Micropillar Surfaces Research Articles

Surface topography effects on dynamic behavior of water droplet over a micro-structured surface using an improved-VOF based lattice Boltzmann method

A volume of fluid-based lattice Boltzmann method (VOF-LBM) is developed, using the open-source parallel lattice Boltzmann solver library (Palabos) to study the wettability of solid surfaces and include the droplets’ dynamic behavior. This improves the accuracy and diminishes limitations on the density and viscosity ratios and the outcoming spurious velocities. An experimental study is performed, and the numerical results are verified through comparison with experimental data. Next, to design novel hydrophobic surfaces with pillar structures, the effect of different pillar patterns and topographies on the contact angle of water droplets and the droplet motion over an inclined surface are investigated. Hydrophobic micro-pillar surfaces are designed to enhance the ease of droplets movement on the slope. The effect of contact angle, wetted area, and surface energy on the sliding behavior of droplets is examined and presented. Results indicate that the new VOF-LBM method can simulate high density and viscosity ratios (1000 and 1.81×10-2 respectively), allowing real-world problems to be analyzed with appropriate accuracy. This reduces the spurious velocities produced at the fluids interface by 19 times compared to the Shan and Chen method. Results demonstrate that the contact angle of the water droplet on the surface with a contact angle of 115° can reach 160° by changing the surface topography. Also, a hydrophobic micro-pillar surface is designed in which the droplet moves nearly 12% faster than the flat surface.

Critical heat flux prediction model of pool boiling heat transfer on the micro-pillar surfaces

An pool boiling heat transfer experimental investigation was studied on the surfaces with micro-pillars using FC-72 as working fluid. The pool boiling experiment was conducted under the saturated and subcooled conditions (ΔTsub = 25 K). It can be found that the critical heat flux (CHF) of surfaces wtih micro-pillars is higher than that of smooth chip, and the surface area enhancement ratio and the arrangement of the micro-pillars have significant effects on the CHF. The replenished liquid velocity ul is proposed to evaluate the wicking effect. A new prediction model is established to predict the CHF of surfaces with micro-pillars considering the effect of replenished liquid velocity ul, vapor column radius rv and vapor column spacing λb. The results show that the new CHF prediction model can effectively reflect the impact of the micro-pillars on CHF, and the experimental data can be predicted with an error band of ± 10%, indicating a good predicted ability.

Unified descriptor for enhanced critical heat flux during pool boiling of hemi-wicking surfaces

Boiling heat transfer is dictated by interfacial phenomena at the three-phase contact line where vapor bubbles form on the surface. Structured surfaces have shown significant enhancement in critical heat flux (CHF) during pool boiling by tailoring interfacial phenomena. This CHF enhancement has been primarily explained by two structural effects: roughness, which extends the contact line length at the bubble base, and wickability, the ability to imbibe liquid through surface structures by capillary pumping. In this work, we show that CHF enhancement on structured surfaces cannot be described by roughness or wickability alone. This result was confirmed using systematically designed micropillar surfaces with controlled roughness and wickability. Further, we performed a scaling analysis and derived a unified descriptor, which represents the combined effects of thin film density and volumetric wicking rate. This unified descriptor shows a reasonable correlation with CHF values with our experiments and literature data. This work provides important insights in understanding the role of surface structures on CHF enhancement, thereby providing guidelines for the systematic design of surface structures for enhanced pool boiling heat transfer.

Condensation Frosting on Micropillar Surfaces - Effect of Microscale Roughness on Ice Propagation.

Microscale surface structures have been widely explored as a promising tool for antifreezing or frost avoidance on heat transfer surfaces. Despite studies of many surface feature designs, the mechanisms associated with condensation freezing and ice propagation on microstructured surfaces have yet to be thoroughly elucidated, espectially when it comes to quantitative understanding. In this work, condensation freezing on circular micropillar surfaces is investigated, with varying pillar spacing and height (layout/microscale roughness) but a constant pillar diameter. The pillar layout is found to have significant effects on both liquid nucleation and neighboring droplet interactions, as reflected by the condensation droplet distribution prior to soilidification and eventually the freezing front propagation area velocity. In general, nucleation is preferred on the pillar top rather than the bottom of the pillared surface unless there is a large distance between the pillars. Interactions between neighboring droplets solely on pillar tops (or bottom surfaces) can induce heterogeneity in the droplet distribution and slow freezing front propagation. Based on the roles the pillars play in nucleation, droplet coalescence, and ice bridging, four different condensation states are identified and related to the layout of the pillars, and the freezing front area propagation velocity is found to be different in each state. The findings provide a quantitative basis for designing antifreezing surfaces, applicable to a wide range of thermal systems.

Bubble dynamics and heat transfer performance on micro-pillars structured surfaces with various pillars heights

Micro-pillars structured surfaces have been found to enhance nucleate boiling significantly, as compared with smooth surfaces. In this study, a VOF-based (volume of fluid) numerical model is developed to investigate the single-bubble dynamics on micro-pillars structured surfaces in nucleate boiling, aiming to understand the effect of pillar height (h) on boiling enhancement. The model is solving by the commercial CFD (computational fluid dynamics) solver Fluent. The results show that increasing h modifies remarkably the bubble morphology. An interesting phenomenon is observed for larger h that the bubble bottom undergoes expansion, shrinkage, and re-expansion in gaps between micro-pillars in the growth stage, so that a mushroom-like bubble is generated, with a small root and a large head. The modified bubble morphology promotes the bubble departure, leading to a shorter departure time. Moreover, increasing h also enhances both evaporation rate in the microlayer and evaporation rate on the liquid-vapor interface, and thereby increasing bubble departure diameter. Thus, the shorter departure time and larger departure diameter are responsible for the enhanced boiling heat transfer on micro-pillar structured surfaces with larger h.

Enhancement of Magneto-Mechanical Actuation of Micropillar Arrays by Anisotropic Stress Distribution.

Magnetically active shape-reconfigurable microarrays undergo programmed actuation according to the arrangement of magnetic dipoles within the structures, achieving complex twisting and bending deformations. Cylindrical micropillars have been widely used to date, whose circular cross-sections lead to identical actuation regardless of the actuating direction. In this study, micropillars with triangular or rectangular cross-sections are designed and fabricated to introduce preferential actuation directions and explore the limits of their actuation. Using such structures, controlled liquid wetting is demonstrated on micropillar surfaces. Liquid droplets pinned on magnetic micropillar arrays undergo directional spreading when the pillars are actuated as depinning of the droplets is enabled only in certain directions. The enhanced deformation due to direction dependent magneto-mechanical actuation suggests that micropillar arrays can be fundamentally tailored to possess application specific responses and opens up opportunities to exploit more complex designs such as micropillars with polygonal cross sections. Such tunable wetting of liquids on microarray surfaces has potential to improve printing technologies via contactless reconfiguration of stamp geometry by magnetic field manipulation.

Experimental investigation of the characteristics of thermosyphon with flat evaporator and micro-pillar arrays

This study experimentally investigated the cascaded start-up performance of thermosyphon with a flat evaporating surface combined with different sizes of micro-pillars. The thermosyphon was composed of a condenser section and an evaporator section. Different sizes of micro-pillar were processed on the surface of the evaporator bottom plate to enhance the heat transfer. A smooth flat surface was used as the standard reference for comparison with the micro-pillar surfaces. Deionized water was used as the working fluid to cool the condenser section of the thermosyphon. The operating pressure of thermosyphon was measured using a pressure sensor. Additionally, the evaporator section wall temperature and the temperature and flow rate of the cooling water were measured at nine different heat fluxes. The start-up performances of the thermosyphon with a smooth evaporator bottom plate (THSE) and thermosyphon with a micro-pillar in the evaporator bottom plate (THMPE) were compared. The THSE had a longer start-up time and a higher start-up temperature than the THMPE. When the heat flux was 68.6 W/cm2, the start-up time of the THMPE with a micro-pillar width of 0.2 mm and a height of 0.8 mm was reduced by 57.1% compared to THSE, and the start-up temperature was reduced by 28%. Thermosyphons with different micro-pillar widths showed various start-up performances for different heat fluxes. For a narrow micro-pillar width, THMPEs with larger micro-pillar heights had a lower start-up temperature and a shorter start-up time. As the micro-pillar width increased, the micro-pillar height had a different and more complex effect on the start-up performance of the thermosyphon. When the micro-pillar heights were 0.2 mm and 0.4 mm, the thermosyphon with wider micro-pillars had a longer start-up time with a lower heat flux but a shorter start-up time with a higher heat flux. When the micro-pillar heights were 0.6 mm and 0.8 mm, varying the micro-pillar width had a smaller influence on the start-up performance of the thermosyphon, but was dependent on the heat flux.

Visualization-based nucleate pool boiling heat transfer enhancement on different sizes of square micropillar array surfaces

Visualization-based nucleate pool boiling experiments are carried out on different sizes of square micropillar array surfaces (SMAS) and plain surface using deionized water as working fluid at atmospheric pressure. The quantitative measurements of the bubble dynamics, including the nucleation site density, the bubble departure diameter and the frequency, are obtained using a high-speed camera with microscope. The obtained results show that the SMAS, with square micropillar in the range of 0.2–0.8 mm height and width (the pitch between the micropillar is equal to the micropillar width), has a considerably lower wall superheat, which enhanced the heat transfer coefficient (HTC) by 32–203% compared to the plain surface. Generally, the boiling heat transfer intensifies with the increase in the square micropillar height keeping the width constant. But the tendency reverses at high heat flux levels. The high-speed visualization reveals that the higher square micropillar has faster bubble departure frequency at low heat flux, because higher micropillar is conducive to induce capillary flow and aids in separating the paths of departing bubble and replenishment liquid. However, the increase of bubble diameter at high heat flux restrains bubble departure with higher square micropillar. In addition, the increase of square micropillar width is conducive to enhancing the boiling heat transfer and the tendency is reversed at high heat flux levels when the square micropillar height is kept constant. The bubble dynamics are compared with the predictions using various correlations from the literature.

Wetting Dynamics of a Water Droplet on Micropillar Surfaces with Radially Varying Pitches

The wetting dynamics of a sessile droplet on square micropillar substrates with radially varying pitches, prepared on silicon wafers using a photolithography technique, is investigated experimentally. Two configurations are considered, namely, substrates with radially increasing pitch and radially decreasing pitch. The droplet initially placed at the center experiences a wettability gradient because of the variation in pitch of the micropillar substrate leading to complex wetting dynamics. We observed that the droplet remains in the Cassie-Baxter state in the case of a radially increasing pitch and exhibits a higher contact angle than that on a smooth surface during its spreading stage. In contrast, the droplet experiences the Wenzel condition in the case of a radially decreasing pitch and assumes a lower contact angle relative to that observed on a smooth surface. The wetted diameter of the droplet in the radially decreasing pitch configuration is found to be smaller than that observed in the radially increasing pitch configuration. Our study also reveals that increasing the size of the pillars increases the wetted diameter of the droplet in both configurations. Theoretical models developed using the Cassie-Baxter and Wenzel states for the radially increasing and radially decreasing pitches satisfactorily predict the experimental behaviors.

Pool boiling heat transfer on silicon chips with non-uniform micro-pillars

This paper investigated the pool boiling heat transfer performance of non-uniform micro-pillar surfaces. Three kinds of uniform micro-pillar surfaces (named U-S, U-M and U-L) and two kinds of non-uniform micro-pillar surfaces (named R-SML and R-SM) were fabricated to be tested, and a smooth surface (named SS) was prepared as a comparison. Experiments were performed in FC-72 at the subcoolings of 0 K and 25 K in both horizontal and vertical orientations. The results showed that the heat transfer coefficient (HTC) and critical heat flux (CHF) of surfaces with micro-pillars have been greatly enhanced compared to those of the smooth surface. Active nucleation site radius was discussed to explain the advanced ONB (onset of nucleate boiling) of U-M, R-SML and R-SM, then SEM images were token to prove this theory. Boiling images of 6 W/cm2 at 25 K subcooling were token by a high-speed camera to analyze the HTC of different chips. Surface area enhancement ratio and wickability were used for the explanation of CHF comparison. The results showed that surface area enhancement ratio and wickability are not the decisive factors that influences CHF for FC-72, and the uniformity of microstructures should be considered when predicting the CHF. Non-uniform micro-pillars with small surface area enhancement ratio and wickability can have larger CHF than that of uniform micro-pillars with large surface area enhancement ratio and wickability.

Influence of Micropillar Induced Deformation on Chromatin Architecture in Regulating Stem Cell Differentiation

Surface topography induced nuclear deformation in stem cells has previously shown to influence differentiation outcomes. Mechanical forces resulting from topographical interventions get transduced into the nucleus through the cytoskeleton, causing changes in chromatin architecture and gene expression. However, their effect on genome architecture and gene expression in regulating stem cell differentiation is not well understood. Here, we study the influence of nuclear deformation induced by micropillar surfaces composed of methacrylated poly (1,8-octamethylene citrate) on chromatin architecture and gene expression during human mesenchymal stem cell differentiation (hMSC) towards osteogenic and adipogenic lineages. Differentiation assays reveal that these micropillars facilitate osteogenic differentiation, and attenuate adipogenic differentiation. It has been shown that changes in chromatin nanoarchitecture regulate global gene expression patterns and that an increase in chromatin packing scaling corresponds to an increase in the dynamic range and variations in gene expression. We hypothesized that micropillars enhance osteogenic differentiation by limiting the transcriptional accessibility of the initially plastic stem cells to explore their genomic landscape, primarily, by modulating their chromatin packing scaling. Using Partial Wave Spectroscopic (PWS) microscopy, we show that hMSCs cultured on micropillars have decreased chromatin packing scaling compared to those cultured on flat surfaces. Moreover, corresponding RNA-Seq studies show significant downregulation of genes associated with cell proliferation, and, of those involved in inhibition of osteogenic differentiation in MSCs seeded on micropillars compared to the flat surface. Therefore, by combining nanoscale-imaging experiments involving PWS and Electron Microscopy with enhanced chromatin contrast (ChromEM), and sequencing performed on stem cells induced with osteogenic and adipogenic medium, this study aims to understand the relationship between nuclear deformation induced by micropillars, chromatin architecture, and expression of differentiation-specific transcriptional programs.

Protective stainless steel micropillars for enhanced photocatalytic activity of TiO2 nanoparticles during wear

Enhanced photocatalytic activity of liquid flame spray (LFS) deposited TiO2 nanoparticles under tribological wear was investigated on metal injection molded (MIM) stainless steel substrates having micropillars as the load carrying support. A combination of LFS with MIM microtextures on the surface is a simple and cost-efficient way for manufacturing efficient photocatalytic substrates. Computer controlled microworking technique in combination with MIM was used to produce stainless steel micropillar surfaces that were functionalized by the LFS deposition of photocatalytically active TiO2 nanoparticles. The photocatalytic activity was measured in gas-phase with an in-house built photoreactor. Our results show that micropillars with controlled spacing we can control the surface area and increased photocatalytic activity of the micropillar substrate was observed compared to flat reference. The wear test confirmed that micropillars not only increase the surface area but they also provide protective support against wear whereas flat reference substrate lost the photocatalytic activity completely during wear.

Guided growth with aligned neurites in adult spiral ganglion neurons cultured in vitro on silicon micro-pillar substrates

Objective. Assessment of the relationship between the topographical organization of silicon micro-pillar surfaces (MPS) on guidance and neural alignment of adult spiral ganglion neurons (SGN) and use of the otosurgical approach as an alternative for the extraction and isolation of SGNs from adult guinea pigs. Approach. SGNs from adult guinea pigs were isolated using conventional and otosurgical approach for in vitro cell culturing on MPS of various micro-pillar widths (1–5.6 µm) and spacing (0.6–15 µm). Cell cultures were compared morphologically with neuronal cultures on control glass coverslips. Main results. We found enhanced SGN in vitro cultures in MPS areas with small and intermediate inter-pillar spacing (from 0.6 µm to 3.2 µm) as well as in MPS areas with wider pillars (from 1.8 µm to 4 µm) compared to MPS flat zones and control glass coverslips. Scanning electron microscopy (SEM) images highlighted how neurites of SGNs follow straight lines by growing on top and between micro-pillars. Only micro-pillars with small and intermediate pillar spacings favor neurite alignment along preferred angles (30°, 90°, and 150°), while pillars with wider spacing produced less aligned neurites. We found propensity of adult SGNs grown on MPSs to attain more bipolar and multipolar morphologies. Additionally, we observed reduced interaction between neuronal and glial cells compared to control glass coverslips. Finally, we found that the otosurgical approach was more beneficial for SGN survival on glass coverslips and MPS flat surfaces than the conventional method. Significance. MPS with specific architecture supports the guided growth of adult SGNs in vitro and controls adult SGN development and behavior.

Highly dispersed porous polydimethylsiloxane for boosting power-generating performance of triboelectric nanogenerators

One of the common approaches in improving the output performance of triboelectric nanogenerators (TENGs) is to increase triboelectric charge density by enlarging the surface area of triboelectric materials. Herein, highly porous polydimethylsiloxane (PDMS) introduced by nanograss silicon mold is developed to improve the output performance of PDMS-based TENGs in contact-separation mode. With an area of 100 cm2, the power generating performance of porous PDMS-based TENG (S1) is greatly enhanced and the maximum voltage of 3200 V and current of 94 μA are obtained. It also delivers a maximum power density of 0.58 W m−2 at a load resistance of 10 MΩ, offering almost three times power improvement compared with the TENGs with micro-pillar (S2) and flat (S3) PDMS surfaces. The boosting power-generating performance is mainly attributed to an increase surface roughness and additional triboelectric charges induced on the surfaces of the inner pores. Additionally, the S1 TENG is successfully used as a power source to directly drive 400 series-connected light emitting diodes (LEDs) to high brightness. This developed TENG also provides enough energy for 10 μF capacitor to power a wireless transmitter within 10 s. This work presents a new porous patterned PDMS-based TENG, which not only can be readily fabricated but also considerably delivers the high output performance toward the realization of self-powered and sustainable electronic devices.

Wetting transition on heated microstructrured superhydrophobic surface

Superhydrophobic surface has attracted significant attention since their potentiality to industrial and academic applications. Moreover, superhydrophobic surface wettability at non-ambient temperature, especially at high temperature (but not boiling) was of great importance in many industrial processes. In this paper, we designed and fabricated 4 series superhydrophobic micro-pillar surfaces on the Silicon wafers to investigate wettability at different temperatures. These micro-pillar surfaces were fabricated by photolithography and ICP etching technologies. The temperature-dependent wettability of DI water droplets was characterized using contact angle measurements. The wetting behavior was observed to be different on the surfaces, and the wetting transition occurred at a specific temperature.

Observation of sessile droplet freezing on textured micropillar surfaces via visualization and thermography

Sessile droplet freezing on textured micropillar surfaces was observed visually and thermographically. Hydrophobic surfaces with three different topographies were selected, the contact angles of water on which were 141°, 102°, and 138°. Droplet surface temperature distribution measurements from lateral view enabled quantitative evaluation of the freezing stages. According to the experiment, it was found that the surface hydrophobicity decreases with the increasing pillar distance. Only a slight increase in hydrophobicity was found with the decreasing pillar diameter. The liquid water droplet experienced five successive stages to become fully frozen. It was also confirmed that the droplet’s opaque appearance is caused by the sudden ice shell formation at the ice incipience. Besides, the solidification frontier (SF) movement inside the droplet was analyzed. The freezing rate was higher at the SF edge. Moreover, the textured surfaces with higher hydrophobicity were prone to further postpone the droplet’s freezing onset. However, surface topography showed minor impact on the droplet surface recalescence stage duration. Additionally, the droplet internal solidification stage lasted longer on samples with higher hydrophobicity.

Ultra-repellency of Al surfaces: design and evaluation

Aluminum (Al) surfaces with ultra-repellency as well as desirable robustness were designed and fabricated. With photolithographic patterning of a thick SU-8 layer and sputtering of a thin Al film, re-entrant micro-pillar textured Al surfaces were prepared. After derivatization with perfluoroalkyl phosphoric acid (FPA), the textured Al surfaces showed ultra-repellency for a wide variety of liquids. The contact angles (CAs) of deionized (DI) water, hexadecane and dodecane were larger than 150°, and those of methanol and ethanol were larger than 100°. The sliding angles (SAs) of DI water, hexadecane and dodecane were 5°, 10°, and 10°, respectively, showing excellent superamphiphobicity. The SAs of methanol and ethanol were in the range of 20°–30°. The robustness of the ultra-repellent Al surface was evaluated by three parameters: robust height (H*), robust angle (T*) and robust factor (A*). For the DI water probing, the values of the parameters are H* ≈ 403, T* ≈ 119 and A* ≈ 92, respectively, indicative of a desirable robustness. We clarified that only re-entrant structures can support composite liquid–solid–vapor interfaces when the corresponding Young’s CAs are smaller than 90°, and the function of the nanometer structures of the hierarchical textures which were widely adopted to fabricate superamphiphobic surfaces is to help construct re-entrant structures. FPA derivatization is effective in lowering the surface energy of Al surfaces, combining with re-entrant textures to provide a simple and high throughput approach to ultra-repellency for a wide variety of liquids.

Profile Characterization and Temperature Effect on the Wettability of Microstructured Surfaces

Wetting is one of the omnipresent phenomena governed via natural laws. Moreover, surface wettability at non-ambient temperature especially at high temperature (30°C to 90°C) is of great importance in many industrial processes. In this study, Si wafers with various structures were fabricated to investigate wettability at different temperatures. Three shapes with micro-pillar structured surfaces were designed and fabricated. Pillar-structured surfaces were fabricated by photolithography and ICP etching. The temperature-dependent wettability of single-phase regime droplets was characterized using contact angle measurements. The wetting behavior of a water droplet was observed.

An ultrasensitive micropillar-based quartz crystal microbalance device for real-time measurement of protein immobilization and protein-protein interaction

A new sensing device was developed to achieve ultrahigh sensitivity, by coupling polymer micropillars with a quartz crystal microbalance (QCM) substrate to form a two-degree- of-freedom resonance system (QCM-P). The sensitivity of these QCM-P devices was evaluated by measuring mass changes for both deposited gold film and adsorption of bovine serum albumin (BSA), respectively, on poly(methyl methacrylate) (PMMA) micropillar surfaces, as well as assessing ligand-analyte binding interactions between anti-human immunoglobulin G (anti-hIgG) and human immunoglobulin G (hIgG). The anti-hIgG and hIgG binding results show QCM-P achieved an eightfold improvement in sensitivity relative to conventional QCM sensors. In addition, the binding affinity obtained from the QCM-P device for anti-hIgG and hIgG proteins was found in good agreement with that measured by surface plasmon resonance (SPR) for the same binding reaction.

Isolation, Detection, and Antigen‐Based Profiling of Circulating Tumor Cells Using a Size‐Dictated Immunocapture Chip

AbstractEven though the diagnostic and prognostic value of circulating tumor cells (CTCs) has been demonstrated, their clinical utility and widespread adoption have been limited. Herein, we describe a new device, size‐dictated immunocapture chip (SDI‐Chip), for efficient, sensitive, and spatially resolved capture and detection of CTCs. SDI‐Chip enables selective, frequent, and extended interaction of CTCs with hydrodynamically optimized immunocoated micropillar surfaces. CTCs with different antigen expression levels can be efficiently captured and spatially resolved around the micropillars. Capture efficiency greater than 92 % with a purity of 82 % was achieved with blood samples. CTCs were detected in non‐metastasis colorectal (CRC) patients, while none was detected from healthy volunteers. We believe that SDI‐Chip will facilitate the transition of tumor diagnosis from anatomical pathology to molecular pathology in localized CRC patients.