Cylindrical Micropillars Research Articles

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.

Jet ejection following drop impact on micropillared hydrophilic substrates

The jetting phenomenon from impinging droplets on partially wetting hydrophilic substrates composed of cylindrical micropillars is studied. Impact velocity and fluid viscosity are varied to characterize the jets. It is found that the jetting phenomenon arises for certain ranges of Weber and Ohnesorge numbers. Jet speed, height, and diameter scale linearly with the Weber number. The scaling analysis indicates that the jet is produced by pure inertial focusing of radial flow due to the collapse of an air cavity formed at the center of the drop during the recoiling phase of the impact.

Ferroelastic and Plastic Behaviors in Pseudo-Single Crystal Micropillars of Nontransformable Tetragonal Zirconia

The orientation-dependent micromechanical properties of nontransformable tetragonal (t') zirconia, which diffusionlessly transformed from the fluorite cubic phase and does not show stress-induced phase transformation, were characterized via pseudo-single crystal micropillar compression and electron microscopy. The t' zirconia sample was obtained via atmospheric plasma spraying of 4.5 mol% yttria-stabilized zirconia (YSZ) powders into liquid nitrogen and consolidated into a bulk state via hot pressing at 1100°C. Dense and cylindrical micropillars were fabricated using a focused ion beam from pseudo-single crystalline regions, which had a nanodomain microstructure of three t' variants partitioned by {1 0 1}c twin boundaries with 90° symmetry, and were compressed using a flat-end diamond indenter. Near- c compressions were attributed to ferroelastic domain switching and subsequent {1 0 1}c and/or {1 1 1}c slips. In ferroelastic deformation, a certain t' variant diminished and a binary domain microstructure developed with c axes perpendicular to the compressive direction. Near- c compressions were governed by {0 0 1}c slips accompanied by strain hardening with negligible ferroelasticity, resulting in buckling deformation with rotational kinking. Crack-tolerant plasticity was ascribed to ferroelastic toughening, where t' variants with c axes across crack planes developed to relieve stress concentrations around the crack tips in both orientations. In contrast, pseudo-cleavage fractures on low-index planes were observed in near- c compressions. Crack-tolerant behavior was not observed in the cubic counterpart with a domain-free microstructure (8.0 mol% YSZ), which demonstrated catastrophic fractures. Hence, ferroelastic toughening is viewed as the origin of enhanced toughness in t' zirconia.

Low-temperature rheology of calcite

SUMMARY Low-temperature plastic rheology of calcite plays a significant role in the dynamics of Earth's crust. However, it is technically challenging to study plastic rheology at low temperatures because of the high confining pressures required to inhibit fracturing. Micromechanical tests, such as nanoindentation and micropillar compression, can provide insight into plastic rheology under these conditions because, due to the small scale, plastic deformation can be achieved at low temperatures without the need for secondary confinement. In this study, nanoindentation and micropillar compression experiments were performed on oriented grains within a polycrystalline sample of Carrara marble at temperatures ranging from 23 to 175 °C, using a nanoindenter. Indentation hardness is acquired directly from nanoindentation experiments. These data are then used to calculate yield stress as a function of temperature using numerical approaches that model the stress state under the indenter. Indentation data are complemented by uniaxial micropillar compression experiments. Cylindrical micropillars ∼1 and ∼3 μm in diameter were fabricated using a focused ion beam-based micromachining technique. Yield stress in micropillar experiments is determined directly from the applied load and micropillar dimensions. Mechanical data are fit to constitutive flow laws for low-temperature plasticity and compared to extrapolations of similar flow laws from high-temperature experiments. This study also considered the effects of crystallographic orientation on yield stress in calcite. Although there is a clear orientation dependence to plastic yielding, this effect is relatively small in comparison to the influence of temperature.

Hollow Micropillar Array Method for High-Capacity Drug Screening on Filter-Grown Epithelial Cells.

New high-throughput assay formats and innovative screening technologies are needed for miniaturized screens using small quantities of near-native, patient-derived cells. Here, we developed a hollow micropillar array method to screen compounds using epithelial cells cultured on a porous support, with the goal of screening thousands of compounds using a single 24 mm diameter transwell filter containing cultured cells. Test compounds (∼1 nL) in an alginate hydrogel were printed by microinjection in hollow cylindrical micropillars (height = 150 μm, inner diameter = 100 μm) spaced 300 μm apart in a square array configuration. Compounds were delivered by positioning the array near the surface of a cell layer, with 5-10 μm of distance between the micropillars and cell surface. Micropillar array geometry, and the viscosity of the hydrogel and overlying solutions, were optimized computationally and experimentally to produce sustained exposure of cells to test compounds with minimal cross-talk from compounds in neighboring micropillar wells. The method was implemented using a 10 × 10 micropillar array (size = 3 × 3 mm) on CFTR-expressing epithelial cells, in which CFTR chloride channel function was measured from fluorescence in response to iodide addition using a genetically encoded cytoplasmic yellow fluorescent protein halide indicator. The hollow micropillar array platform developed here should be generally applicable for high-capacity drug screening using small numbers of cells cultured on solid or porous supports.

Исследования конденсата поляритонов в микрорезонаторных микростолбиках в сильных магнитных полях -=SUP=-*-=/SUP=-

AbstractThe photoluminescence of a nonequilibrium polariton condensate in cylindrical and rectangular micropillars etched on the surface of a high- Q GaAs microcavity is investigated in magnetic fields of up to 12 T. The measurements are carried out under different levels of nonresonant optical pumping with nanosecond laser pulses for a wide range of cavity detuning. As far as nonresonant excitation produces a high density of excitons in a reservoir, it should be expected that the exciton–polariton interaction, which depends on the pump level, has a considerable effect on the Zeeman splitting and polarization of the condensate. However, measurements of the Zeeman splitting and polarization in high magnetic fields demonstrate that only minor changes take place up to the highest available pump levels. This means that, in the case under study, the effect of exciton–polariton interaction on the polariton system is insignificant. At the same time, the data obtained provide an estimate for the exciton density in the reservoir. In contrast to cylindrical micropillars, the photoluminescence of the condensate in rectangular micropillars consists of two perpendicularly linearly polarized lines which retain a high degree of linear polarization even in a field as high as 12 T. The Zeeman splitting in this case is nearly independent of the pump power. The degrees of both circular and linear polarization change with pump power, but these changes are noticeably smaller than the ones predicted theoretically. This indicates that the system of polaritons in micropillars deviates considerably from thermodynamic equilibrium.

Investigation of a Polariton Condensate in Micropillars in a High Magnetic Field

The photoluminescence of a nonequilibrium polariton condensate in cylindrical and rectangular micropillars etched on the surface of a high-Q GaAs microcavity is investigated in magnetic fields of up to 12 T. The measurements are carried out under different levels of nonresonant optical pumping with nanosecond laser pulses for a wide range of cavity detuning. As far as nonresonant excitation produces a high density of excitons in a reservoir, it should be expected that the exciton–polariton interaction, which depends on the pump level, has a considerable effect on the Zeeman splitting and polarization of the condensate. However, measurements of the Zeeman splitting and polarization in high magnetic fields demonstrate that only minor changes take place up to the highest available pump levels. This means that, in the case under study, the effect of exciton–polariton interaction on the polariton system is insignificant. At the same time, the data obtained provide an estimate for the exciton density in the reservoir. In contrast to cylindrical micropillars, the photoluminescence of the condensate in rectangular micropillars consists of two perpendicularly linearly polarized lines which retain a high degree of linear polarization even in a field as high as 12 T. The Zeeman splitting in this case is nearly independent of the pump power. The degrees of both circular and linear polarization change with pump power, but these changes are noticeably smaller than the ones predicted theoretically. This indicates that the system of polaritons in micropillars deviates considerably from thermodynamic equilibrium.

Mechanical failure of metal/ceramic interfacial regions under shear loading

Shear failure of the interfacial regions of CrN/Cu/Si and CrN/Ti/Si ceramic-coating/metal-adhesion-layer/substrate systems was measured quantitatively and observed concurrently through instrumented compression of cylindrical micro-pillars in-situ a scanning electron microscope. Results indicate that shear failure of the interfacial region occurred in two stages: an initial shear deformation of the entire metal interlayer followed by an unstable shear-off close to the metal/ceramic interface. The shear-off is suggested to be concomitant with the metal/ceramic interface going from being “locked”, with no relative displacement between materials on the two sides of the interface, to being “unlocked”, with significant relative displacements. Density functional theory and molecular dynamics studies on a related metal/ceramic interface, Ti/TiN, showed that a weak interaction plane exists in the metal layer near the chemical interface in a coherent Ti/TiN structure. Consequently, the free energy and theoretical shear strength of the semi-coherent Ti/TiN interface is found to depend on the physical location of the misfit dislocation network (MDN). The minimum energy and strength of the interface occur when the MDN is near, but not at the chemical interface. The relevance of the present work to other metal/ceramic interfaces is discussed.

Investigation of a nonequilibrium polariton condensate in cylindrical micropillars in a strong magnetic field

We analyze the photoluminescence of a nonequilibrium polariton condensate in cylindrical micropillars etched on the surface of a high-Q GaAs microcavity in a wide range of detunings in a magnetic field up to 12 T for various levels of nonresonant laser pumping by nanosecond pulses. With such a method of excitation, a considerable effect of the interaction of the reservoir of photoexcited excitons with the condensate on the Zeeman splitting of the polariton condensate levels can be expected, which can lead to a decrease in its value and even to sign reversal. However, the measurements of photoluminescence in a wide range of optical excitation densities show that Zeeman splitting weakly depends on the optical pumping (its variation does not exceed 15% of the splitting in a field of 12 T). The estimation of the exciton density in the reservoir based on these data gives a value lower than 108 cm–2. In addition, a noticeable decrease (by a factor of about 1.8) in the polariton condensation threshold in a magnetic field is detected.

Energy Dissipation of Moving Drops on Superhydrophobic and Superoleophobic Surfaces.

A water drop moving on a superhydrophobic surface or an oil drop moving on a superoleophobic surface dissipates energy by pinning/depinning at nano- and microprotrusions. Here, we calculate the work required to form, extend, and rupture capillary bridges between the protrusions and the drop. The energy dissipated at one protrusion WS is derived from the observable apparent receding contact angle Θrapp and the density of protrusions n by Ws = γ(cos Θrapp + 1)/n, where γ is the surface tension of the liquid. To derive an expression for Ws that links the microscopic structure of the surface to apparent contact angles, two models are considered: A superhydrophobic array of cylindrical micropillars and a superoleophobic array of stacks of microspheres. For a radius of a protrusion R and a receding materials contact angle Θr, we calculate the energy dissipated per protrusion as Ws = πγR2[A - ln(R/κ)]f(Θr). Here, A = 0.60 for cylindrical micropillars and 2.9 for stacks of spheres. κ is the capillary length. f(Θr) is a function which depends on Θr and the specific geometry, f ranges from ≈0.25 to 0.96. Combining both equations above, we can correlate the macroscopically observed apparent receding contact angle with the microscopic structure of the surface and its material properties.

Controlled switching between quantum states in the exciton–polariton condensate

Optically controlled switching between modes of a polariton laser having different symmetries has been demonstrated experimentally. The microscopic shift of the optical excitation spot dramatically changes the shape of the polariton condensate formed in a cylindrical micropillar on the basis of the planar semiconductor microcavity. Switching between the ring and lobed condensate is achieved owing to the violation of the cylindrical symmetry of the effective potential formed by the lateral surface of the pillar and by the cloud of incoherent excitons created by optical pumping.

Squeeze flow between a sphere and a textured wall

The motion of a millimetric sphere, translating in a viscous fluid towards a wettable textured wall, is investigated experimentally. The textures consist of square arrays of cylindrical or square micro-pillars, the height, width, and spacing of which are varied, keeping the periodicity small compared to the sphere radius. An interferometric device is used to measure the sphere vertical displacement, for distances between the sphere and the base of the pillars smaller than 0.1 sphere radius, and with a resolution of 200 nm. At a given distance from the top of the pillars, the sphere velocity is found to be significantly larger than the corresponding velocity for a smooth solid wall. A squeeze flow model of two adjacent fluid layers is developed in the lubrication approximation, one fluid layer having an effective viscosity that reflects the viscous dissipation through the array of pillars. The pressure field in the gap between the sphere and the textured surface is then used to obtain the drag force on the sphere and hence its velocity. Adjustment of the model to the velocity measurements yields the effective viscosity for a given texture. Finally, a correlation between the effective viscosity and the geometry of the pillar array is proposed.

Measuring Plasticity in Confined Cu Thin Films at Different Geometries

We report quantitative measurement of plasticity in confined Cu thin films with a new micro-pillar testing protocol. Polycrystalline Cu and CrN thin films were sequentially sputter deposited onto Si (100) substrates, forming thin film assemblies in which polycrystalline Cu thin films of various thicknesses were confined between non-deforming Si and CrN. Cylindrical micro-pillars of CrN/Cu/Si were fabricated through scripted focused Ga+ ion beam milling, with the interfaces either normal to the axial direction or at a 45° inclination. The CrN/Cu/Si micro-pillars were compression loaded in the axial direction with a flat diamond punch on an instrumented nanoindenter. The axial compression loading caused extensive plasticity within the thin Cu interlayers with the interfaces in both the normal and inclined orientations, but with distinctly different responses. We show that significant plastic flow occurs within the confined Cu thin films in both normal compression and shear loading. The flow stress of the confined Cu films is dependent on the Cu layer thickness and the deformation geometry. The presently described micro-pillar testing protocol offers quantitative evaluation of the plastic response of thin metal films under different deformation geometries. The present results offer new experimental examples of scale-dependent plasticity in thin metal films, and new experimental test cases for non-local plasticity theories.

Micro-pillar measurements of plasticity in confined Cu thin films

Sputtering was utilized to deposit polycrystalline Cu and CrN films sequentially onto Si(100) substrates, forming specimen assemblies in which predominantly 〈111〉 oriented Cu thin films of varying thicknesses were confined between Si and CrN. Cylindrical micro-pillars of CrN/Cu/Si(100) were fabricated through focused ion beam milling, with the interfaces either normal to the axial direction or at an inclination of 45°. Axial compression loading of the micro-pillars produced extensive plasticity within the thin Cu interlayers in both cases, but with distinctly different responses involving combined compression and shear when the interfaces are normal to the compression axis and constrained shear when the interfaces are inclined. Significant size effects were observed, offering new experimental evidence of scale-dependent plasticity for thin film layers, and new experimental test cases for non-local plasticity theories.

The Five Ws (and one H) of Super-Hydrophobic Surfaces in Medicine

Super-hydrophobic surfaces (SHSs) are bio-inspired, artificial microfabricated interfaces, in which a pattern of cylindrical micropillars is modified to incorporate details at the nanoscale. For those systems, the integration of different scales translates into superior properties, including the ability of manipulating biological solutions. The five Ws, five Ws and one H or the six Ws (6W), are questions, whose answers are considered basic in information-gathering. They constitute a formula for getting the complete story on a subject. According to the principle of the six Ws, a report can only be considered complete if it answers these questions starting with an interrogative word: who, why, what, where, when, how. Each question should have a factual answer. In what follows, SHSs and some of the most promising applications thereof are reviewed following the scheme of the 6W. We will show how these surfaces can be integrated into bio-photonic devices for the identification and detection of a single molecule. We will describe how SHSs and nanoporous silicon matrices can be combined to yield devices with the capability of harvesting small molecules, where the cut-off size can be adequately controlled. We will describe how this concept is utilized for obtaining a direct TEM image of a DNA molecule.

Deformation response of ferrite and martensite in a dual-phase steel

Deformation response of ferrite and martensite in a commercially produced dual-phase sheet steel with a nominal composition of 0.15% C–1.45% Mn–0.30% Si (wt.%) was characterized by nanoindentation and uniaxial compression of focused ion beam-milled cylindrical micropillars (1–2μm diameter). These experiments were conducted on as-received and pre-strained specimens. The average nanoindentation hardness of ferrite was found to increase from ∼2GPa in the as-received condition to ∼3.5GPa in the specimen that had been pre-strained to 7% plastic tensile strain. Hardness of ferrite in the as-received condition was inhomogeneous: ferrite adjacent to ferrite/martensite interface was ∼20% harder than that in the interior, a feature also captured by micropillar compression experiments. Hardness variation in ferrite was reversed in samples pre-strained to 7% strain. Martensite in the as-received condition and after 5% pre-strain exhibited large scatter in nanoindentation hardness; however, micropillar compression results on the as-received and previously deformed steel specimens demonstrated that the martensite phase in this steel was amenable to plastic deformation and rapid work hardening in the early stages of deformation. The observed microscopic deformation characteristics of the constituent phases are used to explain the macroscopic tensile deformation response of the dual-phase steel.

Uniaxial Tension of a Class of Compressible Solids With Plastic Non-Normality

Abstract Motivated by a model that qualitatively captured the response of vertically aligned carbon nanotube (VACNT) pillars in uniaxial compression, we consider the uniaxial tensile response of a class of compressible elastic-viscoplastic solids. In Hutchens et al. [“Analysis of Uniaxial Compression of Vertically Aligned Carbon Nanotubes,” J. Mech. Phys. Solids, 59, pp. 2227–2237 (2011), Erratum 60, 1753–1756 (2012)] an elastic viscoplastic constitutive relation with plastic compressibility, plastic non-normality, and a hardening-softening-hardening hardness function was used to model experimentally obtained uniaxial compression data of cylindrical VACNT micropillars. Complex deformation modes were found in uniaxial compression, which include a sequential buckling-like collapse of the type seen in experiments. These complex deformation modes led to the overall stress-strain signature of the pillar not being of the same form as the input material hardness function. A fundamental question that motivates exploring the deformation of this class of materials—both experimentally and theoretically—is how to extract the intrinsic material response from simple tests. In this study we explore the relation between the input material response and the overall stress strain behavior in uniaxial tension using the constitutive framework of Hutchens et al. A simple one-dimensional analysis reveals the types of instability modes to be expected. Dynamic, finite deformation finite element calculations are carried out to explore the dependence of diffuse necking, localized necking, and propagating band deformation modes on characteristics of the hardness function. Attention is devoted to uncovering implications for obtaining intrinsic material properties of complex hierarchical structures; for example, vertically aligned carbon nanotubes (VACNTs), from uniaxial tension experiments.

Embossed superhydrophobic polymer surfaces with topological variances

In this report, we compare hot and ultraviolet (UV) embossing processes to fabricate hydrophobic surfaces with topological variance. In the hot embossing process, polypropylene was pressurized on a nickel mold with heat, while UV-curable polyurethane acrylate (PUA) was dispensed on the mold followed by UV exposure during the UV embossing process. Cylindrical micro-pillars were successfully replicated using two types of embossing processes with the same mold. The contact angles were then compared at various pillar diameters and with different embossing processes. Although the materials and embossing techniques were different, we achieved a contact angle (CA) enhancement of about 40%. The water contact angle of a transferred substrate, fabricated by the UV embossing process, was 138°. This was much higher than that of flat UV-cured PUA (78°). In addition, we fabricated reverse pillar-shape packing materials for good wettability and then compared the CA values. With the reverse-shape model, we were able to enhance the wettability and decrease the degree of hydrophobicity. This will be beneficial for lipophilic packing materials. This oleophilic characteristic demonstrates enhanced lubrication capabilities as a retort curry packing material. Open image in new window

Enhancement of exciton radiative recombination for In-doped ZnO nanowires with aluminum cylindrical micropillars

Zinc oxide (ZnO) has attracted intensive research effort in recent years, due to its unique properties and versatile applications. Recent work on the conservation of surface plasmon (SP) and light through period metal arrays has elucidated the propagation of SP resonance behavior. In this paper, we discuss the enhancement of exciton radiative recombination of the ZnO nanowires with Al cylindrical micropillars. Optical characterization of exciton interacted with SP resonance for indium-doped ZnO nanowires with Al cylindrical micropillars has been also investigated. From photoluminescence spectra of In-doped ZnO nanowires, it is found that the In-doped ZnO nanowires have a blue emission at 425nm, which resulted from the ZnO band-to-band transition. Prior to the arrays of samples were annealed, a broad green emission centered at 500nm was observed, which is attributed to ZnO native point defects. The relatively strong green band emission results from the radiative recombination that arises from the ionized oxygen vacancy and surface-defect related luminescence. Compare the In-doped ZnO on Si substrate, the enhancement of PL intensity for In-doped ZnO with deposited Al pattern film can be attributed to strong interaction with SP resonance and exciton over a broad temperature range. These experimental results indicate that Al cylindrical micropillars can significantly enhance carrier confinement and increase the quantum efficiency of In-doped ZnO/Al heterostructures due to the interaction of SP resonance between the In-doped ZnO nanowires and Al cylindrical micropillar structures, the surface-defect related luminescence, and the auxiliary test structures with variable micropillar parameters.

Directional motion of evaporating droplets on gradient surfaces

Evaporation of sessile water droplets on surfaces with wettability gradients was studied. The wettability gradient was generated by fabricating non-uniformly distributed cylindrical micropillars on silicon surfaces. During the evaporation, it was found that the center of mass of the droplet moved either in or against the direction of the wettability gradient, depending on the configuration of the micropillars. An energy-based theoretical criterion was derived to predict the moving direction. The theoretical predications agreed well with the experimental observations. The results provide a parametric design basis to control the contact line dynamics and directional transport of evaporating droplets.