Micropillar Spacing Research Articles

Dropwise condensation on subcooled micropillar surfaces with 3D lattice Boltzmann method

In this investigation, an alternative geometric formula is proposed to address the force between fluid nodes and fluid ghost nodes, with the aid of which the contact angles can be varied in the range of 48.3°∼131.9°. This formula is applied to the three-dimensional double-distributed thermal lattice Boltzmann method being proved to be accurate and reliable by single droplet condensation. The effects brought by varying micropillar size on the kinetic properties of condensed droplets, including nucleation, growth, coalescence and jumping, are investigated in detail. The results show that the droplet wetting state tends to be the suspended Cassie state as the width and spacing of the micropillars are decreased, and the condensed droplets can merge and jump off the micropillar surface. In the meantime, the average droplet number increases, the average diameter and the diameter of dominant droplets decrease, thus reducing the condensate coverage. When the micropillar spacing is small, increasing the micropillar height results in the condensed droplet state being changed from Wenzel to Cassie state, and the percentage of small droplets also increases. Instead, when the micropillar spacing is large, by increasing micropillar height, droplets can nucleate in the middle of micropillars, and the percentage of large droplets is improved due to increased heat transfer area. In this study, the surface self-cleaning capability is strongest with the combination of dimensionless pillar height 0.4, spacing 0.1 and width 0.1, which reduces the condensate coverage by 66 % compared to its plain competitor.

Fabrication of superamphiphobic micro-reentrant structures by inverted electrochemical deposition

Due to excellent oil resistance and shortened liquid-solid contact time, superamphiphobic surfaces have promising application prospects in oil transportation, and petroleum pollution control. Although superamphiphobic surfaces have been constructed on various substrates, there is still a lack of high-efficient methods for fabricating micro-reentrant structures on engineering metal substrates. This paper proposes a new method combining laser processing and inverted electrochemical deposition techniques to achieve high-efficient and large-scale fabrication of superamphiphobic surfaces on copper substrates. Firstly, micropillar array structures are constructed by laser processing, and the structures exhibit excellent superhydrophobicity after low surface energy modification. Then, inverted electrochemical deposition is performed after polishing the top of the array, and local angle of the gas/liquid interface can be controlled by adjusting the distance between the top of the micropillar and the electrolyte level, thus regulating micro-morphology of the roof structure. The influences of electrochemical deposition parameters on characteristics of the micro-reentrant structures are systematically investigated. Finally, superamphiphobic micro-reentrant structures (roof diameter > 190 μm, micropillar spacing <250 μm) with water and oil contact angles exceeding 150° are prepared after lowering the surface energy of the deposited structures. After anti-fouling, blowing, ultrasonic vibration, and tape adhesion tests, the prepared superamphiphobic micro-reentrant structures maintained good water and oil repellency, showing excellent chemical stability and durability. The research may provide a simple, efficient, and low-cost method for high-efficient constructing superamphiphobic surfaces on engineering metal substrates, enriching the fabrication techniques of micro-reentrant structures and facilitating their practical applications.

Fabrication of a three-dimensional micro/nanocarbon structure with sub-10 nm carbon fiber arrays based on the nanoforming and pyrolysis of polyacrylonitrile-based jet fibers

To produce a three-dimensional micro/nanocarbon structure, a manufacturing design technique for sub-10 nm carbon fiber arrays on three-dimensional carbon micropillars has been developed; the method involves initiating electrostatic jetting, forming submicron-to-nanoscale PAN-based fibers, and maximizing the shrinkage from polyacrylonitrile (PAN)-based fibers to carbon fibers. Nanoforming and nanodepositing methods for polyacrylonitrile-based jet fibers as precursors of carbon fibers are proposed for the processing design of electrostatic jet initiation and for the forming design of submicron-to-nanoscale PAN-based fibers by establishing and analyzing mathematical models that include the diameter and tensile stress values of jet fibers and the electric field intensity values on the surfaces of carbon micropillars. In accordance with these methods, an array of jet fibers with diameters of ~80 nm is experimentally formed based on the thinning of the electrospinning fluid on top of a dispensing needle, the poking of drum into an electrospinning droplet, and the controlling of the needle–drum distance. When converting thin PAN-based jet fibers to carbon fibers, a pyrolysis method consisting of the suspension of jet nanofibers between carbon micropillars, the bond between the fibers and the surface of the carbon micropillar, and the control of micropillar spacing, stabilization temperature, and carbonation rate is presented to maximize the shrinkage from PAN-based fibers to carbon fibers and to form sub-10 nm carbon fiber arrays between three-dimensional carbon micropillars. The manufacturing design of a three-dimensional micro/nanocarbon structure can produce thin PAN-based jet nanofibers and nanofiber arrays aligned on micropillar surfaces, obtain shrinkage levels reaching 96% and incorporate sub-10 nm carbon fibers into three-dimensional carbon micropillars; these actions provide new research opportunities for correlated three-dimensional micro/nanocarbon structures that have not previously been technically possible.

Condensation mode transition and droplet jumping on microstructured surface

Condensation enhancement on the micropillar structured surfaces is studied using a three-dimensional lattice Boltzmann model, and the optimal micropillar configuration is assessed. The impacts brought by varying micropillar geometric parameters on heat transfer and droplet dynamics during dropwise-to-filmwise transition are explored. Increasing micropillar width and spacing or decreasing height accelerates nucleation and improves nucleation density in the dropwise condensation stage, while radial contraction of the liquid film is hindered and the heat transfer area is increased in the filmwise condensation stage. Compared with its plain competitor, the maximum number of the isolated droplets is increased by 525%, the nucleation time is advanced by 57.1% and the heat flux is increased by 187.4% on the optimal micropillar surface. The entire condensation process spanning over nucleation, coalescence and jumping of multiple condensate droplets on the micropillar surface is realized using the present three-dimensional lattice Boltzmann model for the first time. The effects of height and spacing of micropillar arrays on jumping height and velocity are analyzed. The results show that the micropillar height poses little effect on the jumping height and velocity when the micropillar spacing is 0.1. When the micropillar spacing is 0.15, the jumping height and velocity first increase but then decrease with the increase of the micropillar height. Finally, the condensation heat transfer mechanism under different subcooling on the hierarchical surface is analyzed in detail.

Coalescence-induced droplet jumping on superhydrophobic surfaces with non-uniformly distributed micropillars

The phenomenon of droplet jumping induced by coalescence holds significant potential for application in diverse areas such as water collection, self-cleaning, and thermal management of electronic devices. In comparison to a flat surface, a superhydrophobic surface with structure can affect the interaction between the droplet and the surface, thereby affecting parameters such as the droplet jumping velocity. In this study, numerical simulation was used to examine the similarities and differences of coalescence-induced droplet jumping on superhydrophobic surfaces with uniform and non-uniform micropillars, as well as the effect of micropillar non-uniformity on droplet jumping. The results indicated that the non-uniform distribution of micropillars could lead to asymmetries in the shrinkage of the liquid bridge, the morphology of the droplet when it jumps off the surface, and the distribution of the high-pressure region at the bottom of the droplet. Additionally, it decreased the droplet’s vertical velocity while substantially increasing the horizontal velocity. When the droplet radius was constant, decreasing the micropillar width or increasing the micropillar spacing on the sparse side was advantageous for the droplet jumping on a superhydrophobic surface with non-uniform micropillars. By defining the solid fraction on the micropillar surface with non-uniform distribution, the coupling effect of the width and spacing of the micropillars on the droplet jumping was thoroughly analyzed, and the optimal solid fraction for the best directional transport effect of droplets was determined. This study revealed the mechanism underlying the effect of surface micropillar structure on coalescence-induced droplet jumping, and the findings could guide the design of superhydrophobic surface microstructures, which was of great significance for the advancement of droplet jumping applications.

Vapor condensation heat transfer on two-tier hierarchical microstructured surface

Condensation heat transfer on the hierarchical microstructured surfaces is simulated using a three-dimensional phase-change lattice Boltzmann method. Parametric effects raised by widely varying primary micropillar spacing, secondary micropillar position and height, surface wettability and subcooling on the condensation heat transfer are discussed in detail. The condensation heat flux first increases but then decreases with the increase of spacing between primary micropillars. Droplets are prone to nucleate on the top sides of primary micropillars with smaller pillar spacing, and they tend to maintain the Cassie‐Baxter state when secondary micropillars are arranged on the lateral sides of primary micropillars. As the height of secondary micropillar increases, the heat flux first increases but then decreases thereafter. When the contact angle rises from 57° to 130°, the state of condensate droplet changes from the Wenzel state to the partial wetting state. It is also found that there exist four types of droplet nucleation sites on the hierarchical microstructured surface: at the bottom of secondary micropillars, at the corner between primary micropillars and substrate, between two adjacent primary micropillars, and at the center of substrate between four primary micropillars. The increase of subcooling is favorable for nucleation, and the heat flux and condensation rate increase in accordance.

Vapor condensation on micropillar structured surface with lattice Boltzmann method

Pure vapor condensation process and heat transfer on micropillar structured surfaces with various wettabilities are simulated numerically based on an improved three-dimensional phase-change lattice Boltzmann method (LBM). Condensed droplet dynamic behaviors containing nucleation, growth and departure are mainly explored. Two preferable droplet nucleation sites are detected successfully: at the corner between micropillar sides and subcooled substrate, and at the center of substrate. The effects of contact angle, micropillar spacing and height on droplet nucleation time, departure diameter and departure frequency are investigated quantitatively. Results indicate that as the contact angle increases, nucleation of condensate is delayed, while the departure diameter is reduced and departure frequency is increased. The increase in micropillar spacing, height and contact angle changes droplet from Wenzel state to partial wetting state. Moreover, it is also shown that the highest temperature and maximum local heat flux appear at the three-phase contact line. Despite the very complex roles of micropillar structures and wettability on condensation heat transfer, an optimal heat transfer surface with configurations of contact angle of 90°, micropillar spacing of 12 and height of 18 is presented. This study provides a fundamental understanding in regards to vapor condensation on microscale structured surface from numerical view.

Binary mixture droplet wetting on micro-structure decorated surfaces

Liquid surface tension as well as solid structure play a paramount role on the intimate wetting and non-wetting regimes and interactions between liquids droplets and solid substrates. We hypothesise that the coupling of these two variables, independently addressed in the past, eventually offer a wider range of understanding to the surface science and interfacial communities.In this work, intrinsically hydrophobic micro-pillared surfaces varying in the spacing between structures, and pure ethanol, pure water and their binary mixtures (as well as acetone–water and ethylene glycol–water mixtures) are utilised, accessing a wide range of substrate solid fractions and liquid surface tensions experimentally. Wettability measurements are carried out at different azimuthal directions to exemplify the wetting/non-wetting behaviour as well as the droplet asymmetry function of both liquid composition and structure spacing.Our findings reveal that high water concentration droplets, i.e., high surface tension fluids, sit in the Cassie-Baxter regime while partial non-wetting Wenzel or mixed-mode regimes with enhanced droplet asymmetry ensuing for medium and high ethanol concentrations, i.e., low surface tension fluids, below certain micropillar spacing. Beyond micropillar spacing s ≥ 40 µm, the impact of the surface structure on the droplet shape is negligible, and droplets adopt a similar contact angle and circular shape as on a flat smooth hydrophobic surface. Wetting and non-wetting regimes are then supported by classical wetting theories and equations. A wetting regime map for a wide range of surface tension fluids and/or their mixtures on a wide domain of solid fractions is then proposed.

Enhancement of vapor condensation heat transfer on the micro- and nano-structured superhydrophobic surfaces

Recently, micro- or nano- structured surfaces haven been developed to enhance condensation heat transfer, water harvesting and self-cleaning. However, at large subcoolings, condensate floods the subcooled substrate, thus deteriorating the heat transfer efficiency. Here, the superhydrophobic surfaces with micropillared and nanopillared structures are proposed to enhance heat transfer at large subcoolings. The influence of micropillar spacing and surface subcooling on the droplet dynamics and heat transfer performance is experimentally investigated using microscopic visualization techniques. In addition, the microscopic modeling of condensation heat transfer on the microstructured surfaces is performed using the mesoscopic lattice Boltzmann method. The results demonstrate that the droplet size distribution on the micropillared surface is significantly smaller over that of the nanostructured surface. The heat transfer coefficient decreases with the increase of micropillar spacing. As the subcooling rises, although the condensate floods the substrate, the heat transfer coefficient of the S10R30 (S10R30 represents the micropillar arrays with s = 10 μm and 2r = 60 μm) surface is enhanced by 26.4% compared to the hydrophobic nanostructured surface. This is because the height of liquid film is the same of order of magnitude as the micropillars, reducing the thermal resistance caused by the liquid layer. Combining environmental scanning electron microscope (ESEM) observations and LB simulation results, it is concluded that the droplets first nucleate at the bottom corner of micropillars. In addition, the condensate droplets merge to form a film, fill the micropillar gaps, and cover the entire micropillars, leading to a sharp decrease in heat flux. These findings provide a theoretical and experimental guidance for the development of condensing surfaces to enhance heat transfer.

Finite element modeling and simulation of ultrasensitive film bulk acoustic resonator enabled by micropillar structure

Portable and ultra-sensitive film bulk acoustic resonator (FBAR) is a promising device to satisfy the requirement of detecting gas and biological molecule. In this work, a novel sensing device was developed to achieve ultrahigh sensitivity, by coupling polymer micropillars with a FBAR substrate to form a two-degrees-of-freedom resonance system (FBAR-micropillars). We systematically investigated the effects of micropillar structure on the characteristics of FBAR-micropillars device by finite element method (FEM). It was found that the resonant frequency shift increased with increasing the height of micropillars (h) within a certain range, and the FBAR-micropillars device displayed nonlinear frequency response, which was opposite to the linear response of conventional FBAR devices. In addition, a positive resonant frequency shift was captured near the “coupled resonant point” of the FBAR-micropillars device. The geometric parameters of micropillars, including micropillar diameter and micropillar spacing could also cause a change of Q-factor and mass sensitivity. The optimized design of the proposed device achieved a threefold improvement in sensitivity relative to conventional FBAR without pillars, suggesting a feasible method to improve the mass sensitivity of acoustic resonators.

Guided cell migration on a graded micropillar substrate

Cell migration is facilitated by the interaction of living cells and their local microenvironment. The local topography is one of the key factors regulating cell migration. Interaction between the surface topography and the cell behaviors is critical to understanding tissue development and regeneration. In this study, a dynamic mask photolithography technique has been utilized to fabricate a surface with graded micropillars. It has been demonstrated that the cells have been successfully guided to migrate from the sparse zone to the dense zone. The cell polarization angle has been characterized in both sparse zone and the dense zone. Compared to the dense zone, the cells in the sparse zone are more aligned along the direction of the micropillar spacing gradient, which enables the guided cell migration. Moreover, the effects of the micropillar spacing gradient, micropillar diameter, and micropillar height have been investigated in terms of the cell migration speed and cell spreading area. Finally, two issues significantly affecting the cell migration have been discussed: trapped cells between the micropillars and cell clusters.

Integration of Mesenchymal Stem Cells into a Novel Micropillar Confinement Assay.

Mechanical cues such as stiffness have been shown to influence cell gene expression, protein expression, and cell behaviors critical for tissue engineering. The mechanical cue of confinement is also a pervasive parameter affecting cells in vivo and in tissue-engineered constructs. Despite its prevalence, the mechanical cue of confinement lacks assays that provide precise control over the degree of confinement induced on cells, yield a large sample size, enable long-term culture, and enable easy visualization of cells over time. In this study, we developed a process to systematically confine cells using micropillar arrays. Using photolithography and polydimethylsiloxane (PDMS) molding, we created PDMS arrays of micropillars that were 5, 10, 20, or 50 μm in spacing and between 13 and 17 μm in height. The tops of micropillars were coated with Pluronic F127 to inhibit cell adhesion, and we observed that mesenchymal stem cells (MSCs) robustly infiltrated into the micropillar arrays. MSC and nucleus morphology were altered by narrowing the micropillar spacing, and cytoskeletal elements within MSCs appeared to become more diffuse with increasing confinement. Specifically, MSCs exhibited a ring of actin around their periphery and punctate focal adhesions. MSC migration speed was reduced by narrowing micropillar spacing, and distinct migration behaviors of MSCs emerged in the presence of micropillars. MSCs continued to proliferate within micropillar arrays after 3 weeks in culture, displaying our assay's capability for long-term studies. Our assay also has the capacity to provide adequate cell numbers for quantitative assays to investigate the effect of confinement on gene and protein expression. Through deeper understanding of cell mechanotransduction in the context of confinement, we can modify tissue-engineered constructs to be optimal for a given purpose. Impact Statement In this study, we developed a novel process to systematically confine cells using micropillar arrays. Our assay provides insight into cell behavior in response to mechanical confinement. Through deeper understanding of how cells sense and respond to confinement, we can fine tune tissue-engineered constructs to be optimal for a given purpose. By combining confinement with other physical cues, we can harness mechanical properties to encourage or inhibit cell migration, direct cells down a particular lineage, induce cell secretion of specific cytokines or extracellular vesicles, and ultimately direct cells to behave in a way conducive to tissue engineering.

Effect of Micropillar Spacing and Temperature of Substrate on Contact Angle Dynamics

ABSTRACTContact angle dynamics of droplets deposited on a structured surface were studied in this work and the effects of substrate microstructure and temperature were investigated. Microstructures consisting of uniformly-sized, cubic micropillars with varying pillar spacings were constructed by microfabrication. Droplets (of the order of tens of microlitres in volume) were deposited on these surfaces and dynamic contact angles were observed using various techniques. Advancing and receding contact angles were measured using tilting of the surfaces or by injection and aspiration of fluid from a horizontal droplet by syringe. Droplets on these surfaces appeared to be mainly in the Wenzel state. Contact angle hysteresis was obtained as a function of pillar spacing or, equivalently, surface roughness. Depinning force was deduced and a linear dependence on maximal three phase contact line was found. The techniques of tilting the surface on which the droplet was deposited and uniformly increasing and reducing the volume of the droplet via the syringe both gave the same contact angle hysteresis for a given micropillar spacing. The effect of temperature was then assessed using a heated tilting plate. Contact angle hysteresis was found to increase with temperature. Further work to elucidate mechanisms governing this dependence will be undertaken.

How microstructures affect air film dynamics prior to drop impact.

When a drop impacts a surface, a dimple can be formed due to the increased air pressure beneath the drop before it wets the surface. We employ a high-speed color interferometry technique to measure the evolution of the air layer profiles under millimeter-sized drops impacting hydrophobic micropatterned surfaces for impact velocities of typically 0.4 m s(-1). We account for the impact phenomena and show the influence of the micropillar spacing and height on the air layer profiles. A decrease in pillar spacing increases the height of the air dimple below the impacting drop. Before complete wetting, when the impacting drop only wets the top of the pillars, the air-droplet interface deforms in between the pillars. For large pillar heights the deformation is larger, but the dimple height is hardly influenced.

Wetting behavior on hybrid surfaces with hydrophobic and hydrophilic properties

Hybrid surfaces consisting of a micropillar array of hydrophobic and hydrophilic sites were designed and fabricated to understand the effects of their unique surface morphology and chemistry on droplet condensation. Droplet impingement experiments have revealed that hybrid surfaces exhibit high contact angles, which is characteristic of purely hydrophobic surfaces. However, little is known about the wetting behavior of droplets that nucleate and grow on hybrid surfaces during condensation. In fact, condensed droplets display a distinct wetting behavior during the droplet growth phase which cannot be reproduced by simply impinging droplets on hybrid surfaces. In this study, hybrid surfaces with three different spacing ratios were subjected to condensation tests using an environmental scanning electron microscopy (ESEM) and a condensation cell under ambient conditions. For hybrid surfaces with spacing ratio below 2, droplets were observed to form on top and sides of the micropillars, where they grew, coalesced with adjacent droplets, and shed after reaching a given size. After shedding, the top surface remained partially dry, which allowed for immediate droplet growth. For hybrid surfaces with spacing ratio equal to 2, a different wetting behavior was observed, where droplets basically coalesced and formed a thin liquid film which was ultimately driven into the valleys of the microstructure. The liquid shedding process led to the renucleation of droplets primarily on top of the dry hydrophilic sites. To better understand the nature of droplet wetting on hybrid surfaces, a surface energy-based model was developed to predict the transition between the two observed wetting behaviors at different spacing ratios. The experimental and analytical results indicate that micropillar spacing ratio is the key factor for promoting different wetting behavior of condensed droplets on hybrid surfaces.

Effects of the Hierarchical Structure of Rough Solid Surfaces on the Wetting of Microdroplets

We used the lattice Boltzmann method to investigate how the hierarchical structure of a rough solid surface, which in this work is modeled as the microstructure (micropillars) covered with nanostructures (nanopillars), affects the contact angle of microdroplets atop of the solid surface and the wetting transition between the Wenzel and Cassie states. Our simulation results show that the Wenzel-to-Cassie state transition can be achieved by decreasing the fluid-solid attraction, increasing the micropillar spacing, or coating the microstructures with nanostructures. For the effect of the hierarchical structure on the contact angle, we find that the micropillars show a negligible effect on the contact angle, but they may affect the sliding angle. In contrast, it is the nanostructure that determines the contact angle. The contact angle increases with the nanopillar length until reaching a maximal value, but its dependence on the nanopillar spacing becomes more complicated. The contact angle may first increase with the nanopillar spacing and then decreases, or decreases monotonously, depending on whether the liquid enters the nanostructure or not. In this work, we also demonstrate in the presence of contact line pinning, that the pinning effect affects the apparent contact angle.

Droplet contact angle behavior on a hybrid surface with hydrophobic and hydrophilic properties

A hybrid surface consisting of an array of hydrophobic and hydrophilic sites was designed and fabricated in an effort to better understand the effects of microscale surface features and chemistry on wettability. A model based on energy minimization was developed to design and predict the wettability of hybrid surfaces. Measured advancing, receding, and equilibrium contact angles fit the proposed model well. Experiments show that a higher degree of hydrophobicity results in higher contact angles and that contact angle hysteresis increases with decreasing micropillar spacing (b/a). Moreover, measured roll-off angle as an indicator of droplet shedding, decreases with b/a.

Effect of micropillar electrode spacing on the performance of electrohydrodynamic micropumps

Electrohydrodynamic (EHD) micropumps with three-dimensional 50 μm × 50 μm micropillar electrodes were fabricated and tested in this study. Two basic electrode configurations were investigated: (i) micropillar emitter and collector electrodes (symmetric) and (ii) micropillar emitter and planar collector electrodes (asymmetric). The micropumps were fabricated by integrating chromium/gold planar electrodes with electroplated 3-D Nickel micropillars on a glass substrate with a 100 μm high PDMS microchannel. The effect of the spanwise micropillar spacing on the pump performance was determined. The pumps were tested using HFE-7100 as the working fluid for the maximum pressure generation under a no flow condition. The micropumps with the asymmetric electrode design generated a significantly higher pressure head than the corresponding micropumps with symmetric electrode configuration for the same applied voltage, with lower power consumption. A decrease in the spanwise spacing of the micropillar electrodes increased the pump performance for the symmetric configuration, while the performance decreased for the asymmetric configuration.

Nanoscale Patterning of Microtextured Surfaces to Control Superhydrophobic Robustness

Most naturally existing superhydrophobic surfaces have a dual roughness structure where the entire microtextured area is covered with nanoscale roughness. Despite numerous studies aiming to mimic the biological surfaces, there is a lack of understanding of the role of the nanostructure covering the entire surface. Here we measure and compare the nonwetting behavior of microscopically rough surfaces by changing the coverage of nanoroughness imposed on them. We test the surfaces covered with micropillars, with nanopillars, with partially dual roughness (where micropillar tops are decorated with nanopillars), and with entirely dual roughness and a real lotus leaf surface. It is found that the superhydrophobic robustness of the surface with entirely dual roughness, with respect to the increased liquid pressure caused by the drop evaporation and with respect to the sagging of the liquid meniscus due to increased micropillar spacing, is greatly enhanced compared to that of other surfaces. This is attributed to the nanoroughness on the pillar bases that keeps the bottom surface highly water-repellent. In particular, when a drop sits on the entirely dual surface with a very low micropillar density, the dramatic loss of hydrophobicity is prevented because a novel wetting state is achieved where the drop wets the micropillars while supported by the tips of the basal nanopillars.

Wrinkled, Dual-Scale Structures of Diamond-Like Carbon (DLC) for Superhydrophobicity

We present a simple two-step method to fabricate dual-scale superhydrophobic surfaces by using replica molding of poly(dimethylsiloxane) (PDMS) micropillars, followed by deposition of a thin, hard coating layer of a SiO(x)-incorporated diamond-like carbon (DLC). The resulting surface consists of microscale PDMS pillars covered by nanoscale wrinkles that are induced by residual compressive stress of the DLC coating and a difference in elastic moduli between DLC and PDMS without any external stretching or thermal contraction on the PDMS substrate. We show that the surface exhibits superhydrophobic properties with a static contact angle over 160 degrees for micropillar spacing ratios (interpillar gap divided by diameter) less than 4. A transition of the wetting angle to approximately 130 degrees occurs for larger spacing ratios, changing the wetting from a Cassie-Cassie state (C(m)-C(n)) to a Wenzel-Cassie state (W(m)-C(n)), where m and n denote micro- and nanoscale roughness, respectively. The robust superhydrophobicity of the Cassie-Cassie state is attributed to stability of the Cassie state on the nanoscale wrinkle structures of the hydrophobic DLC coating, which is further explained by a simple mathematical theory on wetting states with decoupling of nano- and microscale roughness in dual scale structures.