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We studied the effects of vertical vibrations on a water drop that was pinned to the sharp edges of a rectangular post. By varying the frequency and amplitude of the vertical displacement, distinct resonance peaks were observed using a simple optical technique. The vibrational spectra of the first two modes exhibited two closely spaced peaks, which corresponded to standing waves that exist along the major and minor contour lengths of the drops. The values of the resonance frequencies can be explained rather well by a simple model, which was originally proposed for axially symmetric drops. [ABSTRACT FROM AUTHOR]

The movement of liquid droplets on the energy gradient surface has attracted extensive attention inspired by biological features in nature, such as the periodic spindle-shaped nodes in spider silks and conical-like barbs of cacti, and the structure–property–function relationship of multifunctional gradient surfaces. In this study, a series of specific patterns are fabricated with 3D printing technology, followed by modification via the atmospheric pressure plasma treatment and liquid phase chemical deposition, resulting in enhancing the ability of water droplets of 5 μL to travel 18.47 mm on a horizontal plane and 22.75 mm against gravity at up to a 20° tilting angle. Additionally, analysis techniques have been employed, including a contact angle analyzer, ESCA, and a laser confocal microscope to evaluate the sample performance. This work could further be applied to many applications related to microfluidic devices, drug delivery and water/fog collection.

We studied the effects of vertical vibrations on a water drop that was pinned to the sharp edges of a rectangular post. By varying the frequency and amplitude of the vertical displacement, distinct resonance peaks were observed using a simple optical technique. The vibrational spectra of the first two modes exhibited two closely spaced peaks, which corresponded to standing waves that exist along the major and minor contour lengths of the drops. The values of the resonance frequencies can be explained rather well by a simple model, which was originally proposed for axially symmetric drops.

This paper introduces a method for the bottom‐up fabrication of bio‐inspired hierarchical patterns having tunable anisotropic wetting properties. The method exploits the surface instability of bilayers comprising a gold nanofilm attached to a substantially prestretched elastomer substrate. Upon film formation, highly aligned wrinkles spontaneously form on the surface owing to the surface instability driven by the compressive residual stress in the film. Thereafter, uniaxial compressive strain is applied to the film by prestretch relaxation of the substrate, which generates an array of high‐aspect‐ratio ridges on the surface. Consequently, hierarchical patterns comprising unidirectionally aligned ridges covered with wrinkles are obtained. Water droplets placed on surfaces having the aforementioned hierarchical patterns show direction‐dependent contact angles, resulting in elongated shapes, which indicates the presence of anisotropic wetting properties. The magnitude of wetting anisotropy can be tuned by simple control of the applied compressive strain and film thickness. [ABSTRACT FROM AUTHOR]

Abstract This paper introduces a method for the bottom‐up fabrication of bio‐inspired hierarchical patterns having tunable anisotropic wetting properties. The method exploits the surface instability of bilayers comprising a gold nanofilm attached to a substantially prestretched elastomer substrate. Upon film formation, highly aligned wrinkles spontaneously form on the surface owing to the surface instability driven by the compressive residual stress in the film. Thereafter, uniaxial compressive strain is applied to the film by prestretch relaxation of the substrate, which generates an array of high‐aspect‐ratio ridges on the surface. Consequently, hierarchical patterns comprising unidirectionally aligned ridges covered with wrinkles are obtained. Water droplets placed on surfaces having the aforementioned hierarchical patterns show direction‐dependent contact angles, resulting in elongated shapes, which indicates the presence of anisotropic wetting properties. The magnitude of wetting anisotropy can be tuned by simple control of the applied compressive strain and film thickness.

The wetting physics and wetting transition on micro-wavy and micro V-grooved surfaces of similar topographic scales are investigated and compared with an emphasis on the droplet stability and anisotropic wetting that promotes liquid drainage. To develop 3D models of liquid droplets and to assess the morphology and spreading of liquid droplets across a wide range of parametric space, Surface Evolver (SE) has been used. The effect of roughness parameters of micro V-grooved surfaces such as groove height, groove width, pillar width, and groove angle on the wetting behavior is analyzed and compared to the same for the corresponding dimensional variation of pitch, amplitude, and depth of the asperities of micro-wavy surfaces. For all the comparable configurations, surfaces with micro-wavy textures exhibit higher wettability and smaller stable droplets than micro V-grooved surfaces. In addition, after specific threshold values of the aforementioned topographic parameters, the Cassie-Wenzel wetting transition is observed. Although wettability on micro-scale wavy and V-grooved surfaces have been widely examined because of their obvious importance, comparison of wetting physics and droplet energetics on them for similar roughness scale has not been conducted before. Therefore, the developed model may be useful in designing micro-structured V-grooved and wavy surfaces with tunable directional wetting properties.

In this report, the surface topography of crescent array in the Nepenthes slippery zone is measured and bionic fabricated by two‐photon polymerization (TPP) technology. A subregion outside‐in scanning method (SOSM) is proposed to solve the photoresist converge and large area surface positioning problem existing in bionic crescent array surface (BCAS) fabrication. The geometric parameters of BCAS, including area fraction, height, offset, and outer–inner radius ratio, are systematically investigated to reveal their effects on the hydrophobic performance. Perfluorinated polyether oil and fluorocarbon solvent were used to increase the hydrophobicity and homogeneity of bionic surface. The contact angles of BCASs coated with and without the solution are measured. The maximum contact angle can reach to 152.1°. What's more, considering the anisotropy of BCAS, anisotropic wetting research is conducted. Offset and outer–inner radius ratio are found to have obvious effects on the contact angle differences between convexity and concavity directions. The maximum contact angle difference can reach to 5.1°. Spreading and pinning phenomenon caused by droplet anisotropic wetting are observed and explained. This work is expected to provide references for the BCAS to be applied in super‐hydrophobic and droplets motion fields. [ABSTRACT FROM AUTHOR]

In recent years, there has been an increasing interest in methods to fabricate hydrophobic surfaces. Hydrophobic surfaces have been used in multiple applications in microfluidic devices to control fluid flow, as self-cleaning surfaces and also in de-icing or for drag reduction. Conventionally, hydrophobic surfaces were created by laser processing, self-assembly and other chemical processing methods. However, in many of these methods, hydrophobicity of the surface cannot be maintained for an extended time or restricted to limited set of materials. In some applications, creation of anisotropy in the hydrophobic property and the ability to pattern it, is important. Here, a low-cost, high-throughput method to generate highly hydrophobic and anisotropic surface has been developed. This method uses Computer Numerical Control machining employing diamond tools whose tips have been micro-structured using Focused Ion Beam that enables parallelization and achieves at least four times higher machining speed compared with other methods. The versatility of this method has been demonstrated by machining both metal and polymeric materials. Significant anisotropic wetting has been observed on the machined surface with the anisotropy in directional contact angle of up to 71.6°. Highly-hydrophobic surfaces with contact angle of 163.1° on 6061 Aluminum Alloy and 155.7° on polymethyl methacrylate surface were created.

A bio-inspired approach to fabricate robust superhydrophobic (SHB) surfaces with anisotropic properties replicated from a leek leaf is presented. The polydimethylsiloxane (PDMS) replica surfaces exhibit anisotropic wetting, anti-icing, and light scattering properties due to microgrooves replicated from leek leaves. Superhydrophobicity is achieved by a novel modified candle soot (CS) coating that mimics leek's epicuticular wax. The resulting surfaces show a contact angle (CA) difference of ≈30° in the directions perpendicular and parallel to the grooves, which is similar to the anisotropic properties of the original leek leaf. The coated replica is durable, withstanding cyclic bending tests (up to 10 000 cycles) and mechanical sand abrasion (up to 60 g of sand). The coated replica shows low ice adhesion (10 kPa) after the first cycle; and then, increases to ≈70 kPa after ten icing-shearing cycles; while, anisotropy in ice adhesion becomes more evident with more cycles. In addition, the candle soot-coated positive replica (CS-coated PR) demonstrates a transmittance of ≈73% and a haze of ≈65% at the wavelength of 550 nm. The results show that the properties depend on the replicated surface features of the leek leaf, which means that the leek leaf appears to be a highly useful template for bioinspired surfaces., (© 2024 The Author(s). Small published by Wiley‐VCH GmbH.)

The movement of liquid droplets on the energy gradient surface has attracted extensive attention inspired by biological features in nature, such as the periodic spindle-shaped nodes in spider silks and conical-like barbs of cacti, and the structure-property-function relationship of multifunctional gradient surfaces. In this study, a series of specific patterns are fabricated with 3D printing technology, followed by modification via the atmospheric pressure plasma treatment and liquid phase chemical deposition, resulting in enhancing the ability of water droplets of 5 μL to travel 18.47 mm on a horizontal plane and 22.75 mm against gravity at up to a 20° tilting angle. Additionally, analysis techniques have been employed, including a contact angle analyzer, ESCA, and a laser confocal microscope to evaluate the sample performance. This work could further be applied to many applications related to microfluidic devices, drug delivery and water/fog collection.

The directed self-assembly (DSA) of block copolymers (BCPs) is a promising next-generation lithography technique for high-resolution patterning. However, achieving lithographically applicable BCP organization such as out-of-plane lamellae requires proper tuning of interfacial energies between the BCP domains and the substrate, which remains difficult to address effectively and efficiently with high-χ BCPs. Here, we present the successful generation of anisotropic wetting by plasma treatment on patterned spin-on-carbon (SOC) substrates and its application to the DSA of a high-χ Si-containing material, poly(1,1-dimethylsilacyclobutane)- block -polystyrene (PDMSB- b -PS), with a 9 nm half pitch. Exposing the SOC substrate to different plasma chemistries promotes the vertical alignment of the PDMSB- b -PS lamellae within the trenches. In particular, a patterned substrate treated with HBr/O 2 plasma gives both a neutral wetting at the bottom interface and a strong PS-affine wetting at the sidewalls of the SOC trenches to efficiently guide the vertical BCP lamellae. Furthermore, prolonged exposure to HBr/O 2 plasma enables an adjustment of the trench width and an increased density of BCP lines on the substrate. Experimental observations are in agreement with a free energy configurational model developed to describe the system. These advances, which could be easily implemented in industry, could contribute to the wider adoption of self-assembly techniques in microelectronics, and beyond to applications such as metasurfaces, surface-enhanced Raman spectroscopy, and sensing technologies.

Superhydrophobic surface with anisotropic wetting and no low‐surface‐energy modification is investigated in which laser ablation, ultrasonic cleaning with degreasing solution, and electrodeposition are used sequentially. The macrogrooves are prepared by laser, and the micro–nanoscale structures on macrogrooves are prepared by electrodeposition, which is important to study the effect of hierarchical structures on anisotropic wetting. Herein, the surface morphology and surface crystal structure are discussed with respect to the laser‐ablated surface and the subsequent degreasing solution super‐cleaning/electrodeposited surfaces. At the same time, stainless steel surfaces with grooves of different periodic intervals (30–150 μm) and depths (10–40 μm) are systematically fabricated by laser only. When the periodic interval is smaller (<=50 μm), the static anisotropy is not obvious (5°–7°). However, the smaller the groove periodic interval and the greater the depth (<=30 μm), the more obvious the sliding anisotropy. Finally, dense and uniform micro–nanostructures on laser‐ablated grooves are prepared by electrodeposition. The micro–nanohierarchical structures prepared by electrodeposition not only can make the droplets have superhydrophobic properties but also enhance the water drop's anisotropic sliding ability. [ABSTRACT FROM AUTHOR]

The anisotropic wetting behaviours of water droplets on six surfaces with different solid–liquid interaction intensities are systemically investigated by molecular dynamics simulation. The six surfaces include Smooth, Square, Triangle, Sin, Sin-plus and Cylinder surfaces, and the interaction potentials between surface atoms and water molecules are adjusted to obtain various surface free energies. Moreover, the condensation process on those surfaces is investigated under a fixed solid-liquid interaction intensity. The wetting results indicate that surface energy plays an essential role in anisotropic wetting of the six surfaces. Moreover, under the fixed surface energy, the anisotropic wetting behaviours of water droplets are determined by the atom potential energy barrier and top area of solid substrate. The droplet on Square surface has most obvious anisotropic wetting behaviour because of its substantial atom potential energy barrier and large top area to support water droplet. In the condensation process, droplets nucleation, growth and coalescence processes are all visualised and quantitatively recorded, we find that texture creates lower atom potential energy on the surface and water molecules tend to nucleate on the area with low atom potential energy. Furthermore, the texture hinders the reunion of small clusters and consequently reduces the maximum cluster size. [ABSTRACT FROM AUTHOR]

Droplet impingement and anisotropic wetting on chemically heterogeneous stripe‐patterned surfaces is simulated by means of many‐body dissipative particle dynamics. The ratio of the stripe width and initial droplet diameter, defined as β, ranges from 0.5 to 1.0 so that the wetting process is in the Beyond–Cassie–Baxter regime and is highly anisotropic. At zero Weber number (that is, without considering droplet inertia) and with superhydrophobic stripes, β is the only factor affecting the droplet perpendicular contact angle and aspect ratio. For inertial droplets, β and the Weber number are found to have an effect on the eventual droplet morphology on multi‐striped surfaces. These morphologies include elongated shape, split‐off, and "butterfly" shape. A correlation for critical split‐off conditions has been determined. An energy analysis of droplet impingement shows that the normalized surface energy of the droplet is independent of the Weber number if the droplet is elongated or butterfly‐shaped. [ABSTRACT FROM AUTHOR]

Anisotropic wetting is important in industrial, medical and agricultural applications. In this paper, a novel microstructure performing prominent ultra-anisotropic wetting properties is proposed, with MEMS fabrication process. This microstructure possesses both sharp edge and sharp corner (SESC) in topography. The sharp edge can hinder liquid spreading significantly, whereas the sharp corner enhance spreading. By lithography technology on polymer film, Parylene-C, the microstructure can be tailored into different shapes to achieve various liquid spreading properties, two of which are demonstrated here. 2-D SESC microstructure illustrates ultra-high contact angle difference in parallel and perpendicular directions as high as 93.8°, as well as bidirectional wetting properties. Another microstructure, namely 3-D SESC microstructure, demonstrates specific unidirectional wetting properties. The unique microstructure proposed in this paper provide new ideas for application of controlling liquid spreading on surface. [2019-0077] [ABSTRACT FROM AUTHOR]

The droplet anisotropic wetting on chemically heterogeneous stripe-patterned substrates after impact is studied by using many-body dissipative particle dynamics in this work. After a low-velocity impact on substrates with different length ratio (ratio of stripe width to the initial droplet size) and Cassie area fraction, the droplet can show various shapes and contact angles in parallel and orthogonal directions to the stripes. The elongation of the droplet can be increasingly evident when increasing the length ratio. Also, the contact angles at both directions follow the Cassie-Baxter predicted values well at low length ratio while deviate from them at high length ratio. Both the capillary and kinetic effects have a significant influence on the anisotropic wetting. When impact with higher velocity, the droplet leaves some residues on the hydrophilic stripes of the substrates. Surprisingly, the residues for substrates with certain length ratio (≤0.316) are distributed in circular regions with almost identical radii, indicating the spreading stages of them are independent on the surface properties and dominated by the kinetic effect. When the droplet reaches a stable state, the main body shows anisotropic wetting behavior, indicating the capillary effect dominates the wetting instead of the kinetic effect at the retraction stage. Unlabelled Image • Combined effect of length ratio, Cassie area fraction and initial kinetic energy is considered in spreading on substrates. • Critical length ratio is identified to separate the anisotropy and isotropy of wetting. • The droplet shows no anisotropic effect in spreading stage when given a high initial impact velocity. • A simple algorithm is proposed to calculate the contact angle of the droplet with distorted contact line. [ABSTRACT FROM AUTHOR]

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.

This work presents a practical methodology to produce multilevel hierarchical structures with precise control of the structural geometry at every level by combining photo and nanoimprint lithography processes. The method involves sequential steps of nanoimprinting of a first deposited polymer layer followed by nanoimprinting of a second deposited layer of a photoresin and, afterward, performing on this layer optical lithography by means of a maskless laser writer to pattern micrometer-size features. A hierarchical topography is consequently obtained comprising nanopatterns and micropatterns at different levels designed independently with very high feature control. The process can be repeated sequentially employing hierarchical working molds produced on a previous fabrication cycle to produce multilevel self-similar hierarchical topographies in a sort of fractal growing manner. The patterning method has broad applicability, as exemplary demonstration, superhydrophobicity, and anisotropic wetting behavior are revealed. [ABSTRACT FROM AUTHOR]

As two typical aspects of the wettability based on bio-inspired functional surfaces, superhydrophobic and anisotropic wetting properties have attracted much interest in practical applications. However, how to use a simple approach to realize those properties is still a huge challenge; simultaneously, the corrosion resistance is also important when the surfaces are introduced into more complex environments. In this paper, a simple and highly effective two-step approach for the fabrication of bio-inspired superhydrophobic and anisotropic wetting surfaces is reported. Aluminum alloy (Al) plates were first structured using nanosecond laser, and then were placed in an oven for low-temperature annealing (100 °C). The results show that the nanosecond laser ablation not only builds dual- and three-level structures inspired by lotus leaf and rice leaf but also oxidizes the Al materials, resulting in an oxide film covering on the surface. After the low-temperature annealing, the as-fabricated surfaces display the superhydrophobic and anisotropic wetting properties by adjusting the scan spacing of adjacent microgrooves. The potentiodynamic polarization and electrochemical impedance spectroscopy tests show that the as-fabricated surfaces have excellent corrosion resistance that can provide an effective protection for the Al materials. In addition, this approach can be applied to a broad range of metals (stainless steel, copper and titanium). This research will be instructive for the applications of functional materials with the properties of superhydrophobicity and anisotropic wetting. • We present a simple two-step approach to achieve bio-inspired superhydrophobic and anisotropic wetting surfaces. • This approach does not involve low surface energy modification. • The superhydrophobic and anisotropic wetting surfaces show excellent corrosion resistance. • This technology can be applied to a broad range of metals (stainless steel, copper and titanium). [ABSTRACT FROM AUTHOR]

Abstract The present work introduces the single step facile fabrication of semi-hemispherical end micro-grooved textures on Ti-6Al-4V flat surfaces using ball-end micromilling and their impact on surface morphology and direction dependent dynamic wetting of ultrapure deionized water nanodroplet. Directional dynamic wetting was analyzed from both parallel and perpendicular viewing directions of the micro-grooved textures using measurement of the dynamic contact angles. It was further utilized to predict Young's equilibrium contact angle which was also compared with Wenzel and Cassie-Baxter wetting models. Predicted values of Young's equilibrium contact angle in both the directions were also correlated with the geometrical parameters of the fabricated micro-grooved textures. The semi-hemispherical end micro-groove geometry enhances the surface wetting whereas, micro-grooved textures confine the droplet spreading along its length leading to directional wetting. The nanodroplet stretches along micro-grooves and distorted in transverse direction leading to comparatively lower contact angle in perpendicular viewing direction to that of parallel viewing direction. Results reveal that there exists a strong anisotropic wetting nature for the low pitch and high depth of micro-grooved texture. Anisotropic wetting almost disappears for 300 μm pitch due to minimum contact between the nanodroplet and micro-grooved textures where the dominance of un-textured ridge portion is significant. Graphical abstract Unlabelled Image Highlights • Semi-hemispherical end micro-grooves were fabricated by ball-end micromilling. • Semi-hemispherical end groove geometry produced better hydrophilicity. • Anisotropy of micro-groove was examined by dynamic wetting of the nanodroplet. • Anisotropy causes droplet distortion and stretching and depends on groove geometry. • Wetting in perpendicular direction was more than that in parallel direction. [ABSTRACT FROM AUTHOR]

Anisotropic wetting shows potential application in advanced microfluidics and biosensors. This study proposes a three-dimensional (3-D) methodology using a finite element method to investigate the anisotropic wettability of striped surfaces. The variations of free energy (FE), free energy barrier (FEB), contact angle (CA), contact angle hysteresis (CAH), droplet shape and the three-phase contact line shape of an anisotropic droplet during the spreading process are explored. It is found that the droplet length and three-phase contact line length fluctuate by the increase of the droplet width and three-phase contact line width during the spreading process of droplet. In addition, a relatively small stripe width with higher free energy is necessary to achieve a large equilibrium CA and a small CAH or easily reach the equilibrium CA. The present methodology can be potentially used for designing architected surfaces with anisotropic wetting properties in microfluidic devices, lab-on-a-chip systems, and self-cleaning surfaces. [ABSTRACT FROM AUTHOR]

Abstract: Smart pH‐responsive surfaces that could autonomously induce unidirectional wetting of acid and base with reversed directions are fabricated. The smart surfaces, consisting of chemistry‐asymmetric “Janus” silicon cylinder arrays (Si‐CAs), are prepared by precise modification of functional groups on each cylinder unit. Herein, amino and carboxyl groups are chosen as typical pH‐responsive groups, owing to their protonation/deprotonation effect in response to pH of the contacted aqueous solution. One side of the Si‐CAs is modified by poly(2‐(dimethylamino)ethyl methacrylate), while the other side is modified by mixed self‐assembled monolayers of 1‐dodecanethiol and 11‐mercaptoundecanoic acid. On such surfaces, it is observed that acid and base wet in a unidirectional manner toward corresponding directions that are modified by amino or carboxyl groups, which is caused by asynchronous change of wetting property on two sides of the asymmetric structures. The as‐prepared Janus surfaces could regulate the wetting behavior of acid and base and could direct unidirectional wetting of water with reversed directions when the surfaces are treated by strong acid or base. Due to the excellent response capability, the smart surfaces are potential candidates to be applied in sensors, microfluidics, oil/water separation, and smart interfacial design. [ABSTRACT FROM AUTHOR]

There are many different methods to improve and control surface wettability. One of these methods is the creation of micro- and nanotextures or multiscale textures on the surfaces. In this research, the process of ultrasonic vibration-assisted face-turning (UVAT) was used in order to improve surface wettability of Al7075-T6. To this end, the effect of various parameters in UVAT process including the vibration modes of the tool (CT, LVT, EVT, 3D-VT), cutting speed, and feed rate was examined on the roughness and topography of surfaces and thus on the surface wettability. Surface wettability was estimated through measuring water droplet contact angle in the two directions of cutting speed (θc) and feed rate (θf). Analysis of the variance of experimental results showed that among the existing parameters, the type of vibration mode, with 65.56 and 60.84% effectiveness, has the greatest impact on the surface wettability (θc, θf), while feed rate, with 95.39% effectiveness, has the greatest impact on anisotropic wettability (Δθ). [ABSTRACT FROM AUTHOR]

Abstract: Smart surfaces with tunable wettability have aroused much attention in the past few years. However, to obtain a surface that can reversibly transit between the lotus‐leaf‐like superhydrophobic isotropic and rice‐leaf‐like superhydrophobic anisotropic wettings is still a challenge. This paper, by mimicking microstructures on both lotus and rice leaves, reports such a surface that is prepared by creating micro/nanostructured arrays on the shape memory polymer. On the surface, the microstructure shapes can be reversibly changed between the lotus‐leaf‐like random state and the rice‐leaf‐like 1D ordered state. Accordingly, repeated switch between the superhydrophobic isotropic and anisotropic wettings can be displayed. Research results indicate that the smart controllability is ascribed to the excellent shape memory effect of the polymer, which endows the surface with special ability in memorizing different microstructure shapes and wetting properties. Meanwhile, based on the smart wetting performances, the surface is further used as a rewritable functional platform, on which various droplet transportation programmes are designed and demonstrated. This work reports a superhydrophobic surface with switchable isotropic/anisotropic wettings, which not only provides a novel functional material but also opens a new avenue for application in controlled droplet transportation. [ABSTRACT FROM AUTHOR]

This study investigated the wetting behavior of Ti6Al4V surfaces that were groove-structured by means of femtosecond laser irradiation. The material was treated under ambient air conditions by use of a laser wavelength of 1030 nm and a pulse duration of 300 fs. Highly accurate structures with a gap width of 20 µm, a gap depth of 10 µm, and varying strut widths (1−300 µm) were generated and the contact angles in parallel and perpendicular direction were determined using sessile drop method with ultrapure water 1, 8, and 15 days after irradiation. All deterministic surfaces exhibited a pronounced contact angle change over time. The structures showed a strong anisotropic wetting behavior with a maximum contact angle aspect ratio of 2.47 at a strut width of 40 µm and a maximum difference between the parallel and perpendicular contact angle of 47.9° after 1 day.

This paper reports the directional wetting behavior of water on a patterned heterogeneous surface, which is integrated by two asymmetric nanostructures with different directions. The asymmetric nanostructures are Janus silicon cylinder arrays (Si-CAs) modified by molecules with distinct surface energies on two sides. Through a photolithography process, the two asymmetric nanostructures are integrated in a single surface to form chessboard patterns with different directions. When water is injected onto the patterned surface, water droplets wet in a unidirectional manner, and the wetting direction is along resultant direction of the two wetting directions of water on the two asymmetric patterns. This paper investigates the factors which may influence the wetting behavior of water on the surface, experimental results show that patterned asymmetric surfaces prepared from different chessboard pattern size, Si-CAs with different diameters, and different morphologysymmetric structures do not affect the wetting property, but the molecules modified on two sides do. Larger surface energy difference between the modified molecules improves the ability of the surfaces induce water moving. It is believed that the patterned asymmetric nanostructure arrays provide a new strategy to modulate the wetting behavior of water, which will show potential applications in microfluidics and others involving unidirectional liquid manipulation. [ABSTRACT FROM AUTHOR]

A grooved hybrid surface that has silane-coated surface (hydrophobic surface) on ridges and uncoated surface (hydrophilic surface) inside groove was fabricated on a Pyrex glass substrate through the laser-induced backside wet etching (LIBWE) process. The wettability characteristics of the grooved hybrid surface were evaluated from static contact angles (CAs) measured in parallel and orthogonal directions. With an increase in the width of the hydrophobic ridge, the CAs increased from 24° to 104° in the parallel direction. Consequently, the anisotropic wetting property of the fabricated hybrid surface was confirmed. Results of moisture condensation experiments indicated that moisture condensed rapidly in the hydrophilic grooves. Furthermore, the diffuse reflection of light was prevented when the groove was completely filled with water. [ABSTRACT FROM AUTHOR]

This manuscript describes the preparation of hierarchically, i.e. micro and nanometer scale ordered surfaces with variable functionality by simultaneously combining the microfabrication using Laser Interference Lithography (LIL) and the self-assembly of block copolymers occurring at the nanometer scale. The block copolymers employed in this study are a double hydrophobic poly(2,3,4,5,6-pentafuorostyrene)- block -polystyrene (P5FS 31 - b -PS 21 ) and an amphiphilic polystyrene- block -poly(acrylic acid) (PS 20 - b -PAA 9 ) diblock copolymer. The incorporation of these block copolymers in the photosensitive mixture resulted in surfaces with hierarchically micro and nanostructured interfaces with either hydrophobic or hydrophilic surface chemical composition. Moreover, in comparison with microstructured surfaces prepared using statistical copolymer with similar composition, the use of block copolymers enhanced both the surface wettability changes and the final anisotropic wetting. We evidenced herein that by preparing unidirectional micrometer size patterns with either P5FS 31 - b -PS 21 or PS 20 - b -PAA 9 the wetting anisotropy increases with the hydrophobicity or the hydrophilicity of the surface. [ABSTRACT FROM AUTHOR]

Directional liquid transport is expected via adjusting chemical components, surface morphology, and external stimuli and is critical for practical applications. Although many studies have been conducted, there are still challenges to achieving real-time transformation of liquid transport direction on the material surface. Herein, we demonstrate a strategy to achieve curvature responsive anisotropic wetting on the elastic film with V-shaped prism microarray (VPM) microstructure, which can be used to control the direction of liquid transport. The results reveal that the curvature change of an elastic film can adjust the arrangement of V-shaped prisms on the elastic film. Correspondingly, the liquid wetting trend will change and even the moving direction reverses with varying arrangements of the V-shaped prisms on the elastic film. Meanwhile, surface hydrophobicity of the VPM elastic film also affects the liquid wetting trend and even shows the opposite transport direction of the liquid, which is up to the water wetting state on the VPM elastic film. Based on these results, the VPM elastic film can serve as a valve to control the liquid transport direction and is promising in the application of liquid directional harvest, chemical reaction, microfluidic, etc .

It is well-known that the bio-inspired sharkskin covering the original pattern has the apparent drag reduction function in the turbulent flowing stations, which can be regarded as “sharkskin effect”, and it has progressively been put application into the fluid engineering with obtaining great profits. In this paper, the anisotropic wetting phenomena on sharkskin are discovered, the contact angles and rolling angles on different orientations are not the same. In addition, the hydrodynamic experiments on different sharkskin surfaces are conducted, and the experimental results illustrate that the super-hydrophobic and drag-reducing properties on deformed biological surfaces are improved to some extent compared to the original morphology, which has important significance to expand its practical applications. [ABSTRACT FROM AUTHOR]

We introduce a simple yet robust method for fabrication of wrinkled polydimethylsiloxane (PDMS) surface exhibiting tunable wettability under applied mechanical strain. Submicrometer-scale periodic grooves were prepared by casting the optical discs such as compact disc (CD) and digital versatile disc (DVD) patterns on PDMS. On wrinkled PDMS, the liquid droplet exhibits an anisotropic wetting resulting in a higher contact angle (CA) in the direction perpendicular to the wrinkle axis. The peaks and valleys in the surface topology of PDMS lead to the wetting anisotropy of 14° for CD templated PDMS and 7° for DVD templated PDMS. The wetting anisotropy for a fixed Young's CA (110°) was found to depend on the wrinkles periodicity ( λ ) and amplitude (2 A ). A mechanical strain in the direction perpendicular to the wrinkle axis was applied on CD templated PDMS to tune λ value from 1.49 μm to 1.88 μm. The wetting anisotropy decrease with an increase in λ value and the water droplet becomes isotropic for 30% strain value. Comparison with existing theoretical Johnson and Dettre (J-D) model attributes the anisotropic shape of the droplet and contact angle variations to surface roughness. The multiple-cycle switching between anisotropic and isotropic wetting was achieved. [ABSTRACT FROM AUTHOR]

The liquid anisotropic wetting properties have garnered considerable interest due to the rapidly rising usage of functional micro-nano fabrics in sanitary product applications. However, designing and preparing such materials in a manner that is easy, safe, and vast in a large scale remains a problematic issue. In this work, a poly (lactic acid) / poly (ethylene glycol) (PLA/PEG) micro-nano fabric with aligned fibrous roughness inspired by bamboo leaves was designed and prepared via a simple double-stage drafting melt blowing process. The results indicate that aligned fibrous roughness and homogenized fiber diameters ranging from 4.4 to 2.7 µm were successfully tailed by adjusting the drafting ratios from 1.0 to 3.0. Additionally, benefiting from the aligned fibrous roughness, the PLA/PEG micro-nano fabric demonstrated excellent anisotropic wetting performance, evidenced by a clear contrast between parallel static water contact angles (θ //) and perpendicular (θ ⊥) to the longitudinal direction, as well as remarkable contact angle hysteresis. Additionally, the samples had a significant aspect ratio of 3.16. As a result, this PLA/PEG micro-nano fabric may be an alternative for medical and health applications such as medical dressings and meshes. [Display omitted] • A sustainable micro-nanofibrous fabric with a high aligned fibrous roughness was developed inspired by the bamboo leaves. • A large wetting aspect ratio is effectively obtained by facile tailing the aligned structure during the double-stage drafting melt blowing process. • This green fabric with an excellent anisotropic wetting performance shows a potential application in sanitary products. [ABSTRACT FROM AUTHOR]

Anisotropic wetting of machined surfaces is widely applied in industries which can be greatly affected by roughness and solid's chemical properties. However, there has not been much work on it. A free-energy thermodynamic model is presented by analyzing geometry morphology of machined surfaces (2-D model surfaces), which demonstrates the influence of roughness on anisotropic wetting. It can be concluded that the energy barrier is one of the main reasons for the anisotropic wetting existing in the direction perpendicular to the lay. In addition, experiments in investigating anisotropic wetting, which was characterized by the static contact angle and droplet's distortion, were performed on machined surfaces with different roughness on hydrophilic and hydrophobic materials. The droplet's anisotropy found on machined surfaces increased with mean slope of roughness profile Kr . It indicates that roughness on anisotropic wetting on hydrophilic materials has a stronger effect than that on hydrophobic materials. Furthermore, the contact angles predicted by the model are basically consistent with the experimentally ones. [ABSTRACT FROM AUTHOR]

Effect of microgroove geometry on the frost formation and frost melt-water drainage is experimentally investigated on a number of aluminum surfaces. Condensation and frost formation processes are also examined on a set of microgrooved copper surfaces for which the dimensions of the microgroove geometry are varied in a cyclic manner. These metal surfaces are studied because of their technical importance as working materials and are fabricated by two different techniques, photolithography and wet etching, respectively. The morphology, distribution and growth pattern of the condensed and frozen water droplets are found to be considerably different on the microgrooved surfaces from that on the flat baseline surfaces. While the amount of frost melt-water retention on the flat surfaces is found to increase in the subsequent refrost cycles and is highest in the last frost cycle, the microgrooved aluminum surfaces consistently exhibit improved frost-water drainage in all frost cycles and under different operating conditions. Findings of this study will be useful in designing microgrooved metal surfaces operating under frosting/defrosting conditions to have improved frost-water drainage properties. [ABSTRACT FROM AUTHOR]

The surface of the medical grade titanium alloy (Ti–6Al–4V) was modified using a millisecond laser to alter the topography and wetting behavior. We combined anisotropic surface patterning with a nitriding process which is essential for the protection of dental implants against corrosion and wear. While a homogenous nitride layer was achieved, the laser induced a surface topography composed of both micro- and nano-structures. The surface topography leads to anisotropy in wetting which is desired in next generation implant applications such as nature-analogue anisotropic dental restoration. [ABSTRACT FROM AUTHOR]

This article introduces surface textured polymer fibers as a new platform for the fabrication of affordable microfluidic devices. Fibers are produced tens of meters-long at a time and comprise 20 continuous and ordered channels (equilateral triangle grooves with side lengths as small as 30 micrometers) on their surfaces. Extreme anisotropic spreading behavior due to capillary action along the grooves of fibers is observed after surface modification with polydopamine (PDA). These flexible fibers can be fixed on any surface-independent of its material and shape-to form three-dimensional arrays, which spontaneously spread liquid on predefined paths without the need for external pumps or actuators. Surface textured fibers offer high-throughput fabrication of complex open microfluidic channel geometries, which is challenging to achieve using current photolithography-based techniques. Several microfluidic systems are designed and prepared on either planar or 3D surfaces to demonstrate outstanding capability of the fiber arrays in control of fluid flow in both vertical and lateral directions. Surface textured fibers are well suited to the fabrication of flexible, robust, lightweight, and affordable microfluidic devices, which expand the role of microfluidics in a scope of fields including drug discovery, medical diagnostics, and monitoring food and water quality. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports enhanced anisotropic wettability using two-level hierarchical micro-structures. First-order micro-channels with superimposed second-order micro-textures were machined on aluminum surfaces using ultrasonic elliptical vibration cutting (EVC). Controllable sinusoidal micro-textures with a wavelength one order of magnitude smaller than the widths of the first-order micro-channels were applied to the surface. Anisotropic wettability was evaluated by water contact angle measurements in two orthogonal directions. The results have shown that the generated hierarchical micro-structures have nearly doubled the anisotropic contact angle in comparison to one-level structures. A relationship between anisotropic wetting characteristics and process parameters is also presented. [Copyright &y& Elsevier]

Surfaces with anisotropic wettability, widely found in nature, have inspired the development of one-dimensional water control on surfaces relying on the well-arranged surface features. Controlling the wetting behavior of organic liquids, especially the motion of oil fluid on surfaces, is of great importance for a broad range of applications including oil transportation, oil-repellent coatings, and water/oil separation. However, anisotropic oil-wetting surfaces remain unexplored. Here, the unique skin of a filefish Navodon septentrionalis shows anisotropic oleophobicity under water. On the rough skin of N. septentrionalis, oil droplets tend to roll off in a head-to-tail direction, but pin in the opposite direction. This pronounced wetting anisotropy results from the oriented hook-like spines arrayed on the fish skin. It inspires further exploration of the artificial anisotropic underwater oleophobic surfaces: By mimicking the oriented hook-like microstructure on a polydimethylsiloxane layer via soft lithography and subsequent oxygen-plasma treatment to make the PDMS hydrophilic, artificial fish skin is fabricated which has similar anisotropic underwater oleophobicity. Drawn from the processing of artificial fish skin, a simple principle is proposed to achieve anisotropic underwater oleophobicity by adjusting the hydrophilicity of surface composition and the anisotropic microtextures. This principle can guide the simple mass manufacturing of various inexpensive high surface-energy materials, and the principle is demonstrated on commercial cloth corduroy. This study will profit broad applications involving low-energy, low-expense oil transportation, underwater oil collection, and oil-repellant coatings on ship hulls and oil pipelines. [ABSTRACT FROM AUTHOR]

Wrinkled polymer surfaces have previously been investigated for a range of applications in microfluidics and technology, but defects in the surface structure have largely been seen as a hindrance to the application of nano-structured surfaces. In this study, poly(dimethylsiloxane) (PDMS), was treated with a combination of air-plasma (P), strain (S), and relaxation (R) to create a more diverse range of surface structures in addition to wrinkled PDMS. SPR treatments produced the usual, wrinkled surface with the presence of perpendicular cracks whereas PSR and PSPR resulted in a range of more bi-directional surface structures. The behaviour of water droplets on the surfaces was also investigated, with it being shown that the surface chemistry directed water more strongly than the physical structures. As a result, the hydrophobic cracks on SPR surfaces cause significant anisotropy perpendicular to the strain, with similar observations made for PSPR. However, PSR surfaces induced anisotropy parallel to the direction of strain, showing the potential to tune surface behaviour depending on the application. [Display omitted] • Wrinkled elastomer surfaces are produced using combinations of stretching, plasma oxidation and relaxation steps. • Depending on the order of operations, different surface structures are produced. • Defects in surface structures act as pinning points to direct wetting on these surfaces. • In most instances, chemical heterogeneity dominates over structural features. [ABSTRACT FROM AUTHOR]

Abstract: The study of the wetting characteristics and motion of condensed droplets is important in any multiphase flow applications. The present work focuses on condensate morphology and growth on cooled horizontal substrates featuring microgrooves. Microfabrication techniques are employed to produce chemically homogeneous and heterogeneous substrates with microgrooves 20–40μm in spacing and 20–180μm in depth. Strong anisotropic wetting behavior was observed on the chemically heterogeneous sample whereas isotropic wetting appeared on the homogeneous samples. Groove geometry is found to have a profound impact on the drainage behavior of condensed droplets but is less important for deposited droplets. Isolated drop growth in microgrooves was simulated numerically to study various wetting modes. The simulation results show that the critical volume for droplets to change morphology decreases with the increase in the contact angle of surface materials in chemically homogeneous grooves. The critical volume for droplets on the chemical heterogeneous sample is much smaller than those on the homogeneous surfaces. [Copyright &y& Elsevier]

Abstract: The directional wetting behavior of chemically defined stripe-patterned anisotropic surfaces is presented. The equilibrium shapes of asymmetric droplets, arising from patterns of alternating hydrophilic and hydrophobic stripes with dimensions in the low-micrometer range, are investigated in relation to the stripe widths. Owing to the well-defined small droplet volume, the equilibrium shape as well as the observed contact angles exhibit unique scaling behavior. Additionally, we investigate the motion of liquid from surface areas with low macroscopic wettability toward areas with a higher wettability. The density of self-assembled fluoroalkylsilane monolayers in terms of the number and width of the stripes, as defined by the chemical patterning, proves to be of paramount importance. Linear and radial patterns are presented, which induce liquid movement along the chemically defined stripes giving rise to a macroscopic gradient in surface energy. [Copyright &y& Elsevier]

Abstract: Liquids spread anisotropically on grooved surfaces; potential applications include self-cleaning surfaces and digital microfluidics. However, the wetting of grooved surfaces is generally only qualitatively understood. We simulate equilibrium drop shapes in the Cassie (suspended) state on grooved surfaces. It is shown that while inertia is required for drops to advance from one groove to the next, drops recede to near-circular shapes under the influence of surface energy alone. Given the number of grooves on which an advancing drop resides, its shape can be quantitatively predicted and matched to experiment. Such drops spread parallel to the grooves as if on a one-dimensional Cassie surface, which can be used to infer the intrinsic contact angle from the drop shape and groove dimensions. Tapered grooves allow self-propelled unidirectional motion. [Copyright &y& Elsevier]

Abstract: A free energy-minimizing simulation that allows computation of the shapes of sessile liquid drops on chemically patterned surfaces is described. A set of nondimensional results for drops on surfaces with a single wetting stripe is presented. Quantitative agreement is found with published experimental data for drops on many-striped surfaces, and a simple model for anisotropic wetting is evaluated. [Copyright &y& Elsevier]

Abstract: We report the experimental and modeling results of water contact angle (CA) on anisotropic surfaces with aligned fibers fabricated by electrospinning. The aligned fibers with radius of several micrometers can effectively stop the three-phase contact line (TPCL) and lead to a very high “static” CA as well as stick–slip patterns. The maximal CA, which appears during the advancing CA measurement, is nearly the same on those surfaces with different fiber densities. A thermodynamic model was applied to describe the free energy change and the location of metastable states. It is found that the energy barrier of potential wells is precisely linear to the fiber radius. By adjusting the reference energy of the model, the calculated result can fit the experimental ones well. Results also indicate that the process for static CA measurement is similar to that for advancing CA to some extent but without much driving force. Static CA in the direction parallel to the fibers is very different from that perpendicular to the fibers since there is no obvious barrier for the TPCL to advance along the fibers. These experimental and modeling results provide some insights into the CA on rough surfaces as well as the stick–slip phenomenon. Relationship between the metastable potential wells and the external energy determines when the “slip” or “stick” happens. Furthermore, the results may be helpful for designing of microfluidic system. [Copyright &y& Elsevier]

Abstract: This paper reports an experimental investigation of low Weber number water drops impacting onto solid surfaces exhibiting anisotropic wetting. The wetting anisotropy is created by patterning the solid surfaces with unidirectional parallel grooves. Temporal measurements of impacting drop parameters such as drop base contact diameter, apparent contact angle of drop, and drop height at the center are obtained from high-speed video recordings of drop impacts. The study shows that the impact of low Weber number water drops on the grooved surface exhibits beating phenomenon in the temporal variations of the dynamic contact angle anisotropy and drop height at the center of the impacting drop. It is observed that the beating phenomenon of impacting drop parameters is caused by the frequency difference between the dynamic contact angle oscillations of impacting drop liquid oriented perpendicular and parallel to the direction of grooves on the grooved surface. The primary trigger for the phenomenon is the existence of non-axisymmetric drop flow on the grooved surface featuring pinned and free motions of drop liquid in the directions perpendicular and parallel to the grooves, respectively. The beat frequency is almost independent of the impact drop Weber number. Further experimental measurements with solid surfaces of different groove textures show that the grooved surface with larger wetting anisotropy may be expected to show a dominant beating phenomenon. The phenomenon is gradually damped out with time and is fully unrecognizable at higher drop impact Weber numbers. [Copyright &y& Elsevier]

A novel surface capable of controlling the directional sliding velocity of a liquid is presented by utilizing partially-exposed nano-bump morphologies. Hierarchical micro-stripe structures with overhang cross-sectional shapes demonstrate two remarkable properties, namely, "anisotropic omniphobicity" with DI water (γ w = 72.1 mN/m) and mineral oil (γ o = 28 mN/m) and manipulation of the droplet sliding velocity by tuning the level of nano-roughness. [Display omitted] • Omniphobic nano-bump surface shows capable of controlling directional sliding velocity of a liquid droplet. • The novel hierarchical micro-stripe structures are fabricated by utilizing partially-exposed nano-bump morphologies. • Droplet sliding velocity is controlled by tuning the level of nano-roughness. Directional liquid manipulation surfaces have been widely used in advanced digital microfluidic platforms for biological and chemical analyses. However, studies on surfaces capable of controlling the droplet behavior of various liquids, including oil as well as water, through sliding velocity have been rare. This article reports the development of an omniphobic surface capable of controlling the directional sliding velocity of a liquid by using partially exposed nano-bump morphologies. A robust and reliable fabrication criterion is discussed for the fabrication of a novel functional liquid sliding surface through the ultraviolet ozone partial etching technique of a UV-curable resin that contains inorganic oxide nanoparticles. The fabricated hierarchical micro-stripes with overhang cross-sectional structures are omniphobic and exhibit droplet sliding velocity control. The velocities of various liquid droplets with varying surface tensions can be controlled by adjusting the level of nano-roughness formed on the surface of the micro-line structure. [ABSTRACT FROM AUTHOR]