Unidirectional Surfaces Research Articles

Peristome‐mimetic Surfaces Fabricated by Nanosecond Laser for Uni‐directional Liquid Spreading

Unidirectional liquid transport, which operates without external energy input, has garnered remarkable interests in the global energy sector. A crucial breakthrough in this field involves understanding how liquids spread on the peristome surface of Nepenthes alata. This study, inspired by the microstructural traits of the Nepenthes peristome, has brought about the development of a biomimetic surface on metal substrates using nanosecond laser processing on inclined bases. This process replicates the natural peristome’s microstructure, which allows for controlled liquid spread on the surface. The research also examines how wettability influences unidirectional liquid spread. Additionally, it involves adjusting the microstructure parameters on the biomimetic surface, which leads to variations in edge curvature and microcavity wedge angles. More importantly, this study comprehensively investigates the relationship between these structural changes and the phenomenon of unidirectional liquid transport. The findings suggest that altering edge curvature and micro‐cavity wedge angles can manage the directional spread of liquids. The optimized surface, with ideal curvature and wedge angles, achieves liquid movement speeds up to 12.03mm/s. This represents a notable improvement, 2.2 and 2.7 times faster than the original structure, respectively. These findings provides valuable insight into the design of energy‐independent unidirectional liquid spreading surfaces on metal substrates.This article is protected by copyright. All rights reserved.

Three-Dimensional Maskless Fabrication of Bionic Unidirectional Liquid Spreading Surfaces Using a Phase Spatially Shaped Femtosecond Laser.

Ubiquitous biological processes exhibit the ability to achieve spontaneous directionally guided droplet transport. Maskless three-dimensional (3D) fabrication of various miniature bionic structures, a method applicable to various materials, is subject to processing method limitations. This remains a large obstacle to realizing self-driven, continuous, and controllable unidirectional liquid spreading. Thus, we present a flexible maskless 3D method for fabricating bionic unidirectional liquid spreading surfaces by using a phase spatially shaped femtosecond laser. The laser can be transformed from having Gaussian distributions to having 3D bionic structure field distributions. Furthermore, we fabricated Syntrichia caninervis bionic structures with a spiculate end for unidirectional water spreading; 1 μL droplets had a 16 mm flow length on Si surfaces when the S. caninervis single structure was 34 (length), 8 (width), and 12 μm (height). Furthermore, various bionic structures-Nepenthes, cactus, and moth structures-were fabricated on Si, SiO2, and Ti. We also demonstrated the measurability of two-dimensional (S-shaped) curved flows on Si wafers as well as 3D curved flows on a Ti pipe turning 120° within 2320 ms. Our method can realize high-efficiency maskless 3D processing of various materials and structures (especially asymmetric structures); it is both flexible and fast, effectively expanding the processing capacity of micro-/nanostructures on patterned surfaces. This is of great significance to various domains such as microfluids, fog collection, and chemical reaction control.

Controllable fabrication of unidirectional liquid spreading surface through confining plasma eruption and femtosecond laser double pulses

The efficient and simple fabrication of a self-driven, continuous, and controllable unidirectional liquid spreading surface is an enormous challenge. In this paper, we present a maskless, one-step, and controllable method to fabricate unidirectional high-speed liquid spreading surfaces. The method confines the eruption of plasma by using oblique glass and femtosecond (fs) laser double pulses. The oblique glass is used to regulate plasma density via plasma confinement, and the double-pulse is used to control the final processing energy by changing pulse delay. The gradient structure size can be controlled by adjusting the laser energy, pulse delay and the tilted glass angle. Moreover, It has even can both achieved a high-speed and unidirectional spreading. A 1 μL droplet flows approximately 7 mm in 640 ms with no increase of the corresponding width using the gradient grating structure fabricated under double pulses with a pulse delay of 1 ps. Using different gradient channel assembly, a selective flow along designed direction can be achieved. The selective flow phenomenon is observed through customized liquid diode. By manufacturing continuous and adjustable gradient structure, the method is potential to control liquid flow in microsystem.

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.

Enhanced slip properties of lubricant-infused grooves

We ascertain the enhanced slip properties for a liquid flow over lubricant-infused unidirectional surfaces. This situation reflects many practical settings involving liquid flows past superhydrophobic grooves filled with gas, or past grooves infused with another, immiscible, liquid of smaller or equal viscosity, i.e. where the ratio of lubricant and liquid viscosities, $\mu \leq 1$. To maximize the slippage, we consider deep grooves aligned with the flow. The (normalized by a texture period $L$) effective slip length, $b_{\mathrm{eff}}$, is found as an expansion to first order in protrusion angle $\theta$ about a solution for a flat liquid-lubricant interface. Our results show a significant increase in $b_{\mathrm{eff}}$ with the area fraction of lubricant, $\phi$, and a strong decrease with $\mu$. By contrast, only little influence of $\theta$ on $b_{\mathrm{eff}}$ is observed. Convex meniscus slightly enhances, and concave - slightly reduces $b_{\mathrm{eff}}$ relative the case of a flat liquid-lubricant interface. The largest correction for $\theta$ is found when $\mu = 0$, it decreases with $\mu$, and disappears at $\mu = 1$. Finally, we show that lubricant-infused surfaces of small $\theta$ can be modeled as flat with patterns of local slip boundary conditions, and that the (scaled with $L$) local slip length at the liquid-lubricant interface is an universal function of $\phi$ and $\mu$ only.

Modelling and hovering control for a coaxial unmanned helicopter using sliding mode

PurposeTo facilitate the nonlinear controller design, dynamic model of a novel coaxial unmanned helicopter (UH) is established and its coupling analysis is presented.Design/methodology/approachThe chattering-free sliding mode controller (SMC) with unidirectional auxiliary surfaces (UASs) is designed and implemented for the coaxial ducted fan UH.FindingsThe coupling analysis based on the established model show severe coupling between channels. For coaxial UH’s special model structure, UAS-SMC controller is proposed to reduce the coupling characteristics between channels of the UH by setting controllers’ output calculation sequence.Originality/valueThe flight control law and control logic are successfully tested in numerical simulation and hardware in the loop (HIL) simulation. The results show best hovering performances without chattering problem, even under the bounded internal dynamics and external disturbances.

Anisotropic Electrowetting on Wrinkled Surfaces: Enhanced Wetting and Dependency on Initial Wetting State.

Electrowetting on dielectric (EWOD) on unidirectional microstructured surfaces has recently evoked significant interest as they can modulate the effect of electrowetting, and can thus find applications in directional wetting in microfluidic systems. However, the dependency of such EW phenomenon on their initial state of wetting and anisotropy is far from being well understood. The current study addresses the initial wetting states and their implication on the anisotropic electrowetting using a wrinkled EWOD platform. Herein we demonstrate a facile stampless and maskless structure generation technique to fabricate wrinkles of varying topography. Further, we have demonstrated alteration in the interfacial wetting conditions by modulating the wrinkle topography, and its effect on the droplet behavior during electrowetting. The capillary wicking-assisted electrowetting on these wrinkled surfaces is in specific direction dictated by the ordered wrinkles and prompts enhanced spreading of the droplet. We also demonstrate that while the enhancement of unidirectional electrowetting is stronger in conformal wetting state surfaces, composite wetting state surfaces depict a reversal in anisotropy.

Boundary conditions at the gas sectors of superhydrophobic grooves

The hydrodynamics of liquid flowing past gas sectors of unidirectional superhydrophobic surfaces is revisited. Attention is focussed on the local slip boundary condition at the liquid-gas interface, which is equivalent to the effect of a gas cavity on liquid flow. The system is characterized by a large viscosity contrast between liquid and gas, $\mu/\mu_g \gg 1$. We interpret earlier results, namely the dependence of the local slip length on the flow direction, in terms of a tensorial local slip boundary condition and relate the eigenvalues of the local slip length tensor to the texture parameters, such as the width of the groove, $\delta$, and the local depth of the groove, $e(y, \alpha)$. The latter varies in the direction $y$, orthogonal to the orientation of stripes, and depends on the bevel angle of groove's edges, $\pi/2 - \alpha$, at the point, where three phases meet. Our theory demonstrates that when grooves are sufficiently deep their eigenvalues of the local slip length tensor depend only on $\mu/\mu_g$, $\delta$, and $\alpha$, but not on the depth. The eigenvalues of the local slip length of shallow grooves depend on $\mu/\mu_g$ and $e(y, \alpha)$, although the contribution of the bevel angle is moderate. In order to assess the validity of our theory we propose a novel approach to solve the two-phase hydrodynamic problem, which significantly facilitates and accelerates calculations compared to conventional numerical schemes. The numerical results show that our simple analytical description obtained for limiting cases of deep and shallow grooves remains valid for various unidirectional textures.

Analytical formulae for longitudinal slip lengths over unidirectional superhydrophobic surfaces with curved menisci

This paper reports new analytical formulae for the longitudinal slip lengths for simple shear over a superhydrophobic surface, or bubble mattress, comprising a periodic array of unidirectional circular menisci, or bubbles, protruding into, or out of, the fluid. The accuracy of the formulae is tested against results from full numerical simulations; they are found to give small relative errors even at large no-shear fractions. In the dilute limit the formulae reduce to an earlier result by Crowdy (Phys. Fluids, vol. 22, 2010, 121703). They also extend analytical results of Sbragaglia & Prosperetti (Phys. Fluids, vol. 19, 2007, 043603) beyond the limit of a small protrusion angle.

Invariant set based sliding mode control for near-space vehicles with attitude constraints

In this article, a robust attitude control scheme, which is called sliding mode control with unidirectional auxiliary surfaces (UAS-SMC), is proposed for the near-space vehicle with attitude constraints and time-varying unknown disturbance. Firstly, the positively invariant sets are designed with the UAS-SMC method. The invariant property of these sets can be utilized to guarantee the attitude constraints. Secondly, a UAS nonlinear disturbance observer is firstly developed for the NSV with time-varying unknown disturbance. By introducing the information of UAS surfaces, the performance of this observer is enhanced. The convergence of all closed-loop signals is rigorously proved via Lyapunov analysis method under the developed robust UAS-SMC scheme. Finally, simulation results are given to illustrate the effectiveness of the proposed attitude control scheme.

Terminal Sliding Mode Control with Unidirectional Auxiliary Surfaces for Hypersonic Vehicles Based on Adaptive Disturbance Observer

A novel flight control scheme is proposed using the terminal sliding mode technique, unidirectional auxiliary surfaces and the disturbance observer model. These proposed dynamic attitude control systems can improve control performance of hypersonic vehicles despite uncertainties and external disturbances. The terminal attractor is employed to improve the convergence rate associated with the critical damping characteristics problem noted in short-period motions of hypersonic vehicles. The proposed robust attitude control scheme uses a dynamic terminal sliding mode with unidirectional auxiliary surfaces. The nonlinear disturbance observer is designed to estimate system uncertainties and external disturbances. The output of the disturbance observer aids the robust adaptive control scheme and improves robust attitude control performance. Finally, simulation results are presented to illustrate the effectiveness of the proposed terminal sliding mode with unidirectional auxiliary surfaces.

Subsurface characteristics of a Fe–0.4 wt%C martenstic steel abraded with nanoindentation and cross-sectional TEM techniques

The present investigation examined the unidirectional abraded surfaces of a martensitic (0.4wt%C) steel with silicon (1.5wt%Si), chromium (1.5wt%Cr) and molybdenum (1.9wt%Mo) additions in order to elucidate the work hardening and softening near the surface layer caused by abrasion, particularly its relation to wear behavior. The abrasion testing was performed on a pin-abrasion apparatus, and a small pin of the specimen was ground on silica abrasive paper at an applied load of 2.1N and a sliding speed of 0.66m/s. The abraded surfaces were examined on a nanometer scale with a nanoindentation apparatus to evaluate the changes in nanohardness as the sliding time progressed. A cross-sectional transmission electron microscope (TEM) technique was also employed to clarify the structural changes in the region close to the abraded surface. The abrasion-induced work hardening with sliding time was observed in the case of chromium and molybdenum addition steels. A fine dispersion of molybdenum carbide (Mo2C) was observed in the surface of the molybdenum steel after abrasion. Mo2C precipitates at approximately 550°C, indicating that surface and near-surface temperatures exceeded the carbide formation temperature after abrasion-induced frictional heating. In contrast, the silicon addition steel softened, which seems to be caused by abrasion heating that leads to some tempering effects. Work hardening and softening, which are caused by abrasion-induced subsurface deformation and frictional heating, respectively, seem to take place simultaneously and thus counteract each other's effect. Metallurgical reaction, such as precipitation and temper by frictional heating, has been found to play an important role in controlling the wear characteristics of steels. The present article discusses the influence of the alloying element addition on the wear response of the martensitic steel.

Chattering-free condition for sliding mode control with unidirectional auxiliary surfaces

A chattering-free condition is proposed as a sufficient condition for the sliding mode control with unidirectional auxiliary surfaces (UAS-SMC) strategy in a class of non-linear systems. With this condition, the UAS-SMC controller would enjoy the following properties: 1) chattering-free property of the control input; 2) enhanced approaching speed on the switching surfaces (except the original point); 3) no requirement to measure the derivatives; 4) a positively invariant set formed by UAS. The stability of the closed-loop system is demonstrated with a chattering-free UAS-SMC controller. Numerical simulation is given to illustrate the benefits and properties of the proposed controller and algorithm.

Chattering‐free sliding mode control with unidirectional auxiliary surfaces for miniature helicopters

PurposeThis article proposes a chattering‐free sliding mode control scheme with unidirectional auxiliary surfaces (UAS‐SMC) for small miniature autonomous helicopters (Trex 250).Design/methodology/approachThe proposed UAS‐SMC scheme consists of a nested sequence of rotor dynamics, angular rate, Euler angle, velocity and position loops.FindingsIt is demonstrated that the UAS‐SMC strategy can eliminate the chattering phenomenon exhibiting in the convenient SMC method and achieve a better approaching speed.Originality/valueThe proposed control strategy is implemented on the helicopter and flight tests clearly demonstrate that a much better performance could be achieved, compared with convenient SMC schemes.

Hygrothermal effects on painted carbon fibre composite surfaces

In this article, the effect of hygrothermal aging on the painted surface finish of unidirectional and fabric carbon fibre composite laminates, with and without surfacing film was investigated. The results highlighted the importance of ensuring that the composite surface directly beneath the paint layer is made from a uniform material with a consistent thickness in order to minimise surface defects from occurring during aging. The surfacing film was found to minimise the print through development on the painted unidirectional and twill composite surfaces. However, the surfacing film layer was found to intermingle with the carbon fibre plies during cure, which resulted in an uneven film thickness that caused increased levels of orange peel. The twill laminate painted surface produced high levels of print through and surface waviness that was caused by the large resin rich regions located within the tow intersections at the surface which enlarged due to thermal expansion and swelling of the matrix with hygrothermal aging. It was also noted that the small resin rich regions between the individual carbon fibres on the unidirectional composite surface were sufficiently large to print through the painted surface.

Bioinspired Directional Surfaces for Adhesion, Wetting, and Transport

In Nature, directional surfaces on insect cuticle, animal fur, bird feathers, and plant leaves are comprised of dual micro-nanoscale features that tune roughness and surface energy. This feature article summarizes experimental and theoretical approaches for the design, synthesis and characterization of new bioinspired surfaces demonstrating unidirectional surface properties. The experimental approaches focus on bottom-up and top-down synthesis methods of unidirectional micro- and nanoscale films to explore and characterize their anomalous features. The theoretical component of the review focuses on computational tools to predict the physicochemical properties of unidirectional surfaces.

Frictional slip lengths for unidirectional superhydrophobic grooved surfaces

The exact solutions due to Philip [ZAMP 23, 353 (1972)] for Stokes shear flow over a periodic array of no-shear slots embedded in a no-slip surface are generalized to account for an arbitrary pattern of no-shear slots in each period window. The slots, or grooves, in each period window run parallel to each other, and are of infinite length, but their widths and separations can be specified arbitrarily. Explicit solutions are found both for longitudinal and transverse flows over the composite grooved surface. Analytical expressions for the transverse and longitudinal slip lengths associated with the microstructured surfaces are then found as functions of the geometrical parameters characterizing the surface. The formulae are relevant to a wide class of flow geometries and are expected to provide a useful tool in the design, analysis, and optimization of the friction properties of grooved microstructured superhydrophobic surfaces. The results are used to show that introducing even a very small wetted region in a no-shear slot can have a significant influence on the effective slip length.

The role of surface texture on friction and transfer layer formation during repeated sliding of Al–4Mg against steel

In the present investigation, various kinds of textures, namely, unidirectional, 8-ground, and random were attained on the die surfaces. Roughness of the textures was varied using different grits of emery papers or polishing powders. Then pins made of Al-4Mg alloys were slid against steel plates at various numbers of cycles, namely, 1, 3, 5, 10 and 20 using pin-on-plate reciprocating sliding tester. Tests were conducted at a sliding velocity of 2 minis in ambient conditions under both dry and lubricated conditions. A constant normal load of 35 N was applied in the tests. The morphologies of the worn surfaces of the pins and the formation of transfer layer on the counter surfaces were observed using a scanning electron microscope. Surface roughness parameters of the plates were measured using an optical profilometer. In the experiments, it was observed that the coefficient of friction and formation of the transfer layer depend on the die surface textures under both dry and lubricated conditions. More specifically, the coefficient of friction decreases for unidirectional and 8-ground surfaces while for random surfaces it increases with number of cycles. However, the coefficient of friction is highest for the sliding perpendicular to the unidirectional textures and least for the random textures under both dry and lubricated conditions. The difference in friction values between these two surfaces decreases with increasing number of cycles. The variation in the coefficient of friction under both dry and lubrication conditions is attributed to the change in texture of the surfaces during sliding. (C) 2011 Elsevier B.V. All rights reserved.

Interfacial propulsion by directional adhesion

The rough integument of water-walking arthropods is well-known to be responsible for their water-repellency [1–4]; however, water-repellent surfaces generally experience reduced traction at an air–water interface [5–8]. A conundrum then arises as to how such creatures generate significant propulsive forces while retaining their water-repellency. We here demonstrate through a series of experiments that they do so by virtue of the detailed form of their integument; specifically, their tilted, flexible hairs interact with the free surface to generate directionally anisotropic adhesive forces that facilitate locomotion. We thus provide new rationale for the fundamental topological difference in the roughness on plants and water-walking arthropods, and suggest new directions for the design and fabrication of unidirectional superhydrophobic surfaces.

Studies on friction and formation of transfer layer when Al–4Mg alloy pins slid at various numbers of cycles on steel plates of different surface texture

In the present investigation, various kinds of textures, namely, unidirectional, 8-ground, and random were attained on the die surfaces. Roughness of the textures was varied using different grits of emery papers or polishing powders. Then pins made of Al–4Mg alloys were slid against steel plates at various numbers of cycles, namely 1, 2, 6, 10 and 20 under both dry and lubricated conditions using an inclined pin-on-plate sliding tester. The morphologies of the worn surfaces of the pins and the formation of transfer layer on the counter surfaces were observed using a scanning electron microscope. Surface roughness parameters of the plate were measured using an optical profilometer. It was observed that the coefficient of friction and formation of transfer layer during the first few cycles depend on the die surface textures under both dry and lubricated conditions. It was also observed that under lubricated condition, the coefficient of friction decreases with number of cycles for all kinds of textures. However, under dry condition, it decreases for unidirectional and 8-ground surfaces while for random surfaces it increases with number of cycles.