Square Array Of Pillars Research Articles

Quasi-superhydrophobic microscale two-dimensional phononic crystals of stainless steel 304

Fabrication of metallic phononic and photonic crystals of characteristic size between 10 and 1000μm remains a challenge in precision using the conventional machining processes or too tedious for the cleanroom-based processes. We report the fabrication and elastodynamic bandgaps of two-dimensional phononic crystals (PhCs) machined on stainless steel 304 (SS304) substrates using the wire-electrochemical micromachining (wire-ECMM) process. Square arrays of pillars of length 400μm and cross section either 350×350μm2 or 250×250μm2 with periods 650 and 550μm, respectively, were micromachined on an SS304 homogeneous substrate. Based on these arrays, three types of PhCs were considered: air-SS304, water-SS304, and epoxy-SS304, where air, water, and epoxy are the hosts and SS304 pillars are the scatterers. We found that texturing the surface increased the contact angle of a 5-μl-water-droplet from 97.9° for an untextured SS304 substrate to a maximum of 145° for SS304 PhCs, making the latter quasi-superhydrophobic. Dispersion relations evaluated using the finite-element method revealed the presence of partial bandgaps in the 0.1–2.7 MHz for all PhCs and a complete bandgap for the epoxy-SS304 PhCs. Transmittance spectrums for incident plane waves also provided evidence for the occurrence of bandgaps. Furthermore, the buckling analysis indicated that these pillars do not undergo buckling until yield—making them mechanically robust.

Liquid film on a hydrophobic radome or roof top in rain

The water film due to rain falling on a radome surface causes severe losses in radio wave transmission. Hydrophobic coatings have been applied as a remedy to reduce the film thickness and to minimize the losses. However, quantitative accounts of the wave scattering are mostly based on empirical estimates of the film thickness. We describe a fluid-mechanical theory for the film under steady rain falling on a textured surface formed by a square array of pillars. Assuming the water surface on top of the pillars to be in the Cassie–Baxter state, the analysis is carried out by making use of the sharp contrast of length scales between the film thickness and the radome radius. The textured surface is viewed as a periodic array of cells around pillars. The macro-scale flow is simple and linear but the micro-scale flow in a typical lattice period is fully nonlinear. These two problems are coupled and are solved iteratively to obtain the slip length and the spatial variation of the film thickness. Numerical results are presented to show the effect of solid fraction on local flow field, the slip length and the non-uniform reduction of the film thickness. To examine the influence of the macro-scale geometry on film formation, the theory is also modified for a hydrophobic roof top formed by two inclined planes.

Interaction of Love waves with coupled cavity modes in a 2D holey phononic crystal

The interaction of Love waves with square array of pillars deposited on a cavity defined in a 2D holey phononic crystal is numerically investigated using Finite Element Method. First, the existence of SH surface modes is demonstrated separately for phononic crystals that consist of square arrayed holes, or rectangular arrayed Ni pillars, respectively in, or on, a SiO2 film deposited on a ST-cut quartz substrate. The coupling between SH modes and torsional mode in pillars induces a transmission dip that occurs at a frequency located in the range of the band-gap of the holey phononic crystal. Second, a cavity is constructed by removing lines of holes in the holey phononic crystal and results in a transmission peak that matches the dip. The optimal geometrical parameters enable us to create a coupling of the cavity mode and the localized pillar mode by introducing lines of pillars into the cavity, which significantly improved the efficiency of the cavity without increasing the crystal size. The obtained results will pave the way to implement advanced designs of high-performance electroacoustic sensors based on coupling modes in phononic crystals.

Coalescence-induced jumping of immersed and suspended droplets onmicrostructured substrates

The coalescence-induced jumping of liquid droplets on superhydrophobic structured substrates is investigated numerically using a three-dimensional multiphase lattice Boltzmann method. The numerical experiments on evolution of droplets during jumping process show higher jumping velocity and height from superhydrophobic substrates structured with a periodic array of square pillars, than flat superhydrophobic substrateswith an equilibrium contact angle of 180◦. The results further reveal a strong effect of pillars on the vertical jumping velocity and the final quasiequilibrium height of the merged droplet as a function of air and liquid viscosity, as well as air inertia. As for substrate wettability, it is found that, compared to the flat superhydrophobic substrate, the critical contact angle where the merged droplet jumps away from substrate is reduced for pillared substrate and is about 120◦. It is also observed that the droplet initial placement on a substrate with a square array of pillars has an important effect on the spontaneous jumping of the coalesced droplet, and a Wenzel–Cassie wetting transition upon coalescence is observed for droplets that are initially immersed within the pillars.

Design of superhydrophobic pillars with robustness

Superhydrophobic surfaces have huge applications in wetting fields. However, compared with natural biological surfaces, artificial superhydrophobic surfaces still show serious deficiencies in the theoretical design and durability. In this work, a robust superhydrophobic surface consisting of a regular array of square pillars with secondary structures on the sidewalls was prepared using low-speed wire electrical discharge machining. We developed the design criteria of regular arrays of square pillars for achieving superhydrophobicity based on interface characteristics and force balance. A series of samples were also prepared to validate the design criteria. The results indicate that the overlap part between the theoretical superhydrophobicity region before and after abrasion leads to stable superhydrophobicity. In addition, specific surfaces transform from the Wenzel state to the Cassie state after abrasion and the contact angle shows a large increase. The design criteria based on the research of predecessors are useful to design superhydrophobic surfaces with robustness.

Numerical study of laminar boundary-layer flows over a superhydrophobic plate

Liquid flow in the laminar boundary layer over a flat plate with a superhydrophobic surface formed by a square array of pillars is studied theoretically. Assuming the water surface on top of the pillars to be in the Cassie-Baxter state, asymptotic analysis is first carried out to separate the micro-scale flow in a typical cell surrounding a pillar and the macro-scale development of the laminar boundary layer of Blasius type. The 3-D cell problem and the 2-D boundary layer problem are solved together iteratively, yielding the slip length and the entire flow field. Numerical results are presented to examine the effect of solid fraction, pillar-to-pillar spacing, and the speed of the ambient flow on drag reduction. It is shown that the slip length is practically constant, while the boundary layer thickness grows monotonically downstream so that hydrophobicity affects drag reduction primarily over the leading portion of a long surface.

Modeling extracellular matrix viscoelasticity using smoothed particle hydrodynamics with improved boundary treatment

We propose viscoelastic smoothed particle hydrodynamics (SPH) with extended boundary conditions as a new method to model the extracellular matrix (ECM) in contact with a migrating cell. The contact mechanics between a cell and ECM is modeled based on an existing boundary method in SPH that corrects for the well-known missing kernel support problem in Fluid Structure Interactions (FSI). This boundary method is here extended to allow the modeling of moving boundaries in contact with a viscoelastic solid. To validate the method, simulations are performed of tractions applied to a viscoelastic solid, Stokes flow around an array of square pillars, and indentation of a viscoelastic material with a circular indenter. By drop out of the inertial terms in the SPH equations of motion, the new SPH formulation allows to solve problems in a low Reynolds environment with a timestep independent of the particle spacing, permitting to model processes at the cellular scale (i.e. μm-scale). The potential of the method to capture cell–ECM interactions is demonstrated by simulation of a self propelling object that locally degrades the ECM by fluidizing it to permit migration. This should enable us to model and understand realistic cell–matrix interactions in the future.

Magnetostatic Interactions in Self-Assembled CoxNi1-xFe2O4/BiFeO3 Multiferroic Nanocomposites.

Self-assembled vertically aligned oxide nanocomposites consisting of magnetic pillars embedded in a ferroelectric matrix have been proposed for logic devices made from arrays of magnetostatically interacting pillars. To control the ratio between the nearest neighbor interaction field and the switching field of the pillars, the pillar composition CoxNi1-xFe2O4 was varied over the range 0 ≤ x ≤ 1, which alters the magnetoelastic and magnetocrystalline anisotropy and the saturation magnetization. Nanocomposites were templated into square arrays of pillars in which the formation of a "checkerboard" ground state after ac-demagnetization indicated dominant magnetostatic interactions. The effect of switching field distribution in disrupting the antiparallel nearest neighbor configuration was analyzed using an Ising model and compared with experimental results.

Inertial effects on thermal transport in superhydrophobic microchannels

This paper presents a numerical investigation on the effects of inertia for laminar fully developed flow in superhydrophobic microchannels where the surface structure consists of a square array of square pillars aligned with the flow direction. Infinite parallel plate channel flow is considered where the flow is assumed to be non-wetting and the meniscus is idealized as flat. A constant heat flux condition is imposed at the tops of the posts whereas the gas/liquid interface is assumed to be adiabatic. A wide range of Peclet numbers (1–10,000), relative channel spacing sizes (3 orders of magnitude), and solid fractions (0.02–0.9) are considered. The effect of these on friction factor-Reynolds number product, Nusselt number, hydrodynamic slip length and temperature jump length is explored. Frictional drag and convective heat transfer are reduced for all cases explored. These reductions are greater for small solid fractions, small relative channel sizes, and low Peclet numbers. Interestingly, over a wide range of explored parameters the results correspond well with the analytical diffusion dominated results present in the literature. It is only at the highest Peclet numbers and smallest relative channel sizes that the results deviate significantly from the diffusion dominated Stokes flow scenario. A mapping is presented that illustrates for which parameter ranges the influence of inertia and relative channel size become significant.

Analytic correlation for the capillary pressure of micro-square-pillar arrays

In this study, the capillary pressure of hexagonal arrays of micro-square-pillars is investigated analytically. The effects of spacing between pillars (30 ≤ s ≤ 90 μm) and contact angle (10° ≤ θ ≤ 50°) on the capillary pressure are considered. In order to estimate the capillary pressure, the shape of liquid interface in the micro-square-pillar array is estimated numerically using a surface-energy-minimization algorithm. The capillary pressure is subsequently obtained as the change in interfacial energy per unit volume. The results indicate that the capillary pressure is directly proportional to the cosine of the contact angle, and monotonically decreases as the spacing between neighboring pillars. Based on the numerical results, a semi-analytic correlation for predicting the capillary pressure of hexagonal arrays of micro square pillars is proposed as a nondimensional form. The hexagonal arrays of square pillars are shown to have 13% higher capillary pressure compared with hexagonal arrays of circular pillars with same solid fraction and pitch.

Vibration-induced Wenzel-Cassie wetting transition on microstructured hydrophobic surfaces

Wetting transitions were studied with vertically vibrated droplet on various polydimethylsiloxane surfaces with square array of pillars. Our experiments show that the increase of the pillar spacing, at each given frequency, leads to a decrease of the critical amplitude (which is required to achieve a Wenzel to Cassie transition). The physical mechanism of Wenzel-Cassie transition by vibration is presented. Compared with other studies of wetting transition by vibration, we demonstrate that Wenzel-Cassie transition comes up when R(θ)fn2Acr2≈const (where fn is the resonance frequency, Acr is the threshold amplitude, and R(θ) is the initial radius of the droplet).

Pinning and wicking in regular pillar arrays.

Pinning and wicking of a liquid meniscus in a square array of pillars is investigated in numerical energy minimizations and compared to wetting experiments. Our combined study shows that criteria for spontaneous film formation, based on thermodynamic considerations as well as on simple geometric modelling of the meniscus shape, are insufficient to predict the onset of wicking. High aspect ratio pillars with a square cross-section may display a re-entrant pinning regime as the density of the pillars is increased, a behaviour that is captured by neither of the aforementioned models. Numerically computed energy landscapes for the advancing meniscus allow us to explain the re-entrant behaviour in terms of energy barriers between topologically different meniscus shapes. Our numerical results are validated by wicking experiments where for the material contact angle θ0 = 47° the re-entrant behaviour is present for square pillars and absent for pillars with circular cross section.

Pillars and pinning centers in superconducting prisms

We solve the Ginzburg-Landau equations for to study the vortex configurations in a superconducting prism with a square array of pillars or holes in the presence of a uniform applied magnetic field. The presence of the pillars (holes) changes the vortex structures in the superconducting prism considerably. We calculate magnetization, free energy and vorticity curves, which shows the transition between different vortex configurations as a function of magnetic fields.

Evidence of Ultrasonic Band Gap in Aluminum Phononic Crystal Beam

In this paper, we prove theoretically and experimentally the existence of complete ultrasonic band gap in phononic crystal beam. The phononic beam structure studied is composed of a linear lattice array of square pillars on a beam, made with aluminum-fortal easily machinable at centimetric scale. Ultrasonic characterization of phononic beam guides shows the existence of a frequency range where the transmitted signals are strongly attenuated, due to the presence of ultrasonic band gap, in agreement with theoretical results predicted by finite element simulation. These structures present a potential for the use as energy loss reduction in micromechanical resonators at high frequency regime.

Dynamic Behavior of Water Droplet Impact on Microtextured Surfaces: The Effect of Geometrical Parameters on Anisotropic Wetting and the Maximum Spreading Diameter

Textured silicon surfaces decorated by square arrays of pillars with adjustable pitch were fabricated. The wetting behavior, especially for direction-dependent water contact angles on textured silicon surfaces after silanization, was investigated by incorporating the contact line fraction into a modified Wenzel model. Also, the effect of geometrical parameters on the anisotropic wetting behavior of water was examined with respect to water droplet impact on the textured surface. Moreover, the maximum spreading factor was studied theoretically in terms of energy conservation, allowing for surface topography and viscous friction of the liquid flowing among the arrays of the posts. Theoretical models were found to be in good agreement with experimental data.

Wetting state transition on hydrophilic microscale rough surface

The wetting process and the wetting state transition stages are studied on hydrophilic rough surfaces covered with microscale pillar arrays of different geometrical morphologies and distributions. The effects of geometrical morphology, distribution, parameters, hydrophilicity, and contact angle hysteresis of pillar arrays on wetting state transition are analyzed by energy method. The results indicate that, on the hydrophilic rough surface covered with hexagonal arrays of square pillars, the water droplet tends to stay at a stable Cassie state, or the wetting state transition is only from a Cassie state to an intermediate state. Moreover, smaller pillar interval, larger square pillar width or diameter of cylinder, higher pillar height, strong hydrophilicity are beneficial to the stability of Cassie state, therefore, the wetting state could be prevented from transforming to pseudo-Wenzel state or Wenzel state. However, smaller area fraction of solid-liquid interface under the water drop and weaker hydrophilicity are beneficial to increasing the apparent contact angle. Therefore, the stability of wetting state and the large apparent contact angle should to be considered in hydrophilic surface design. The contact angle hysteresis plays an opposite effect on wetting state stability and the hydrophobicity or superhydrophobicity of rough solid surface. The results provide a theoretical foundation for the design of substrates covered with hydrophilic rough structures, on which the water droplet will obtain a stable Cassie state.

EFFECT OF TOPOGRAPHICALLY PATTERNED POLY(DIMETHYLSILOXANE) SURFACES ON Pseudomonas aeruginosa ADHESION AND BIOFILM FORMATION

Bacterial pathogens, such as Pseudomonas aeruginosa, readily form biofilms on surfaces, limiting the efficacy of antimicrobial and antibiotic treatments. To mitigate biofilm formation, surfaces are often treated with antimicrobial agents, which have limited lifetime and efficacy. Recent studies have shown that well-ordered topographic patterns can limit bacterial attachment to surfaces and limit biofilm formation. In this study, nano and microscale patterned poly(dimethylsiloxane) surfaces were evaluated for their ability to affect adhesion and biofilm formation by Pseudomonas aeruginosa. Feature size and spacing were varied from 500 nm to 2 μm and included repeating arrays of square pillars, holes, lines and biomimetc Sharklet™ patterns. Bacterial surface adhesion and biofilm formation was assessed in microfluidic flow devices and under static conditions. Attachment profiles under static and fluid flow varied within topography types, sizes and spacing. Pillar structures of all sizes yielded lower surface attachment than line-based patterns and arrays of holes. This trend was also observed for biomimetic Sharklet™ patterns, with reduced bacterial attachment to "raised" features as compared to "recessed" features. Notably, none of the topographically patterned surfaces outperformed smooth surfaces (without topography) for resisting cell adhesion. Initial surface attachment patterns were indicative of subsequent biofilm formation and coverage, suggesting a direct role of surface topography in biofilm-based biofouling.

Imbibition of Solids in Nanopillar Arrays

The imbibition of a solid island on a substrate with a square array of pillars is studied by means of kinetic Monte Carlo simulations. Imbibition is found to occur via an intermediate state where an island sits on the film. Two dynamical regimes are identified depending on the geometry of the substrate: a stochastic regime, where the dynamics are controlled by the nucleation-limited motion of the imbibition front across the array of pillars, and a deterministic regime limited by the diffusion of atoms on top of the imbibition film.

Dynamic Behavior of the Water Droplet Impact on a Textured Hydrophobic/Superhydrophobic Surface: The Effect of the Remaining Liquid Film Arising on the Pillars' Tops on the Contact Time

We have fabricated a series of textured silicon surfaces decorated by square arrays of pillars whose radius and pitch can be adjusted independently. These surfaces possessed a hydrophobic/superhydrophobic property after silanization. The dynamic behavior of water droplets impacting these structured surfaces was examined using a high-speed camera. Experimental results validated that the remaining liquid film on the pillars' tops gave rise to a wet surface instead of a dry surface as the water droplet began to recede away from the textured surfaces. Also, experimental results demonstrated that the difference in the contact time was subjected to the solid fraction referred to as the ratio of the actual area contacting with the liquid to its projected area on the textured surface. Because the mechanism by which the residual liquid film emerges on the pillars' tops can essentially be ascribed to the pinch-off of the liquid threads, we further addressed the changes in the contact time in terms of the characteristic time of pinch-off of an imaginary liquid cylinder whose radius is related to the solid fraction and the maximum contact area. The match of the theoretical analysis and the experimental results substantiates the assumption aforementioned.

Fully Integrated Three-Dimensional Electrodes for Electrochemical Detection in Microchips: Fabrication, Characterization, and Applications

A scalable and rather inexpensive solution to producing microanalytical systems with "on-chip" three-dimensional (3D) microelectrodes is presented in this study, along with applicability to practical electrochemical (EC) detection scenarios such as preconcentration and interferant removal. This technique to create high-aspect-ratio (as much as 4:1) gold microstructures in constrained areas involved the modification of stud bump geometry with microfabricated silicon molds via an optimized combination of temperature, pressure, and time. The microelectrodes that resulted consisted of an array of square pillars approximately 18 microm tall and 20 microm wide on each side, placed at the end of a microfabricated electrophoresis channel. This technique increased the active surface area of the microelectrodes by as much as a factor of 50, while mass transfer and, consequently, preconcentration collection efficiencies were increased to approximately 100%, compared to approximately 30% efficiency for planar nonmodified microelectrodes (samples that were used included the neurotransmitters dopamine and catechol). The 3D microelectrodes were used both in a stand-alone configuration, for direct EC detection of model catecholamine analytes, and, more interestingly, in dual electrode configurations for EC sample processing prior to detection downstream at a second planar electrode. In particular, the 3D electrodes were shown to be capable of performing coulometry or complete (100%) redox conversion of analyte species over a wide range of concentrations, from 4.3 microM to 4.4 mM, in either plug-flow or continuous-flow formats.