Anisotropic Superhydrophobic Surfaces Research Articles

Application of 3D printing for fabrication of superhydrophobic surfaces with reversible wettability.

Control of surface wettability is needed in many applications. The potential use of 3D printing technology to gain control over wettability remains largely unexplored. In this paper, Fused Deposition Molding (FDM) 3D printing technology was utilized to print polylactic acid (PLA) microplate array structures to generate superhydrophobic surfaces with reversable wetting properties. This was achieved by spraying polydimethylsiloxane (PDMS) and silica (SiO2) solutions, over microplate surfaces. Anisotropic wetting properties were also achieved based on the surface structure design. Due to the shape memory properties of PLA, the morphology of the microplate arrays could be switched between the original upright shape and deformed shape. Through alternating pressing and heating treatments, the microplate arrays showed anisotropic wettability switching. The difference between the contact angle (CA) and sliding angle (SA) of water droplets on the original surface parallel to and perpendicular to the microplate array direction were ΔCA = 7° and ΔSA = 3° respectively, and those on the surface of the deformed microplate array were ΔCA = 7° and ΔSA = 21°, respectively. This process enabled reversible alteration in the wetting behavior of water droplets on the original and deformed surfaces between sliding and sticking states. PLA-based shape memory anisotropic superhydrophobic surfaces with tunable adhesion were successfully applied to rewritable platforms, micro droplet reaction platforms, and gas sensing.

Preparation and analysis of tridirectionally anisotropic superhydrophobic surface with biomimetic wedge-shaped structure

A novel biomimetic wedge-shaped structure (BWSS) has been fabricated using nanosecond laser etching method. The BWSS as-processed exhibits superhydrophilicity (contact angle (CA) = 0°) and transforms to superhydrophobicity (CA > 160°) after chemical modification. Sliding angles (SAs) on the superhydrophobic surface show the tridirectional anisotropy: SAs in perpendicular anti outside radial direction (⊥Anti-RO) > SAs in perpendicular outside radial direction (⊥RO) > SAs in parallel direction (∥). Mechanisms of tridirectional anisotropy are discussed in detail. The original phenomenon indicates the superhydrophobic surface with BWSS possesses excellent tridirectionally anisotropic wettability, which can provide enormous potential applications in microfluidic fields.

Controlling High-Speed Droplet Splashing and Superspreading Behavior on Anisotropic Superhydrophobic Leaf Surfaces by Ecofriendly Pseudogemini Surfactants.

Efficient deposition of high-speed droplets on superhydrophobic leaf surfaces remains an important challenge. Especially for the anisotropic wired superhydrophobic leaf surface, the splashing phenomenon is more serious, which leads to the low effective utilization of pesticides by biological targets. The lost pesticides have caused serious ecological environment pollution. Therefore, it is urgent to develop a green and sustainable strategy in a cost-effective to achieve efficient deposition of high-speed droplets on anisotropic superhydrophobic leaf surfaces at low dosage. A kind of green pseudogemini surfactants is constructed based on fatty acids and hexamethylenediamine by electrostatic interaction to control the splashing and spreading of high-speed droplets on superhydrophobic surfaces. The formed surfactant can not only achieve completely inhibition of the bouncing of droplets, but also promote the rapid spreading on superhydrophobic leaf surfaces at a very low usage. The efficient deposition and superspreading phenomenon are attributed to the rapid migration and adsorption of the surfactant from the dynamic spherical micelles at the newly formed solid-liquid interface, the network-like aggregated spherical micelles, and the Marangoni effect caused by surface tension gradient. Moreover, the surfactant shows excellent synergistic effect with herbicides to control weeds by inhibiting droplets splashing. This work provides a more simple, effective and sustainable approach to utilize aggregated spherical micelles rather than conventional vesicles or wormlike micelles to improve the droplet deposition on superhydrophobic leaf surfaces and reduce the impact of surfactants and pesticides on the ecological environment. This article is protected by copyright. All rights reserved.

Stainless steel anisotropic superhydrophobic surfaces fabrication with inclined cone array via laser ablation and post annealing treatment

Stainless steel anisotropic superhydrophobic surfaces fabrication with inclined cone array via laser ablation and post annealing treatment

Fabrication of anisotropic superhydrophobic surface based on the Nepenthes slippery zone

Depending on its highly evolved structures that consist of microscale lunate cells and nanoscale wax coverings, the slippery zone of Nepenthes alata shows significant anisotropic superhydrophobicity, which has gradually become the biomimetic prototype for designing superhydrophobic surfaces. In this study, the authors constructed the structures of the slippery zone into equidistantly distributed greenhouses and array of cylinders, therefore obtaining a biomimetic model of an anisotropic superhydrophobic surface. The greenhouses were printed using ultraviolet-cured material, via 3D printing, and then flake graphite was selected as a substitute for the array of cylinders (wax coverings) and was absorbed onto the printed greenhouses by using high-voltage electrostatic absorption technology. The contact/sliding angle was measured to verify the anisotropic superhydrophobic effect of the fabricated sample. The contact angle increases significantly with an increase in the greenhouse density (l/L value) and achieves a value of 152.6 ± 0.6° when l/L is 0.8, and the sliding angle toward bottom and top shows values of 3.07 ± 0.26° and 5.69 ± 0.24°, respectively. These results indicate that the fabricated sample has anisotropic superhydrophobicity. Therefore, this study provides a simple and low-cost approach for the biomimetic fabrication of anisotropic superhydrophobic surfaces.

Anisotropic Superhydrophobic Properties of Bioinspired Surfaces by Laser Ablation of Metal Substrate inside Water

AbstractWater droplet unidirectional adhesion, wetting, and transport on the asymmetric superhydrophobic surfaces have attracted much research interest in theory analyses and applications. In this study, inspired by the butterfly wings and lotus leaf, a novel anisotropic superhydrophobic surface with different tilt angles of the wedge‐shaped structures is prepared on aluminum surfaces via laser ablation inside water. It is demonstrated experimentally that the resistant force of water droplets on the bioinspired surfaces is different along the tilt direction of the wedge‐shaped structure and the inverse direction. The sliding and bouncing behaviors of water droplets on different bioinspired wedge‐shaped structure surfaces are simulated and tested to understand better the mechanism of the rolling and bouncing droplets induced by asymmetric wedge‐shaped structures. Furthermore, the effect of the tilt wedge‐shaped structures on the anisotropic superhydrophobic properties is revealed from the mechanical point of view. The relationship between the anisotropic superhydrophobic properties and the tilt angle of the wedge‐shaped structure is investigated.

Biased Motions of a Droplet on the Inclined Micro-conical Superhydrophobic Surface.

Anisotropic superhydrophobic surfaces that have many superior properties, such as directional self-cleaning, droplet transport, heat transfer, and so on, are widely used in various fields. Different from symmetric surfaces, a water droplet often shows directional spreading, moving, and bouncing on asymmetric surfaces. To investigate the mechanisms and achieve controllability of droplet motions on asymmetric surfaces, a series of surfaces with inclined micro-conical arrays are fabricated by integrating the methods of soft lithography, hot-pressing, and crystal growth. We found that the droplet would spread along the reverse direction of micro-cone's orientation but bounce and detach off the surface and move toward the direction of micro-cone's orientation. To understand these interesting performances, a mathematical model is established from the perspective of force balance, and a series of numerical simulations are performed. Additionally, the relationship between the droplet motions and the micro-structural parameters, including the inclined angle, line space, and height, are studied. This work may provide useful insights into droplet controlling, anisotropic surface designing, and its applications.

A superhydrophobic surface patterned with hydrophilic channels for directional sliding control and manipulation of droplets

Rapid and controllable sliding of droplets on anisotropic superhydrophobic surface has significant meaning in advanced science and technologies, thus being widely studied in recent years. Herewith, we reported a fabrication of micro- and nano- fractal superhydrophobic surface patterned with micro hydrophilic channels using laser direct writing method. The sliding angle of droplets in the direction parallel to the channels is as low as 8°. While, in the perpendicular direction, droplets exhibit rather high sliding angle, even being pinned to the surface that is tilted to fully upright (sliding angle to be 90°). Such significant anisotropic property enables droplets slide along the pre-assigned stripe-shaped patterns rapidly and steadily, when a droplet is released on a slope. The guidance action still works even when the surface is inverted. Consequently, the patterned surface is suitably designed as a novel droplet manipulate device, which easily achieves effective weight lossless transportation of droplets.

Tunable self-jumping of melting frost on macro-patterned anisotropic superhydrophobic surfaces

Controllable fast self-removal of frost pieces from surfaces can effectively reduce energy consumption and residual droplets during defrosting. In this work, a tuning strategy of melting frost directional self-jumping on regular macro-patterned anisotropic superhydrophobic surface (MASS) was proposed. Under high-rate cooling condition, patterned frost on MASS with significantly different thickness distribution was observed, due to the water vapor gradient in and out of the deep grooves. During defrosting process, L-shaped melting frost on MASS shrank independently with two arms retracting inward, which further caused the melting frost's centroid/mass center to shift out of the pillar top. Partial contact with pillar tops and circular fillet introduced directional lifting force and enabled the directional self-jumping of the melting frost. Influencing factors on directional self-jumping of melting frost on MASS were also discussed. Utilizing macro-patterned anisotropic structure, the frost splitting process and its jumping direction were both tuned, which could promote the self-jumping of melting frost and thus improve the efficiency of defrosting.

Fabrication of biomimetic anisotropic super-hydrophobic surface with rice leaf-like structures by femtosecond laser

It was found that the anisotropic hydrophobicity of rice leaves was caused by the multi-level grooves and tiny papillary microstructures on their surface. Inspired by rice leaves, a rice leaf-like anisotropic super-hydrophobic aluminum surface was obtained by femtosecond laser raster scanning. The surface structure, chemical composition, contact angle and sliding angle of the structured aluminum surface have been investigated in detail. The results show that structured surface appears super-hydrophobic with sliding anisotropy. The contact angle reaches 153° for 5 μl water droplet. The sliding angle is about 9° along the groove direction and about 34° perpendicular to the groove direction for 10 μl water droplet. • The surface structures of the rice leaves were investigated and the mechanism of anisotropic hydrophobicity of rice leaves was revealed. • It achieves the processing of hierarchical grooves and tiny papillary microstructures in “one step”. • The dominant mechanism for the directional wetting was discussed.

Large-area fabrication and applications of patterned surface with anisotropic superhydrophobicity

Recently, anisotropic superhydrophobic surfaces have aroused much attention due to their promising applications. However, large-area fabrication of low-cost superhydrophobic surfaces with obvious anisotropicity is still a huge challenge. In this work, a home-made melt hot-pressing method was proposed to facilely fabricate aligned micro-patterns on high-density polyethylene surface. Such surface displays anisotropic superhydrophobic characteristic with a static water contact angle of 152°and 145° respectively in parallel and perpendicular direction. Moreover, it has low surface adhesion and decent mechanical stability, showing promising applications for hydraulic triboelectric generator with self-cleaning ability, milk residue reduction and drag reduction. Considering the facile fabrication process and inexpensive thermoplastic polymer used, this study not only provides the possibility of large-area and low-cost preparation of orderly patterned surfaces with anisotropic superhydrophobicity, but also opens a new pathway toward wide application of anisotropic superhydrophobic surface.

Anisotropic, adhesion-switchable, and thermal-responsive superhydrophobicity on the femtosecond laser-structured shape-memory polymer for droplet manipulation

Smart superhydrophobic surfaces with reversible wettability are attracting increasing attention. However, most of the reported smart superhydrophobic surfaces switch their wettability through reversibly changing surface chemistry rather than surface microstructure. In this paper, femtosecond laser direct writing was exploited to create microgroove array structure on the shape-memory polymer (SMP) substrates. The groove-structured surface exhibited superhydrophobicity and anisotropic wettability after fluoroalkylsilane modification. Due to the excellent shape-memory property of the SMP substrate, the morphology of the microgroove array could be transformed between original stand shape and deformed shape in response to heat. Correspondingly, water droplet on the surface was reversibly changed between sliding and sticky states through alternant pressing and heating treatments, indicating switchable adhesion of the laser-induced microgroove array. Meanwhile, the anisotropic wettability could also recover with the restoration of the grooved microstructure. Such wettability conversion on the resultant surface could be cycled 10 times without decline of the superhydrophobicity and anisotropic wettability. The anisotropic superhydrophobic SMP surface with both ultralow and ultrahigh water adhesion was successfully applied in rewritable liquid pattern, droplet-based microreactor, and gas sensing. By taking advantage of femtosecond laser, reversible microstructures and surface wettability can be obtained on various SMP substrates. We believe that such surfaces with thermal-responsive superhydrophobicity, anisotropic wettability, and switchable adhesion will have enormous potential applications in non-loss droplet transfer, biological detection, and lab-on-chip devices.

Drag Reduction of Anisotropic Superhydrophobic Surfaces Prepared by Laser Etching.

In this research, the anisotropic superhydrophobic surface is prepared on a stainless steel surface by laser etching, and the drag reduction property of the anisotropic surface is studied by a self-designed solid-liquid interface friction test device. Periodic arrangement structures of quadrate scales with oblique grooves are obtained on a stainless steel surface by a laser. After modification by fluoride, the surface shows superhydrophobicity and anisotropic adhesive property. Here, the inclined direction of grooves and the inverse direction are defined as RO and OR, respectively. By changing the inclination of the grooves, a surface is obtained with a contact angle of 160° and a rolling angle difference of 6° along the RO and inverse RO direction. It is verified by numerical simulation and experiment that the subjected force of water droplets on the surface is different along the RO and inverse RO direction. Furthermore, the as-prepared surface has different drag reduction effects along the two directions. With the increase of velocity, the drag reduction effect of the superhydrophobic surface decreases against the RO direction, while the drag reduction effect along the RO direction is almost unchanged. We believe the anisotropic surface will be helpful in novel microfluid devices and shipping transportation.

MODELLING AND INVESTIGATION OF THE DEPENDENCE OF SUPERHYDROPHOBIC PROPERTIES OF NANOSURFACES ON THE TOPOLOGY OF MICROCHANNELS

In the Cassie-Baxter state anisotropic superhydrophobic surfaces have high lubricating properties. Such superhydrophobic surfaces are used in medical implants, aircraft industry, vortex bioreactors etc. In spite of the fact that quantitative understanding of fluid dynamics on anisotropic superhydrophobic surfaces has been broadened substantially for last several years, there still are some unsolved problems in this field. This work investigates dynamics of a liquid on unidirectional superhydrophobic surfaces in the Cassie-Baxter state, when surface texture is filled with gas and, consequently, the liquid virtually is located on some kind of an air cushion. Energy of the interphase boundary liquid-gas is much smaller than energy of the interphase boundary solid-liquid, that is why the contact angle at wetting such surfaces differs a lot from the Young contact angle and depends on contact area ratio of liquid-gas and liquid-solid in visible contact of liquid and surface. Considering difference in energy obtained if we slightly shift the three-phase contact line, expression for macroscopic equilibrium contact angle, which describes the Cassie-Baxter state, can be deduced. In the work the design formula for computing local-slip length profiles of liquid on the considered superhydrophobic surfaces is obtained.

Droplet Impact on Anisotropic Superhydrophobic Surfaces.

A droplet impacting on a superhydrophobic surface exhibits complete bouncing. The impacting process usually consists of spreading and retracting stages, during which the droplet contacts the underlying substrate. Recent research has been devoted to reducing the contact time using textured surfaces with different morphologies or flexibilities. Here, we design submillimeter superhydrophobic ridges and show that impacting droplets bounce off the surface immediately after capillary emptying in a petal-like shape at a certain Weber number range. The absence of a horizontal retraction process in two directions leads to ∼70% reduction in contact time. We demonstrate that the petal bouncing is attributed to the synergistic cooperation of the hierarchical structures and anisotropic property, which endows effective energy storage and release. When touching the bottom of the grooves, obvious flying wings appear along the ridges with a velocity component in the vertical direction, which help the energy releasing process in achieving fast droplet detachment. At higher Weber numbers, the anisotropic surface distorts the mass distribution and promotes uniform fragmentation of the droplet, and therefore the overall contact time is dramatically reduced. Simple analyses are proposed to explain these phenomena, showing a good agreement with the experimental results. The contact time reduction on anisotropic superhydrophobic surfaces is expected to have a great influence on the design and fabrication of anti-icing and self-cleaning surfaces.

Fabrication of an Anisotropic Superhydrophobic Polymer Surface Using Compression Molding and Dip Coating

Many studies of anisotropic wetting surfaces with directional structures inspired from rice leaves, bamboo leaves, and butterfly wings have been carried out because of their unique liquid shape control and transportation. In this study, a precision mechanical cutting process, ultra-precision machining using a single crystal diamond tool, was used to fabricate a mold with microscale directional patterns of triangular cross-sectional shape for good moldability, and the patterns were duplicated on a flat thermoplastic polymer plate by compression molding for the mass production of an anisotropic wetting polymer surface. Anisotropic wetting was observed only with microscale patterns, but the sliding of water could not be achieved because of the pinning effect of the micro-structure. Therefore, an additional dip coating process with 1H, 1H, 2H, 2H-perfluorodecythricholosilanes, and TiO2 nanoparticles was applied for a small sliding angle with nanoscale patterns and a low surface energy. The anisotropic superhydrophobic surface was fabricated and the surface morphology and anisotropic wetting behaviors were investigated. The suggested fabrication method can be used to mass produce an anisotropic superhydrophobic polymer surface, demonstrating the feasibility of liquid shape control and transportation.

Fabrication of anisotropic PTFE superhydrophobic surfaces using laser microprocessing and their self-cleaning and anti-icing behavior

Due to excellent stability and intrinsic low surface energy, polytetrafluoroethylene(PTFE) is widely utilized to fabricate biomimetic superhydrophobic surfaces. We proposed a facile and rapid method to obtain outstanding superhydrophobic surfaces by CO2 laser microprocessing PTFE plate. Scanning electron microscope(SEM) and contact angle measuring instrument were employed to characterize the structure and wettability. The effects of different micro channels spacing, different processing times and different processing power on wettability and morphology were studied. The self-cleaning and anti-icing properties of the prepared PTFE superhydrophobic surface were investigated. It was demonstrated that the maximum static contact angle was up to 168.36°. The degree of anisotropy could be controlled by adjusting the micro channels spacing. The as-prepared PTFE superhydrophobic surfaces exhibited self-cleaning properties, and water droplets were easy to take away dusts attached to the surfaces. Especially, it was found using the self-made icing monitoring system that as-prepared PTFE superhydrophobic surface showed an outstanding dynamic anti-icing performance even if the surface temperature was as low as −25°C and surrounded by ice and frost. The present study therefore suggests that CO2 laser microprocessing could be applied to large-scale preparation of superhydrophobic surface for practical applications in different fields of self-cleaning, anti-icing, microfluidic devices, lab-on-chips system, directional water harvesting, drag reduction and other fields.

Reducing the contact time using macro anisotropic superhydrophobic surfaces \u2014 effect of parallel wire spacing on the drop impact

Surfaces designed to reduce the contact time of impacting droplets are potentially of great importance for fundamental science and technological applications, for example, anti-icing, self-cleaning and heating transfer applications. Previous studies have shown that the contact time can be reduced via introducing one or several crossing macroscale wires on superhydrophobic surfaces (SHSs). However, the impacts that strike far from the wires (off-center impacts) have contact times that are equal to those obtained on SHSs. Here we demonstrate that this problem can be largely solved by using macro anisotropic SHSs (macro-aniso-SHSs)—in which the wires are parallel and macroscaled. The droplet contact time depends on the spacing between the macrostripes and is remarkably reduced by 40–50% when the spacing is comparable to the droplet size. Obvious differences in the contact time are not observed for impacts that are centered on the stripe and in the groove. The impacts centered in the groove produce new hydrodynamics that are characterized by extended spreading, easy break up and bouncing in a flying-eagle configuration. The study discusses the underlying mechanisms of the impact processes. Moreover, the effect of parallel wires on the contact time is discussed by comparing the droplet impact data for grooved rice leaves and non-grooved cabbage leaves. The enhanced drop mobility associated with the macro-aniso-SHSs should be very useful in many industrial applications.

Two-Pronged Jet Formation Caused by Droplet Impact on Anisotropic Superhydrophobic Surfaces

When a liquid droplet impacts a superhydrophobic surface with anisotropic surface patterning in the form of alternating ribs and cavities, the rebounding droplet may exhibit a unique two-pronged jet emission. Droplet impact experiments with 11 different fluids of viscosity that varied by more than three orders of magnitude were conducted, and this paper quantifies the Capillary number, Ca, and Ohnesorge number, Oh, ranges over which the two-pronged phenomenon occurs. For Oh > 0.0154, the behavior was never observed, while at lower values of Oh, the behavior occurs for an intermediate range of Ca that depends on Oh.

Effects of isotropic and anisotropic slip on droplet impingement on a superhydrophobic surface

The dynamics of single droplet impingement on micro-textured superhydrophobic surfaces with isotropic and anisotropic slip are investigated. While several analytical models exist to predict droplet impact on superhydrophobic surfaces, no previous model has rigorously considered the effect of the shear-free region above the gas cavities resulting in an apparent slip that is inherent for many of these surfaces. This paper presents a model that accounts for slip during spreading and recoiling. A broad range of Weber numbers and slip length values were investigated at low Ohnesorge numbers. The results show that surface slip exerts negligible influence throughout the impingement process for low Weber numbers but can exert significant influence for high Weber numbers (on the order of 102). When anisotropic slip prevails, the droplet exhibits an elliptical shape at the point of maximum spread, with greater eccentricity for increasing slip and increasing Weber number. Experiments were performed on isotropic and anisotropic micro-structured superhydrophobic surfaces and the agreement between the experimental results and the model is very good.