Pillar Surface Research Articles

Microfluidic Pump Driven by Anisotropic Phoresis

Fluid flow along microchannels can be induced by keeping opposite walls at different temperatures, and placing elongated tilted pillars inside the channel. The driving force for this fluid motion arises from the anisotropic thermophoretic effect of the elongated pillars that generates a force parallel to the walls, and perpendicular to the temperature gradient. The force is not determined by the thermophilic or thermophobic character of the obstacle surface, but by the geometry and the thermophoretic anisotropy of the obstacle. Via mesoscale hydrodynamic simulations, we investigate the pumping properties of the device as a function of the channel geometry, and pillar surface properties. Applications as fluidic mixers, and fluid alternators are also outlined, together with the potential use of all these devices to harvest waste heat energy. Furthermore, similar devices can be also built employing diffusiophoresis or electrophoresis.

Effect of bumps on the stability of water droplets on the pillar surface

The effect of small bumps on the state of water droplets was investigated experimentally on the hydrophobic surfaces of pillars with a controlled size. The bump-introduced pillar surfaces were fabricated by the UV-imprinting technique on the SiO2 nanoparticle/resin composite and the partial removal of the resin by O2 etching. Water droplets showed a large contact angle on the pillar surfaces with the bumps. However, on the pillar surfaces without the bumps, the contact angle decreased with the increase in the distance between pillars. These results clearly indicate that the bumps stabilize hydrophobicity on the pillar surface.

Toward a Better Understanding of Hemiwicking: A Simple Model to Comprehensive Prediction.

The hemiwicking state has attracted much interest because of numerous important potential applications in inking, printing, boiling heat transfer, and condensation. However, the mechanism of the emergence of hemiwicking has not been well understood, especially the effects of geometry of patterned surfaces on the hemiwicking state has not been systematically investigated. Here, we presented a new method to study the critical conditions for hemiwicking on patterned surfaces. By minimizing the variation of the free energy, we obtain the corresponding stable height of the hemiwicking film and find that it is easier for a droplet to be in the hemiwicking state if the pillar surface has small spacing, large radius and height, and a small intrinsic contact angle. Our established model is applied to a flat-topped cylindrical pillar-patterned surface, and the modeling results are in well agreement with experiments and other existing theories. Besides, our model is also applied to other kinds of patterned surfaces including hemispherical-topped cylindrical and conical pillars, about which the other existing theories are deficient. Our theoretical results not only are in well agreement with the experimental observations but also provide some important predictions, which implies that the established model could be applicable to understanding the basic physical mechanism of the hemiwicking state and be useful in guiding the design and fabrication of hemiwicking surfaces.

3D lattice Boltzmann investigation of nucleation sites and dropwise-to-filmwise transition in the presence of a non-condensable gas on a biomimetic surface

Condensation in the presence of non-condensable gas on a biomimetic pillared subcooled surface with hybrid wettability (with hydrophilic top and hydrophobic side and bottom) is investigated using a newly developed 3D multi-component multiphase lattice Boltzmann model. Three preferred nucleation sites with different surface wettability contrasts are found: (i) at the corner of side and bottom hydrophobic surface; (ii) at the center of the bottom hydrophobic surface, and (iii) on the top hydrophilic surface of the pillar. Influencing factors, such as pillar geometrical parameters, subcooling degree and non-condensable gas concentration, on dropwise-to-filmwise condensation transition are examined. For dropwise condensation on a top hydrophilic pillar surface, the droplet undergoes a two-stage growth pattern from “changing contact line” to “constant contact line”. Non-condensable gas is found to aggregate near the condensing interface in the vapor phase and at corners of pillar side surface and bottom surface. Increasing pillar width or pillar height (with other geometric parameters remained unchanged), as well as decreasing degree of wall subcooling or non-condensable gas concentration, can delay transition from dropwise to filmwise condensation.

Effect of low air pressure on mechanical properties and shrinkage of concrete

Recently, it was observed that cracking on the surface of concrete bridge pillars is more significant in Chinese high-elevation regions, where the climatic character is low air pressure. Many regions in the world are located in low air pressure climate conditions; however, the influence of air pressure on concrete has rarely been studied before. In this paper, mechanical properties and shrinkage of concrete exposed to different air pressures and relative humidities for 1 year were studied. Effects of air pressure on developments of mechanical properties and shrinkage properties were analysed. A modified environmental coefficient for predicting shrinkage of concrete was proposed. Under lower air pressure, lower mechanical strength and greater shrinkage strain were observed, especially after 28 d. The value of the ratio of shrinkage strains was almost constant during the aging of C30 and C50 concretes. The ratio was larger when air pressure was lower. The modified environmental coefficient, taking account of air pressure, was used to predict concrete shrinkage. This coefficient can be used in existing models in place of the original relative humidity coefficient.

A study of surface subsidence and coal pillar safety for strip mining in a deep mine

Some villages and bridges are located on the ground surface of the working district no. 7 in the Wanglou Coal Mine. If longwall mining is adopted, the maximum deformation of the ground surface will exceed the safety value. Strip mining is employed for the working district no. 7 which is widely used to reduce surface subsidence and the consequent damage of buildings on the ground surface. To ensure the safety of coal pillars and improve the recovery coefficient, theoretical analysis and numerical simulation (FLAC 3D) were adopted to determine the coal pillar and mining widths and to discuss the coal pillar stress distribution and surface subsidence for different mining scenarios. The results revealed that the width of coal pillars should be larger than 162 m, and the optimized mining width varies from 150 to 260 m. As the coal seam is exploited, vertical stress is mainly applied on the coal pillar, inducing stress changes on its ribs. The coefficient of mining-induced stress varies from 2.02 to 2.62 for different mining scenarios. The maximum surface subsidence and horizontal movement increase as the mining width increases. However, when the mining width increases to a certain value, increasing the pillar width cannot significantly decrease the maximum subsidence. To ensure the surface subsidence less than 500 mm, the mining width should not be larger than 200 m. Considering the recovery coefficient and safety of the coal pillar, a pillar width of 165 m is suggested.

Micro-textured stainless steel material towards enhancement for adhesion of red blood cell

In this research, the stainless steel sheet (EN-1.4301) is selected to investigate the adhesive interaction of the RBC to the textured and non-textured surfaces. The sheet sample is equally divided into two sides, one is textured with pillar patterns and the other side remains non-textured as conventionally finished surface (smooth surface). The pillar patterns were fabricated by laser surface texturing (LST) technology with a picosecond laser. Atomic force microscopic (AFM) single-cell indentation tests are performed on the RBC to the counter-surfaces. Based on the experimental results, Force-Piezo Displacement curves under all three press distances (30, 50, 80%) of the RBC thickness, show strong adhesion signals of the RBC to pillar textured stainless steel surface. Compared to non-textured stainless steel surface, such adhesive behavior is not observed. From the perspective of contact mechanics, these phenomena can be explained based on the change of bending energy, interaction energy and surface contact area of cell membrane due to the micro-deformations among the contact regions of the RBC and pillar patterns. These findings expand our understanding of RBC adhesive behavior (possibly to all cell types) to the micro-textured counter-surface.

Fabrication of radially aligned electrospun nanofibers in a three-dimensional conical shape

Abstract This paper reports on the rapid fabrication of radially-aligned, three-dimensional conical structures by electrospinning. Three different polymers, Polyvinylpyrrolidone, Polystyrene and Polyacrylonitrile were used to electrospin the cones. These cone structures are spreading out from a vertical conductive pillar, which can be arbitrarily placed on specific part of the collector. The lower part of the cone is clearly defined on the collector, and the cone has a relatively uniform radius around the pillar. The cones are constituted of fibers that are radially aligned towards the top of the pillar, but there is no apex and the fibers fall flat on the top of the pillar surface. A parametric study has been performed to investigate the effects of the pillar morphology (height and thickness) and the electrospinning parameters (applied voltage and working distance) on the overall shape and size of the cone structure, as well as the fiber alignment. The pillar morphology influences directly the cone diameter and height. The electrospinning parameters have little effect on the cone structure. The formation mechanism has been identified to be related to the shape of the electric field, which has been systematically simulated to understand the effect of the electric field lines on the final dimensions of the cone structure.

Experimental investigation on enhancement of nucleate pool boiling heat transfer using hybrid wetting pillar surface at low heat fluxes

Hybrid wetting surfaces have the significant potential to enhance the performance of boiling heat transfer. In this paper, the hybrid wetting surfaces with pillars in millimeters are fabricated by the chemical deposition approach. Next the heat transfer coefficients for these fabricated surfaces are measured in nucleate pool boiling heat transfer experiment at low heat fluxes ranging from 3.75 to 18 W/cm2. Under the same geometric size and heating power conditions, it is found that the heat transfer coefficients of the hybrid wetting surfaces are higher than the values of the spatially uniform wetting surfaces, regardless of hybrid modes. Additionally, the enhancement performance of boiling heat transfer among the hybrid wetting surfaces with different geometric sizes are distinctly different. Finally, the orthogonal array experiments relating to influential factors including wetting modes and geometric factors of the pillar surfaces are implemented, and further reveal the effects of the combination of these various influential factors on the performance of nucleate pool boiling heat transfer.

Wetting characteristics of a water droplet on solid surfaces with various pillar surface fractions under different conditions

A numerical study was carried out using a molecular dynamics program to examine the wetting characteristics of nano-sized water droplets on surfaces with various pillar surface fractions under different conditions. Square-shaped pillars had surface fractions that increased from 11.1 % to 69.4 %. The pillars had 4 different heights and 3 different surface energies. When the pillar surface fraction changed, the contact angle of a water droplet also changed due to the attraction between the droplet and the pillar surface or the inner attraction of the water molecules. The pillar height also has different effects on the water droplet depending on the magnitude of surface energy.

Synthesis of plasma polymerized PEM with pillar surface structure and its fuel cell performance

Proton exchange membrane (PEM) is synthesized successfully by plasma polymerization technique in RF plasma discharge. The chemical composition of the prepared membranes is examined by FTIR analyses. SEM and AFM analysis of the membrane show the formation of a prominent pillar surface structure at high RF power. The pillar structure of the membrane also plays an indispensable role in the cell performance of the plasma polymerized PEM. The physical properties of the membrane developed under different plasma conditions are investigated in the range of RF power from 20 to 80 W. The membrane synthesized at 50W shows better cell performance that it develops 610mWcm−2 of power density and 0.68Acm−2 of current density at 0.6volts in fuel cell than other membranes synthesized at different plasma conditions.

Damage-Free Smooth-Sidewall InGaAs Nanopillar Array by Metal-Assisted Chemical Etching.

Producing densely packed high aspect ratio In0.53Ga0.47As nanostructures without surface damage is critical for beyond Si-CMOS nanoelectronic and optoelectronic devices. However, conventional dry etching methods are known to produce irreversible damage to III-V compound semiconductors because of the inherent high-energy ion-driven process. In this work, we demonstrate the realization of ordered, uniform, array-based In0.53Ga0.47As pillars with diameters as small as 200 nm using the damage-free metal-assisted chemical etching (MacEtch) technology combined with the post-MacEtch digital etching smoothing. The etching mechanism of InxGa1-xAs is explored through the characterization of pillar morphology and porosity as a function of etching condition and indium composition. The etching behavior of In0.53Ga0.47As, in contrast to higher bandgap semiconductors (e.g., Si or GaAs), can be interpreted by a Schottky barrier height model that dictates the etching mechanism constantly in the mass transport limited regime because of the low barrier height. A broader impact of this work relates to the complete elimination of surface roughness or porosity related defects, which can be prevalent byproducts of MacEtch, by post-MacEtch digital etching. Side-by-side comparison of the midgap interface state density and flat-band capacitance hysteresis of both the unprocessed planar and MacEtched pillar In0.53Ga0.47As metal-oxide-semiconductor capacitors further confirms that the surface of the resultant pillars is as smooth and defect-free as before etching. MacEtch combined with digital etching offers a simple, room-temperature, and low-cost method for the formation of high-quality In0.53Ga0.47As nanostructures that will potentially enable large-volume production of In0.53Ga0.47As-based devices including three-dimensional transistors and high-efficiency infrared photodetectors.

Spatial Control of Condensation on Chemically Homogeneous Pillar-Built Surfaces.

The random nature of dropwise condensation impedes spatial control hereof and its use for creating microdroplet arrays, yet here we demonstrate the spatial control of dropwise condensation on a chemically homogeneous pillar array surface, yielding ∼8000 droplets/mm2 under normal atmospheric pressure conditions. The studied pillar array surface is defined by photolithography and etched in silicon by deep reactive ion etching. Subsequently, the surface is covered with a self-assembled monolayer of perfluorodecyltrichlorosilane (FDTS) to render the surface hydrophobic. To obtain a perfect droplet array, with one droplet per pillar, we exploit a phenomenon where the water vapor flux is focused on the apexes of surface asperities by diffusion while matching the nucleation point density to the array dimensions. Matching is here achieved through the variation of interpillar distance and vapor flow conditions.

Following or Against Topographic Wettability Gradient: Movements of Droplets on a Micropatterned Surface.

Two kinds of possible movements of droplets (i.e., following or against the topographic wettability gradient) on a micropatterned surface are investigated by using a particle-based method: many-body dissipative particle dynamics (MDPD). The displacement along the wettability gradient and contact angles on both sides of the droplet are analyzed. The results show that the migration trajectory of the droplet is determined by the coexistence of Cassie and Wenzel states and the unbalanced Young's force, which are related to the impact velocity, pillar height, and surface tension. The droplet remains in the Cassie state and advances spontaneously following the wettability gradient under a small impact velocity, high pillar height, and large surface tension. On the contrary, when the coexistence of Cassie and Wenzel states appears, the contact line on the side of the Cassie state retracts and that on the side of the Wenzel state pins, inducing droplet movement against the wettability gradient. Additionally, the critical impinging velocity, which determines the migration direction of the droplet, also depends on the pillar height and surface tension. The outcomes are helpful in designing surfaces with topographical wettability gradients for droplet transportation.

Surface wetting of the C/SiC brake lining with micro-scale heat dissipation fins to cool off the brake system: Influence of the fibre ending orientation and fin interval

Abstract The micro-scale heat dissipation fins significantly contribute to cool off a brake system. However, micro-scale heat dissipation fins will change the surface wetting and then change the humidity of components in the brake system. A higher humidity is helpful for improving the thermal conductivity coefficient of the C/SiC component, which aids in further improving the cooling performance. To the best knowledge of the authors, little attention has been devoted to improving the humidity of the C/SiC brake lining by micro-scale fins. The aim of this study is mainly to discuss the surface wetting of the porous C/SiC brake lining with micro-scale heat dissipation fins to facilitate heat dissipation in the brake system by increasing the humidity. In this study, micro-scale heat dissipation fins with various intervals were fabricated by a laser on three typical C/SiC surfaces. Surface wetting was characterized by the spreading time of the water droplets. The theoretical model of the water droplet spreading time was established by a Washburn-type equation. Both the experimental and theoretical results indicated that: (1) a hydrophilic C/SiC surface could be achieved by fabricating micro-scale heat dissipation fins on a circular fibre-ending surface compared with a pillar fibre-ending surface; (2) wetting control of the C/SiC surface is not obvious by changing of the micro-fin interval on the order of micrometers; and (3) the surface wetting of the pillar fibre-ending C/SiC surface was more sensitive to increased repetitions of laser scanning. In the current stage, the experimental results presented a stochastic surface wetting. In this regard, more details on the irregularity of micro-scale heat dissipation fins resulting from a laser process are discussed. The conclusions can be extended to optimize the heat dissipation fin arrangement of the C/SiC brake lining and optimize the overall cooling performance of the brake system.

Nanopillar Surface Topology Promotes Cardiomyocyte Differentiation through Cofilin-Mediated Cytoskeleton Rearrangement.

Nanoscaled surface patterning is an emerging potential method of directing the fate of stem cells. We adopted nanoscaled pillar gradient patterned cell culture plates with three diameter gradients [280-360 (GP 280/360), 200-280 (GP 200/280), and 120-200 nm (GP 120/200)] and investigated their cell fate-modifying effect on multipotent fetal liver kinase 1-positive mesodermal precursor cells (Flk1+ MPCs) derived from embryonic stem cells. We observed increased cell proliferation and colony formation of the Flk1+ MPCs on the nanopattern plates. Interestingly, the 200-280 nm-sized (GP 200/280) pillar surface dramatically increased cardiomyocyte differentiation and expression of the early cardiac marker gene Mesp1. The gradient nanopattern surface-induced cardiomyocytes had cardiac sarcomeres with mature cardiac gene expression. We observed Vinculin and p-Cofilin-mediated cytoskeleton reorganization during this process. In summary, the gradient nanopattern surface with 200-280 nm-sized pillars enhanced cardiomyocyte differentiation in Flk1+ MPCs.

Size-dependent plastic deformation of twinned nanopillars in body-centered cubic tungsten

Compared with face-centered cubic metals, twinned nanopillars in body-centered cubic (BCC) systems are much less explored partly due to the more complicated plastic deformation behavior and a lack of reliable interatomic potentials for the latter. In this paper, the fault energies predicted by two semi-empirical interatomic potentials in BCC tungsten (W) are first benchmarked against density functional theory calculations. Then, the more accurate potential is employed in large scale molecular dynamics simulations of tensile and compressive loading of twinned nanopillars in BCC W with different cross sectional shapes and sizes. A single crystal, a twinned crystal, and single crystalline nanopillars are also studied as references. Analyses of the stress-strain response and defect nucleation reveal a strong tension-compression asymmetry and a weak pillar size dependence in the yield strength. Under both tensile and compressive loading, plastic deformation in the twinned nanopillars is dominated by dislocation slip on {110} planes that are nucleated from the intersections between the twin boundary and the pillar surface. It is also found that the cross sectional shape of nanopillars affects the strength and the initial site of defect nucleation but not the overall stress-strain response and plastic deformation behavior.

Effect of argon plasma treatment on the output performance of triboelectric nanogenerator

Physical and chemical properties of the polymer surface play great roles in the output performance of triboelectric nanogenerator (TENG). Specific texture on the surface of polymer can enlarge the contact area and enhance the power output performance of TENG. In this paper, polydimethylsiloxane (PDMS) films with smooth and micro pillar arrays on the surface were prepared respectively. The surfaces were treated by argon plasma before testing their output performance. By changing treatment parameters such as treating time and plasma power, surfaces with different roughness and their relationship were achieved. The electrical output performances of the assembled TENG for each specimen showed that argon plasma treatment has a significant etching effect on the PDMS surface and greatly strengthen its output performance. The average surface roughness of PDMS film increases with the etching time from 5mins to 15 mins when the argon plasma power is 60W. Nevertheless, the average surface roughness is inversely proportional to the treatment time for the power of 90W. When treated with 90W and 5mins, many uniform micro pillars appeared on the both PDMS surface, and the output performance of the TENG for plasma treated smooth surface is 2.6 times larger than that before treatment. The output voltage increases from 42V to 72V, and the short circuit current increases from 4.2μA to 8.3μA after plasma treatment of the micro pillar array surface. However, this plasma treatment has time-efficient due to the hydrophobic recovery property of Ar plasma treated PDMS surface, both output voltage and short circuit current decrease significantly after 3 months.

Static wetting characteristics of micro-textured stainless steel surfaces under uniaxial loading condition

The wetting behaviour associated with the surface micro asperities is investigated on the groove and pillar textured SS304 solid surface which is widely used in the flight vehicles where the stresses are induced by uniaxial compressive loads for both positive and negative curvature. By varying the applied load on the groove-textured surfaces in the direction perpendicular to grooves, the positive curvature shows a decrease in static contact angle initially then increases for further increase in deflection; however, the negative curvature induced by the same applied load shows an opposite trend. At the same time, in the pillar-textured surface, the static contact angle decreases with increase in applied load for the positive curvature and the same wetting parameter shows an opposite trend for the negative curvature. This phenomenon is mainly attributed to the pinning behaviour of the three phase contact line of the liquid drop on these two surfaces.

Gas sensor based on ZnO film/silica pillars

Silica submicron pillars are used as substrate for Zinc oxide (ZnO) gas sensor for the first time. The submicron pillars with the large surface ratio can improve the gas sensing performance obviously. Silicon pillars are fabricated by cesium chloride (CsCl) self-assembly lithography and inductively coupled plasma dry etching as substrate, the ZnO film is deposited on the pillars surface by RF magnetron sputtering. With this method, the pillar based gas sensor has the higher gas response, the shorter response and recovery time than the planar one with different working temperatures, different gas concentrations. 300 °C is the best working temperature, for planar gas sensor, the gas response is 22.81 for 1520 ppm ethanol, the response time is 55 s and the recovery time is 169 s. While for the pillar based one, the gas response is 28.20, the response time is 51 s and the recovery time is 92 s.