Micropillar Surface Research Articles

Remarkable bactericidal traits of a metal-ceramic composite coating elated by hierarchically structured surface.

SummaryA ceramic-based coating with a hierarchical surface structure was synthesized via solid-state reaction enabled by a double cathode glow discharge technique. This innovative coating comprises two distinct layers, specifically an outer layer with a well-aligned micro-pillar array and a dense inner layer. Both are composed of a face-centered cubic Cu(Co,Ni,Fe) solid solution phase together with a spinel-type Fe(Al,Cr)2O4 oxide. This coating exhibits superhydrophobicity and, yet, a very strong adhesion to water, i.e., the so-called “rose petal effect”. This coating also exhibits highly efficient antibacterial ability against both Staphylococcus aureus and Escherichia coli bacteria under both dark and visible light conditions. The excellent antibacterial property originates from the synergistic effects through the release of Cu ions coupled with photothermal activity upon light activation.

Durable poly(N-isopropylacrylamide) grafted PDMS micropillared surfaces for temperature-modulated wetting

Surface grafting of polymer brushes has become an area of significant research to deliver desired material properties such as smart wetting. In this research the grafting of Poly(N-isopropylacrylamide)(PNIPAAm) by benzophenone-initiated UV polymerization on micro-pillared surfaces was performed. By increasing polymerization time PNIPAAm first coats the top of the mushroom pillars then bridges the gap between pillars rather than penetrating the inter-pillar spaces due to the intrinsic features of Poly(dimethylsiloxane) (PDMS) pillars. This enables controlled grafting via control of polymerization time. Above the lower critical solution temperature (LCST), the grafted PNIPAAm brushes are immiscible with water and the surfaces displayed a strongly hydrophobic state with a large contact angle of 120°. Below the LCST, the grafted PDMS micropillars surface swells and absorbs water significantly faster than flat grafted surfaces, displaying a hydrophilic state with a small contact angle 33°. Interestingly, grafting on micropillars as opposed to flat surfaces enhanced the magnitude of the contact angle change across PNIPAAm’s hydrophobicity-hydrophilicity switch and enhances the speed of graft hydration. The grafted surface was further found to be durable under strong water jetting. Thus, a significant synergetic effect of the grafted PNIPAAm and micropillars delivered a durable temperature-modulated smart wetting property with a transition from the hydrophobic to hydrophilic state.

Publisher’s Note: “Slippery and magnetically responsive micropillared surfaces for manipulation of droplets and beads” [AIP Adv. 10, 085021 (2020)

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Super-wetting enabled by an array of SU-8 micro-pillars etched with ion-beam

The wettability transition of an ion-beam-etched micro-pillar array surface is reported in the present study. The micro-pillar array, made of negative photoresist SU-8, was fabricated by ultraviolet lithography. The results indicate that the combination of the micro-pillar array and ion-beam etching technique can easily regulate surface wettability, which can be changed from hydrophobicity to super-hydrophilicity. Moreover, the super-hydrophilic property of the textured surface can be significantly affected by the etching time and etching energy, because the ion-beam etching could alter the shape and height of the micro-structure as well as the chemical composition of the surface. When a 2 µl water droplet spreads on the etched micro-pillar array surface, the contact angle (CA) decreases to almost 0° within 144 ms, indicating ultrafast spreading of the water droplet on the textured surface. It was found that the CA of the fabricated surface remained the same after two weeks’ exposure in the air, showing undependable wettability stability of the textured surface. The results of the present study can open a gate for potential applications of proposed super-wetting surfaces for micro-fluidics and anti-counterfeiting technologies.

Numerical study of droplet evaporation on heated flat and micro-pillared hydrophobic surfaces by using the lattice Boltzmann method

The evaporation processes of sessile droplets on both heated flat and structured hydrophobic surfaces are numerically studied by using a thermal multi-phase lattice Boltzmann model. The contact-line dynamics and heat transfer regarding the evaporation of droplets are simulated and the effects of the heat conductivity, wettability, and roughness of the substrate on the droplet evaporation are investigated. The results indicate that the different evaporation modes of the droplets on the rough surfaces are attributed to the microscopic “stick–slip” motion of the contact line. The constant contact angle (CCA) mode of droplet evaporation on the rough hydrophobic surface can be regarded as a macroscopic average of the microscopic “stick–slip” processes with a short stick period and small change in apparent contact angle. The weaker stick effect of the surface with a higher hydrophobicity and smaller solid fraction makes the droplet evaporation more likely to occur in the CCA manner.

Slippery and magnetically responsive micropillared surfaces for manipulation of droplets and beads

Stimuli-responsive surfaces are of practical importance for applications ranging from enhanced mixing of reagents in lab-on-a-chip systems until probing cellular traction forces. Non-destructive reversible bending of cilia-inspired magnetic pillars can be used for controlled transportation of non-magnetic objects and bio-inspired sensing. Magnetic actuation of micropillars suspended in liquids allows controlled mixing, propelling, and stirring of fluids as well as droplet manipulation, which are important for various applications including generation of cell spheroids and droplet coalescence in microfluidic systems. In order to expand their practical applications, fabrication processes capable of rapid prototyping have to be developed. Inspired by biological cilia and their functionalities, actuating hairy surfaces are herein fabricated and implemented to manipulate both microbeads and droplets. The artificial cilia are based on microscale magnetic pillar arrays made of flexible polydimethylsiloxane functionalized with magnetic microparticles. The arrays are fabricated by a new method using patterned molds that relies on cryogenic separation to produce transparent cilia-inspired arrays without requiring manual interference to clean the templates during the process. Magnetic actuation of the pillar arrays is demonstrated in isopropanol and silicone oil. Filling with oil yields magnetically responsive slippery lubricated surfaces allowing directional motion of droplets by repetitive bending and recovery of the flexible magnetic pillars. The achieved structures allow manipulation of microbeads and droplets which is uncommon even at the sub-mm scale; directional motion is demonstrated for 250 μm–550 μm sized droplets. Droplet transportation is facilitated by extremely low hysteresis and a high degree of omnidirectional bending of the pillar array.

Superwicking on Nanoporous Micropillared Surfaces.

Engineering surfaces with excellent wicking properties is of critical importance to a wide range of applications. Here, we report a facile method to create superhydrophilic nanoporous micropillared surfaces of silicon and their applicability to superwicking. Nanopores with a good control of the pore depth are realized over the entire surface of three-dimensional micropillar structures by electrochemical etching in hydrofluoric acid. After rinsing in hydrogen peroxide, the nanoporous micropillared surface shows superhydrophilicity with the superwicking effect. The entire spreading process of a water droplet on the superhydrophilic nanoporous micropillared surface is completed in less than 50 ms, with an average velocity of 91.2 mm/s, which is significantly faster than the other wicking surfaces reported. Owing to the presence of nanopores on the micropillar array, the wicking dynamics is distinct from the surfaces decorated only by micropillar arrays. The spreading dynamics of a water droplet shows two distinct processes simultaneously, including the capillary penetration between micropillars and the capillary imbibition into the nanopore's interior. The wicking dynamics can be described by the two stages separated by the time when the contact line starts to recede. The transition between the two wicking regimes is due to the increasing effect of the imbibition of the bulk droplet by the nanopores. While a similar transition of the wicking dynamics is shown on the surfaces with different pore depths, the nanopore structure with a greater depth causes a greater amount of imbibition to slow down the spreading and promote superwicking.

Effect of gravity in the Cassie-to-Wenzel transition on a micropatterned surface

When the Cassie–Baxter and Wenzel states coexist for a liquid droplet on a micropatterned surface, the Cassie-to-Wenzel transition takes place if the energy barrier is overcome. Although multiple metastable states coexist due to the micropattern, this paper presents a simple Cassie-to-Wenzel transition of a 2 µL water droplet on a particular micropillared surface: When the droplet is gently deposited above the surface, it equalizes to the Cassie state at zero gravity; however, it transitions to the Wenzel state at the terrestrial gravity, in which the gravitational potential energy overcomes the energy barrier between the Cassie and Wenzel states.

Thickness-dependent fast wetting transitions due to the atomic layer deposition of zinc oxide on a micro-pillared surface.

Smart surfaces promote the fundamental understanding of wetting and are widely used in practical applications for energy and water collection. Light-induced switchable wettability facilitated by ZnO coatings, for instance, was developed for liquid manipulation at the surface. However, the transition of wetting states was reported to follow a hydrophobic–hydrophilic cycle in an hour, which is very long and may limit its future applications. We recently discovered that the cycle of the wetting state transitions on inorganic coatings can be shortened to less than 100 seconds by using ALD-coated ZnO on a pillared surface. However, the mechanisms are still unclear. Here, we investigated the effects of coating thickness on the transition speed and found that it significantly depended on the thickness of the coating with the optimal thickness less than 50 nm. We found that the minimum critical time for a wetting state transition cycle was less than 50 seconds with a thickness of 40 nm. Although the transition time of surfaces with coatings over 70 nm thickness remained constant at 10 min for a cycle, it was shorter than those of other deposition techniques for a coarse surface. Here, we propose a “penetration–diffusion” model to explain the fast and thickness-dependent wetting transitions. Our study may provide a new paradigm for fast wetting transition surfaces with cycle time within tens of seconds using a homogeneous thin layer coated on a rough surface.

Flow micropillar array electroporation to enhance size specific transfection to a large population of cells

Despite serving as a popular non-viral delivery approach, electroporation carries several drawbacks in its current configurations. We developed a Flow Micropillar-array Electroporation (FME) system to wisely regulate an important transmembrane-determining factor, namely cell size variations among individual cells, to achieve effective transfection. In FME, cells flow through a slit-type microfluidic channel on which carbon electrodes with well-patterned micropillar array texture are integrated as the top and bottom wall. Gravity helps bring cells to the micropillar array surface so that the permeable area on cells in different size populations is specified by their size regardless their random location fact. Without sacrificing cell viability, we demonstrate this FME concept by delivering DNA plasmids to several mammalian cell lines with obvious transfection enhancement when compared to a commercial system (K562: 3.0 folds; A549: 3.3 folds; HeLa: 1.8 folds, COS7: 1.7 folds; 293T: 2.9 folds; mES: 2.5 folds). Moreover, carbon-based electrodes are less expensive, more durable, and convenient for integration with a microfluidic setup which enables rapid and massive transfection capability that many therapeutic application needs. The success of FME may benefit many emerging biological studies and clinical practice that requires effective transfection to a large population of cells in limited processing time.

Wetting on Micropatterned Surfaces: Partial Penetration in the Cassie State and Wenzel Deviation Theoretically Explained.

A liquid droplet on a micropatterned substrate equalizes into either the Cassie-Baxter (also called Cassie for short) or the Wenzel state. This paper investigates the wetting phenomena on ideal micropatterned surfaces consisting of straight micropillars at different pillar dimensions and spacings (the word "ideal" refers to being chemically homogeneous and free of submicron-scale roughness all over the micropatterned surface). Two modeling approaches are used: (1) a thermodynamic approach analyzing the Gibbs energy of the droplet-solid-gas system and (2) a computational fluid dynamics (CFD) approach studying the three-dimensional dynamic wetting process to validate the results of the first approach. The thermodynamic approach incorporates three creative submodels proposed in this paper: (i) a sagging model explaining the pillar edge effect, (ii) a touchdown model transitioning the droplet's partial penetrating condition toward its full penetrating condition, i.e., the Wenzel state, and (iii) a liquid-volume model dynamically computing the liquid volume between the pillar valleys while in the partial penetrating condition or in the Wenzel state. The results of the thermodynamic approach reveal (1) a small energy barrier between the Cassie and Wenzel states, (2) no partial penetration and sagging of the liquid in the Cassie state on the ideal straight micropillared surface, and (3) that the apparent contact angle in the most stable Wenzel state can be 5° or more lower than the prediction of the Wenzel equation when the pillar height is equal or greater than 75 μm. To the best of our knowledge, this paper presents the theoretical explanation of this Wenzel deviation on micropatterned surfaces for the first time in the literature. Utilizing the state-of-the-art continuum model developed by the authors in previous studies, the CFD approach investigates the same wetting conditions and confirms the same findings.

Nanopattern-Embedded Micropillar Structures for Security Identification.

A novel method was developed for fabricating nanopatterns embedded on micropillar-structured surfaces using nanowelding technology for security identification. Commonly used substrates, that is, polyethylene films, glass wafers, Si wafers, and curved surfaces, were employed and their characteristics were evaluated. Cr was deposited onto the selected substrate to strengthen the adhesion force, and an adhesive layer of ultra-thin metal was deposited on top of the Cr layer. Lastly, nanopatterns were embedded on the substrates by nanowelding. The morphologies, cross sections, and three-dimensional (3D) images of the fabricated nanostructures were evaluated, and their crystalline structures and compositions were analyzed. Using the same method, nanopatterns embedded on micropillar-structured surfaces were fabricated for the first time as security patterns to improve security identification. The fabricated security patterns were characterized in three stages. First, micropillar structures and structural color were simply observed via optical microscopy to achieve a preliminary judgment. The appearance of structural color was due to the nanostructures fabricated on the micropillar surface. Next, the designed nanopatterns on the micropillar-structured surfaces were observed by scanning electron microscopy. Lastly, the changes in the spectral peaks were precisely observed using a spectrometer to achieve an enhanced security pattern. The fabricated security patterns can be suitable for valuable products, such as branded wines, watches, and bags. In addition, the proposed method offers a simple approach for transferring metal nanopatterns to common substrates. Moreover, the fabricated security patterns can have potential applications in semiconductor electrodes, transparent electrodes, and security identification codes.

A coupled wicking and evaporation model for prediction of pool boiling critical heat flux on structured surfaces

Boiling is an effective heat transfer mechanism that is central to a variety of industrial processes including electronic systems, power plants, and nuclear reactors. Micro-/nano-structured surfaces have been demonstrated to significantly enhance the critical heat flux (CHF) during pool boiling, but there is no consensus on how to predict the structure-induced CHF enhancement. In this study, we develop an analytical model that takes into consideration key mechanisms that govern CHF during pool boiling on structured surfaces, namely, capillary wicking and evaporation of the liquid layer underneath the bubble. The model extends existing wicking-based CHF theories by introducing the competing evaporation mechanism. The model reveals a wicking-limited regime where CHF increases monotonically with the wicking flux, and an evaporation-limited regime where additional increases in the wicking flux do not significantly affect CHF. The model predictions are shown to agree with experimental CHF data from the literature for boiling of water on surfaces structured with square micropillar arrays. A parametric study is performed for such micropillared surfaces and has identified the optimal structure based on the competition between wicking and evaporation, capillary pressure and viscous resistance, and conduction and liquid-vapor interfacial resistance.

Superamphiphobic Cu/CuO Micropillar Arrays with High Repellency Towards Liquids of Extremely High Viscosity and Low Surface Tension

For almost all the research of super anti-wetting surfaces, pure liquids like water and n-hexadecane are used as the probes. However, liquids of diverse compositions are used in academic research, industrial production and our daily life. Obviously, the liquid repellency of super anti-wetting coatings is highly dependent on properties of the liquids. Here, we report the first superamphiphobic surface with high repellency towards liquids of extremely high viscosity and low surface tension. The surfaces were prepared by constructing a hierarchical micro-/nanostructure on the Cu micropillar arrays followed by modification with perfluorosilane. The surfaces are superamphiphobic towards the liquids with extremely high viscosity and low surface tension because of (i) the micro-/nanostructured surface composed of micropillars with proper pillar distance and CuO nano-flowers, and (ii) the abundant perfluorodecyl groups on the surface. The contact angles, sliding angles, apparent contact line at the solid-liquid interface and adhesion forces are the end products of micropillar distance, viscosity and surface tension. Smaller micropillar distance, higher viscosity and higher surface tension contribute to reducing the adhesion force. We in situ observed the process of microcapillary bridge rupture for the first time using highly viscous liquids. We also successfully reduced the adhesion forces and enhanced the average rolling velocity of liquids with extremely high viscosity and low surface tension by regulating the micropillar distance.

Effect of wetting states on frequency response of a micropillar-based quartz crystal microbalance

A new acoustic wave resonance device induced by coupling micropillars with a quartz crystal microbalance (QCM-P) was developed for potential use in a wide range of applications such as drug discovery and development. The effect of wetting states of liquid on the micropillars of the QCM-P devices becomes essential in understanding the frequency signals from QCM in these applications. Euler-Bernoulli beam theory-based models were developed to establish the relationship between the resonance frequency shift of QCM-P and different wetting states of liquid on the micropillar surface including Cassie and Wenzel states. To validate the models, micropillars were fabricated on a QCM substrate by using nanoimprinting lithography (NIL) and liquids with different viscosity and surface tension, as well as different surface treatments were utilized to achieve Cassie and Wenzel states. The experimental results show that both wetting states and micropillar height have profound impact on the frequency shift of the QCM devices and the model can accurately capture the resonance of the QCM-P device and predict the surface-liquid interactions with a reasonable accuracy.

Lattice Boltzmann simulation of nucleate boiling in micro-pillar structured surface

A two-particle distribution functions lattice Boltzmann method is used to simulate periodic bubble nucleation, growth and departure from a micro-pillar structured surface. Bubble growth process and the effects of micro-pillar character scales on bubble departure period are investigated in this paper. Fluid hydrodynamic and thermodynamic are characterized by the density distribution function and the temperature distribution function respectively. Pseudo-potential model and Peng-Robinson equations of state are selected in our simulations. Constant temperature boundary condition is applied to the bottom solid. It is found that bubble generated in the vicinity of the micro-pillars, but in the same superheat, the smooth surface has no phase change phenomenon. This result indicates that micro-structure can contribute to the nucleation of the fluid phase change. Geometrical dimensions of micro-pillars have obvious effects on the growth of the bubble. Specifically, we investigated the influence of the distance between two pillars and the height of the pillars on the bubble growth. Results show that, to some extent, the distance of two micro-pillars “n” and the height of micro-pillar “h” have obvious effects on the bubble growth. For the height “h”, the smaller of “h”, the bigger of heat flux and the smaller of bubble departure periodic. For the distance “n”, it has no obvious effect on surface heat flux and bubble departure periodic but has an important effect on deciding whether it can form an activated nucleate site. Only proper “n” and “h” can be chosen to ensure an activated nucleation site according to our simulation results.

Wetting characteristic of bubble on micro-pillar structured surface under a water pool

In this study, the contact angle of air bubbles on a micro-structured surface under a water pool is experimentally investigated. A previous study “Kang and Jacobi, Equilibrium Contact Angles of Liquids on Ideal Rough Surfaces, Langmuir (2011)” adopted the work of adhesion as the additional work of the process of droplet contact on a textured surface, and it explained the reason for failure of the classical theoretical model (Wenzel equation) in real wetting tests. We extended the above concept to bubble interaction with the textured surface. First, the bubble contact angles on the textured surface were modeled based on the concept of the work of adhesion in the free energy calculation. Second, the contact angle of air bubbles was measured on the bottom of test surfaces under a water pool. The test-section surfaces have micro-pillars with a size of 5–40 μm prepared by the microelectromechanical systems (MEMS) technique. The bubble contact angles were compared with both the modeled bubble shape and classical prediction (Wenzel & Cassie–Baxter equations). In addition, detail wetting features which shows wetting transition and critical wetting behavior of bubble were discussed.

Spontaneous wetting transition of droplet coalescence on immersed micropillared surfaces

• The possibility of spontaneous Wenzel-to-Cassie transition is verified by an energy analysis. • The wetting transition of droplets during coalescence depends on small interpillar spacing and wettability. • The dynamical contact angle hysteresis of droplets can be decreased via spontaneous Wenzel-to-Cassie transition. • Robust superhydrophobicity and efficient droplet transportation rely on surface geometry and wettability. The spontaneous wetting transition of droplet coalescence on immersed micropillared surfaces was investigated by using many-body dissipative particle dynamics (MDPD). The simulation results show that wetting states are influenced by both the interpillar spacing and intrinsic contact angle of surfaces during droplet coalescence. The overestimation of the contact angle by the Cassie equation was modified by introducing a penetration factor, and the energy analysis suggests that the spontaneous Wenzel-to-Cassie transition could be predicted from critical contact angles. For surfaces with small interpillar spacing and large intrinsic contact angle, the spontaneous Wenzel-to-Cassie transition occurs during droplet coalescence, irrespective of the size of the immersed area. The contact angle hysteresis was also studied, in which the droplet was driven on immersed micropillared surfaces. It was found that reducing interpillar spacing is a feasible option to maintain robust superhydrophobicity, and that it is possible to realize the spontaneous Wenzel-to-Cassie transition. Otherwise, larger interpillar spacing and larger size of the immersed region will result in longer contact lines pinned by pillars if the spontaneous Wenzel-to-Cassie transition fails to occur. Meanwhile, it leads to the weakening of droplet mobility during transportation. The outcomes demonstrate that appropriate geometrical structures and wettability properties should be taken into consideration in order to achieve robust superhydrophobicity and high efficiency of the droplet transportation.

A representative volume element model for the adhesion between a micro-pillared surface and a compliant substrate

A representative volume element (RVE) model was developed to understand how the adhesion response between an effectively rigid substrate with micro-pillar arrays and a compliant substrate is affected by the spacing and aspect ratio of pillars. A number of verification and validation steps were taken to ensure the RVE model was robust. We found that the pull-off force decreases when pillar spacing is small enough such that the local deformation within the compliant substrate around one pillar is affected by neighboring pillars. Additionally, pull-off force can increase dramatically if the compliant substrate makes contact with the back surface between pillars, which occurs when the pillar spacing is large and pillar aspect ratio is small. Our RVE model can provide useful insights and quantitative data for understanding the adhesive contact mechanics between micro-patterned surfaces and soft substrates.

Effect of micropillar surface texturing on friction under elastic dry reciprocating contact

Surface texturing is considered to be a promising method to improve the tribological properties. Depending upon the experimental conditions, the effect of texturing varies from favourable to unnoticeable to detrimental. In this work, surfaces with micropillars are studied under elastic dry reciprocating contact. An array of micropillars with different pillar heights are generated on stainless steel using wire-cut electrical discharge machining. The effect of stiffness of the micropillars on friction is investigated, keeping the number of micropillars in contact with a flat aluminium alloy (Al6061) slider and contact geometry constant. Reciprocating experiments are carried out against a flat surface such that about 81 micropillars are in contact. From the experimental results, it is found that the coefficient of friction is independent of the stiffness of the texture elements. However, work done per cycle significantly varied with the stiffness of texture element and applied normal load. A lumped system model with Coulomb friction shows that the work done per cycle varies quadratically with the normal load. The experimental results agree with this simplified model except in the incipient sliding regime. These results show how the work done per cycle varies, for different contact stiffness under elastic contact even though the coefficient of friction remains constant. The implication of this study for a macroscopic measured coefficient of friction as a function of microscopic asperity level friction is discussed.