Low Surface Tension Liquids Research Articles

Facile preparation of super-oleophobic TiO2/SiO2 composite coatings by spraying method

Re-entrant geometries can effectively trap air pockets, increase air/liquid interface fraction, prevent the penetration of low surface tension organic liquids, and achieve super-oleophobic performance. However, the creation of re-entrant geometries usually involving expensive lithographic tools, or complicated chemical procedures, and cannot be applied on a large scale, such super-oleophobic surfaces are unsuitable for practical application. Constructing hierarchical micro/nanostructure on substrates is a feasible solution to achieve super-oleophobicity through facile spraying method. Herein, we report a super-oleophobic TiO2/SiO2 composite coatings by taking advantage of an exceedingly simple spraying method. The dendritic structures produced by spraying the TiO2/SiO2 mixture play the critical role in obtaining the re-entrant geometry required for oleophobic surface, and the introduction of micron-scale TiO2 particles and nano-scale SiO2 particles provide an enough dual-scale rough structure for the coating. The TiO2/SiO2 composite coatings exhibit superior repellence to high and low surface tension liquids as well long-term durability. This design strategy may provide useful guidelines for the fabrication of particle-based spray-coated super-oleophobic surfaces.

Macrodrop-Impact-Mediated Fluid Microdispensing.

High‐resolution fluid dispensing techniques play a critical role in modern digital microfluidics, micro‐biosensing, and advanced fabrication. Though most of existing dispensers can achieve precise and high‐throughput fluid dispensing, they suffer from some inherent problems, such as specially fabricated dispensing micronozzles/microtips, large operating systems, low volume tunability, and poor performance for low surface tension liquids and liquids containing solid/liquid additives. Herein, the authors propose a facile, low‐frequency micro dispensing technique based on the Rayleigh–Plateau instability of singular liquid jets, which are stimulated by the air cavity collapse arising in the impact of microliter drops on non‐wetting surfaces. This novel dispensing strategy is capable to produce single microdrops of low‐viscosity liquids with a tunable volume from picoliters to nanoliters, and the operational surface tension range covers most laboratory solvents. The dispensing function is implemented without using small‐dimension nozzles/tips and enables handling diverse complex liquids. Moreover, the rather simple operating platform allows the integration of the whole dispensing function into a handy portable device with a low cost. Employing this microdispensing technique, the authors have controlled microchemical reactions, handled liquid samples in biological analysis, and fabricated smart materials and devices. The authors envision that this rational microdrop generator would find applications in various research areas.

Investigation of Flow Pattern and Void Fraction of Air and Low Surface Tension Liquid in A 30° Inclined Small Pipe

Two-phase flow in the mini pipe is applied in wide fields. The most common of two-phase flow is a couple of gas and liquid. The essential properties of the liquid are density, viscosity, and surface tension. There are many variations of the flow direction, horizontal, incline, and vertical, in terms of orientation. The two-phase investigation of flow pattern and void fraction of air and low surface tension liquid in a 30° inclined small pipe has been carried out. Dry air was used as a gas phase, while the liquid was the mixture solution of distilled water and 3% (by volume) of butanol. Butanol addition aimed to decrease the surface tension, which became 42.9 millinewton/meter, instead of 71 mN/m when using distilled water. The test section was a 130 mm length, 1.6 mm inner diameter circular glass pipe. The rig used was equipped with the air compressor, pressure tank, high-speed camera, liquid flow meter, and gas flow meter. The liquid was fed to the test section by the pressurized tank, instead of directly pumped, to avoid pulsation. Ranges of gas and liquid superficial velocities were 0.025 – 66.3 m/s and 0.033 – 4,935 m/s, respectively. Flow patterns were obtained from the captured high-speed video. Meanwhile, the void fractions were acquired by image processing of the video. As a result, five distinctive flow patterns were observed: plug, slug-annular, churn, bubbly, and annular. The separated flow was absent. The change of the liquid surface tension affected the shifting of some transition boundary lines in the flow pattern map. The transition line between slug-annular and annular against churn flow was shifted to the lower side or toward lower JL when the liquid surface tension decreased. In short, the churn flow was easier to be formed when the liquid surface tension was lower.

Liquid Wells as Self-Healing, Functional Analogues to Solid Vessels.

Liquids are traditionally handled and stored in solid vessels. Solid walls are not functional, adaptive, or self-repairing, and are difficult to remove and re-form. Liquid walls can overcome these limitations, but cannot form free-standing 3D walls. Herein, a liquid analogue of a well, termed a "liquid well" is introduced. Water tethered to a surface with hydrophobic-hydrophilic core-shell patterns forms stable liquid walls capable of containing another immiscible fluid, similar to fluid confinement by solid walls. Liquid wells with different liquids, volumes, and shapes are prepared and investigated by confocal and Raman microscopy. The confinement of various low-surface-tension liquids (LSTLs) on surfaces by liquid wells can compete with or be complementary to existing confinement strategies using perfluorinated surfaces, for example, in terms of the shape and height of the confined LSTLs. Liquid wells show unique properties arising from their liquid aggregate state: they are self-healing, dynamic, and functional, that is, not restricted to a passive confining role. Water walls can be easily removed and re-formed, making them interesting as sacrificial templates. This is demonstrated in a process termed water-templated polymerization (WTP). Numerical phase-field model simulations are performed to scrutinize the conditions required for the formation of stable liquid wells.

Transparent and Robust Amphiphobic Surfaces Exploiting Nanohierarchical Surface-grown Metal–Organic Frameworks

Highly amphiphobic (repelling both water and low surface tension liquids) and optically transparent surface treatments have widespread demand. By combining a rational growth of metal–organic frameworks (MOFs) with functionalization by environmentally safe, flexible alkyl groups, here we present surfaces with nanohierarchical morphology, comprising two widely differing nanoscale features. These nanohierarchical MOF films show excellent amphiphobicity. We further present three key features. First, we demonstrate the need to use flexible alkyl chains to achieve low drop sliding angles and self-cleaning. Second, our thin (∼200 nm) MOF films display excellent optical transparency and robustness. Third, the nanohierarchical morphology enables a unique combination of additional desirable properties, e.g., resistance to high-speed liquid impact (up to ∼35 m/s, Weber number >4 × 104), thermal stability up to 200 °C, scratch resistance, low ice adhesion for >10 icing/deicing cycles, stability in harsh acidic and basic environments, and capability to remove carcinogenic pollutants from water.

Dynamically oleophobic epoxy coating with surface enriched in silicone

We successfully prepared a dynamically oleophobic epoxy coating with low contact angle hysteresis (≤ 5.2°) for different low surface-tension liquids. Due to significant enrichment of covalently bonded polydimethylsiloxane (PDMS) at the coating surface, the obtained coating displays remarkable repellency against various low surface-tension liquids, including hexadecane, dodecane, decane, and soybean oil, as well as anti-smudge property and strong interfacial adhesion towards various substrates. Moreover, these excellent surface/interfacial properties have demonstrated to be very robust after being subjected to different environments (low pH, high pH, heating, and UV exposure). This type of coating may find promising applications in a broad range of fields where oil repellency is desired.

A Conductive Hydrophobic Polyaniline Sandwiched Polyvinylidene Fluoride Membrane for Early Detection of Surfactant-Induced Wetting in Membrane Distillation Using Impedance

Wetting of hydrophobic membranes is considered to be one of the major limitations that must be overcome to further the development of membrane distillation technology. Low surface tension liquids c...

Realizing surface amphiphobicity using 3D printing techniques: A critical move towards manufacturing low-cost reentrant geometries

Amphiphobic surfaces are obtained by lowering the surface energy through changes in surface geometry. These changes can be designed on the surface, thereby altering its wettability, and in turn rendering it amphiphobic. The main geometrical entities behind this phenomenon are reentrant geometries which prevent the solid-liquid interface tension from breaking, thereby resulting in contact angles greater than 90°. The science behind modelling and manufacturing of these reentrant geometries is well established apart from manufacturing them via extrusion-based 3-Dimensional printing processes. This review paper in identifying this gap summarizes various characterization parameters for surface wettability followed by identifying the role of surface reentrant geometries to introduce superamphiphobicity in polymers. The focus of the paper then moves towards achieving amphiphobicity using 3D printing processes where the current state of research is discussed in terms of reentrant profiles and achievement of high static contact angles. Role of the most common yet rarely reported Fused Deposition Modelling technique is discussed in more detail and a preliminary investigation based on characteristics flow and printing parameters used in Fused Deposition Modelling has been presented. The surface amphiphobicity is achieved in a one-step process characterized by high static contact angles with low and high surface tension liquids owing to air entrapment in characteristic layer-by-layer deposition features obtained in Fused Deposition Modelling.

Oleophobic textiles with embedded liquid and vapor hazard detection using differential planar microwave resonators

Protective clothing must repel hazardous liquids such as oils, acids, and solvents, which often exhibit low surface tension. The low surface tension liquid repellency of textiles is currently characterized qualitatively, considering only the first thirty seconds of wetting. This study demonstrates that embedded sensors within protective fabrics can more fully characterize liquid repellency while simultaneously detecting the hazardous substance. The liquid repellency of oleophobic textiles was detected in-situ using differential planar microwave resonator structures. A differential split ring resonator was designed with resonant responses at 4.4 and 4.6 GHz with a sensitivity of 50 MHz per unit ε. Fabrics were rendered oleophobic by dip-coating. The liquid repellency was monitored in-situ using droplets of heptane, octane, decane, dodecane, and water. Wetting transitions and droplet evaporation were identified in real time. The 4.4 GHz resonance peak’s shift was used to measure the liquid repellency, whereas the 4.6 GHz resonator monitored the liquid’s vapor as it absorbed into a gas-sensitive elastomer. The microwave response was tracked over 10 h every 15 s, and this transient data could identify the liquids based on their wetting and evaporation rates. Such sensors could be readily embedded in oleophobic textiles and enhance personal protective equipment.

Fabrication of superamphiphobic surface with hierarchical structures on metal substrate

The superamphiphobic surface has attracted wide attention because of its excellent liquid-repellent property. Low sliding angle and mechanical stability are the key requirements for superamphiphobic surface to realize its potential application in industry and life. Here, the superamphiphobic surface with hierarchical micro-nano rough structures has been prepared on metal substrate by laser etching and spraying methods. The surface can repel both water and low surface tension liquids (eg. ethylene glycol, rapeseed oil, cyclohexane). The superamphiphobic surface with excellent liquid-repellent property has certain anti-fouling and drag reduction function. The prepared surface has good robustness. It shows good liquid-repellent property even if it suffers physical damage or acid/base attacks. We envision that the superamphiphobic surface will be used in a wide range of applications such as anti-fouling and liquid transportation.

(Invited) Anodizing of Metals for Fabrication of Functional Nanostructured Surfaces

Anodizing of metals, including aluminum, titanium, and iron, is a simple and const-effective process to form various nanostructured oxide films, such as nanoporous, nanotubular, and nanowire films. Such nanostructured anodic films have attracted much attention for many potential functional applications as well as fundamental interests on the mechanism of self-organized nanostructure formation. Here, our recent studies on the formation of nanostructured anodic films on aluminum, copper, and zinc and their application to controlled wettability will be outlined.Anodizing of zinc in KHCO3 electrolyte produces a nanowire-type anodic film on entire metal surface. The formation of the nanowire film involves the initial pitting corrosion; pits are developed on the zinc metal surface, and embryos of nanowires are formed within the pits. When the pits are covered with a thin air-formed or electropolishing film, the nanowire formation is promoted as a consequence of the increased concentration of dissolved zinc species within the pits. Then, the nanowires grow toward the outside of the pits and cover the entire metal surfaces. Because of this growth process, the nanowires grow radial direction before covering the entire surface. The nanowire surface is one of the promising geometry for superhydrophobicity. In fact, anodic nanowire film surface becomes superhydrophobic after fluoroalkylsilane monolayer coating.The anodic nanowire film is also developed on copper by anodizing in KOH electrolyte. The copper surface can be completely covered with the nanowires only for 30 s at a constant current density of 10 mA cm–2. The nanowires consist of a single crystalline Cu(OH)2 phase, and the nanowires on copper are formed uniformly without forming pits. The nanowires are reduced readily to copper metal with the nanowire morphology remaining almost unchanged, although the copper metal nanowires are polycrystalline. The copper metal nanowires are also superhydrophobic after fluoroalkylthiol monolayer coating. The copper substrate with the superhydrophobic metal nanowires maintain the high thermal conductivity and effectively removes the condensed water droplet, being very suitable for heat exchanger applications with high efficiency.Aluminum is the most suitable metal to control the surface morphology by wet process. The author’s group successfully fabricated hierarchically micro-/nano-porous aluminum surface by a combination of chemical etching and anodizing in acid electrolytes. Micropits are developed by chemical etching containing HCl solution and the HCl concentration control the size of pits. Nanopores are generated by anodizing in sulfuric acid or oxalic acid, the pore widening process can be applied to control the pore size and porosity. The hierarchically porous aluminum surface is used to form super-repellency even for low surface tension liquids, including rapeseed oil (surface tension of 35 mN m–1), hexadecane (28 mN m–1), octane (22 mN m–1), and even hexane (18 mN m–1). The nanopores can be used to infiltrate the self-healing agent of organic coating, so that self-healing super-liquid-repellency occurs repeatedly even if the thin organic coating is damaged. The porous aluminum surface is also suitable for fabricating the slippery liquid infused porous surface (SLIPS) by impregnating a lubricant liquid into the pores. The SLIPS on aluminum show excellent liquid slipping property, anti-snow-sticking property, and high corrosion resistance.References T. Inoue, A. Koyama, D. Kowalski, C. Zhu, Y. Aoki and H. Habazaki, physica status solidi (a), 1900836 (2020). K. Nakayama, A. Koyama, C. Zhu, Y. Aoki and H. Habazaki, Advanced Materials Interfaces, 5, 1800566 (2018). K. Nakayama, E. Tsuji, Y. Aoki, S.-G. Park and H. Habazaki, The Journal of Physical Chemistry C, 120, 15684 (2016).

Molecularly grafted PVDF membranes with in-air superamphiphilicity and underwater superoleophobicity for oil/water separation

Microporous polyvinylidene fluoride (PVDF) based membranes are widely used in separation and purification applications, including oil/water separation. However, due to hydrophobic nature, these membranes are prone to high fouling and wetting by low surface tension liquids and substances, thereby limiting its full potential in many important applications such as oil/water separation. Herein, we present a simple and straight forward method to modify and produce in-air superamphiphilic and under-water superoleophobic PVDF membranes by molecular grafting using three different amino silanes. The molecular grafting imparted novel properties due to the combined effect of chemistry, nanostructuring, and roughness. After grafting, the membranes exhibited in-air superamphiphilicity in both dry and wet conditions. In water, the membrane does not allow the attachment of oil droplets, making it superoleophobic. However, when placed under oil, the membrane showed superhydrophobicity but still allowed the adhesion of water droplets. After thorough characterization for structure, modification, and physicochemical properties, the membranes were tested for oil/water mixture separation. The functionalized membranes showed > 99% oil rejection and flux recovery. These results showed great promise for molecular grafting of membranes for separation and purification applications.

Droplet Evaporation Dynamics of Low Surface Tension Fluids Using the Steady Method.

Droplet evaporation governs many heat- and mass-transfer processes germane in nature and industry. In the past 3 centuries, transient techniques have been developed to characterize the evaporation of sessile droplets. These methods have difficulty in reconciling transient effects induced by the droplet shape and size changes during evaporation. Furthermore, investigation of evaporation of microdroplets residing on wetting substrates, or fluids having low surface tensions (<30 mN/m), is difficult to perform using established approaches. Here, we use the steady method to study the microdroplet evaporation dynamics of low surface tension liquids. We start by employing the steady method to benchmark with water droplets having base radii (20 ≤ Rb ≤ 260 μm), apparent advancing contact angle (45° ≤ θa,app ≤ 162°), surface temperature (30 < Ts < 60 °C), and relative humidity (40% < ϕ < 60%). Following validation, evaporation of ethanol (≈22 mN/m), hexane (≈18 mN/m), and dodecane (≈25 mN/m) were studied for 90 ≤ Rb ≤ 400 μm and 10 < Ts < 25 °C. We elucidate the mechanisms governing the observed behavior using heat and mass transport scaling analysis during evaporation, demonstrating our steady technique to be particularly advantageous for microdroplets, where Marangoni and buoyant forces are negligible. Our work not only elucidates the droplet evaporation mechanisms of low surface tension liquids but also demonstrates the steady method as a means to study phase change processes.

Robust anti-icing superhydrophobic aluminum alloy surfaces by grafting fluorocarbon molecular chains

Infusion of low surface tension liquids in nanostructured surfaces is currently used to promote an anti-icing response, although the long term stability of these systems is often jeopardized by losses of the infused liquid. In this work, we propose an alternative to the infusion procedure to induce a more effective and long lasting anti-icing capacity. The method consists of a combination of surface nanostructuration with the chemical grafting of fluorocarbon molecules. Al6061 substrates have been subjected to laser roughening and further modified with a nanostructured Al2O3 thin film to achieve a dual roughness and porous surface state. These surfaces have been subjected to a grafting treatment with perfluorooctyltriethoxysilane (PFOTES) vapor or, for comparative purposes, infused with a low surface tension liquid. A comparative analysis of the wetting, water condensation and anti-icing properties of these two systems showed an outstandingly better performance for the grafted surfaces with respect to the infused ones. Grafted surfaces were markedly superhydrophobic and required higher water vapor pressures to induce condensation. When looking for their anti-icing capacity, they presented quite long freezing delay times for supercooled water droplets (i.e. almost four hours) and exhibited a notably low ice accretion in a wind tunnel test. The high aging resistance and durability of these grafted surfaces and the reproducibility of the results obtained when subjected to successive ice accretion cycles show that molecular grafting is an efficient anti-icing methodology that, in aggressive media, may outperform the classical infusion procedures. The role of the fluorocarbon chains anchored on the surface in inducing an anti-icing functionality is discussed.

Monolayer Nanoparticle‐Covered Liquid Marble Production with Low Surface Tension Liquids

AbstractAs an emerging droplet system, monolayer nanoparticle‐covered (mNPc) liquid marbles have shown fascinating properties and application values. Their formation process has been clear, but liquid availability for production has yet to be clarified. This study addresses this issue from the angle of surface tension, revealing critical surface tension and contact angle values for mNPc liquid marble production with both rolling and compression methods. Water‐ethanol and water‐surfactant systems are employed as testing liquids. With irreversible pinning as criterion, critical surface tension values that allow mNPc liquid marble formation are obtained with the two kinds of liquids, forming ranges from 38.6 to 31.8 mN m−1, and from 36.7 to 27.6 mN m−1, for the rolling and compression methods, respectively. Correspondingly, contact angles in the ranges from 132° to 126° and from 116–124° are regarded as the lower limits for mNPc liquid marble production with the rolling and compression methods, respectively. In addition, rupture and deformation behaviors under low surface tension are systematically investigated and a depinning approach that facilitates marble formation is proposed. This study could provide important guidance for various applications that require low surface tension liquids to produce mNPc liquid marbles.

Polydopamine‐Mediated Superlyophobic Polysiloxane Coating of Porous Substrates for Efficient Separation of Immiscible Liquids

AbstractLyophobic membrane has the potential to separate immiscible liquids. However, the multistep fabrication and limited option of materials have created major challenges for their practical application. Here, a strategy to fabricate superlyophobic materials is presented by using polydopamine (PDA) as an intermediate coating. The results demonstrate that PDA nanoparticles deposited on diverse substrates can direct the subsequent formation of polysiloxane microsheets (PSMSs), which increases surface roughness and results in superlyophobicity towards high surface tension (ST) liquids and superlyophilicity to liquids with ST of 33 mN m‐1 and below. Superlyophobic membranes fabricated from stainless steel mesh, cotton fabric, and filter papers show high efficiency for continuous separations of chloroform/water (&gt;96%) and carbon tetrachloride/formamide (&gt;97%). In addition, superlyophobic melamine foam and polyurethane foam can absorb various low ST liquids with capacities ranging from 53 to 120 g g‐1 and from 26.5 to 52.5 g g‐1, respectively, revealing the efficient separation of a broad range of immiscible liquids. All of the as‐prepared superlyophobic materials show excellent regeneration with insignificant change of separation efficiency over 10 cycles. The preparation route for superlyophobic materials in this work introduces a new strategy for the separation of immiscible liquids by both batch absorption and continuous filtration treatment.

Versatile, mechanochemically robust, sprayed superomniphobic coating enabling low surface tension and high viscous organic liquid bouncing

Superomniphobicity repelling both water and organic liquids is an important feature of special wettability material, which has received great attention in surface/interface area. The bottleneck in the real application is how to develop robust superomniphobic surface by using cost-effective, scalable, facile materials and methods. Here, we report a superomniphobic coating by using low-cost and scalable spraying suspensions method. The sprayed superomniphobic coating assembles into multi-leveled micro/nanostructures with very low apparent surface free energy of 12.98 mN m−1. The coating is capable of diverse organic liquids bouncing with liquid surface tension low to 21 mN m−1 and viscosity high to 297.8 mPa s. Remarkably, the coating can be applied in different substrates due to the in-situ formed adhesive and demonstrates strong mechanical robustness under high pressure water impact (27.8 m s−1), abrasion and tape-peeling cycles, and sustains exposure to highly corrosive aqua regia, extreme low/high temperature and UV environments. The organic liquid bouncing behavior related to impact velocity and liquid properties were also studied. With facile fabrication and multifaceted robustness, our coating paves the way for real industrial applications.

Enhanced Nucleate Pool Boiling by Coupling the Pinning Act and Cluster Bubble Nucleation of Micro-nano Composited Surfaces

A new idea by coupling the pinning act of the micro-pin-fins and the cluster bubble nucleation of nonuniform nanowalls, was proposed to improve the boiling performance on micro-nano composited surface in the low surface tension liquid. To realize this idea, the uniform/nonuniform nanowalls were composited on the smooth surface and micro-pin-finned surfaces. The pool boiling performance of a smooth surface, the uniform/nonuniform nanowall surfaces, micro-pin-finned surface, and micro-nano composited surfaces with uniform/nonuniform nanowalls were tested and compared with each other. Both the heat transfer coefficient (HTC) and critical heat flux (CHF) of the micro-nano composited surface with nonuniform nanowalls (named PFNW2) are significantly increased compared to the micro-pin-finned surface. In addition, small bubble departure diameters, high bubble departure frequency and velocity were also found on PFNW2. A unique bubble behavior, named microbubble explosive emission, was observed, which provides a strong evidence for the effectiveness of the method to enhance boiling heat transfer by coupling the pinning act of the micro-pin-fins and the cluster bubble nucleation. It is suggested that high liquid subcooling and modulated nanostructures for bubble departure promotion are both indispensable for further enhancement of HTC and CHF in low surface tension liquid.

Enhanced omniphobicity of mullite hollow fiber membrane with organosilane-functionalized TiO2 micro-flowers and nanorods layer deposition for desalination using direct contact membrane distillation

In this study, a novel omniphobic mullite hollow fiber membrane (HFM) was successfully fabricated to reduce the fouling and wetting propensity of direct contact membrane distillation (DCMD). The surface of the mullite hollow fiber membrane was functionalized with flower-like (FL) and rod-like (RL) TiO2 rough layer. Then, surface fluorination was carried out using 1H,1H,2H,2H-perfluorodecyltriethoxysilane (97%) (C8). The modified mullite hollow fiber membrane, which was initially hydrophilic in nature, exhibited high liquid repellency towards water and low surface tension liquids such as ethylene glycol (47.3 mN/m) and olive oil (32 mN/m). The order of the membranes in terms of wetting resistance for low surface tension liquids is as follows: C8-FL/TiO2-HFM > C8-RL/TiO2-HFM > C8-HFM. The omniphobic membrane that was fabricated with TiO2 micro-flowers (C8-FL/TiO2-HFM) was the only membrane that exhibited superomniphobic properties towards ethylene glycol (~150°) and was nearly superomniphobic towards olive oil (~140°). In addition, the formation of air layers was observed on submerged C8-FL/TiO2-HFM and C8-RL/TiO2- HFM, which was proven to significantly reduce organic fouling even after 500 min of DCMD with an aqueous NaCl (3.5%) feed solution containing humic acid (10 mg/L). At the macroscopic level, no significant fouling was observed for the C8-FL/TiO2-HFM. This could be attributed to the hierarchical structure induced by TiO2 micro-flowers, which played a critical role in achieving excellent anti-fouling properties. This has also translated into the high flux stability of the C8-FL/TiO2-HFM. Rise in permeate conductivity was observed for the C8-HFM but not for C8-FL/TiO2-HFM and C8-RL/TiO2- HFM. These results suggest that the fabricated omniphobic mullite hollow fiber membrane with flower-like structure is promising for a robust DCMD process, even for the desalination of real seawater that contains organic contaminants.

Macro-corrugated and nano-patterned hierarchically structured superomniphobic membrane for treatment of low surface tension oily wastewater by membrane distillation

A hierarchically assembled superomniphobic membrane with three levels of reentrant structure was designed and fabricated to enable effective treatment of low surface tension, hypersaline oily wastewaters using direct contact membrane distillation (DCMD). The overall structure is a combination of macro corrugations obtained by surface imprinting, with the micro spherulites morphology achieved through the applied phase inversion method and nano patterns obtained by fluorinated Silica nanoparticles (SiNPs) coating. This resulted in a superomniphobic membrane surface with remarkable anti-wetting properties repelling both high surface tension water and low surface tension oils. Measurements of contact angle (CA) with DI water, an anionic surfactant, oil, and ethanol demonstrated a robust wetting resistance against low surface tension liquids showing both superhydrophobicity and superoleophobicity. CA values of 160.8 ± 2.3° and 154.3 ± 1.9° for water and oil were obtained, respectively. Calculations revealed a high liquid-vapor interface for the fabricated membrane with more than 89% of the water droplet contact area being with air pockets entrapped between adjacent SiNPs and only 11% come into contact with the solid membrane surface. Moreover, the high liquid-vapor interface imparts the membrane with high liquid repellency, self-cleaning and slippery effects, characterized by a minimum droplet-membrane interaction and complete water droplet bouncing on the surface within only 18 ms. When tested in DCMD with synthetic hypersaline oily wastewaters, the fabricated superomniphobic membrane demonstrated stable, non-wetting MD operation over 24 h, even at high concentrations of low surface tension 1.0 mM Sodium dodecyl sulfate and 400 ppm oil, potentially offering a sustainable option for treatment of low surface tension oily industrial wastewater.