Re-entrant Texture Research Articles

The evolution of crystal‐poor rhyolite in the middle Okinawa Trough and its implications for the state of magma chamber

The crystal‐poor rhyolitic pumice is the product of extreme magma differentiation and is characterized by highly vesicular, honeycomb‐like structure. However, little is known about the evolutionary history and the time of volatile saturation of the pumice‐forming melt. A crystal‐poor rhyolitic pumice (TVG7‐1) has been collected from the Yoron Hole hydrothermal field in the middle Okinawa Trough, a young continental margin back‐arc basin in the western Pacific. This study conducted a detailed analysis on the texture and in situ chemical composition of plagioclase, orthopyroxene and Fe‐Ti oxides to reveal the evolution process, storage state and eruption triggers of eruptible silicic melt. Texture and zoning of specific minerals and mineral populations suggest a magmatic evolution history of open system. Additionally, the hollow reentrant texture, as well as results from plagioclase‐liquid hygrometers, confirm a water‐rich melt pocket. The exsolution of volatiles (H2O, etc.), in this melt pocket, will contribute to local oxidation conditions, which may have been recorded by the concordant behaviour of FeO and An in plagioclase. Then, the overpressure caused by volatile exsolution would destabilize the magma chamber, which will eventually be broken by magma mixing/recharge, and lead to violent melt evacuation. Consequently, crystal size distribution (CSD) provides a new perspective on understanding the kinetic effect of magma mixing/recharge in the Okinawa Trough, especially on the mineral populations. Our study reveals the petrogenesis of crystal‐poor rhyolite within the framework of the mush model, refines the magma evolution history and demonstrates that pumice magma reaches volatile saturation before the eruption.

Unprecedented superoleophobicity achieved with fluorinated wrinkle mesoporous silica

Superoleophobic surfaces can be facilely fabricated by utilizing fluoroalkyl-modified wrinkled mesoporous silica (F-WMS). Initially, a hydrophilic wrinkled mesoporous silica (WMS) (average particle size ∼400 nm) with radial pores and narrow silica walls was prepared, and subsequently modified with a fluoroalkylsilane, specifically 1H,1H,2H,2H-heptadecafluorodecyl trimethoxysilane (FAS-17), to impart low surface energy. Preservation of the WMS pore structure was observed even at a 20 mol% FAS-17 feeding ratio; thus, the resulting F-WMS was designated as F20-WMS. An aluminum substrate was coated with an epoxy resin (EP) for adhesion, followed by spray-coating the F20-WMS suspension onto the EP layer. The resulting EP/F20-WMS coating displayed a notably high static water contact angle (CA) of 172.3° and a low roll-off angle (RA) of 1° Notably, the coating exhibited exceptional oil repellency, showcasing a hexadecane CA of 165.1° and an RA of 4.3°, surpassing the performance of most reported superoleophobic coatings. Plotting cos (CAs on EP/F20-WMS) against (CAs on a flat surface coated with FAS-17), utilizing CA data with various liquids (water – dodecane, 72.1 – 25.3 mN/m, respectively), indicated that the liquids were consistently in Cassie states with a contacting fraction of approximately 0.03 between the liquid droplets and the surface. The unprecedented superoleophobicity of EP/F20-WMS was attributed to the extremely low contact fraction and the presence of a re-entrant texture. Moreover, the coatings exhibited exceptional mechanical durability in tape-peeling tests (50 cycles) and achieved the highest adhesion rating (5 B) in cross-cut tests. This superior oleophobicity, coupled with high durability, opens up opportunities for real-life applications necessitating liquid-free surfaces.

Direct-micro-fabrication of Hydrophobic Surface with Re-entrant Texture on Metal Produced by Femtosecond-pulsed Laser

Re-entrant textures are promising geometries for hydrophobic surfaces, however a direct processing method of microscale re-entrant textures applicable for general industrial materials such as metals has yet to be established. The purpose of this study was to demonstrate a possibility of direct processing method of microscale re-entrant textures by using a femtosecond-pulsed laser. We designed a novel and simple optical unit including a pair of step mirrors and a newly designed aspherical condenser lens that enable processing of reverse-tapered uniaxial grooves. A maximum reverse-taper angle of 20° was achieved on stainless steel using a femtosecond-pulsed laser that could be controlled linearly with the step mirror angles. Four types of test-pieces with re-entrant texture composed of reverse-tapered grooves were fabricated with reverse-tapered angles of 5 – 20°. It was demonstrated that the apparent contact angle exhibited an increase in the processed angle of the re-entrant texture. The re-entrant structures on stainless steel achieved a hydrophobicity over 140° of apparent contact angle with good stability, and allowing water droplets to slide off.

Porcupine quills-like-structures containing smart PVDF/chitosan hybrids for anti-fouling membrane applications and removal of hazardous VOCs

The separation materials with enhanced transport and separation in membrane distillation (MD) and antifouling features were generated. The membranes were systematically characterized to understand how the covalent modification process, i.e., silanization and functionalization with chitosan, impacts the membrane performance. The material modification was done by the biomimicry of desert beetle (Stenocara gracilipes) armor and porcupine quills. The hydrophobic-hydrophilic structure of beetle armor was mimicked with the application of PVDF membrane as a hydrophobic surface and chitosan (CS) as hydrophilic nodes. By changing the experimental protocol, unique quill-like structure morphology was generated. The introduction of pristine and partially hydrophobized chitosan increased roughness significantly from 70 nm to 386 nm and 440 nm, causing contact angle improvement from 114.6° to 157.4° and 168.2°, respectively. Four times increment of transport was observed after adding CS. MD membranes with superior wetting resistance (critical surface tension ca. 20 mN m−1) were created by combining hierarchically structured membranes with re-entrant texture. The selection of fluorine-free modifiers allows the synthesis of environmentally friendly membranes and reduces the problem of bioaccumulation and acute toxicity occurring with long-chain fluorinated modifiers. Materials were tested in the air gap membrane distillation (AGMD) to remove hazardous volatile organic compounds from water. Data were analyzed by applying Hansen’s Solubility Parameters. Membranes were also applied for raw apple juice dewatering with fouling study during the osmotic MD process.

Long-Chain PFASs-Free Omniphobic Membranes for Sustained Membrane Distillation.

Omniphobic membranes possessing high wetting resistance have been created for the treatment of challenging hypersaline feedwaters with low surface tension through membrane distillation (MD). However, virtually all such membranes are fabricated with long-chain per- and polyfluoroalkyl substances (PFASs, ≥8 fluorinated carbons). The environmental risks and high bioaccumulation potential of long-chain PFASs have raised increasing concerns. Developing highly wetting-resistant MD membranes while avoiding the use of long-chain PFASs is essential to improve the viability of MD for resilient and sustainable water purification. We demonstrate that MD membranes with exceptional wetting resistance can be designed through the combination of hierarchically structured membranes consisting of re-entrant texture at different length scales and (ultra)short-chain fluorocarbons, which have lower acute toxicity and bioaccumulation potentials than long-chain PFASs. Our hierarchically structured membrane with three-tier micro/nanostructure fabricated with short-chain fluorocarbon possesses superior wetting resistance, which is comparable to or higher than the long-chain PFASs-based omniphobic membranes reported in the literature. Furthermore, the hierarchically structured membranes fabricated with ultrashort-chain fluorocarbons display improved wetting resistance against feedwaters with low surface tension. Our findings indicate that long-chain PFASs are not required when designing wetting-resistant membranes and that the balance between sustainability and wetting resistance should be tailored to the wetting potential of the feedwater.

Construction of omniphobic PVDF membranes for membrane distillation: Investigating the role of dimension, morphology, and coating technology of silica nanoparticles

Surface deposition of silica nanoparticles (SiNPs) is widely used for constructing omniphobic membranes for membrane distillation (MD) with an aim of creating hierarchical and reentrant textures. Herein, we report a comprehensive analysis of the effects of physical properties of SiNPs on membrane performance from the viewpoint of their dimensions and morphologies. An emerging fabrication technology, i.e., the electrospray method, was also investigated and systematically compared to the conventional solution deposition. SiNPs with spherical dimensions of 30 nm aggregated significantly in solution deposition process, forming a surface layer with uncontrollable defects and becoming vulnerable to inorganic fouling. While membranes utilizing SiNPs with a diameter of 200 nm showed severe pore blocking, which resulted in much lower operational fluxes. However, the fluxes could be improved by 20–50% using hollow mesoporous SiNPs, where the intrinsic cavities provided extra channels for vapor transport. More importantly, using a high-voltage electric field in the electrospray procedure resulted in a uniform distribution of SiNPs, thereby endowing an ideal membrane surface layer for a robust MD operation. The proposed study provides new insights into the effects of the physical properties of SiNPs, and shows the advantage of electrospray technology in fabricating defect-free MD membranes for sustainable applications.

Femtosecond Laser-Assisted Top-Restricted Self-Growth Re-Entrant Structures on Shape Memory Polymer for Dynamic Pressure Resistance.

Bioinspired surface material with re-entrant texture has been proven effective in exhibiting good pressure resistance to droplets with low surface tension under static conditions. In this work, we combined femtosecond laser cutting with shape memory polymer (SMP) and tape to fabricate re-entrant micropillar arrays by proposing a top-restricted self-growth (TRSG) strategy. Our proposed TRSG strategy simplifies the fabrication process and improves the processing efficiency of the re-entrant structure-based surface material. The structural parameters of the re-entrant micropillar array (microdisk diameter D, center-to-center distance P, and height H) can be adjusted through our TRSG processing method. To better characterize the anti-infiltration ability of various re-entrant micropillars, we studied the dynamic process of ethylene glycol droplet deformation by applying external vertical vibration to the surface material. Three parameters (vibration mode, amplitude, and frequency) of the external excitation and structural parameters of the re-entrant micropillar array were systemically investigated. We found that the surface material had better dynamic pressure resistance when P and D of the re-entrant texture were 650 and 500 μm, respectively, after heating for 6 min. This work provides new insights into the preparation and characterization of the surface material, which may find potential applications in microdroplet manipulation, drug testing, and biological sensors.

Bioinspired Superhydrophobic Surface Constructed from Hydrophilic Building Blocks: A Case Study of Core–Shell Polypyrrole-Coated Copper Nanoneedles

Hydrophilic polypyrrole-coated copper nanoneedles (PPy-CuNDs) were synthesized and utilized to construct a superhydrophobic surface on a polyethylene terephthalate fabric (PET) by using the spray-coating technique. The morphology of the as-synthesized PPy-CuNDs can be facilely tuned by changing the concentration of the reducing agent: hydrazine monohydrate. The CuNDs with well-defined nanocrystalline structures and nanoscale thick, rough PPy coating layers were formed simultaneously in one pot. The PPy-CuNDs self-assembled into an entangled, stacking nanocarpet on the surface of the PET fabric, and they eventually formed a reentrant surface texture similar to that of chrysanthemum leaves. The PPy-CuND-PET surface initially showed good superhydrophobic properties, but a fast transition from the superhydrophobic state to the highly adhesive state was observed. The underlying mechanism of this transition and its potential applications were proposed in the context.

Microfabrication of re-entrant surface with hydrophobicity/oleophobicity for liquid foods

Re-entrant texturing may potentially improve the hydrophobicity and oleophobicity of a surface. The food industry requires a microfabrication method to keep surfaces clean without leaving a packaging residue for applications such as food bottles, food containers, and preservation bags. The goal of this study is thus to establish a microfabrication method for re-entrant texturing with spherical curvature to produce hydrophobic/oleophobic surfaces for liquid foods, such as soy sauce and canola oil. Samples with a spherical curvature are created from an ultra-violet-cure (UV-cure) resin and poly (tetrafluoroethylene) (PTFE) microbeads with diameters between 2.26 to 1,353 microns by spin coating on a glass substrate. The resin thickness, the mass and diameter of the microbeads, and the spin coater rotation speed are used as the microfabrication parameters. A side view of samples showing the spherical curvature reveals that a re-entrant texture indeed forms. Distilled water, soy sauce, and canola oil are dropped softly onto the re-entrant surface, however, the droplets cannot be placed stably. For appropriate microbead diameters, the apparent contact angles of soy sauce and canola oil showed 130.2 and 119.4 degrees, respectively. This facile fabrication method for re-entrant surfaces could prove useful for generating hydrophobic/oleophobic surfaces for Newtonian liquid foods.

Ultra-repellency of Al surfaces: design and evaluation

Aluminum (Al) surfaces with ultra-repellency as well as desirable robustness were designed and fabricated. With photolithographic patterning of a thick SU-8 layer and sputtering of a thin Al film, re-entrant micro-pillar textured Al surfaces were prepared. After derivatization with perfluoroalkyl phosphoric acid (FPA), the textured Al surfaces showed ultra-repellency for a wide variety of liquids. The contact angles (CAs) of deionized (DI) water, hexadecane and dodecane were larger than 150°, and those of methanol and ethanol were larger than 100°. The sliding angles (SAs) of DI water, hexadecane and dodecane were 5°, 10°, and 10°, respectively, showing excellent superamphiphobicity. The SAs of methanol and ethanol were in the range of 20°–30°. The robustness of the ultra-repellent Al surface was evaluated by three parameters: robust height (H*), robust angle (T*) and robust factor (A*). For the DI water probing, the values of the parameters are H* ≈ 403, T* ≈ 119 and A* ≈ 92, respectively, indicative of a desirable robustness. We clarified that only re-entrant structures can support composite liquid–solid–vapor interfaces when the corresponding Young’s CAs are smaller than 90°, and the function of the nanometer structures of the hierarchical textures which were widely adopted to fabricate superamphiphobic surfaces is to help construct re-entrant structures. FPA derivatization is effective in lowering the surface energy of Al surfaces, combining with re-entrant textures to provide a simple and high throughput approach to ultra-repellency for a wide variety of liquids.

Homogenization in Hydrodynamic Lubrication: Microscopic Regimes and Re-Entrant Textures

The form of the Reynolds-type equation which governs the macroscopic mechanics of hydrodynamic lubrication interfaces with a microscopic texture is well-accepted. The central role of the ratio of the mean film thickness to the texture period in determining the flow factor tensors that appear in this equation had been highlighted in a pioneering theoretical study through a rigorous two-scale derivation (Bayada and Chambat, 1988, “New Models in the Theory of the Hydrodynamic Lubrication of Rough Surfaces,” ASME J. Tribol., 110, pp. 402–407). However, the resulting homogenization theory still remains to be numerically investigated. For this purpose, after a comprehensive review of the literature, three microscopic regimes of lubrication will be outlined, and the transition between these three regimes for different texture types will be extensively demonstrated. In addition to conventional textures, representative re-entrant textures will also be addressed.

Mechanical characterization of auxetic stainless steel thin sheets with reentrant structure

Smart materials in auxetic form present a great potential for various medical applications due to their unique deformation mechanisms along with durable infrastructure. Both analytical and finite element (FE) models are extensively used in literature to characterize mechanical response of auxetic structures but these structures are mostly thick enough to be considered as bulk material and 3D inherently. Auxetic plates in very thin form, a.e. foil, may bring numerous advantages such as very light design and better biodegradability when needed. However, there is a gap in literature on mechanical characterization of auxetic thin plates. In this study, structural analysis of very thin auxetic plates under uniaxial loading is investigated using both FE method and experimental method. 25 μm thick stainless steel (316L) plates are fabricated with reentrant texture for three different unit cell dimensions and tested under uniaxial loading using universal testing machine. 25 and 50 μm thick sheets with same cell dimensions were analyzed using implicit transient FE model including strain hardening and failure behaviors. FE results cover all the deformation schemes seen in actual tests and total deformation level matches with test results. Effect of plate thickness and cell geometry on auxetic behavior is discussed in detail using FE results. Finally, based on FE analysis results, an optimum geometry for prolonged auxetic behavior, high flexibility and high durability is suggested for future potential applications.

Free-Standing, Flexible, Superomniphobic Films.

Fabrication of most superomniphobic surfaces requires complex process conditions or specialized and expensive equipment or skilled personnel. In order to circumvent these issues and make them end-user-friendly, we developed the free-standing, flexible, superomniphobic films. These films can be stored and delivered to the end-users, who can readily attach them to virtually any surface (even irregular shapes) and impart superomniphobicity. The hierarchical structure, the re-entrant texture, and the low solid surface energy render our films superomniphobic for a wide variety of liquids. We demonstrate that our free-standing, flexible, superomniphobic films have applications in enhanced chemical resistance and enhanced weight bearing.

Designing Robust Hierarchically Textured Oleophobic Fabrics

Commercially available woven fabrics (e.g., nylon- or PET-based fabrics) possess inherently re-entrant textures in the form of cylindrical yarns and fibers. We analyze the liquid repellency of woven and nanotextured oleophobic fabrics using a nested model with n levels of hierarchy that is constructed from modular units of cylindrical and spherical building blocks. At each level of hierarchy, the density of the topographical features is captured using a dimensionless textural parameter D(n)*. For a plain-woven mesh comprised of chemically treated fiber bundles (n = 2), the tight packing of individual fibers in each bundle (D2* ≈ 1) imposes a geometric constraint on the maximum oleophobicity that can be achieved solely by modifying the surface energy of the coating. For liquid droplets contacting such tightly bundled fabrics with modified surface energies, we show that this model predicts a lower bound on the equilibrium contact angle of θ(E) ≈ 57° below which the Cassie–Baxter to Wenzel wetting transition occurs spontaneously, and this is validated experimentally. We demonstrate how the introduction of an additional higher order micro-/nanotexture onto the fibers (n = 3) is necessary to overcome this limit and create more robustly nonwetting fabrics. Finally, we show a simple experimental realization of the enhanced oleophobicity of fabrics by depositing spherical microbeads of poly(methyl methacrylate)/fluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS) onto the fibers of a commercial woven nylon fabric.

The design and applications of superomniphobic surfaces

Surfaces that display contact angles >150° along with low contact angle hysteresis with essentially all high and low surface tension liquids, including water, oils and alcohols, are known as superomniphobic surfaces. Such surfaces have a range of commercial applications, including self-cleaning, non-fouling, stain-free clothing, drag reduction, corrosion prevention and separation of liquids. Such surfaces have thus generated immense academic and industrial interest in recent years. In this review, we discuss the systematic design of superomniphobic surfaces. In particular, we discuss the significance of surface energy, roughness and the critical role of re-entrant texture in obtaining the so-called Cassie–Baxter state with low surface tension liquids. We also discuss how hierarchical scales of texture can yield high contact angles and decrease the contact angle hysteresis of superomniphobic surfaces by reducing the solid–liquid contact area. On the basis of this understanding, we discuss dimensionless design parameters that allow for the systematic design of superomniphobic surfaces. We also review the current literature on superomniphobic surfaces, paying particular attention to surfaces that demonstrate good mechanical, chemical and radiation durability—traits that are essential for any commercial application of superomniphobic surfaces. Finally, we conclude by identifying the unresolved challenges in the fabrication of durable superomniphobic surfaces and highlight the future needs in the field. Surfaces that display contact angles >150° along with a low contact angle hysteresis for both low and high surface tension liquids are known as superomniphobic surfaces. Such surfaces have several applications, including self-cleaning, non-fouling, stain-free clothing, drag reduction, corrosion prevention and separation of liquids. In this review, we discuss the design criteria, recent studies, applications, challenges and potential of superomniphobic surfaces. Surfaces that strongly repel low surface tension liquids (e.g. oils and alcohols) are classified as superoleophobic and those that strongly repel high surface tension liquids (e.g. water) are classified as superhydrophobic. However, if a surface shows both these characteristics, it can be considered superomniphobic. Liquid droplets on superomniphobic surfaces roll off very easily and, as a result, these surfaces are attractive as non-fouling coatings, self-cleaning surfaces and liquid-separation techniques. Anish Tuteja at the University of Michigan, United States, and colleagues review the design principles required to fabricate superomniphobic surfaces. They discuss surface energy, roughness and hierarchical scales of surface texture, and recognize the importance of re-entrant textures (i.e. convex topography) to achieve superomniphobicity. Several examples of superomniphobic surfaces are presented with particular focus on the need to improve chemical and mechanical durability, to realize their full potential in commercial and industrial applications.

Tuning Omniphobicity via Morphological Control of Metal–Organic Framework Functionalized Surfaces

Fabrication of microstructures for imparting omniphobicity to a surface generally requires the use of lithographic techniques and specialized equipment. We report instead a simple strategy for the synthesis of microstructured surfaces via metal–organic framework (MOF) self-assembly. Our method allows us to localize epitaxial growth of MOF at the tips of needle crystals to create mushroom-shaped structures, thus conferring re-entrant textures to the MOF-functionalized surfaces. These structures synthesized via wet chemistry were found to have omniphobic properties due to the resulting re-entrant texture.

Superoleophobic surfaces: design criteria and recent studies

Surfaces that display contact angles greater than 150° along with a low contact angle hysteresis for low surface tension liquids such as oils and alcohols are known as superoleophobic surfaces. Such surfaces are of interest for a diverse array of applications including self-cleaning, nonfouling, stain-free clothing and spill-resistant protective wear, drag reduction, microrobots for aqueous and chemical environments and icephobicity. Recently, significant advances have been made in understanding the criteria required to design superoleophobic surfaces. In this article, the authors discuss the roles of surface energy and roughness, the critical role of re-entrant texture and the role of hierarchical structure in fabricating superoleophobic surfaces. On the basis of this understanding, the authors also discuss two design parameters that allow for the systematic design of superoleophobic surfaces. The authors also summarize the recent studies on superoleophobic surfaces and emphasize the need for careful and diligent characterization. Finally, the authors conclude with the major challenges and opportunities for research on superoleophobic surfaces.

Superomniphobic surfaces: Design and durability

Abstract

Superomniphobic Surfaces for Effective Chemical Shielding

Superomniphobic surfaces display contact angles >150° and low contact angle hysteresis with essentially all contacting liquids. In this work, we report surfaces that display superomniphobicity with a range of different non-Newtonian liquids, in addition to superomniphobicity with a wide range of Newtonian liquids. Our surfaces possess hierarchical scales of re-entrant texture that significantly reduce the solid-liquid contact area. Virtually all liquids including concentrated organic and inorganic acids, bases, and solvents, as well as viscoelastic polymer solutions, can easily roll off and bounce on our surfaces. Consequently, they serve as effective chemical shields against virtually all liquids--organic or inorganic, polar or nonpolar, Newtonian or non-Newtonian.

Counterion Exchange to Achieve Reversibly Switchable Hydrophobicity and Oleophobicity on Fabrics

We describe a simple layer-by-layer (LbL) technology and counterion exchange procedure to tune the liquid wettability of commercially available cotton fabrics. A polyelectrolyte multilayer is deposited on the fabric surface by the LbL technology, and counterion exchange is used to control the surface composition and thereby to modulate the solid surface energy. The tunability of the solid surface energy, along with the inherent re-entrant texture of the cotton fabric, results in simultaneously switchable wettability between a nonwetting state and a fully wetted state for water and hexadecane. This switchable hydrophobicity and oleophobicity can be explained within a robustness factor, which is a quantitative criterion for the transition between the two states. The counterion exchange can be confirmed by X-ray photoelectron spectroscopy analysis.