Pillar Surface Research Articles

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.

The Combined Effect of Substrate Stiffness and Surface Topography on Chondrogenic Differentiation of Mesenchymal Stem Cells.

Stem cell differentiation is guided by contact with the physical microenvironment, influence by both topography and mechanical properties of the matrix. In this study, the combined effect of substratum nano-topography and mechanical stiffness in directing mesenchymal stem cell (MSC) chondrogenesis was investigated. Three polyesters of varying stiffness were thermally imprinted to create nano-grating or pillar patterns of the same dimension. The surface of the nano-patterned substrate was coated with chondroitin sulfate (CS) to provide an even surface chemistry, with cell-adhesive and chondro-inductive properties, across all polymeric substrates. The surface characteristic, mechanical modulus, and degradation of the CS-coated patterned polymeric substrates were analyzed. The cell morphology adopted on the nano-topographic surfaces were accounted by F-actin distribution, and correlated to the cell proliferation and chondrogenic differentiation outcomes. Results show that substratum stiffness and topographical cues affected MSC morphology and aggregation, and influenced the phenotypic development at the earlier stage of chondrogenic differentiation. Hyaline-like cartilage with middle/deep zone cartilage characteristics was generated on softer pillar surface, while on stiffer nano-pillar material MSCs showed potential to generate constituents of hyaline/fibro/hypertrophic cartilage. Fibro/superficial zone-like cartilage could be derived from nano-grating of softer stiffness, while stiffer nano-grating resulted in insignificant chondrogenesis. This study demonstrates the possibility of refining the phenotype of cartilage generated from MSCs by manipulating surface topography and material stiffness.

Investigation on Ge surface diffusion via growing Ge quantum dots on top of Si pillars

We report on a simple and intuitionistic experimental method to quantitatively measure surface diffusion lengths of Ge adatoms on Si(001) substrates and its activation energy Ea, which is achieved by growing Ge quantum dots (QDs) on top surfaces of Si pillars with different radii and taking an advantage of preferential nucleation and growth of Ge QDs at the top surface edge of the pillars. Diffusion length of Ge adatom can directly be measured and determined by the radius of the pillar below which no QDs will nucleate and grow at the central region of the top surface of the Si pillar. With a growth rate v fixed at 0.1 Å/s, by changing the growth temperature, the diffusion lengths at different temperatures would be obtained. Arrhenius plot of diffusion length as a function of growth temperature gives the value of Ea of 1.37 eV. Likewise, with a growth rate v fixed at 0.05 Å/s, the Ea value is obtained to be 1.38 eV. Two Ea values agree well with each other, implying that the method is reliable and self-consistent. Moreover, for a fixed growth temperature, the surface diffusion lengths are found to be directly proportional to 1/ν. It also agrees well with the theoretical prediction, further demonstrating the reliability of the method.

Water condensation on ultrahydrophobic flexible micro pillar surface

We investigated the growth dynamics of water drops in controlled condensation on ultrahydrophobic geometrically patterned polydimethylsiloxane (PDMS) cylindrical micro pillars. At the beginning, the condensed drops size is comparable to the pattern dimensions. The interesting phenomenon we observe is that, as the condensation progresses, water drops between the pillars become unstable and enforced to grow in the upward direction along the pillars surface. The capillary force of these drops is of the order of and acts on neighboring pillars. That results into bending of the pillars. Pillars bending enhances the condensation and favors the most energetically stable Wenzel state.

FIB-induced dislocations in Al submicron pillars: Annihilation by thermal annealing and effects on deformation behavior

In-situ transmission electron microscopy (TEM) annealing of submicron Al pillars (∼300–450 nm in diameter) fabricated by focused ion beam (FIB) shows that the dislocation loops formed by the high energy Ga+ ion beam impact near the material surface can be removed by annealing at around half of the melting point (Tm). Quantitative analysis of real-time TEM data reveals that the dislocation loops first show a ripening behavior at around 0.4Tm, i.e. larger loops coarsen at the expense of smaller ones. Subsequently, the ripened loops start to shrink and are eventually annihilated at ∼0.5Tm by the diffusion of thermally activated vacancies. Microcompression tests on the as-fabricated and annealed pillars suggest that the FIB-induced defects, particularly dislocation loops, distinctively affect the deformation behavior of submicron Al pillars; while the yield and flow stresses appear unaffected by the annealing, strain bursts are larger and more frequent in the annealed pillars compared to those of as-fabricated samples. In-situ TEM compression revealed that the initial plastic deformation and subsequent plastic flow of Al pillars are significantly altered by the presence of FIB-induced dislocation loops, as they actively respond to the applied stress. We observe that the initial dislocation activity in most FIB-prepared pillars was the glide of FIB-induced dislocation loops. With further straining, the dislocation loops escaped the pillar, leaving slip steps at the pillar surface and/or dislocation debris within the pillar volume via the interaction with other mobile dislocations. The subsequent dislocation slip is mostly localized at these locations, thereby forming large slip steps. Contrarily, in the case of annealed pillars, the deformation was rather homogeneous with the formation of multiple fine slip steps. The present direct TEM observations contribute to the understanding of the nature of FIB-induced defects and reveal their distinct roles in the deformation behaviors of submicron pillars.

Boundary friction force of tree frog’s toe pads and bio-inspired hexagon pillar surface

In our daily life, there is growing use of polymer soft materials, such as rubber and silica gel. In some circumstances, their wet anti-skidding property plays an important role, e.g., rubber tires on car, windscreen wiper and rubber gloves. Its friction under the wet environment is difficult to study since the interaction effect of fluid, capillary effect and soft solid coupling together. As millions years of evolution, creatures have evolved some special soft skins to attach to all kinds of surfaces. Inspired from mother nature, as the toe pad of the tree frog can greatly attach to wet surface, it was used to study the wet friction theory of soft materials. The structure of its toe pad has been characterized by SEM images. The toe surface was constructed by arrayed pillars with a height of 5 μ m, circumcircle diameter of 10 μ m and gap of 1 μ m. About half of these pillars was hexagon, and the rest was pentagon, heptagon, and quadrangle. The friction of toe pads has been tested with consecutive sliding steps on dry substrates. Its friction increases with secretion decreasing and boundary friction formed just like the friction tests on human skin. Under the inspiration of tree frog’s toe pads, we designed and fabricated a bio-inspired soft surface (PDMS) with hexagon pillar array. With liquid between contact area decreasing from wet to dry condition, its friction shows a peak value, which is similar to the friction of tree frog’s toe pads. During boundary friction condition, the value can be 50 times higher than dry friction. The liquid between contact interface forms strong capillary force to rise friction as it can be regarded as extra normal force. Comparing to smooth surface, bio-inspired surface can maintain stable and long term boundary friction since the micro-pillars can separately deform to steady friction and the channels can hold more liquid than smooth surface. As hydrophilic hexagon pillar surface can quickly spread liquid through channels to create uniform thin liquid films between contact interfaces, it exhibits much better boundary friction property than hydrophobic one, whose boundary friction is almost 0 mN. By comparing the boundary and dry friction of four type of surfaces under different weight, the four dry friction coefficients display no change as common materials. However, the hydrophilic surface demonstrated the high boundary friction coefficient with little weight. This results from that capillary force between contact interfaces almost stays the same and only plays a great role when the external pressure between interfaces was small. With external normal force increasing, the affection of normal pressure induced by liquid capillary force will be weaken. The boundary of hexagon pillar surface is also different when sliding in the hexagon angle direction and side direction. This was in consistent with the distribution of tree frog’s toe pad channels. This study explains the mechanism of tree frog’s wet attachment ability. It also further expands the understanding of wet friction mechanism in soft materials. It will be useful for the designing of stronger wet attachable surface or preventing the stick-slide effect on linear guide way.

Prediction and Characterization of Dry-out Heat Flux in Micropillar Wick Structures.

Thin-film evaporation in wick structures for cooling high-performance electronic devices is attractive because it harnesses the latent heat of vaporization and does not require external pumping. However, optimizing the wick structures to increase the dry-out heat flux is challenging due to the complexities in modeling the liquid-vapor interface and the flow through the wick structures. In this work, we developed a model for thin-film evaporation from micropillar array wick structures and validated the model with experiments. The model numerically simulates liquid velocity, pressure, and meniscus curvature along the wicking direction by conservation of mass, momentum, and energy based on a finite volume approach. Specifically, the three-dimensional meniscus shape, which varies along the wicking direction with the local liquid pressure, is accurately captured by a force balance using the Young-Laplace equation. The dry-out condition is determined when the minimum contact angle on the pillar surface reaches the receding contact angle as the applied heat flux increases. With this model, we predict the dry-out heat flux on various micropillar structure geometries (diameter, pitch, and height) in the length scale range of 1-100 μm and discuss the optimal geometries to maximize the dry-out heat flux. We also performed detailed experiments to validate the model predictions, which show good agreement. This work provides insights into the role of surface structures in thin-film evaporation and offers important design guidelines for enhanced thermal management of high-performance electronic devices.

Coal pillar safety and surface deformation characteristics of wide strip pillar mining in deep mine

The wide strip pillar mining of Tangkou coal mine in Eastern China is the engineering background in the paper. Coal pillar safety and surface deformation characteristics of wide strip pillar mining in deep mine are studied by using in situ measurement, three-dimensional similar material simulation test, and theoretical analysis. Results show that in deep mine, the plastic zone’s width of coal pillar becomes larger obviously and the vertical stress of coal pillar presents a saddle-shaped distribution. The lateral deformation of coal pillar mainly concentrates on the area 9.5 m from the edge of coal pillar, which is discontinuous, stepped, and mutational. In addition, if only one working face with strip pillar mining is mined, the influence extent caused by coal mining is weaker and the surface subsidence and curvature are smaller. If multiple working faces with strip pillar mining are mined, the surface subsidence is obvious, the subsidence factor is 0.22, the subsidence basin is larger but gentler and uniform, and the surface deformation is continuous and long term. On the basis of the characteristics of wide strip pillar mining in deep mine, the safety of wide coal pillar in deep mine is checked.

Visualization of the Evaporating Liquid-Vapor Interface in Micropillar Arrays

We captured interesting static and dynamic behavior of the liquid-vapor interface in well-defined silicon micropillar arrays during thermally driven evaporation of water from the microstructured surface. The 3-D shape of the meniscus was characterized via laser interferometry where bright and dark fringes result from the interference of incident and reflected monochromatic light due to a variable thickness thin liquid film (FIG. 1). During steady state evaporation experiments, water was supplied to the sample with a syringe pump at 10 μL/min. FIG. 2a and 2b show a SEM image of a typical fabricated micropillar array and a schematic of the experimental setup, respectively. When water wicks through the micropillar array, the meniscus in a unit cell (four pillars in FIG. 1) assumes an equilibrium shape depending on the location from the liquid source/reservoir and the ambient conditions (ambient evaporation at Qin = 0 W). At this point, the meniscus is pinned at the top of the pillars. As the evaporation rate increases due the applied heat flux, the meniscus increases in curvature, thus increasing the capillary pressure to sustain the higher evaporation rate. This is evidenced by the increasing number of fringes in the unit cell when Qin is increased (0 W, 0.11 W, 0.44 W, and 0.99 W, FIG. 1a-1d respectively). Beyond a maximum curvature, the meniscus de-pins from the pillar top surface and recedes within the unit cell. This occurs when the capillary pressure generated at this curvature, cannot balance the viscous loss resulting from flow through the micropillar array. We observed that this receding shape was independent of the applied heat, and only depended on the micropillar array geometry and the intrinsic wettability of the material. Representative meniscus profiles along the diagonal direction of the unit cell obtained from image analysis of FIG. 1 at various Qin are shown in FIG. 2c.

The effect of eigenstrain induced by ion beam damage on the apparent strain relief in FIB-DIC residual stress evaluation

FIB milling using Ga ions is known to be accompanied by implantation, multiplication of material defects, material property modification (e.g. amorphisation), inelastic shrinking/swelling and residual stress generation. These processes affect the reliability of the micro-ring-core method for residual stress evaluation. Safe use of this technique requires formulating approaches that provide quantitative criteria of the method's validity. In the present study this task is accomplished by proposing a numerical model based on eigenstrain. Parametric simulations were performed to identify the extent to which the FIB-DIC micro-ring-core measurements are affected. As an example of a real and relevant material system, the procedure was applied to silicon material. The curvature of an AFM cantilever due to FIB damage was monitored, and the eigenstrain magnitude determined by matching the model to observations. Using the resulting eigenstrain profile, parametric analysis was performed in terms of the pillar radius, and the elastic strain field calculated at the pillar surface that is monitored in the FIB-DIC micro-ring-core method. An important property of the model is its versatility that allows it to be adapted to different milling conditions and geometries to determine the ultimate spatial resolution limits of the FIB-DIC method.

Effects of Pillar Height and Junction Depth on the Performance of Radially Doped Silicon Pillar Arrays for Solar Energy Applications

The effects of pillar height and junction depth on solar cell characteristics are investigated to provide design rules for arrays of such pillars in solar energy applications. Radially doped silicon pillar arrays are fabricated by deep reactive ion etching of silicon substrates followed by the introduction of dopant atoms by diffusion from a phosphorus oxide layer conformally deposited by low‐pressure chemical vapor deposition. Increasing the height of the pillars has led to doubling of the efficiency from 6% for flat substrates to 12% for 40 μm high pillars with a 900 nm junction depth because of an increase in the total junction area and lower optical reflection. For higher pillars, the current density and efficiency is decreased, which is attributed to the increasing presence of defect states at the surface introduced during the etching process. This effect can be counteracted by an Al2O3 passivation layer on the pillar surface. An optimum efficiency of 13% is found for a junction depth of 790 nm for 40 μm pillar height. At increased junction depths, the efficiency is decreased due to the ever thinner undoped core of the pillars, causing pillars with a large junction depth to become less efficient than flat silicon substrates.

SOFC Anode Based on YSZ Pillars

In anode of solid oxide fuel cells (SOFCs), yttria-stabilized zirconia (YSZ), Ni, pore are distributed, and oxygen ions, electrons, H2 and H2O gases are transported in each path. And the species chemically reacted each other at the boundaries (triple phase boundaries, TPBs). In the view point of tortuosity, aligned YSZ and Ni particles and TPB are promising structures for a low resistances of matter transporting and their chemical reaction. In our previous work [1], it was proposed and developed that Ni particles were aligned by magnetic field during the drying process after screen-printing Ni/YSZ paste. A second step of low-tortuosity path configuration is completely straight along the specie transporting direction. Because the nominal conductivity of oxygen ion in YSZ is the lowest among the specie (ion, electrons, gas) transportation conductivities, YSZ should be straight preferentially. In this study, we prototyped pillar pattern of YSZ, infiltrated Ni, and investigated the effect of pillar width, height, and surface area of pillar corresponding to TPB length on the generation performance. The YSZ pillars were fabricated by an excimer laser. The figure shows that the width were varied 5, 10, 20, 40 μm, and their trenches were fabricated deeper in wider pillar. The trenches of YSZ pattern were infiltrated by Ni particles. The finer pillar resulted in higher generation performance as also shown in the figure. Furthermore, the electrochemical phenomena was discussed by Lattice-Boltzmann Method simulation. [1] K. Nagato, N. Shikazono, A. Weber, D. Klotz, M. Nakao, E. Ivers-Tiffée, ECS Trans. 57, 1307-1311 (2013). Figure 1

Study of the wetting characteristics of water droplet on a heterogeneous pillared surface

We investigated the wetting characteristics of a water droplet on a heterogeneous pillared surface at the nano-scale including contact angle, molecule inflow percentage and density fields and compared them with the wetting characteristics of a water droplet on a homogeneous pillared surface. Molecular dynamics simulations were employed to analyze the wetting behavior of water droplets on surfaces with pillar structures by considering different potential energies including bond, angle, Lennard-Jones and Coulomb to calculate the interacting forces between water molecules and the surface. The heterogeneous surfaces considered had pillars with a different surface energy than the base surface. It was found that the difference in surface energy between the base surface and pillar had little effect on the hydrophobicity of the surface at low pillar heights. However cases with pillar heights over H = 4.24 A, the pillar surface energy has a larger effect on the molecule inflow percentage with the maximum differences in the range from 33.8% to 47.2% depending on the base surface energy. At a pillar height of H = 16.96 A, the pillar surface energy has a large effect on the contact angle of the water droplet with the maximum differences in the range from −26.1% to −40.62% depending on the base surface energy. There was a large variation in the contact angle of the droplet as the pillar height increased when there was a large difference in the surface energies between the base and the pillars.

Active control of the anisotropic wettability of the carbon fiber reinforced carbon and silicon carbide dual matrix composites (C/C–SiC)

Anisotropic wettability characterizes the surfaces of the carbon fiber reinforced carbon and silicon carbide dual matrix composites (C/C–SiC), because the surfaces are composed of different fiber endings. The aim of this investigation is to establish control over the anisotropic wettability on C/C–SiC surfaces. The influence of the groove density on the droplet contact angles (surface wettability) was investigated both on the circle fiber ending surfaces and on the pillar fiber ending surfaces. Based on the groove density, the 14 specimens were divided into 7 groups. Each group was composed of a circle ending surface and a pillar ending surface. Comparison of the droplet contact angles was also established within each group. The results showed that the droplet contact angles were decreased after laser treatment. The droplet contact angles also decrease with the increase of the groove density. The differential of the droplet contact angles within each group was narrowed after laser treatment. The hypothesized explanations were proposed for the observed phenomenon. Some fibers initially covered by the SiC matrix were re-exposed on the groove side wall. The variety of the fiber ending distribution on the groove side wall could possibly introduce a more efficient control over the surface wettability. This method can be extended to optimize the surface wettability and to eliminate the anisotropic effects of the fiber reinforced composites.

Enhancement of plasticity in Zr-based bulk metallic glasses electroplated with copper coatings

Electrodeposition was used to coat copper films on the surface of the BMG pillars (bulk metallic glasses) of Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit. 105) with the film thicknesses of 71.5 and 161.1 μm. The experimental results of the compression tests of the bare Vit. 105 pillars and the coated Vit. 105 pillars revealed that the copper costing increased the density of shear bands in the Vit. 105 pillars formed during the tests, resulting in the improvement of plasticity. The plastic strain was 6.1% for the coated pillars with a coating thickness of 161.1 μm, which is 3.59 times of 1.7% of the bare Vit. 105 pillars. The deformation of the copper films dissipated the strain energy and limited the propagation of shear bands, which led to the initiation and formation of multiple shear bands. The technique developed in this work provides an effective way to enhance the plasticity of BMGs at room temperature.

Effect of crystal orientation on the strengthening of iron micro pillars

Abstract Micro pillar compression testing has been performed on bcc iron single crystals to determine the mechanical properties and deformation mechanisms and their dependency on size and crystal orientation. Slip traces on the surface of post-compressed pillars were scrutinised to determine the operating slip system. The size dependency of the flow curves was investigated and the power law constants, K and n , and the orientation dependency of the resolved shear stresses were determined. The slip trace analysis revealed that the slip follows the { 110 } 〈 111 〉 family of slip systems. The size dependency of the flow stresses follows the power law relation where the power law exponent n is in general between 0.5–0.8 and depend on orientation and the strain. The power law coefficient K increases as a function of strain. The shape is similar for the different loading directions, but varies in magnitude. The shear stress resolved to the ( 1 ¯ 01 ) [ 111 ] slip system shows an orientation dependent relation. Size and strain have little effect on this relation.

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Hierarchical superhydrophobic/hydrophilic substrates based on nanospheres self-assembly onto micro-pillars

We report a novel superhydrophobic/hydrophilic substrate with micro-/nano-hierarchical structures by mimicking the lotus effect. Intrinsic hydrophobic polystyrene nanospheres or intrinsic hydrophilic silica nanospheres, via evaporation-induced self-assembly, are deposited on the surfaces of silicon pillars, including on tops as well as sidewalls. The obtained hierarchical structures with the polystyrene nanosphere deposition could amplify its intrinsic hydrophobicity, because gas interstices between both the nanospheres and micro-pillars jointly enhance the liquid-gas contact fraction significantly. Related theoretical analysis indicates that such structures could easily achieve an apparent contact angle (CA) of higher than 150°. In experiments, we measure the apparent CA of such kinds of hierarchical structures with the silicon pillars in different geometries, and find that the maximum value is up to 163.8°, with a 3.2° slide angle. The hierarchical structures with the silica nanosphere deposition could amplify its intrinsic hydrophilicity as well, because the double structures greatly increase the liquid–solid contact area. The corresponding experiment results show that the apparent CA can be as low as 7.6°.

Laboratory investigation of support mechanism of thin spray-on liner for pillar reinforcement

Thin spray-on liners (TSLs) are attracting attention as effective rock support reinforcement in underground mines. They have the potential to increase roadway development rates and provide resistance at small rock surface displacements. To study the reinforcement provided by a TSL when applied onto a pillar surface, the support mechanism of TSL-coated rock samples was investigated, thereby providing a basis for studying the effect of TSL confinement on rock pillars. A polymeric material liner was applied to three types of rock (siltstone, sandstone and granite), and details of the sample preparation and loading procedures are presented. The results of rock failure tests indicate that significant strength improvement and enhancement of post-failure characteristics developed for the TSL-encapsulated samples. TSL reinforcement of the weaker rocks appears to be better than that of the stronger rocks. It is concluded that effective rock reinforcement occurs in the case when the tensile strength of the TSL material is greater than the tensile strength of the rock; the TSL reinforcement of stronger rock types may not be as effective. The test results obtained are consistent and conclusive.