Non-wetting Property Research Articles

Achieving Super-Metallophobicity on Silicon-based Ceramics at High Temperature.

As a critical concept in physical chemistry, superwettability is widely concerned in both fundamental science and practical engineering in past few decades. Despite this, investigation on high temperature superwettability is still a void, which is significant both in scientific and industrial fields. Herein, a ceramic with specific high temperature non-wetting property, Si2N2O is proposed. Compared with other materials, Si2N2O is elucidated with better practical non-wetting property against various non-ferrous metals. Combining with micro-nanostructures, the metallophobicity is further improved (contact angle >150° and contact angle hysteresis ≈0°). The extraordinary metal repellency is defined as "super-metallophobicity", which is proved to be induced by distinctive thermodynamic and dynamic wetting behavior on the rough surface. The research of super-metallophobicity not only sheds light on superwettability at high temperature, but also offers worthy insights for future potential material design in a wide range of applications, such as metallurgy, 3D printing and semiconductor industry.

Bioinspired Superhydrophobic Nanocoating Based on Polydopamine and Nanodiamonds to Mitigate Bacterial Attachment to Polyvinyl Chloride Surfaces in Food Industry Environments.

Polyvinyl chloride (PVC) is commonly utilized as a food-contact surface by the food industry for processing and storage purposes due to its durability, ease of fabrication, and cost-effectiveness. Herein, we report a composite coating for the superhydrophobization of PVC without the use of polyfluoroalkyl chemistry. This coating rendered the PVC superhydrophobic, exhibiting a static water contact angle of 151.9 ± 0.7° and a contact angle hysteresis of only 3.1 ± 1.0°. The structure of this composite coating, consisting of polydopamine, nanodiamonds, and an alkyl silane, was investigated by utilizing both scanning electron microscopy and atomic force microscopy. Surface chemistry was probed using attenuated total reflectance-Fourier transform infrared, and the surface wetting behavior was thoroughly characterized using both static and dynamic water contact angle measurements. It was demonstrated that the superhydrophobic PVC was cleanable using a food-grade surfactant, becoming wet in contact with high concentration surfactant solutions, but regaining its nonwetting property upon rinsing with water. It was demonstrated that the coating produced a 2.1 ± 0.1 log10 reduction (99.2%) in the number of Escherichia coli O157:H7 cells and a 2.2 ± 0.1 log10 reduction (99.3%) in the number of Salmonella enterica Typhimurium cells that were able to adsorb onto PVC surfaces over a 24 h period. The use of this fluorine-free superhydrophobic coating on PVC equipment, such as conveyor belts within food production facilities, may help to mitigate bacterial cross-contamination and curb the spread of foodborne illnesses.

Silica-nanoparticle reinforced lubricant-infused copper substrates with enhanced lubricant retention for maintenance-free heat exchangers

Copper substrates are widely used in heat exchangers due to their low cost and high thermal conductivity. While copper substrates have been modified to exhibit non-wetting property via lubricant infusion to enhance condensation heat transfer efficiency, these engineered surfaces often lack chemical robustness and lubricant retention, limiting their long-term use without maintenance. In this work, we present a new strategy in which omniphobic and chemically inert fluorocarbon oil is infused into a nanostructured copper substrate reinforced with silica nanoparticles (SiNP) to achieve enhanced durability and acid-resistive properties. We demonstrate that the assembly of SiNP layer prior to lubricant infusion serves as a physical barrier and provides additional anchoring points for the lubricant to retain via capillary force. Moreover, we show that SiNP-reinforced liquid-infused surface (LIS) exhibits excellent non-wetting and self-cleaning properties, leading to enhanced stability against acid exposure as well as dust, oil, and microbial contamination. Based on the excellent long-term stability in heat transfer performance even under harsh environmental challenges, we envision that the SiNP-reinforced LIS presented in this work will provide new insight in the design of robust and maintenance-free lubricant-infused surfaces for energy and environmental applications.

Wodyetia bifurcate structured carbon fabrics with durable superhydrophobicity for high-efficiency oil-water separation

The superhydrophobic fiber-based membranes with features of high separation efficiency and low energy consumption for oil-water separation remains a formidable challenge. In this paper, a robust and durable superhydrophobic cotton-derived carbon fabric (CDCF) with wodyetia bifurcate-like structure is fabricated via in situ cobalt-nickel basic carbonate (CNC) deposition and 1 H, 1 H, 2 H, 2 H-perfluorooctyltriethoxysilane (POTS) coating. The combined action of rough surface structure and low surface energy makes CDCF/CNC/POTS with superhydrophobicity/superoleophilicity, anti-wetting, and self-cleaning performance. Intriguingly, the CDCF/CNC/POTS can keep its superhydrophobicity under of the water droplet impact pressure of 781 Pa. In addition to its robust dynamic superhydrophobicity, CDCF/CNC/POTS can also maintain its non-wetting property under harsh environmental conditions such as mechanical abrasion treatment, acidic, alkaline and salt solutions, and ultraviolet radiation. Importantly, the CDCF/CNC/POTS can separate various oil-water mixtures and emulsions under gravity with ultrahigh oil-water mixtures permeate flux (∼19,126 L/m2h), high surfactant-stabilized emulsion permeate flux (∼821 L/m2h), and high separation efficiency (> 98.60 %). Moreover, remarkable recyclability endow the CDCF/CNC/POTS with promising application in treating oily wastewater. This work may benefit the low-cost mass production of cotton-based carbon fabrics for developing eco-friendly high-efficiency separators.

Fabrication of air-permeable superhydrophobic surfaces with excellent non-wetting property

The superhydrophobic surface (SHS) is attractive for drag reduction, anti-icing, and anti-fouling, but its insufficient non-wetting property has limited its applications. This study aims to fabricate, characterize, and evaluate SHS with excellent non-wetting properties. The SHS is fabricated via electrochemical treatments and silanization, changing superhydrophilic porous titanium substrates into superhydrophobicity. Hierarchical micro/nano-structures and low-energy coating are created on substrates with interconnected micropores, resulting in high static contact angle (SCA) air-permeable SHS (i.e. SCA = 152.5°). The non-wetting property of the SHS, characterized by the interfacial reflection intensity, is reversibly recovered in the turbulent water flow test. An excellent non-wetting property is achieved by actively minimizing the pressure difference across the surface.

Fabrication and properties of symmetrical W/Si3N4/W functionally graded materials by spark plasma sintering

Si3N4 ceramics have shown promising application prospects as the sealing materials for liquid metal batteries (LMBs). However, the joining of Si3N4 to the metal shell of LMBs is rather difficult because of the non-wetting property and mismatch of their thermal expansion coefficients (CTEs). Herein, to solve the above issue, W/Si3N4 functionally graded materials (FGMs) were designed and fabricated via the spark plasma sintering (SPS) process. The effects of layer numbers and layer components on the phase composition, structure, and thermal stress of the FGMs were systematically investigated using both experimental and simulation methods. W/Si3N4 FGMs, with optimal layer numbers, were then utilized for fabricating symmetrical W/Si3N4/W FGMs. The thermal stress of the products can be effectively reduced by adjusting the MgO content of the central Si3N4 layer. As a result, the optimal product displayed good interface bonding between two adjacent layers and also exhibited high flexural strength (943.9 MPa) and electrical resistivity (2.0 × 1012 Ω cm). This route for the fabrication of symmetrical W/Si3N4/W FGMs exhibits the advantage of high efficiency, and the products with excellent properties can be used as sealing materials for LMBs.

Conversion of Polymer Surfaces into Nonwetting Substrates for Liquid Metal Applications.

Liquid metal-based applications are limited by the wetting nature of polymers toward surface-oxidized gallium-based liquid metals. This work demonstrates that a 120 s CF4/O2 plasma treatment of polymer surfaces-such as poly(dimethylsiloxane) (PDMS), SU8, S1813, and polyimide-converts these previously wetting surfaces to nonwetting surfaces for gallium-based liquid metals. Static and advancing contact angles of all plasma-treated surfaces are >150°, and receding contact angles are >140°, with contact angle hysteresis in the range of 8.2-10.7°, collectively indicating lyophobic behavior. This lyophobic behavior is attributed to the plasma simultaneously fluorinating the surface while creating sub-micron scale roughness. X-ray photoelectron spectroscopy (XPS) results show a large presence of fluorine at the surface, indicating fluorination of surface methyl groups, while atomic force microscopy (AFM) results show that plasma-treated surfaces have an order of magnitude greater surface roughness than pristine surfaces, indicating a Cassie-Baxter state, which suggests that surface roughness is the primary cause of the nonwetting property, with surface chemistry making a smaller contribution. Solid surface free energy values for all plasma-treated surfaces were found to be generally lower than the pristine surfaces, indicating that this process can be used to make similar classes of polymers nonwetting to gallium-based liquid metals.

Mechanical Equilibrium Dynamics Controlling Wetting State Transition at Low-Temperature Superhydrophobic Array-Microstructure Surfaces

Superhydrophobic materials are significant for engineering applications in the anti-icing field because of their non-wetting property. The interface physical mechanisms of non-wetting properties are important to promote real applications of superhydrophobic surfaces, especially under low-temperature conditions. Here, we found that low temperature could induce the wetting state transition from a Cassie–Baxter state to a Wenzel state. This transition occurred at 14 °C (and 2 °C) on superhydrophobic surfaces with pillar heights of 250 μm (and 300 μm). As a consequence, the driving-force of the Cassie-Wenzel (C-W) wetting transition was induced by the contraction of air pockets on cooling, and the pressure of air pockets supporting the droplet decreased with the contraction degree. Decreasing the pressure of air pockets broke the mechanical equilibrium at the solid–liquid contact interface, and the continuous contraction overcame the resistance in the C-W wetting transition. Based on the analysis of work against resistance in the C-W wetting transition, lower C-W wetting transition temperature was mainly attributed to a higher pillar, which produced more work against resistance to require more energy. This energy was directly reflected by the energy required for continuous contraction of air pockets. Superhydrophobic surfaces with higher pillar structure remain stable non-wetting property at low-temperature conditions. This work provides theoretical support for the application of superhydrophobic materials in low-temperature environments.

Guiding light via slippery liquid-infused porous surfaces

Slippery liquid-infused porous surfaces (SLIPSs) have been explored for many applications, taking advantage of their highly non-wetting property. In this work, we explore the SLIPS as a cladding material for waveguiding. SLIPSs are prepared by infusing perfluoropolyether oil to hydrophobized nanoporous surfaces of silicon. Power loss and transmission efficiency of an HeNe laser (1.82 mW and 632.8 nm) with varying incident angles were measured through microchannels consisting of the SLIPSs as cladding layers (noil = 1.30) and water (nwater = 1.33) as a core, compared to other cladding types including a planar silicon surface and the nanoporous surfaces in hydrophilic (Wenzel state) and hydrophobic (Cassie–Baxter state) conditions with no oil infused. Agreeing with Snell's law, a total internal reflection occurs at the incident angle as high as 14° for the SLIPSs. The waveguide loss at 14° is only 1.8 dB/cm for the SLIPSs, while those for planar silicon, hydrophilic nanoporous, and hydrophobic nanoporous surfaces are 5.9, 7.4, and 4.9 dB/cm, respectively. The power transmission efficiency of the SLIPSs is independent of the porosity because the surfaces are fully covered with the oil layer, whereas those of hydrophilic and hydrophobic nanoporous surfaces, whose pores are filled with water and air, respectively, depend on the porosity. The significantly lower power loss and the insensitivity to the surface porosity are advantages of the SLIPSs over the other surfaces and can benefit in waveguiding applications such as optofluidics.

Simulation and Experimental study on IMS (Injection Molded Solder) bumping with expanded resist patterning for reinforcement of fine-pitch capability

Abstract Injection Molded Solder (IMS) process has been developed as one of the advanced micro-bumps forming techniques. This paper presents a novel IMS process utilizing a double-layer resist patterning with expanded opening for the reduction of required injection pressure. It consists of upper resist layer with larger opening diameter than lower resist opening. With this configuration, necessary injection pressure is adjusted to upper resist size, while lower layer determines the position and the size of fabricated bump. Furthermore, two different types of resist opening shape, an isolated opening and connected opening are proposed and evaluated in this study from both simulation and experimental perspective. For simulation, CFD analysis is performed to demonstrate the behavior of liquid molten solder flow and its bump forming process. It is revealed that solder bump was successfully formed in accordance with theoretical bump size in both designs. However, the result indicates that insufficient non-wetting property of resist material could lead solder bridging between adjacent bumps in connected design. Experiment is also conducted using the novel resist structure with the dimension correlated to the simulation model which has lower resist diameter of 10 μm, upper resist diameter of 15 μm, and the pitch of 20 μm. IMS is operated and it is confirmed that the uniform bumps can be favorably formed without any defects.

Counterintuitive Ballistic and Directional Liquid Transport on a Flexible Droplet Rectifier.

Achieving the directional and long-range droplet transport on solid surfaces is widely preferred for many practical applications but has proven to be challenging. Particularly, directionality and transport distance of droplets on hydrophobic surfaces are mutually exclusive. Here, we report that drain fly, a ubiquitous insect maintaining nonwetting property even in very high humidity, develops a unique ballistic droplet transport mechanism to meet these demanding challenges. The drain fly serves as a flexible rectifier to allow for a directional and long-range propagation as well as self-removal of a droplet, thus suppressing unwanted liquid flooding. Further investigation reveals that this phenomenon is owing to the synergistic conjunction of multiscale roughness, structural periodicity, and flexibility, which rectifies the random and localized droplet nucleation (nanoscale and microscale) into a directed and global migration (millimeter-scale). The mechanism we have identified opens up a new approach toward the design of artificial rectifiers for broad applications.

Fabrication of hydrophobic PP/CH3SiO2 composite hollow fiber membrane for membrane contactor application

Polymeric membrane wetting by liquid absorbent is one of the major challenges of the gas- liquid membrane contactors. In order to overcome this issue, composite polypropylene (PP) hollow fiber membranes were prepared by incorporation of methyl grafted silica nanoparticles (CH3-g-silica NPs) via two treatment approaches including blending the NPs with dope solution before membrane fabricating (approach 1), and also coating the NPs on the surface of the prepared membrane (approach 2). The fabricated membranes were characterized by using ATR-FTIR, FESEM, TEM, AFM, porosity, contact angle, mechanical strength, breakthrough pressure, wetting resistance and gas permeation test. The obtained results from ATR-FTIR spectra confirmed the successful formation of the PP-CH3SiO2 composite hollow fiber membranes. It was seen that for composite membranes fabricated with approaches 1 and 2 the average contact angle increased from 124° to 145° and 168°, respectively. The fabricated membranes were also investigated for CO2 absorption process in gas- liquid membrane contactors. As a result, the gentle decrease in flux of composite membranes compare to the sharp flux drop of neat one confirmed the effective improvement in the non-wetting property of the membranes in the presence of inorganic particles.

Combinational Biomimicking of Lotus Leaf, Mussel, and Sandcastle Worm for Robust Superhydrophobic Surfaces with Biomedical Multifunctionality: Antithrombotic, Antibiofouling, and Tissue Closure Capabilities.

Surface wetting occurring in daily life causes undesired contaminations, which are critical issues in various fields. To solve these problems, the nonwetting property of a superhydrophobic (SH) surface has proven its utility by preventing contaminant infiltration, serious infections, or malfunction. However, the application of SH surfaces in the biomedical field has been limited due to the weak durability and toxicity of the related components. To overcome these limitations, we developed a robust and biocompatible SH surface through combinational biomimicking of three natural organisms, lotus leaf, mussel, and sandcastle worm, for the first time. Using the water-immiscible and polycationic characteristics of mussel adhesive protein (iMglue), an SH iMglue-SiO2(TiO2/SiO2)2 coating was fabricated by solution-based electrical charge-controlled layer-by-layer growth of nanoparticles (NPs). The fabricated iMglue-SiO2(TiO2/SiO2)2 SH surface showed excellent durable nonwetting properties and was applied to an intracatheter tube coating to develop antithrombotic catheters under blood flow. Furthermore, we developed a iMglue-employed SH patch for a tissue closure bandage by spraying hydrophobic SiO2 NPs on the iMglue-covered cotton pads. The prepared iMglue-employing SH patch showed perfect bifunctionality with excellent antibiofouling and tissue closure capabilities. Our work presents a novel, useful strategy for fabricating a biomedically multifunctional, robust SH surface through combinational mimicking of natural organisms.

Manipulation schemes and applications of liquid marbles for micro total analysis systems

Micro total analysis systems (microTAS) provide the opportunity to create complete analytical microsystems by integrating various functional modules, such as sample preparation, separation and detection, into a single chip-sized microfabricated device. Microfluidics is the enabling technology for implementing the concept of microTAS. Liquid marble (LM), as a promising separate digital microfluidic platform, has the great potential to enhance the broad applications of microTAS. LMs are small liquid droplets encapsulated by multilayered hydrophobic particles and have attracted a great interest from the microfluidics research community due to their non-wetting property. A LM maintains its integrity and exhibits low friction on various carrier surfaces, enabling the LM to be actuated by external electric, magnetic, gravitational and acoustic fields or other manipulation schemes. LMs can thus serve effectively for the storage and transportation of small liquid volumes. In addition, they have been widely used for the quick detection of water pollution or gas emission and, most importantly, micromixing and microreactions for chemical and biomedical purposes. This paper reviews the recent developments in the manipulation techniques and emerging applications of LMs. The review aims to facilitate better understanding of their use as a unique digital microfluidic platform to promote further advancement of microTAS. The paper begins with different manipulation schemes of LMs according to the nature of actuation energy. Next, it summarizes the diverse applications of LMs for various chemical and biological assays. Finally, this paper concludes with future perspectives regarding the research on LMs in microTAS technologies.

Facile preparation of durable superhydrophobic coating with self-cleaning property

A facile one-pot route was proposed to prepare a superhydrophobic P25 (nano TiO2)/ATP (attapulgite)/ER (epoxy resin)/PDMS (polydimethylsiloxane) coating with good durability and self-cleaning property. The influences of the components, e.g. the mass ratio of P25 to ATP, the amount of ER, on the wettability and morphology of the coating surface were investigated. The morphology and the hydrophobicity of the as-prepared P25/ATP/ER/PDMS composite coatings were observed and measured by SEM and contact angle goniometer, respectively. The components of the coating were analyzed by FT-IR. And the abrasion resistance of the coating was carried out by sandpaper abrasion tests. The results showed that the coating exhibited a good superhydrophobicity with a water contact angle (WCA) above 150° and a sliding angle (SA) of 6.7° under the optimal component formula condition, i.e., the mass ratio of ATP to P25 was 1.7:0.3, and amount of ER was 0.28 g. And the coating could maintain the superhydrophobicity even after 280 cm of abrasion length. Additionally, the superhydrophobic P25/ATP/ER/PDMS coating exhibited high thermal stability, good repellency for acidic and basic solutions, and good non-wetting property. Therefore, the as-prepared superhydrophobic P25/ATP/ER/PDMS coating with micro-nanoscale roughness may be a potential candidate to meet the emerging requirements in practical applications, such as self-cleaning, drag reduction, and so on.

Rolling viscous drops on a non-wettable surface containing both micro- and macro-scale roughness

It has previously been shown that when a liquid drop of high viscosity is placed on a non-wettable inclined surface, it rolls down at a constant descent velocity determined by the balance between viscous dissipation and the reduction rate of its gravitational potential energy. Since increasing the roughness of the surface boosts its non-wetting property, the drop should move faster on a surface structured with macrotextures (ribbed surface). Such a surface was obtained from a superhydrophobic soot coating on a solid specimen printed with an extruder-type 3D printer. The sample became superoleophobic after a functionalization process. The descent velocity of glycerol drops of different radii was then measured on the prepared surface for varied tilting angles. Our data show that the drops roll down on the ribbed surface approximately 27% faster (along the ridges) than on the macroscopically smooth counterpart. This faster velocity demonstrates that ribbed surfaces can be promising candidates for drag-reduction and self-cleaning applications. Moreover, we came up with a modified scaling model to predict the descent velocity of viscous rolling drops more accurately than what has previously been reported in the literature.

Magnetic Liquid Metal Marble: Characterization of Lyophobicity and Magnetic Manipulation for Switching Applications

We report a “magnetic liquid metal marble” (MLMM) obtained by coating a liquid metal’s surface with micro/nano-sized ferromagnetic iron (Fe) particles, which enables on-demand, magnetic manipulation of a liquid metal droplet for switching applications. Among liquid metals, gallium-based liquid metal alloys have been developed for a variety of applications. However, most developed applications using the gallium-based liquid metal alloy only work on deformability because of its easy-wetting property stemming from surface oxidation. By coating the oxidized surface with the 45 $\mu \text{m}$ or 45 nm diameter Fe particles, the MLMM exhibits non-wetting property investigated by evaluating apparent contact angles and sliding angles against various surfaces. On the Teflon-coated glass, the largest contact angle was measured to be ~169.0°, and the lowest sliding angle was obtained to be 17.2°, respectively. In order to move the 45- $\mu \text{m}$ diameter Fe particles-coated MLMM, we measured the minimum required magnetic flux density of 150 gauss and demonstrated the magnetic control of the liquid metal marble to turn ON light emitting diodes. In addition, we investigated that hydrochloric acid-vapor treatment on the MLMM enhanced the lyophobicity (sliding angle of 9.4°), reduced the minimum magnetic flux density (150 to 107 gauss) to actuate it, and enabled electrical switching applicability even in silicon oil with shorter delay time and high mobility of the MLMM under the applied magnetic field. [2015-0329]

Fabrication of a superhydrophobic and oleophobic PTFE membrane: An application to selective gas permeation

A superhydrophobic and oleophobic film was fabricated on polytetrafluoroethylene (PTFE) using SiO2 nanoparticles and 1H,1H,2H,2H-perfluorooctyltrichlorosilane (FOTS). The surface of the bare PTFE sheet was first coated with SiO2 nanoparticles by the dip-coating method, and FOTS was subsequently deposited on the SiO2-coated PTFE by either dip-coating or vapor deposition. Both of the FOTS-coated PTFE samples prepared using either dip-coating or vapor deposition initially showed high repellence towards water and hexadecane droplets. The FOTS-PTFE prepared by vapor deposition gradually absorbed hexadecane. By contrast, the dip-coated FOTS-PTFE displayed a durable non-wetting property, even after exposure to acidic/basic solutions and UV light irradiation. Gas permeability of the prepared superhydrophobic and oleophobic PTFE sheets towards CO2 and dimethyl methylphosphonate (DMMP) vapor was also determined. We suggest that our superhydrophobic and oleophobic membrane could be used as the shielding layer of a gas sensor, to repel various liquids and only allow transmission of gases.

Novel transparent, liquid-repellent smooth surfaces with mechanical durability

A simple fabrication method for a smooth and transparent liquid-repellent film has been presented, and the smooth film was successfully obtained through sol–gel process with (3-Glycidyloxypropyl)-trimethoxysilane (GPTMS) followed by surface treatment of air plasma and fluorination. Combination of hydrolyzing of GPTMS and the surface treatment can form covalently interfacial interaction and further enhance surface robustness of the smooth film. The smooth film displays non-wetting behavior towards water and many organic liquids with very low surface tension, such as n-hexane (γlν=18.4mN/m) and petroleum ether (γlν=17.5mN/m); meanwhile, optical transmittance of the smooth film is greater than 86% throughout a broad spectrum of ultraviolet and visible wavelengths. Importantly, mechanical durability of the obtained film surface was proved by tests of rubbing, wiping, thumb pressing, and peeling of adhesive tape. More surprisingly, the rubbing process can enhance surface non-wetting property and transparency of the smooth film to a certain extent. In addition, the smooth surfaces can also be easily fabricated on other different metal substrates. The impressive liquid-repellency of the smooth surface is concluded to be governed primarily not by surface topography but rather by distribution of fluorine compound on the surface.

Mushroom-Shaped Micropillars for Robust Nonwetting Surface by Electrohydrodynamic Structuring Technique

Mushroom-shaped micropillars (MSMPs) have a unique microscopic shape that is suitable to produce a highly robust nonwetting surface. This commercially important property is possible thanks to the specific overhang shape, which has a larger tip diameter than the shaft below. We fabricated such a textured surface using poly(methyl methacrylate) (PMMA) via electrohydrodynamic structuring technique. The method combines hot embossing, electrically induced growth, and electrowetting. Even though PMMA itself is hydrophilic (the intrinsic contact angle is 82°), its modified surface is hydrophobic with an apparent contact angle of 152°. We also studied the robustness of the nonwetting surface by comparing the difference between the static contact angles before and after the samples had been submerged in water. Our results show that only the surface structured with the 43-μm tip-diameter MSMPs (the largest size) can successfully maintain its highly nonwetting property. Because of the large fabrication potential of the MSMPs with a large tip diameter, the described surface structuring technique is a promising candidate for the mass production of artificial and sufficiently robust nonwetting surfaces.