Mesoscale Roughness Research Articles

Capillary Wicking on Heliamphora minor-Mimicking Mesoscopic Trichomes Array.

Liquid spontaneously spreads on rough lyophilic surfaces, and this is driven by capillarity and defined as capillary wicking. Extensive studies on microtextured surfaces have been applied to microfluidics and their corresponding manufacturing. However, the imbibition at mesoscale roughness has seldom been studied due to lacking fabrication techniques. Inspired by the South American pitcher plant Heliamphora minor, which wicks water on its pubescent inside wall for lubrication and drainage, we implemented 3D printing to fabricate a mimetic mesoscopic trichomes array and investigated the high-flux capillary wicking process. Unlike a uniformly thick climbing film on a microtextured surface, the interval filling of millimeter-long and submillimeter-pitched trichomes creates a film of non-uniform thickness. Different from the viscous dissipation that dominated the spreading on microtextured surfaces, we unveiled an inertia-dominated transition regime with mesoscopic wicking dynamics and constructed a scaling law such that the height grows to 2/3 the power of time for various conditions. Finally, we examined the mass transportation inside the non-uniformly thick film, mimicking a plant nutrition supply method, and realized an open system siphon in the film, with the flux saturation condition experimentally determined. This work explores capillary wicking in mesoscopic structures and has potential applications in the design of low-cost high-flux open fluidics.

Impact of nano-scale trabecula size on osteoblastic behavior and function in a meso-nano hybrid rough biomimetic zirconia model.

A novel implant model consisting of meso-scale cactus-inspired spikes and nano-scale bone-inspired trabeculae was recently developed to optimize meso-scale roughness on zirconia. In this model, the meso-spike dimension had a significant impact on osteoblast function. To explore how different nano-textures impact this model, here we examined the effect of different nano-trabecula sizes on osteoblast function while maintaining the same meso-spike conformation. Zirconia disks with meso-nano hybrid surfaces were created by laser etching. The meso-spikes were fixed to 40 μm high, whereas the nano-texture was etched as large and small trabeculae of average Feret diameter 237.0 and 134.1 nm, respectively. A polished surface was also prepared. Rat bone marrow-derived and human mesenchymal stromal cell-induced osteoblasts were cultured on these disks. Hybrid rough surfaces, regardless of nano-trabecula dimension, robustly promoted the osteoblastic differentiation of both rat and human osteoblasts compared to those on polished surfaces. Hybrid surfaces with small nano-trabeculae further enhanced osteoblastic differentiation compared with large nano-trabeculae. However, the difference in osteoblastic differentiation between small and large nano-trabeculae was much smaller than the difference between the polished and hybrid rough surfaces. The nano-trabecula size did not influence osteoblast attachment and proliferation, or protein adsorption. Both hybrid surfaces were hydro-repellent. The atomic percentage of surface carbon was lower on the hybrid surface with small nano-trabeculae. Small nano-trabeculae promoted osteoblastic differentiation more than large nano-trabeculae when combined with meso-scale spikes. However, the biological impact of different nano-trabeculae was relatively small compared with that of different dimensions of meso-spikes.

Roughness Suppression in Electrochemical Nanoimprinting of Si for Applications in Silicon Photonics

Metal-assisted electrochemical nanoimprinting (Mac-Imprint) scales the fabrication of micro- and nanoscale 3D freeform geometries in silicon and holds the promise to enable novel chip-scale optics operating at the near-infrared spectrum. However, Mac-Imprint of silicon concomitantly generates mesoscale roughness (e.g., protrusion size≈45nm) creating prohibitive levels of light scattering. This arises from the requirement to coat stamps with nanoporous gold catalyst that, while sustaining etchant diffusion, imprints its pores (e.g., average diameter≈42nm) onto silicon. In this work, roughness is reduced to sub-10nm levels, which is in par with plasma etching, by decreasing pore size of the catalyst via dealloying in far-from equilibrium conditions. At this level, single-digit nanometric details such as grain-boundary grooves of the catalyst are imprinted and attributed to the resolution limit of Mac-Imprint, which is argued to be twice the Debye length (i.e., 1.7nm)-a finding that broadly applies to metal-assisted chemical etching. Last, Mac-Imprint is employed to produce single-mode rib-waveguides on pre-patterned silicon-on-insulator wafers with root-mean-square line-edge roughness less than 10nm while providing depth uniformity (i.e., 42.9±5.5nm), and limited levels of silicon defect formation (e.g., Raman peak shift<0.1cm-1 ) and sidewall scattering.

A Correlation between Deformation-Induced Surface Roughness and In-Plane Plastic Strain in an Aluminum Alloy at the Mesoscale

Experimental and numerical studies on deformation-induced surface roughening in a commercial purity aluminum alloy are presented and discussed. Mesoscale surface profiles evolving in the experimental and numerical specimens in the course of tension are processed to reveal a correlation between roughness characteristics and in-plane plastic strains at the mesoscale. A dimensionless parameter calculated as a ratio of the rough profile length to the profile evaluation length has been used for quantitative estimations of the mesoscale roughness patterns. The dimensionless roughness parameter is shown to depend exponentially on the in-plane plastic strains at the mesoscale. The results support an early assumption that in-plane plastic strains accumulated in a loaded material can be evaluated from the estimations of mesoscale surface roughness.

The effect of texture sharpness on deformation-induced surface roughening in α-titanium

The deformation behavior of hexagonal metals is known to be strongly affected by a material texture. This paper aims at understanding the effect of a basal texture on the deformation-induced surface roughness in α-titanium. The effect of the basal texture severity on the deformation behavior is estimated for polycrystals characterized by the same grain geometry and different crystallographic orientations. For this purpose, three-dimensional polycrystalline models with the grain morphology and crystallographic orientations integrated explicitly are constructed using the step-by-step packing method. The texture severity is shown to strongly affect mesoscale roughness patterns. A more pronounced basal texture leads to suppression of the microscale surface roughness formed by displacements of individual grains perpendicular to the free surface (the orange peel pattern) and mesoscopic surface roughness formed by the displacements of grain groups. The basal texture reduces the material ability to deform perpendicular to the loading axis, while the prismatic slip gives rise to in-plane shear banding.

Numerical study of the texture effect on deformation-induced surface roughening in titanium polycrystals

The influence of a texture on the mesoscale deformation-induced surface roughening in titanium polycrystals is studied using direct microstructure-based simulations. Relying on the experimental data for commercial purity titanium, equiaxed grain conglomerates are generated by the method of step-by-step packing. The grain constitutive behavior is described in terms of crystal plasticity, with the grain orientations being assigned randomly or representing a basal texture. The micro- and mesoscale roughness evolution in the model polycrystals under uniaxial tension is calculated using ABAQUS/Explicit. In order to quantify mesoscale roughening, a dimensionless roughness parameter is calculated for a set of experimental and numerical roughness profiles evolving in the course of tension. The mesoscale roughness parameter is shown to grow nonlinearly with the plastic strain of the evaluated mesoscale regions. Particularly, it begins increasing rapidly in the region of neck formation well before necking becomes macroscopically evident. The basal texture is shown to significantly affect the plastic strain localization and roughness patterns. The mesoscale roughness, strongly pronounced in the untextured polycrystal, is mostly suppressed in the presence of texture. A set of in-plane shear bands are formed in the textured polycrystal at an angle of 45° to the tensile axis much like those observed experimentally.

Early prediction of macroscale plastic strain localization in titanium from observation of mesoscale surface roughening

The numerical and experimental investigations have been performed to establish a correlation between local strains and mesoscale deformation-induced surface roughening in commercial purity titanium specimens. The evolution of surface profiles in selected regions of the specimens is examined in the course of uniaxial tension. Complementing the experimental studies, crystal plasticity finite-element simulations of deformation-induced surface roughening are performed for a model polycrystal. It has been shown that a series of out-of-plane surface displacements characterized by different amplitudes and frequencies are formed in the experimental and computational specimens under tension. Roughening intensifies on a larger scale when smaller length scale deformation mechanisms are exhausted. In order to quantify the multiscale surface patterns a dimensionless roughness parameter has been proposed. Unlike the traditional methods of roughness assessment, unfiltered profiles are analyzed to take into account the deformation mechanisms that are at work at different length scales. The dimensionless roughness parameter is shown to be sensitive to and well correlated with the local plastic strains. In the region of macroscopic necking, the mesoscale roughness begins to increase nonlinearly well ahead of the visible evidence of the macroscale strain localization, while in the other regions it ceases to increase at all. The results support the assumption that the plastic strain localization and fracture can be predicted early from estimations of mesoscale surface roughness.

Electrochemical nanoimprinting of silicon

Scalable nanomanufacturing enables the commercialization of nanotechnology, particularly in applications such as nanophotonics, silicon photonics, photovoltaics, and biosensing. Nanoimprinting lithography (NIL) was the first scalable process to introduce 3D nanopatterning of polymeric films. Despite efforts to extend NIL's library of patternable media, imprinting of inorganic semiconductors has been plagued by concomitant generation of crystallography defects during imprinting. Here, we use an electrochemical nanoimprinting process-called Mac-Imprint-for directly patterning electronic-grade silicon with 3D microscale features. It is shown that stamps made of mesoporous metal catalysts allow for imprinting electronic-grade silicon without the concomitant generation of porous silicon damage while introducing mesoscale roughness. Unlike most NIL processes, Mac-Imprint does not rely on plastic deformation, and thus, it allows for replicating hard and brittle materials, such as silicon, from a reusable polymeric mold, which can be manufactured by almost any existing microfabrication technique.

Micromechanical model of deformation-induced surface roughening in polycrystalline materials

A micromechanical model has been developed to describe deformation-induced surface roughening in polycrystalline materials. The three-dimensional polycrystalline structure is taken into account in an explicit form with regard to the crystallographic orientation of grains to simulate the micro- and mesoscale deformation processes. Constitutive relations for describing the grain response are derived on the basis of crystal plasticity theory that accounts for the anisotropy of elastic-plastic properties governed by the crystal lattice structure. The micromechanical model is used to numerically study surface roughening in microvolumes of polycrystalline aluminum and titanium under uniaxial tensile deformation. Two characteristic roughness scales are distinguished in the both cases. At the microscale, normal displacements relative to the free surface are caused by the formation of dislocation steps in grains emerging on the surface and by the displacement of neighboring grains relative to each other. Microscale roughness is more pronounced in titanium, which is due to the high level of elastic-plastic anisotropy typical of hcp crystals. The mesoscale roughness includes undulations and cluster structures formed with the involvement of groups of grains. The roughness is quantitatively evaluated using a dimensionless parameter, called the degree of roughness, which reflects the degree of surface shape deviation from a plane. An exponential dependence of the roughness degree on the strain degree is obtained.

Micro- and mesomechanical aspects of deformation-induced surface roughening in polycrystalline titanium

This article addresses the problem of multiscale surface roughening in commercial purity titanium subjected to uniaxial tension. In situ investigations of the evolution of grain- and mesoscale roughness in selected regions of titanium specimens were performed. Based on the experimental data obtained, 3D polycrystalline models with explicit consideration of grain structure were generated and implemented in finite-element calculations. Constitutive models of grains were constructed, using crystal plasticity theory to take into account the elastic-plastic anisotropy on the grain scale. The experimental and numerical results obtained have shown that a series of multiscale surface undulations are formed on the free surface of the specimen subjected to tension. The smallest out-of-plane surface displacements are attributed to intragrain dislocation glide. Larger displacements are associated with relative grain motion. The latter give rise to the formation of an orange peel pattern. The displacements of the two types are a microscale phenomenon. The largest surface displacements formed by the grain groups involved in out-of-plane and in-plane cooperative motion are referred to as the mesoscale roughness. The latter is found to correlate well with local strains of the specimen regions under examination. The main conclusion drawn from the experimental and numerical results is that it is the mesoscale that will furnish a clue to prediction of plastic strain localization and fracture of materials far in advance of the macroscale manifestation of these processes.

In situ analysis of flow dynamics and deformation fields in cutting and sliding of metals

The flow dynamics, deformation fields and chip-particle formation in cutting and sliding of metals are analysed, in situ , using high-speed imaging and particle image velocimetry. The model system is a brass workpiece loaded against a wedge indenter at low speeds. At large negative rake angles, the flow is steady with a prow of material forming ahead of the indenter. There is no material removal and a uniformly strained layer develops on the workpiece surface—the pure sliding regime. When the rake angle is less negative, the flow becomes unsteady, triggered by formation of a crack on the prow free surface and material removal ensuing—the cutting regime. The strain on the prow surface at crack initiation is found to be constant. Chip morphologies, such as discrete particle, segmented chip and continuous chip with mesoscale roughness, are shown to arise from a universal mechanism involving propagation of the prow crack, but to different distances towards the indenter tip. The simple shear deformation in continuous chip formation shows small-angle oscillations also linked to the prow crack. Implications for material removal processes and ductile failure are discussed.

Droplet on a regularly patterned solid. Wenzel's regime and meso-scale roughness.

The applicability of Wenzel's equation to describe a liquid droplet settled on the solid surface regularly patterned with rectangular prisms was examined by means of simulations of the droplet/surface system morphology and energetics. The droplet deposited on the meso-scale surface roughness (i.e. the droplet size was larger than the size of heterogeneities by about an order of magnitude) was considered. Several different approaches to the estimation of the contact angle were employed. The discrepancies between the results of simulated experimental measurements and the predictions based on the Wenzel equation were analyzed and discussed. The influence of three-phase contact line effects on the droplet morphology and the existence of metastable states was shown.

The joint effect of mesoscale and microscale roughness on perceived gloss

Computer simulated stimuli can provide a flexible method for creating artificial scenes in the study of visual perception of material surface properties. Previous work based on this approach reported that the properties of surface roughness and glossiness are mutually interdependent and therefore, perception of one affects the perception of the other. In this case roughness was limited to a surface property termed bumpiness. This paper reports a study into how perceived gloss varies with two model parameters related to surface roughness in computer simulations: the mesoscale roughness parameter in a surface geometry model and the microscale roughness parameter in a surface reflectance model. We used a real-world environment map to provide complex illumination and a physically-based path tracer for rendering the stimuli. Eight observers took part in a 2AFC experiment, and the results were tested against conjoint measurement models. We found that although both of the above roughness parameters significantly affect perceived gloss, the additive model does not adequately describe their mutually interactive and nonlinear influence, which is at variance with previous findings. We investigated five image properties used to quantify specular highlights, and found that perceived gloss is well predicted using a linear model. Our findings provide computational support to the ‘statistical appearance models’ proposed recently for material perception.

Why do rough surfaces appear glossy?

The majority of work on the perception of gloss has been performed using smooth surfaces (e.g., spheres). Previous studies that have employed more complex surfaces reported that increasing mesoscale roughness increases perceived gloss [Psychol. Sci.19, 196 (2008), J. Vis.10(9), 13 (2010), Curr. Biol.22, 1909 (2012)]. We show that the use of realistic rendering conditions is important and that, in contrast to [Psychol. Sci.19, 196 (2008), J. Vis.10(9), 13 (2010)], after a certain point increasing roughness further actually reduces glossiness. We investigate five image statistics of estimated highlights and show that for our stimuli, one in particular, which we term "percentage of highlight area," is highly correlated with perceived gloss. We investigate a simple model that explains the unimodal, nonmonotonic relationship between mesoscale roughness and percentage highlight area.

The Wind Profile in the Coastal Boundary Layer: Wind Lidar Measurements and Numerical Modelling

Traditionally it has been difficult to verify mesoscale model wind predictions against observations in the planetary boundary layer (PBL). Here we used measurements from a wind lidar to study the PBL up to 800 m above the surface at a flat coastal site in Denmark during a one month period in autumn. We ran the Weather Research and Forecasting numerical model with two different roughness descriptions over land, two different synoptic forcings and two different PBL schemes at two vertical resolutions and evaluated the wind profile against observations from the wind lidar. The simulated wind profile did not have enough vertical shear in the lower part of the PBL and also had a negative bias higher up in the boundary layer. Near the surface the internal boundary layer and the surface roughness influenced the wind speed, while higher up it was only influenced by the choice of PBL scheme and the synoptic forcing. By replacing the roughness value for the land-use category in the model with a more representative mesoscale roughness, the observed bias in friction velocity was reduced. A higher-order PBL scheme simulated the wind profile from the west with a lower wind-speed bias at the top of the PBL. For easterly winds low-level jets contributed to a negative wind-speed bias around 300 m and were better simulated by the first-order scheme. In all simulations, the wind-profile shape, wind speed and turbulent fluxes were not improved when a higher vertical resolution or different synoptic forcing were used.

Direct fabrication of double-rough chestnut-like multifunctional Sn@C composites on copper foil: lotus effect and lithium ion storage properties

This paper reports the fabrication of a new, double-rough chestnut-like Sn@C composite in the form of mesospheres with nanohairs directly on a copper surface by a simple and scalable procedure. The hierarchical order in the nanostructure and the composition of the nanocomposite impart at least two functional properties: the lotus effect and reversible storage of lithium ions. The nanohairs of carbon nanotube-encapsulated metallic tin; and the underlying core of carbon-tin mesospheres with rough exterior; give rise to nanoscale and mesoscale roughness not unlike the double-rough surface of a lotus leave (nanohairs on microbumps). Subsequently the modified copper surface is superhydrophobic just like the lotus leaves. On the other hand the ability to directly deposit lithium-active storage compounds (Sn, C) on a current conductor (Cu) has also resulted in an electrode immediately usable as a lithium ion battery anode without additional processing. Indeed electrochemical measurements have shown quite satisfying properties for lithium ion battery applications.

A Study of the Morphology of 3C-SiC Layers Grown at Different C/Si Ratios

A comparative study of the morphology of 3C-SiC films prepared with different C:Si ratios is presented. The silane precursor controls the growth rate at all values of C:Si ratio but combined observations using Atomic Force Microscopy and Scanning Electron Microscopy indicate that the C:Si ratio is critical in determining the grain size and faceting at C:Si values close to 1. Makyoh topography reveals various surface defects, a slight mesoscale roughness and bending of the epiwafers.

Photometric anomalies of the lunar surface studied with SMART-1 AMIE data

We present new results from the mapping of lunar photometric function parameters using images acquired by the spacecraft SMART-1 (European Space Agency). The source data for selected lunar areas imaged by the AMIE camera of SMART-1 and the data processing are described. We interpret the behavior of photometric function in terms of lunar regolith properties. Our study reveals photometric anomalies on both small (sub-kilometer) and large (tens of kilometers) scales. We found the regolith mesoscale roughness of lunar swirls to be similar in Mare Marginis, Mare Ingenii, and the surrounding terrains. Unique photometric properties related to peculiarities of the millimeter-scale regolith structure for the Reiner Gamma swirl are confirmed. We identified several impact craters of subkilometer sizes as the source of photometric anomalies created by an increase in mesoscale roughness within the proximal crater ejecta zones. The extended ray systems reveal differences in the photometric properties between proximal and distant ejecta blankets. Basaltic lava flows within Mare Imbrium and Oceanus Procellarum indicate higher regolith porosity for the redder soils due to differences in the chemical composition of lavas.

A Comparative Study of the Morphology of 3C-SiC Grown at Different C/Si Ratios

A comparative study of the morphology of 3C-SiC films prepared with different C:Si ratios is presented. The silane precursor controls the growth rate at all values of C:Si ratio but combined of observations using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) indicates that the C:Si ratio is critical in determining the grain size and at values of C:Si close to 1 texturing and faceting become evident. Makyoh Topography reveals various surface defects, a slight mesoscale roughness and bending of the epiwafers.