Transform Infrared (FT-IR) Analysis:

Figure 3. FT-IR Spectra of pristine cotton fabric and Janus membrane. FT-IR absorption spectra of pristine cotton fabric and developed Janus membrane were acquired and compared as depicted in Figure 3 to scrutinize the interaction between cotton and PTFE. The


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characteristic peaks at 1317 cm-1, 1156 cm-1, 1103 cm-1 and 1017 cm-1 of the cellulosic chain of cotton substrate were observed in both pristine cotton and Janus membrane which were due to strong infrared absorption corresponding to the bending vibration of C-O bond.21 New peaks at 1217 cm-1 and 1152 cm-1 realized in Janus membrane were due to strong infrared absorption associated with C-F bond stretching, indicating the presence of PTFE on developed Janus membrane.64 Peak at 3273 cm-1 correlates to moderate infrared absorption associated with Hbonded O-H bond stretching of alcohol group inherent to the cellulosic cotton fabric, while peaks at 3876 cm-1, 2892 cm-1 and 1427 cm-1 corresponds to the weak infrared absorption associated with free hydroxyl O-H bond stretching of alcohol group, C-H bond stretching of alkanes and inring C-C bond stretching of aromatic groups respectively present in the cellulosic chain of cotton.65 Although, no chemical interaction was observed between PTFE and cotton substrate suggesting the chemical inertness of PTFE towards cellulosic cotton substrate, but physical adherence was enhanced via heat treatment of coated fabric.


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Wettability behavior of Janus Membrane:

Figure 4. (a) Thermally triggered transition from superhydrophillicity to superhydrophobicity of coated fabric, (b) Water contact angle of Janus membrane (c) Oleophillicity of the Janus membrane with petrol, diesel and engine oil, (d) Water droplets (coloured with red dye) sitting on the superhydrophobic Janus membrane, (e) A jet of water bouncing off the Janus membrane, (f) Janus membrane exhibiting Petal effect, (g) Anti-icing property of Janus membrane at -20oC, condensed water droplets (coloured with red dye) are shed by the applied pressurized airstream of 100 kPa within 8 seconds.


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(1) Effect of roughness of Janus membrane: The wettability behavior of heterogeneous wettable Janus membrane was comprehensively analyzed for both water as well as oil via contact angle measurement. As discussed earlier, the coated membranes dried at ambient temperature exhibited superhydrophillicity and the thermally triggered transition from superhydrophillicity to superhydrophobicity was analyzed by curing the coated fabrics at various temperatures as depicted in Figure 4(a), which suggests the transition at around 120oC attributed to the complete evaporation of water from PTFE dispersion impregnated cotton fabric 60 and curing beyond 120oC does not have a significant effect on the water contact angle of Janus membrane. Although, increase in curing temperature leads to the increase in surface roughness due to increased surface asperity height as suggested by Sahoo et al., but only slight increase in hydrophobicity was observed on curing the membrane beyond 120oC, which might be contemplated to slight increase in surface roughness.66 The as-prepared Janus membrane possessed superhydrophocity (water contact angle: 168o ± 3o) and superoleophillicity (contact angles of petrol, diesel and engine oil: all close to 0o) as shown in Figure 4(b) and (c) respectively, which can be attributed to the complementary effects of hierarchical dual scale morphology of the membrane surface and low surface energy of PTFE.67 Liquids with low surface tension, such as petrol (γlv = 29 mN/m), diesel (γlv = 29.5 mN/m) and engine oil (γlv = 65 mN/m) spread and saturate thoroughly (within 2 secs for petrol and diesel while 30 secs for engine oil) on the surface of Janus membrane. Since, the surface tension of oils is generally much smaller than that of water, superhydrophobicity and superoleophillicity can be realized simultaneously if the surface tension of solid substrate lies between that of oil and water, facilitating the directional fluid gating effect of the membrane whereby it allows oils to permeate but thwarts penetration of water.68,69


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Several wetting regimes have been proclaimed for a hierarchical dual scale rough surface, including Wenzel, Cassie-Baxter and impregnating Cassie wetting regime.70

- 72

According to

Wenzel's regime, when the water droplet comes in contact with the hierarchical rough surface, water impregnates the pores completely and only surface roughness enhance the contact angle, leading to the following equation for contact angle of rough surface73: cos = cos

(1)

Where, θ is the contact angle of rough surface, Rf is the roughness factor defined as a ratio of actual to projected surface area and θo is the Young's contact angle of smooth surface. When water doesn't impregnate the pores of the rough surface, the formation of air pockets between solid-liquid interface transpires which further enhances the contact angle of the hierarchical surface, then the contact angle of such a surface is given by Cassie-Baxter equation74: cos = cos − 1 +

(2)

Where, fSL is the fraction of projected area constituting solid-liquid interface. But, when water partially impregnates the pores of rough surface which leads to a contact angle between Wenzel and Cassie-Baxter, then the surface ensues impregnating Cassie wetting regime and contact angle in given by75: cos = 1 + (cos − 1)

(3)

A superhydrophobic surface exhibiting lotus effect (non-sticking and self-cleaning) ensues Cassie-Baxter regime while that exhibiting petal effect (strong adhesion to water) ensues


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impregnating Cassie wetting regime.71,

72

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In Figure 4(d) a bright and reflective surface

underneath water droplets suggest the presence of air pockets between water and membrane surface. The water jet bounces off the Janus membrane, leaving traces of water as depicted in Figure 4(e), suggesting adhesion between water and membrane surface. The adhesion between water and membrane surface was further confirmed by placing a 10µl water droplet on vertical membrane as depicted in Figure 4(f), illustrating petal effect exhibited by developed Janus membrane ensuing impregnating Cassie wetting regime and demonstrating deviation from conventional Wenzel and Cassie-Baxter wetting regimes. The adhesive properties of the Janus membrane may be attributed to the concave tip of nanospindles generated on the membrane surface as discussed earlier. (2) Effect of concentration of Brine: The wettability behavior of Janus membrane with sodium chloride (NaCl) solutions of various concentrations was also analyzed as depicted in Figure 5(a), which suggests that the membrane retains its innate superhydrophobic characteristic of Janus even in a saturated NaCl solution, however, the water contact angle of the Janus membrane decreases from 168o ± 3o with pure water droplet to 150.8o ± 3o with 40 wt% NaCl solution, which may be attributed to the increase in ionic strength of saline solution causing decrease in solid/liquid phase interfacial tension, that agrees well with the results obtained by Serrano-Saldaña et al.76 This suggests that the developed Janus membrane has a stable superhydrophobic characteristic in both pure water and hypersaline solution, which demonstrates the potency of reported Janus membrane for practical application as oil spill cleanup material in the marine salty atmosphere.


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Figure 5. Superhydrophobic property of Janus membrane in (a) hyper-saline solution, and (b) subzero temperature range. (3) Effect of Sub-zero temeprature: The developed Janus membrane exhibited superhydrophobicity and superoleophillicity at ambient temperature, which can be utilized for oil-water separation, but for utilization of in places where temperature remains in sub-zero range, it is essential to investigate the wettability behavior of membrane at low temperatures. The low-temperature superhydrophocity of the Janus membrane was investigated by regulating surface temperatures from 0 to -20oC and simultaneously measuring contact angle. The water contact angle of the membrane surface decreases with decreasing temperature over the temperature range 0 to -20oC (i.e., from 162o at 0oC to 140o at -20oC) as illustrated in Figure 5(b), which can be attributed to transition from impreganted Cassie condensation at 0oC to Wenzel condensation at -20oC. This occurs due to the low surface temperature of membrane and quick freezing of nanoscale condensed droplets within the nanogaps between nanospinles of Janus membrane. A similar phenomenon was observed by


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Hao, Quanyong, et al., where superhydrophobic copper surfaces exhibited transition from Wensel condensation at -30oC to Cassie condensation at -5oC.77 Anti-icing property of the Janus membrane was also examined by placing the membrane in a NEWTRONIC deep freezer maintained at -20oC. Upon being exposed to the supercooled (-5oC) water droplets of 20µl, ice accrued on the membrane surface. After ~300 s, the progressively increasing pressurized airstream was applied at a rate of 10 kPa/s to fracture solid-ice interface, causing the accrued ice to be detached from the surface. The solid-ice interface was fractured and detached completely from the membrane surface at an airstream pressure of 100 kPa within 8 s, leaving behind small traces of water attributed to the petal effect of membrane as demonstrated in Fig.4(g) (supporting Video S1). The low ice-adhesion ability of the surface can be attributed to the hierarchical dual scale surface texture and closely spaced surface asperities leading to the less saturation of small droplets on the surface.78 The anti-icing property of the developed Janus membrane might prove useful for oil spill cleanups amidst broken sea ice in arctic region where temperature remains sub-zero.

Directional fluid gating mechanism of Janus Membrane: A superhydrophobic membrane in impregnating Cassie wetting regime allows slight water penetration into the membrane in the micron and/or nanostructure but the penetration of water is retarded due to the absence of wettability gradient because of enhanced contact angle of fibers beneath resulting in negative capillary pressure given by Young-Laplace equation. For an idealized circular cross-section pore of porous membrane, Young-Laplace capillary pressure is expressed as79:


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=

cos

(4)

where, θ is the contact angle between the liquid and capillary wall, r is pore radius and γ is the surface tension of liquid. In case of wettable pore surface (θ < 90o), liquid will be drawn into the pore spontaneously due to positive capillary pressure. However, in case of non-wettable pore surface (θ > 90o), liquid will be repelled out of the pore due to negative capillary pressure. In this case, minimal external pressure, i.e., Pbreakthrough is required by the liquid to permeate through the pore which is given by79: = − = −

cos

(5)

This phenomenon governs the directional fluid gating mechanism of the developed Janus membrane by virtue of which it allows the wetting liquids (θ < 90o) to permeate through the membrane due to positive capillary pressure (in our case, oil) while blocks the non-wetting liquids (θ > 90o) from penetrating due to negative capillary pressure (in our case, water) as depicted in Figure 6(a), which can be utilized for separation of oil from oil-water mixture as illustrated in next section. Thus, a membrane exhibiting directional fluid gating ability can be hypothesized as a directional fluid diode80 which allows lower surface tension liquids (wetting liquids) to permeate through it, but prevents penetration of higher surface tension liquids (nonwetting liquids) when no external pressure is applied across it. However, when an external pressure equal to or more than Pbreakthrough is applied across the membrane, it allows both lower and higher surface tension liquids to permeate through it, as depicted in Figure 6(b).


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Figure 6. (a) Directional fluid gating mechanism of Janus membrane, (b) Janus membrane as a directional fluid diode. In principle, pore structure and surface property influences the directional movement of fluid through the capillary channel of the membrane.81 Therefore, the transmembrane breakthrough pressure (Pbreakthrough) for developed Janus membranes, which can be considered as an interwoven mesh with predominant cylindrical texture is expressed as82, 83: =

!"# ($% &'( ))

∗ ∗ * +,!-./0123 % $5 (,!-./0123 % $6 (78 ))

(6)


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Where, θ is the contact angle between the liquid and the membrane surface, D*cylinder is a dimensionless measure of surface porosity considering both pore structure and pore spacing and given by57, 82, 84, 85: ∗ 9: ;

Directional Fluid Gating by Janus Membranes with Heterogeneous

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