Macro-scale Roughness Research Articles

The evolution of stresses and shear resistance on rough faults at large slip

The deviation of natural faults from planarity results in geometric asperities and a heterogeneous stress field, affecting the shear resistance and slip behavior. This study numerically examines the effects of macro-scale roughness, local friction, and wear on the evolution of the fault stresses and shear resistance with fault slip. In contrast to previous analytical and numerical studies, our quasistatic simulations allow for slip at the scale of roughness wavelengths, large roughness levels, and fault opening. The simulations indicate that analytical solutions, which predict a linear increase in shear resistance with slip, significantly overestimate the additional shear resistance from roughness, i.e., the roughness drag, at slips larger than several percent of the minimum roughness wavelength. Starting with mated fault surfaces, the average shear resistance on the fault initially rises rapidly, but the rate of increase diminishes significantly as slip increases and the faults open. While initially showing a larger increase in shear resistance than self-similar faults, at large slip, self-affine faults exhibit significantly smaller shear resistance, and the background stresses are bounded without the need for off-fault inelastic deformation. Although smaller than predicted by analytical solutions, the simulation results show that the additional resistance from macro-scale geometrical irregularities is important, enabling rough faults to operate under shear resistance significantly larger than that from local friction only. The impact of macro-scale roughness increases with the roughness amplitude and the friction coefficient, while wear reduces fault stresses and average shear resistance. The combined effect of local friction and roughness is nonlinear, involving a direct contribution to shear resistance from roughness, as observed in prior studies, and an additional contribution from the increasing average normal stress with slip, which amplifies the resistance from friction and becomes more pronounced at large slip.

Effects of Boulder Arrangement on Flow Resistance Due to Macro-Scale Bed Roughness

Flow resistance in gravel-bed channels is not only affected by the shape and size of the roughness elements, but also by their arrangement on the channel bed surface (position to flow streamlines, spacing between elements, and their protrusion from the channel bed). Many investigations proved that open channel flow resistance can be obtained by integrating the power velocity profile. For a macro-scale roughness condition, this study aims to investigate the effect of different boulder arrangements on flow resistance. First, for each arrangement, the equation relating Γ function of the power velocity profile, the Froude number, and the channel slope was calibrated using available measurements performed in a flume covered by pebbles with “Random”, “Transversal stripe”, and “Longitudinal stripe” arrangements. For each arrangement, the experimental datasets were divided to consider the effects of boulder concentration Ch. Moreover, the relationship obtained for the “Random” arrangement and element concentration lower than 48% was tested using literature measurements performed in a flume covered by coarse elements randomly arranged. Finally, the effects of the different boulder arrangements on the flow resistance law were investigated by imposing a concentration threshold. The results demonstrated that (i) the Darcy–Weisbach friction factor can be accurately estimated by the proposed flow resistance equation, (ii) the flow resistance increases with Ch for low values (<48%) of concentration, while it does not depend on Ch for high concentrations (≥48%), and (iii) the effect of the boulder arrangement on flow resistance law is more evident for low element concentrations.

Roughness of natural aggregates by interferometry and its microstructure

Surface roughness of coarse aggregates, regardless of its known influence in the bonding strength with the cement paste in the hardened state and in the viscosity of the composite in the fresh state, it still a barely researched and discussed topic. In addition, little is known about what rock characteristics influences the multiscale roughness of aggregates. This paper aims to evaluate the roughness of distinct types of aggregates in multiscale (macro and micro scale of roughness) using a 3D interferometer and establish a quantitative relation between the granulometry and mineral composition of the rocks and the surface roughness of aggregates. Six types of aggregates, five crushed and one gravel from riverbed, with sizes ranging from 19 to 25 mm were scanned. One sample from each type was chosen for a petrographic analysis. The six types of aggregates analyzed showed statistical differences between them in macro and micro scales of roughness. In the microscale roughness, basalt was considered the smoothest, and surprisingly, the quartzite (gravel), were one of the roughest. In the macro scale roughness, the quartzite (the gravel) was smoother than all the other samples due its rounded shape, while the gneiss was the most irregular particle and had the highest roughness. The quartzite (gravel) is a good example of how an aggregate could be very rough in a microscale (invisible to the naked eye), comparable to crushed aggregates, while being smooth to the touch and rounded. The weathering seems to affect the roughness at micro(nano)scales, independently of the roundness shape observed at millimetric scale. The micro scale roughness appeared to be related to the average size of the grains, the smaller the average grain size, the lower the micro scale roughness is. Rocks with aphanitic textures tends to fracture in smoother surfaces than rocks with phaneritic textures.

Size, shape and orientation of macro-sized substrate protrusions affect the toe and foot adhesion of geckos.

Geckos are excellent climbers using compliant, hierarchically arranged adhesive toes to negotiate diverse terrains varying in roughness at multiple size scales. Here, we complement advancements at smaller size scales with measurements at the macro scale. We studied the attachment of a single toe and whole foot of geckos on macroscale rough substrates by pulling them along, across and off smooth rods and spheres mimicking different geometric protrusions of substrates. When we pulled a single toe along rods, the force increased with the rod diameter, whereas the attachment force of dragging toes across rods increased from about 60% on small diameter rods relative to a flat surface to ∼100% on larger diameter rods, but showed no further increase as rod diameter doubled. Toe force also increased as the pulling changed from along-rod loading to across-rod loading. When toes were pulled off spheres, the force increased with increasing sphere diameter as observed for along-rod pulling. For feet with separated toes, attachment on spheres was stronger than that on rods with the same diameter. Attachment force of a foot decreased as rod and sphere size increased but remained sufficient to support the body weight of geckos. These results provide a bridge to the macroscale roughness seen in nature by revealing the importance of the dimension, shape and orientation of macroscale substrate features for compliant toe and foot function of geckos. Our data not only enhance our understanding of geckos' environmental adaptive adhesion but can also provide inspiration for novel robot feet in development.

Synthesizing the Roughness of Textured Surfaces for an Encountered-Type Haptic Display Using Spatiotemporal Encoding.

Encountered-type haptic rendering provides realistic, free-to-touch, and move-and-collide haptic sensation to a user. However, inducing haptic-texture sensation without complicated tactile actuators is challenging for encountered-type haptic rendering. In this article, we propose a novel texture synthesizing method for an encountered-type haptic display using spatial and temporal encoding of roughness, which provides both active and passive touch sensation requiring no complicated tactile actuation. Focused on macro-scale roughness perception, we geometrically model the textured surface with a grid of hemiellipsoidal bumps, which can provide a variety of perceived roughness as the user explores the surface with one's bare hand. Our texture synthesis method is based on two important hypotheses. First, we assume that perceptual roughness can be spatially encoded along the radial direction of a textured surface with hemiellipsoidal bumps. Second, perceptual roughness temporally varies with the relative velocity of a scanning human hand with respect to the surface. To validate these hypotheses on our spatiotemporal encoding method, we implemented an encountered-type haptic texture rendering system using an off-the-shelf collaborative robot that can also track the user's hand using IR sensors. We performed psychophysical user tests with 25 participants and verified the main effects of spatiotemporal encoding of a textured model on the user's roughness perception. Our empirical experiments imply that the users perceive a more rough texture as the surface orientation or the relative hand motion increases. Based on these findings, we show that our visuo-haptic system can synthesize an appropriate level of roughness corresponding to diverse visual textures by suitably choosing encoding values.

A molecular level performance manipulation of thermal conductivity and moisture diffusivity through a composite membrane considering interfacial resistance

A molecular dynamics simulation (MDS) technique is used to study the heat and mass transfer inside the composite PVDF (polyvinylidene fluoride)/SiO2 (Si aerogel) bilayer membrane at molecular level. The reverse non-equilibrium molecular dynamics (RNEMD) simulation method is employed to determine the thermal conductivity at each layer and at the interface. Moisture diffusivity is also modeled by the mean square displacements (MSD) of water vapor in the materials. In combination with macro scale resistance models for porous media, the heat conductivity and moisture diffusivity through the whole membrane are calculated. It is found that the interface has a great influence on the membrane overall resistance, especially for thinner membranes. Molecular groups can be manipulated to adjust this resistance. When the hydroxyl groups are increased from 58% to 100%, the membrane overall heat conductivity can be increased by 57%, and the diffusivity can be increased by 48%. The fractional free volume, vibrational density of states (VDOS), adhesion energy, thermal and moisture resistance in each layer and at the interface are investigated to disclose the transport mechanisms. Membranes of low conductivity and high moisture diffusivity that are desired for air humidification applications can be obtained by such molecular level manipulations. By this work, a multi-scale model for interfacial heat and mass transfer resistance between two contacting materials, that considers both macro-scale roughness and porosity and micro-scale molecular interacting, is provided.

Experimental and Numerical Study of Evaporation From Wavy Surfaces by Coupling Free Flow and Porous Media Flow

AbstractThe macroscale roughness of the soil surface has significant influences on the mass/energy interactions between the subsurface and the atmosphere during evaporation. However, most previous works only consider evaporation behavior from flat surfaces. Based on experimental and numerical approaches, the goal of this work is to provide a framework for the understanding of the mechanisms of evaporation from irregular soil surfaces at representative elementary volume scale. A coupling free flow‐porous media flow model was developed to describe evaporation under nonisothermal conditions. For simplicity, sinusoidal‐type wavy surfaces were considered. To validate this modeling approach, an experiment using an open‐ended wind tunnel and soil tank was conducted. The experimental system was instrumented with various environmental sensors to continuously collect atmospheric and subsurface data. Results demonstrate that the surface roughness directly affects both atmospheric and diffusion‐dominated stages I and II evaporation behavior, respectively. The atmospheric conditions directly affect the boundary layer during stage I. The evaporation rate is determined by the diffusion in the boundary layer, but not that in the porous media. The soil properties exert intrinsic influence on the capillary flow and determine the evaporation amount. The complex interaction between capillarity and the boundary layer leads to a heterogeneous distribution of evaporative flux with undulation (i.e., location along the soil surface) and time. Additionally, more and steeper waves indicate more influence from capillary flow, enhancing evaporation compared to a single wave system with the same wave amplitude, while steeper waves also result in a thicker boundary layer and weaken evaporation.

Biomimicking wetting properties of spider web from Linothele megatheloides with electrospun fibers

In this study, we showed a simple approach to biomimic the wetting properties of spider webs, which can be mainly attributed to the geometry of fibers. We created biomimetic fibers using electrospun polyvinylidene fluoride (PVDF) with a wrinkled surface similar to the morphology of spider silk bundles produced by Linothele megatheloides. Without any chemical modification and copying the silk bundles geometry, we successfully translated the similar hydrophobic properties to an electrospun network of fibers. The novelty of this approach lays in obtaining similar macroscale roughness parameters, responsible here for wetting contact angles, due to the substitution of spider silk bundles with individual wrinkled electrospun fibers. The presented methods open new creative solutions for manufacturing anti-wetting surfaces.

Towards the shortest possible contact time: Droplet impact on cylindrical superhydrophobic surfaces structured with macro-scale features

HypothesisRecently, it has been shown that the contact time of impacting water droplets on a superhydrophobic cylindrical surface decreases when its radius becomes comparable to that of the droplet, yet the correlation of this reduction with the impact velocity is unclear. Moreover, on a flat surface, experiments involving the addition of a single macrotexture, along with covering the surface with macroscopic cylindrical ridges (ribbed pattern), have been reported to shorten the contact time. Hence, a ribbed-curved surface with an additional macrotexture may logically lead to a very short contact time.ExperimentsSuch a surface was obtained by utilizing an extruder-type 3D printer and a copper wire was used as the additional macrotexture. The bouncing of water droplets of three different volumes on curved and ribbed-curved samples with two different diameters was investigated for varied impact velocities. FindingsFor the curved surfaces, a scaling model for the contact time reduction with respect to the impact velocity was found. Our data show that adding a wire to the peak of a cylinder containing both micro- and macro-scale roughness (wired ribbed-curved surfaces) yields the contact time even shorter than the inertial-capillary time scale, an unprecedented phenomenon.

Characterizing the Surface Roughness Length Scales of Lactose Carrier Particles in Dry Powder Inhalers.

Surface roughness is well recognized as a critical physical property of particulate systems, particularly in relation to adhesion, friction, and flow. An example is the surface property of carrier particles in carrier-based dry powder inhaler (DPI) formulations. The numerical characterization of roughness remains rather unsatisfactory due to the lack of spatial (or length scale) information about surface features when a common amplitude parameter such as average roughness ( Ra) is used. An analysis of the roughness of lactose carrier particles at three different length scales, designed for specificity to the study of interactive mixtures in DPI, was explored in this study. Three Ra parameters were used to represent the microscale, intermediate scale, and macroscale roughness of six types of surface-modified carriers. Coating of micronized lactose fines on coarse carrier particles increased their microroughness from 389 to 639 nm while the macroroughness was not affected. Roller compaction at higher roll forces led to very effective surface roughening, particularly at longer length scales. Changes in Ra parameters corroborated the visual observations of particles under the scanning electron microscope. Roughness at the intermediate scale showed the best correlation with the fine particle fraction (FPF) of DPI formulations. From the range of 250 to 650 nm, every 100 nm increase in the intermediate roughness led to ∼8% increase in the FPF. However, the effect of surface roughness was greatly diminished when fine lactose (median size, 9 μm) of comparable amounts to the micronized drug were added to the formulation. The combination of roughness parameters at various length scales provided much discriminatory surface information, which then revealed the "quality" of roughness necessary for improving DPI performance.

Rolling viscous drops on a non-wettable surface containing both micro- and macro-scale roughness

It has previously been shown that when a liquid drop of high viscosity is placed on a non-wettable inclined surface, it rolls down at a constant descent velocity determined by the balance between viscous dissipation and the reduction rate of its gravitational potential energy. Since increasing the roughness of the surface boosts its non-wetting property, the drop should move faster on a surface structured with macrotextures (ribbed surface). Such a surface was obtained from a superhydrophobic soot coating on a solid specimen printed with an extruder-type 3D printer. The sample became superoleophobic after a functionalization process. The descent velocity of glycerol drops of different radii was then measured on the prepared surface for varied tilting angles. Our data show that the drops roll down on the ribbed surface approximately 27% faster (along the ridges) than on the macroscopically smooth counterpart. This faster velocity demonstrates that ribbed surfaces can be promising candidates for drag-reduction and self-cleaning applications. Moreover, we came up with a modified scaling model to predict the descent velocity of viscous rolling drops more accurately than what has previously been reported in the literature.

VIRTIS emissivity of Alpha Regio, Venus, with implications for tessera composition

The composition of Venus tessera terrain is unknown. The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) aboard Venus Express (VEx) collects data that yields the surface emissivity at ∼1μm, which contains information convolving a number of surface properties, including composition. We examine the variation of emissivity in the vicinity of Alpha Regio, which is the largest exposure of tessera terrain imaged by VIRTIS. We find that the emissivity of Alpha Regio tessera is lower than adjacent plains materials and the deposits and flows of Eve corona, both of which have previously been interpreted to be basaltic. The emissivity of the bulk of Alpha is also lower than its western boundary, which is interpreted to comprise plains structurally deformed to the same degree as tessera terrain. This suggests that the lower emissivity of Alpha is independent of structural elements, macroscale roughness, or local sedimentation processes, and is due to material properties like composition or grain size. The deviation of the emissivity of Alpha from that of the plains for which a bulk basaltic composition is well supported corresponds to a significant difference in rock type or surface mineral assemblage. The 1μm emissivity of Alpha is consistent with rocks with low ferrous iron content. This includes felsic igneous rocks like granitoids that form under either water-rich or water-poor conditions. A water-rich origin would require both a hydrosphere and a plate recycling mechanism and thus be limited to the lifetime of surface water on Venus. Alternatively, granitoids could form via the differentiation of basaltic melts. The production of all tessera terrain by this mechanism would require the accumulation and preservation of felsic melts from a volume of mafic magma that exceeds what is preserved in the currently observed plains. Both mechanisms of granitoid formation would require that tessera terrain be formed prior to the emplacement of the plains, consistent with their stratigraphic position. Anorthosites also satisfy the emissivity signature and can form from copious amounts of partial melting of a mafic source. Low emissivity values are also consistent with carbonates, sulfates, phyllosilicates and their dehydration products, which may have formed via weathering of basalts under conditions of higher atmospheric PH2O. All of these hypotheses suggest the mineralogy of Alpha tessera records an extinct era of Venus history and is a key target for future exploration.

Influence of growth stress on the surface morphology of N-polar GaN films grown on vicinal C-face SiC substrates

In-situ stress measurements were used to monitor the growth of N-polar GaN films on vicinal C-face SiC substrates using a two-step temperature process. A reduction in compressive stress in the N-polar GaN and a corresponding decrease in surface roughness were observed as the initial growth temperature was reduced from 1000 °C to 900 °C. The results suggest that compressive stress in N-polar GaN promotes step bunching and macroscale roughness in films grown on vicinal substrates. The reduction in compressive stress is proposed to originate from tensile thermal stress induced by the temperature change in the two-step process.

Enhanced Performance of Steel-Aluminium Cast Nodes through Cold Metal Transfer

At present, enhancement of hybrid metal joints for tensile pull-out load through overlap surfaces with a macro-scale roughness is one scope of research. The macro-scale roughness is established through the modified arc-welding process, called “cold metal transfer pin” (CMT-pin), which enables repetitive manufacturing of arrays of metal reinforcements (pins) on parent metal surfaces.Hybrid metal joints between parent steel sleeves and cast aluminium alloy have been investigated. Joint surfaces of parent steel sleeves, which have cylindrical cross-sections, are modified by cylinder and ballhead pins. Cast metal joints were tested under uniaxial tensile loading. At the same time, their load transfer behaviour was determined. Results of tensile tests of hybrid metal joints with different kinds of pins, as well as with different amounts of pins are presented. Comparisons with reference joints without pins or with steel sleeves containing a harmonious triangular polygon cross-section ("P3G") and endings with an enlarged diameter are carried out.The results show an enhanced load transfer performance in the case of hybrid metal joints with pins, as well as enhanced performance measures compared to reference joints. It is also shown that the pin quantity has a major impact on the ultimate joint strength and the irreversible energy absorption density. The use of a certain quantity of pins leads to a change of the failure shape of the joints, which includes steel sleeve pull-out and pin shear to rupture of the cast aluminium.

Dynamic evolution of interface roughness during friction and wear processes

Dynamic evolution of surface roughness and influence of initial roughness (S(a) = 0.282-6.73 µm) during friction and wear processes has been analyzed experimentally. The mirror polished and rough surfaces (28 samples in total) have been prepared by surface polishing on Ti-6Al-4V and AISI 1045 samples. Friction and wear have been tested in classical sphere/plane configuration using linear reciprocating tribometer with very small displacement from 130 to 200 µm. After an initial period of rapid degradation, dynamic evolution of surface roughness converges to certain level specific to a given tribosystem. However, roughness at such dynamic interface is still increasing and analysis of initial roughness influence revealed that to certain extent, a rheology effect of interface can be observed and dynamic evolution of roughness will depend on initial condition and history of interface roughness evolution. Multiscale analysis shows that morphology created in wear process is composed from nano, micro, and macro scale roughness. Therefore, mechanical parts working under very severe contact conditions, like rotor/blade contact, screws, clutch, etc. with poor initial surface finishing are susceptible to have much shorter lifetime than a quality finished parts.

Cells spreading on Micro-fabricated Silica Thin film Coatings

AbstractFrom a biomaterials perspective, it is now understood that success in the osseointegration of a dental implant is conditioned by its “macro”, “micro” and “nano” scale features. Macro-scale roughness is necessary to improve primary stabilization in the post-surgical phase inducing a peri-implant thin fibrous layer. However, the more complex process in the true cell-material interaction is dependent on micro and nano scale phenomena. There is clear evidence that cell adhesion, proliferation, organization and phenotype are modulated at the micro-scale and that protein absorption is fundamentally a process conditioned at nano-scale.

Bidirectional reflectance of rough particulate media: Ray-tracing solution

We investigate the volume and surface shadowing in particulate porous random media with macroscale surface roughness over the full scattering hemisphere using Monte-Carlo ray tracing. We also study the volume and rough-surface effects using two diffuse bidirectional reflectance models, namely, the Lambertian and Lommel–Seeliger models. First, we examine the volume effects arising from the porosity of the loosely packed particulate medium. Next, we consider additional shadowing due to the macroscale surface roughness modelled as two-dimensional random fields following Gaussian correlation and fractional-Brownian-motion statistics. We pay special attention to the relative difference of the reflectance as a function of angles of incidence and emergence from a particulate medium with and without macroscale roughness, since it is the major factor when considering what information can safely be deduced from measured reflectance data. The main conclusions of our study concern the distribution of the reflectance over the scattering hemisphere. The rough-surface shadowing due to macroscale roughness has a relatively small effect on the total reflectance of porous media for small emergent azimuth angles, but the importance of rough-surface shadowing increases for large angles. Also, when compared to a solid rough surface which shows a single extended brightness maximum at the opposition observation geometry, a particulate surface can have two separated maxima along the zero azimuth angle. Thus, observations at the azimuth angle of 180 ∘ and large emergence angle, and observations sampling well the azimuth angle for large values of incidence angle would seem to give optimal data when considering the inversion of the properties affecting the shadowing of regolith-covered surfaces.

Thermal patterns on the smooth and rough walls in turbulent flows

Here an experimental study of temperature fields on the surface of smooth and rough walls is dealt with, in turbulent channel and pipe flows. The hot-foil infrared technique was used to measure the local (instantaneous and time average) temperatures on the walls. The detailed data on mean and fluctuation temperature distributions are presented. The temperature fields in the vicinity of a single macroscale roughness (particle) as well as a lattice of particles are studied.

Constant flux, turbulent convection data using infrared imaging

Here an experimental study of temperature fields on the surface of smooth and rough walls is dealt with, in turbulent channel and pipe flows. The hot-foil infrared technique was used to measure the local (instantaneous and time average) temperatures on the walls. The detailed data on mean and fluctuation temperature distributions are presented. The temperature fields in the vicinity of a single macroscale roughness (particle) as well as a lattice of particles are studied.