Dual-scale Roughness Research Articles

Development of flexible nanocellulose-based composites with enhanced hydrophobicity and improved haze for efficient light management in solar cells

The proliferation of flexible electronic products derived from petroleum substrates in the market has exacerbated environmental pollution stemming from electronic waste generation. As a result, there is an increasing demand for cellulose nanofiber substrates that are green and degradable. However, substantial challenges remain in addressing the hydrophilicity of nanocellulose substrates and optimizing their optical properties for commercial viability. In this study, we propose a multifunctional composite film material through the use of amidation modification and biomineralization technology. We successfully grafted highly carboxylated transparent oxide nanocellulose (TCNF) and octadecylamine (ODA) using carefully designed amidation conditions. Through the carboxyl group after amination, we were able to generate amorphous calcium carbonate (ACC) in situ, resulting in the creation of a flexible, sustainable, and transparent TCNF/ODA/CaCO3 composite film with high hydrophobicity and haze. This was confirmed by a water contact angle value of 138.21° and a haze value of 84.8%. The composite film exhibits low surface energy and high dual-scale surface roughness, while its internal structure is relatively porous and loose, but still possesses multi-scale interface interactions. We demonstrate a strong correlation between the multifunctional properties and structural properties of the composite films in this work, thereby achieving effective light management applications in solar cells. Our findings are expected to provide new insights and ideas for the design of multifunctional green flexible solar cell device substrate materials.

Durable, transparent and superhydrophobic coating with temperature-controlled dual-scale roughness by self-assembled raspberry nanoparticles

This study focuses on creating a superhydrophobic, durable, and exceptionally transparent coating with dual-scale roughness by naturally formed raspberry-like particles. This approach facilitates the management of surface roughness at both single and dual scales through variations in surface functionalization temperature. We illustrated that adjusting the temperature of organosilanes functionalization on the surface allows for various reactions, such as the direct grafting of metallic precursors or their polymerization on the surface, resulting in the formation of large raspberry-like particles.We investigated the impact of nanoparticle concentration, functionalization duration, and reaction temperature on surface properties. Our results reveal that a concentration of 1.5 % SiO2 nanoparticles, combined with surface functionalization using TCMS for 4 h at 3 °C, provides the optimal conditions for creating a surface that combines superhydrophobicity, transparency, and acceptable durability. The resulting surface exhibits an impressive contact angle of 158.9°, a sliding angle of 2°, and a transmittance rate of 82 %.Furthermore, the coating demonstrates remarkable resistance to abrasion for up to 35 cycles and can withstand temperatures up to 280 °C. It also offers enhanced protection against UV radiation for 50 h and improved resistance to sand abrasion for up to 30 s, enduring bombardment pressures of up to 6 bars. Moreover, the coating presents several advantages in terms of surface cleaning.

Fabrication of superhydrophobic surface by spraying hydrophobically modified silica nanoparticles on partially cured polyurethane paint

ABSTRACT There is an increasing demand for superhydrophobic surfaces due to their unique properties like self-cleaning, anti-icing, corrosion resistance, etc. The development of polyurethane (PU)-based superhydrophobic coating has garnered significant attention for aircraft applications. Polyurethane (PU) topcoat is widely used in aircraft and is hydrophilic in nature with a water contact angle (WCA) of 75 ± 3°. In the present work, a hydrophilic PU clear coat on aircraft-grade aluminum substrate is transformed to a superhydrophobic (SHP) surface by spraying environmentally friendly hydrophobically modified silica nanoparticles (HSiNps) oonto a PU layer that was partially cured in a hot air oven at 60°C for 1.5 h. The SHP surface exhibited a WCA of 162 ± 3° and a sliding angle (SA) of 8 ± 3°. With the incorporation of HSiNps, micro – nano dual-scale surface roughness is created and the surface roughness of the PU coating is increased from 0.35 to 2.42 µm. The coating exhibits good adhesion (5B) with excellent water repelling property and remains thermally stable and superhydrophobic even after subjecting it to 100°C for one week. Durability of the coatings is studied by immersing the coatings in various liquids. The retention of superhydrophobic property after anti-fogging, water sprinkler, and sand paper abrasion tests further demonstrate the robustness of the coating.

Wetting of nanoscale water films on hierarchically structured surfaces

Surfaces with hierarchical structures significantly enhance the hydrophobic properties of solids, proving crucial for diverse applications including self-cleaning, anti-icing, and contamination prevention. In this study, we directly observe the dynamic wetting transitions of nanoscale water films on desirable textured surfaces decorated with dual-scale roughness between various wetting states encompassing Cassie–Cassie, Wenzel–Cassie, Cassie–Wenzel, and Wenzel–Wenzel states. Additionally, detailed information on the wetting of the water film on desirable textured surfaces decorated with dual-scale roughness is obtained using atomistic simulations in conjunction with sampling techniques. Through observation of the dynamic wetting transition, two common types of wetting pathways are directly captured, dubbed the preferential primary intrusion and secondary intrusions. The wetting follows which pathway is dependent on Hs/Ss of the small-scale roughness. The mechanisms behind the wetting transitions are revealed based on corresponding free-energy pathways. Moreover, the effect of aspect ratio and intrinsic contact angle on the wetting behavior has been studied. Subsequently, we construct a wetting phase diagram to exhibit all the possible outcomes and identify different wetting regimes. This work paves the way to understanding the wetting mechanisms on nanoscale textured surfaces with two-tier roughness, which can help to design a hydrophobic surface with superior robustness.

Long-lasting low fluorinated stainless steel hierarchical surfaces for omniphobic, anti-fouling and anti-icing applications

Stainless steel (SS) alloys are prevalent in many industries, household appliances or other commodities, where a strict control of surface properties is required to tailor their interaction with the environment. In this work we report a new procedure of stainless steel surface processing that provides a multifunctional response including superhydrophobicity, omniphobicity, self-cleaning, anti-fouling and effective anti-icing capacity, while still preserving a corrosion resistance similar to that of this material in compact form. The method consists of a first nanostructuration step followed by a low fluorination. The nanostructured surfaces presented a dual scale roughness of hierarchical character. The liquid free approach developed in this work to get this singular surface nanostructuration entails a first laser treatment of stainless steel flat substrates, followed by the deposition of a nanostructured thin layer of this material by electron beam evaporation in an oblique angle configuration. The resulting hierarchical surfaces were subjected to fluorination by: (i) the plasma-assisted deposition of a thin Teflon-like coating or (ii) the grafting of fluorinated molecules. The self-cleanable, anti-adherent and ice repellent character of the resulting low fluorinated surfaces outperformed the behaviour of classical slippery surfaces obtained by the infusion of high amounts of fluorinated liquids. These hierarchical SS surfaces withstood mild abrasion tests and the effect of water jets. Moreover, the corrosion behaviour of the fluorinated surfaces determined through their potentiodynamic analysis revealed a similar corrosion resistance than the flat SS substrates. Outstandingly, after these corrosion tests, the fluorinated samples obtained by grafting preserved their surface functionalities without significant degradation. The high mechanical and chemical stability of these low fluorinated samples support their usage for a large variety of applications.

Dual-Coating of Fluorinated Polydimethylsiloxane/Fluorinated SiO2 Nanoparticles for Superhydrophobic and High-Efficiency Bacteriostatic Surface.

A simple two-step spray method is used to prepare superhydrophobic and bacteriostatic surfaces, involving dual-coating with polydimethylsiloxane-normal-fluorine (PDMS-NF) or branched-fluorine (PDMS-BF) in combination with fluorinated silica nanoparticles (FSiO2 -NPs) using a spray technique. This approach has the potential to create surfaces with both water-repellent and antimicrobial properties, which could be useful in a variety of applications. It is noteworthy that the dual-coating on cotton fabric exhibited an impressive dual-scale roughness and achieved superhydrophobicity with a water contact angle of 158° and a hysteresis of less than 3°. Additionally, the coating was subjected to an ultra-high concentration of bacteria (109 CFU/mL) and was still able to inhibit more than 80 % of attachment, demonstrating its effectiveness as a bacteriostatic surface.

A robust superhydrophobic-superoleophilic PDMS/Al2O3/CM composite ceramic membrane: Stability, efficient emulsified oil/water separation, and anti-pollution performance

To address the issue of membrane fouling and low separation efficiency of ceramic membrane (CM) commonly encountered in oil/water separation, the implementation of a robust superhydrophobic coating on the surface of CM represents a viable and efficacious strategy. This study presented a facile approach for the fabrication of a durable superhydrophobic-superoleophilic composite coating on the surface of CM by the in-situ growth of needle-shaped alumina (Al2O3) followed by the impregnation of a polydimethylsiloxane (PDMS) coating. Various characterization techniques, such as FESEM, XPS, CLSM, bubble point porometer, contact angle etc. were used to characterize the microstructure, surface composition, pore size distribution, and surface characteristics of the PDMS/Al2O3/CM. The mechanical, chemical, and thermal stability were evaluated by means of sandpaper abrasion, tape peeling, immersion tests including organic solvent, acid, and base solutions, and heat treatment. The separation performance, anti-pollution property, and reusability of the modified CM were evaluated by oil/water mixture and water-in-oil emulsion. And the oil/water separation model and anti-fouling mechanism was also elaborated. The results indicated that a homogeneous needle Al2O3 cluster with a micro/nano dual-scale hierarchical roughness was grown on the surface of the CM, and the further introduction of PDMS gave the modified CM excellent superhydrophobicity with a water contact angle (WCA) of ∼160° and a water rolling angle (WOA) of 7.6°. The PDMS/Al2O3/CM composite membrane exhibited remarkable capability in segregating oil/water mixtures, including emulsified water-in-oil. The separation efficiency remained above 99 %, even after 15 cycles of separation. This study presents a simple technique for fabricating a robust superhydrophobic coating onto the Al2O3-based CM surface, which exhibits the potential for application to other ceramic membrane substrates. And the anticipated outcome of this developing is a broad spectrum of industrial applications for emulsified oil/water purification.

Laser-textured Pd-Au hierarchical superhydrophobic micro-arrays for ultralow SERS detection

Ultra-trace identification of the limited quantity of diluted analyte solutions requires the condensation of target analytes utilizing the superhydrophobic analyte enrichment effect and are extremely desirable for improving the detection sensitivity. Herein, the identification of ultra-trace analyte concentrations employing laser-textured palladium-gold (Pd-Au) hierarchical superhydrophobic micro-arrays as surface-enhanced Raman sensors (SMA-SERS) is reported. The double ns-laser fast scanning strategy produces hierarchically arranged micro-strip and micro-pyramidal arrays over the large surface areas composed of nano-scale inter-particle gaps, enabling the numerous three-dimensional plasmonic hot-spots due to the uniform formation of Au nanoparticles' aggregates/clusters. Under the optimized fabrication parameters, the resultant hybrid SMA-SERS sensors exhibit periodic micro-pyramidal arrays with excellent inherent superhydrophobicity (Contact angle, CA∼171°) and consequently superior SERS performance with uniform (RSD∼10%) signal detectability down to the ∼34.2 fM (3.42×10−14 M) MB molecules utilizing analyte enrichment effect. The accumulation of analytes within the smaller dried spots (Contact diameter, CD∼1.13 mm) results in increased spatial density of analytes and subsequently enables the ultra-low detectability of MGO, DTTCI, PA, AN, adenine, and l-tryp molecules down to ∼3.57 pM, 0.34 fM, 0.31 nM, 0.44 µM, 0.25 nM, and 97.7 nM concentrations, respectively. The multiplexed identification of target (AN and adenine) analytes is explored utilizing serial-deposition of analytes and depict the simultaneous detection of target analytes down to ∼10−7 M (AN) and 10−9 M (adenine) concentrations. The double ns-laser fast scanning strategy provides dual-scale surface roughness in the SMA-SERS substrates and offers ultrahigh sensitivity for the identification of the low amount of analytes utilizing the superhydrophobicity-induced analyte enrichment effect.

Exalted dual-scale surface roughening in laser ablated aluminum capped with a transparent thin film: Wetting and anti-icing behavior

Near infrared laser ablation of metals, specifically aluminum, has been systematically applied to generate surface roughness. Very high laser fluences may even lead to a so called “explosive” ablation regime where roughness becomes dramatically enhanced. In the present work we have developed an alternative methodology that, utilizing milder laser irradiation conditions (i.e. laser fluences from 0.37 to 0.72 J/cm2), renders aluminum surfaces with a dual-scale roughness character and Sp parameter values twice or even trice the value found in reference samples. This has been possible for aluminum substrates coated with a highly transparent aluminum oxynitride capping layer. The resulting surfaces, consisting of very rough partially oxidized aluminum with negligible amounts of nitrogen species, resulted highly hydrophobic and depicted long icing delay times as required for anti-icing applications. A correlation has been found between the wetting and anti-icing behaviors, the use of a capping layer and the laser irradiation conditions. To account for this exalted roughening phenomenon, we propose that the transparent capping layer confines the laser energy within the aluminum shallow zones, delays the formation of the plasma plume and produces an enhancement in the aluminum ablation, even at relatively low laser fluences.

Surfaces for hearts: Establishing the optimum plasma surface engineering methodology on polystyrene for cardiac cell engineering

Plasma surface modification is a popular method for improving cell culture on surfaces, and polystyrene (PS) is literature’s material of choice. This study identifies the optimum plasma treatment for promoting normal cardiac cell behaviour during culture. PS slides were plasma-treated with O2, N2, O2 + N2 and Ar + N2 for 20 and 30 min in a reactive ion etcher (RIE). SEM reveals that O2 and O2 + N2 plasmas create dual scale roughness, N2 plasma creates oval-shaped structures, while Ar + N2 exhibits no topography. Evaluation by XPS reveals an increase in the atomic percentage of oxygen for all treatments. Contact angle measurements agree as all treatments lead to hydrophilisation, with N2 samples exhibiting long-term stability.Two sources of cells were used to identify the optimum plasma treatment for cardiac cell culture on PS. H9c2 cells exhibit optimal behaviour with N2 and N2 + Ar regarding viability, morphology, and focal adhesion contact. The same was observed for primary cardiomyocytes on N2 samples. For purified cardiomyocytes, immunofluorescence revealed well-organised sarcomeric structure on N2 samples, exhibiting clear improvement compared to control. SEM validated these findings, as cardiomyocytes on N2-treated PS exhibited physiological, elongated shape. These findings provide solid evidence that the optimum treatment for PS is the use of N2 plasma.

Superhydrophobic and Self-Cleaning Aluminum via a Rapid and Controlled Process

Superhydrophobic and self-cleaning materials have been an extensive topic of research owing to their corrosion resistance, antifouling, and anti-icing properties. In the past, superhydrophobicity in aluminum has been obtained by a variety of techniques involving etching by sandblasting, chemical etching, boiling in water, etc. followed by treatment with a fluorochemical and organic solution. However, these techniques are not controlled, might not be easy to implement on a large scale, and mostly make use of fluorochemicals, which are risky for human health and the environment. Therefore, there is a need for a rapid and controlled fabrication process that can provide patterned superhydrophobicity without compromising the environment or the human health. Here, a rapid, controlled, and environmentally friendly procedure for fabricating a superhydrophobic and self-cleaning aluminum surface has been developed, achieving a contact angle of 158.06° and a sliding angle of 1.94°. In the two-step process, a rough surface was obtained using the laser etching technique in under 3 min, which resulted in a precisely controlled pattern with a dual-scale roughness on the surface (∼20–30 μm). Finally, a low surface energy was imparted to the surface by treatment with stearic acid where a low concentration of 2 × 10–4 M and no more than 10 s of exposure was found to be sufficient. Unlike various superficially coated materials, this superhydrophobic surface is durable and does not lose its properties after dipping in water for up to 4 days, encountering fine sand or mud, or at extreme surrounding temperatures of −18 to 100 °C. Thus, this rapid and controlled process for obtaining superhydrophobic aluminum potentially can be deployed for large-scale, structural applications.

Superhydrophobic surfaces with dual-scale roughness and water vapor-barrier property for sustainable liquid packaging applications

There is an ongoing unmet global need to manufacture novel sustainable liquid packaging materials, that are not based on plastic film or aluminum foil. Superhydrophobic coating technologies have been proposed for developing more sustainable packaging materials. In this study, the underlying engineering principles for fabricating superhydrophobic surfaces proposed for liquid packaging are investigated, including but not limited to the substrates used and engineering properties of the surfaces. Specifically, to improve the engineering performance of superhydrophobic paper for use in packaging, the feasibility of combining platy montmorillonite (MMT, for its barrier properties) and nano-rolling-pin-shaped precipitated calcium carbonate (PCC, for its superhydrophobicity) into multifunctional coating layers is investigated. Water droplet evaporation experiments are performed to identify how subtle changes in the morphological structures of as-prepared superhydrophobic paper samples can produce a useful roughness structure for packaging applications. Paperboard, which is widely utilized in packaging, is chosen as a substrate to study the challenges of fabricating superhydrophobic paperboards for use in packaging. The results of this study provide engineering principles for using sustainable paper-based materials with a dual-scale roughness structure and barrier properties in liquid packaging applications.Graphical abstract

Surface Tension and Viscosity Dependence of Slip Length over Irregularly Structured Superhydrophobic Surfaces.

A comprehensive understanding of the slip phenomenon on liquid/solid interfaces is essential for multiple real-world applications of superhydrophobic materials, especially those involving drag reduction. In the current contribution, the so-called “slip-length” on an irregularly structured superhydrophobic surface was systematically evaluated, with respect to varying liquid surface tension and viscosity. The superhydrophobic polymer–nanoparticle composite (SPNC) material used exhibits a dual-scale surface roughness and was fabricated via coating a surface with a mixture of polydimethylsiloxane solution and functionalized silica particles. A cone-and-plate rheometric device was employed to quantify the slip length. To independently study the impact of surface tension and viscosity, three types of aqueous solutions were used: sodium dodecyl sulfate, ethanol, and polyethylene glycol. Our experimental results demonstrate that a decreasing surface tension results in a decreasing slip length when the fluid viscosity is held constant. Meanwhile, the slip length is shown to increase with increasing viscosity when the surface tension of the various liquids is matched to isolate effects. The study reveals a linear relationship between slip length and both capillary length and viscosity providing a reference to potentially predict the degree of achievable drag reduction for differing fluids on SPNC surfaces.

Dropwise condensation on single-micro-scale roughness hydrophobic surfaces

Dropwise condensation (DWC) usually occurs on superhydrophobic surfaces with dual-scale (or multi-scale) roughness in discrete droplets to avoid liquid film formation, enhancing overall heat transfer efficiency. In this study, we demonstrate that the dual-scale roughness hydrophobic surface is not a necessary condition for DWC to occur. A series of single-scale roughness surfaces of regular arrays of square micro-pillars are fabricated to investigate the effect of surface roughness on the wetting behavior of condensed droplets and DWC. A sequence of the wetting behavior of condensed water droplets larger than 200 μm in diameter on single-scale roughness hydrophobic surfaces: Wenzel state → Cassie state → partial Cassie (Cassie-Wenzel mixed) state → Wenzel state (with the irregular shape of three-phase-contact-line) is observed along with increasing surface roughness (pillar height). An in-house design inverted microscopy system is applied for further confirmation of the wetting behavior of condensed droplets. A tiny Wenzel droplet nucleated grow in the microstructure can be dewetted by colliding with a Cassie droplet. This coalescence-induced dewetting transition can effectively make Cassie DWC occur on these single-scale roughness substrates, providing the surface structure design of condensers for Cassie DWC much easier to implement than on dual-scale roughness surfaces.

Mechanically Switchable Wetting Petal Effect in Self-Patterned Nanocolumnar Films on Poly(dimethylsiloxane).

Switchable mechanically induced changes in the wetting behavior of surfaces are of paramount importance for advanced microfluidic, self-cleaning and biomedical applications. In this work we show that the well-known polydimethylsiloxane (PDMS) elastomer develops self-patterning when it is coated with nanostructured TiO2 films prepared by physical vapor deposition at glancing angles and subsequently subjected to a mechanical deformation. Thus, unlike the disordered wrinkled surfaces typically created by deformation of the bare elastomer, well-ordered and aligned micro-scaled grooves form on TiO2/PDMS after the first post-deposition bending or stretching event. These regularly patterned surfaces can be reversibly modified by mechanical deformation, thereby inducing a switchable and reversible wetting petal effect and the sliding of liquid droplets. When performed in a dynamic way, this mechanical actuation produces a unique capacity of liquid droplets (water and diiodomethane) transport and tweezing, this latter through their selective capture and release depending on their volume and chemical characteristics. Scanning electron and atomic force microscopy studies of the strained samples showed that a dual-scale roughness, a parallel alignment of patterned grooves and their reversible widening upon deformation, are critical factors controlling this singular sliding behavior and the possibility to tailor their response by the appropriate manufacturing of surface structures.

Superhydrophobic Polymer Composite Surfaces Developed via Photopolymerization.

Fabrication of superhydrophobic materials using incumbent techniques involves several processing steps and is therefore either quite complex, not scalable, or often both. Here, the development of superhydrophobic surface-patterned polymer–TiO2 composite materials using a simple, single-step photopolymerization-based approach is reported. The synergistic combination of concurrent, periodic bump-like pattern formation created using irradiation through a photomask and photopolymerization-induced nanoparticle (NP) phase separation enables the development of surface textures with dual-scale roughness (micrometer-sized bumps and NPs) that demonstrate high water contact angles, low roll-off angles, and desirable postprocessability such as flexibility, peel-and-stick capability, and self-cleaning capability. The effect of nanoparticle concentration on surface porosity and consequently nonwetting properties is discussed. Large-area fabrication over an area of 20 cm2, which is important for practical applications, is also demonstrated. This work demonstrates the capability of polymerizable systems to aid in the organization of functional polymer–nanoparticle surface structures.

Laser patterned titanium surfaces with superior antibiofouling, superhydrophobicity, self-cleaning and durability: Role of line spacing

Superhydrophobic titanium surfaces with different line spacing (pitch) of 20, 50, 80 and 100 μm are fabricated using nanosecond laser patterning and their antibiofouling, superhydrophobicity, abrasion resistance and self-cleaning properties are studied. The surface morphology , surface profiles and surface roughness are obtained from scanning electron microscopy (SEM), optical microscopy and surface profilometry. Initially, the dual scale roughness structured laser patterned surfaces were hydrophilic but on prolonged storage time in ambient conditions, average water contact angle (WCA) > 160° was obtained due to the accumulation of atmospheric hydrocarbon compounds on the surface which was confirmed by laser Raman spectroscopy (LRS) and X-ray photoelectron spectroscopy (XPS). The laser patterned titanium samples exposed to the Pseudomonas sp ., bacterial culture showed a 4-order reduction in bacterial adhesion as compared to control samples. Furthermore, the superhydrophobic laser patterned samples exhibited excellent water repellency at different ionic strength chloride solutions, photo-catalytic activity under visible light and abrasion resistance. The laser patterned titanium samples are found to regain the superhydrophobicity by replenishing the carbonaceous layer (denuded in organic solvents) on exposure to an open atmosphere for 14 days. This study provides new avenues to engineer the wetting properties of surfaces by varying laser patterning conditions. • Superhydrophobic surface modification of titanium is done by laser patterning • Surface topography and surface chemistry contributes to SHP • SHP titanium samples exhibited excellent repellence to chloride solutions • SHP titanium showed excellent photo-catalytic activity and self-cleaning ability • SHP titanium showed less bacterial adhesion

Recent advances in the mechanical durability of superamphiphobic surfaces: A review

Superamphiphobic (water and oil repellent) surfaces have distinct application areas such as anti-corrosion, anti-friction, anti-icing, self-cleaning, etc. To fabricate these types of surfaces, the base material must have low surface energy and dual-scale roughness (micro and nano levels). The durability of such surfaces is indicated by the duration up to which they remain stable and effective. When mechanical forces interact with these surfaces, it deteriorates the surface integrity and ultimately degrades its superamphiphobic property. Hence, these surfaces have very limited mechanical wear sturdiness and long-term durability. Therefore, in this article, efforts have been made to review the different types of mechanical durability tests performed on superamphiphobic metallic and non-metallic surfaces. Moreover, various kinds of surface texturing techniques in context with durability of such surfaces have been discussed. Finally, from the reported literature, critical conclusions, challenges, and scopes for future work have also been presented.

Metastable wetting model of electrospun mats with wrinkled fibers

Electrospun mats with targeted wettability are a way forward to diversify their mainstream applications and can facilitate in creating a platform for a remarkable set of properties. Limited control over the fiber morphology and surface chemistry, along with the complex architecture of electrospun mats, pose some of the key challenges for maneuvering the wettability. Herein, we present the design principles that underpin the key determinants responsible for a metastable state of a water droplet on electrospun mats possessing wrinkled surface topography. A ‘first-of-its-kind’ analytical model of apparent contact angle based on the modified Cassie-Baxter state has been proposed that accounts for the dual-scale roughness originating from fiber wrinkles and protuberances created by fiber–fiber contacts. Three-dimensional (3D) fiber and structural parameters obtained via X-ray microcomputed tomography (microCT) analysis served as key inputs for predictive modeling. The analytical model of the apparent advancing and receding contact angles has been validated with electrospun mats having well-defined orientation characteristics. In general, the theory and experiments are in reasonable agreement. Although similar magnitudes of static and advancing contact angles of water on surfaces of randomly and aligned electrospun mats have been experimentally observed, the receding contact angles differed significantly.

A durable, fluorine-free, and repairable superhydrophobic aluminum surface with hierarchical micro/nanostructures and its application for continuous oil-water separation

Artificial superhydrophobic surfaces have drawn a lot of attention owing to their multifunctional properties. However, the durability and environmental concerns of such surfaces are critical issues that have limited their widespread utilization in real-life applications. In this study, fluorine-free superhydrophobic aluminum surface was fabricated through a simple and cost-effective method. Its dual-scale micro-/nanostructures (MNS) was fabricated by combining chemical etching and hydrothermal processes. After modification with polydimethylsiloxane coating using a simple vapor deposition method, surface showed excellent superhydrophobicity, with a water contact angle of ~161° and sliding angle of 4°, and sustained its non-wetting properties against various food liquids. The effect of morphological change on the surface's wettability and the durability of the coating materials were investigated in sandpaper abrasion, tape-peeling, and blade-scratching tests. Compared with micro- or nanostructure surfaces, the MNS surface showed better robustness because of its dual-scale roughness. Furthermore, the abraded surface could effectively restore its superhydrophobicity after being recoated with PDMS. The surface also showed long-term superhydrophobic stability in an ambient environment for over 10 months. Finally, the method was applied to Al mesh with superhydrophobic/superoleophilic properties to separate various oil-water mixtures, yielding a high separation efficiency of up to 94% and outstanding reusability.