Microscale Surface Structures Research Articles

Relationship of evolution of slip-surface structure in Indian sandstone to changes in friction coefficient for a wide range of slip rates

Although many experimental studies have investigated the frictional properties of sandstone, few of them have considered the evolution of slip-surface structures with changes of friction coefficient over a wide range of slip rates. Here we report the results of rock-on-rock rotary shear experiments on specimens of Indian sandstone at slip rates from 2 × 10−4 to 1 m s−1. The resultant slip behaviors and microscale surface structures indicate two types of dependency on slip rate. At high slip rates (≥5 × 10−1 m s−1), the friction between the sandstone blocks fluctuated markedly and the surfaces of the blocks were intensively worn. At low slip rates (≤1 × 10−1 m s−1), the friction decreased gradually with increasing slip distance and the slip surfaces became reflective. The high slip-rate experiments produced abundant rock fragments (∼10–100 μm diameter), whereas the slip surfaces after low slip-rate experiments were highly polished fault mirrors accompanied by abundant rounded ultrafine grains (∼100 nm diameter). Such slip-rate-dependent evolution of slip-surface structures in Indian sandstone may arise from the relatively low cohesiveness of that rock. This insight may further our understanding of faulting and sliding mechanisms in sandstone near the ground surface.

GIS-based revision of a WUDAPT Local Climate Zones map of Bern, Switzerland

Urban areas are particularly affected by heatwaves through the intensification of heat stress by the urban heat island effect. For effective climate change adaptation, information about microscale surface cover, structures, and human activity in cities is needed to depict the underlying causes of urban heat stress. The framework of “Local Climate Zones” (LCZs) classifies and standardizes urban areas based on such characteristics. To date, most LCZ mapping workflows use satellite imagery as input. The resulting maps may lack some important details, and thus benefit from the use of additional geodata. We introduce a novel approach that combines the geodata of urban canopy parameters with the remote sensing-based LCZ map of Bern, Switzerland. City-specific urban canopy parameters are calculated and used to adjust established value ranges, if necessary. The most common misclassification patterns are identified and misclassified pixels are corrected using a decision tree and k-nearest-neighbor algorithm. Results show that the conformity with the urban canopy parameter values markedly increased, especially in the distinction of water surfaces, non-built areas, and building height. However, for high-resolution LCZ maps, this also leads to unnecessary heterogeneity, which may require further postprocessing. Given sufficiently available urban canopy parameter data, the proposed workflow is simple and easily adaptable for other cities. It could prove useful in urban climate studies and city planning to enhance an existing LCZ map in a contextualized manner quickly.

Microbial mat compositions and localization patterns explain the virulence of black band disease in corals

Black band disease (BBD) in corals is characterized by a distinctive, band-like microbial mat, which spreads across the tissues and often kills infected colonies. The microbial mat is dominated by cyanobacteria but also commonly contains sulfide-oxidizing bacteria (SOB), sulfate-reducing bacteria (SRB), and other microbes. The migration rate in BBD varies across different environmental conditions, including temperature, light, and pH. However, whether variations in the migration rates reflect differences in the microbial consortium within the BBD mat remains unknown. Here, we show that the micro-scale surface structure, bacterial composition, and spatial distribution differed across BBD lesions with different migration rates. The migration rate was positively correlated with the relative abundance of potential SOBs belonging to Arcobacteraceae localized in the middle layer within the mat and negatively correlated with the relative abundance of other potential SOBs belonging to Rhodobacteraceae. Our study highlights the microbial composition in BBD as an important determinant of virulence.

Hybrid-Light-Source Stereolithography for Fabricating Macro-Objects With Micro-Textures

Abstract Microscale surface structures are commonly found on macroscale bodies of natural creatures for their unique functions. However, it is difficult to fabricate such multi-scale geometry with conventional stereolithography processes that rely on either laser or digital micromirror device (DMD). More specifically, the DMD-based mask projection method displays the image of a cross section of the part on the resin to fabricate the entire layer efficiently; however, its display resolution is limited by the building area. In comparison, the laser-based vector scanning method builds smooth features using a focused laser beam with desired beam-width resolution; however, it has less throughput for its sequential nature. In this paper, we studied the hybrid-light-source stereolithography process that integrates both optical light sources to facilitate the fabrication of macro-objects with microscale surface structures (called micro-textures in the paper). The hardware system uses a novel calibration approach that ensures pixel-level dimensional accuracy across the two light sources. The software system enables designing the distribution and density of specific microscale textures on a macro-object by generating projection images and laser toolpaths for the two integrated light sources. Several test cases were fabricated to demonstrate the capability of the developed process. A large fabrication area (76.8–80.0 mm) with 50 μm micro-features can be achieved with a high throughput.

Investigation and Application of Bio-Inspired Microscale Structures to Turbine Endwall for Enhancing Film Cooling Performance

Abstract In this paper, inspired by micro-scale surface structure patterns of birds’ flight feather, a novel thermal-protection enhancement technique is proposed for a turbine endwall with cavity air leaked from an upstream realistic rim seal. The potential application of the micro-scale surface structures is demonstrated by using well-validated numerical simulations. Overall film cooling performance and total thermodynamic loss are examined to comprehensively evaluate the feasibility of the micro-scale surface patterns for three cavity air flowrates. Detailed thermal fields and flow structures involved in the coolant–mainstream interactions are also presented to reveal physical flow mechanisms behind. Comparisons with a smooth endwall and previously used ribbed endwall cases show that applying the micro-scale structures onto the turbine endwall generates a substantial increase in cooling effectiveness with a very slight aerodynamic penalty. Despite of enhanced heat transfer levels, the resulting heat loads into the endwall are significantly reduced. Detailed thermal fields and flow structures revealed that the addition of the micro-scale structures induces counter-rotating vorticities that mitigate the passage vortex and hence prevents the accumulation of coolant near the suction side. Besides, it is also noted that the grooves of the micro-scale structures have a transport effect that can convey the coolant to the far downstream areas of the endwall, and thus considerably increase the coverage in the chordwise direction. The findings of this work provide an evidence that the bio-inspired, micro-scale surface structures could be an alternative for enhancing turbine endwall cooling performance.

Condensation Frosting on Micropillar Surfaces - Effect of Microscale Roughness on Ice Propagation.

Microscale surface structures have been widely explored as a promising tool for antifreezing or frost avoidance on heat transfer surfaces. Despite studies of many surface feature designs, the mechanisms associated with condensation freezing and ice propagation on microstructured surfaces have yet to be thoroughly elucidated, espectially when it comes to quantitative understanding. In this work, condensation freezing on circular micropillar surfaces is investigated, with varying pillar spacing and height (layout/microscale roughness) but a constant pillar diameter. The pillar layout is found to have significant effects on both liquid nucleation and neighboring droplet interactions, as reflected by the condensation droplet distribution prior to soilidification and eventually the freezing front propagation area velocity. In general, nucleation is preferred on the pillar top rather than the bottom of the pillared surface unless there is a large distance between the pillars. Interactions between neighboring droplets solely on pillar tops (or bottom surfaces) can induce heterogeneity in the droplet distribution and slow freezing front propagation. Based on the roles the pillars play in nucleation, droplet coalescence, and ice bridging, four different condensation states are identified and related to the layout of the pillars, and the freezing front area propagation velocity is found to be different in each state. The findings provide a quantitative basis for designing antifreezing surfaces, applicable to a wide range of thermal systems.

Active Surface with Dynamic Microstructures and Hierarchical Gradient Enabled by in situ Pneumatic Control.

An active surface with an on-demand tunable topography holds great potential for various applications, such as reconfigurable metasurfaces, adaptive microlenses, soft robots and four-dimensional (4D) printing. Despite extensive progress, to achieve refined control of microscale surface structures with large-amplitude deformation remains a challenge. Moreover, driven by the demand of constructing a large area of microstructures with increased complexity—for instance, biomimetic functional textures bearing a three-dimensional (3D) gradient—novel strategies are highly desired. Here, we develop an active surface with a dynamic topography and three-tier height gradient via a strain-tunable mismatching-bonding process. Pneumatic actuation allows for rapid, reversible and uniform regulation of surface microstructures at the centimeter scale. The in-situ modulation facilitates large-amplitude deformation with a maximum tuning range of 185 μm. Moreover, the structural gradient can be modulated by programming the strain value of the bonding process. With our strategy, another two types of surfaces with a four-tier gradient and without gradient were also prepared. By providing active modulation and design flexibility of complicated microstructures, the proposed strategy would unlock more opportunities for a wealth of novel utilizations.

Rational Design of Transparent Nanowire Architectures with Tunable Geometries for Preventing Marine Fouling

AbstractMarine biofouling is a sticky global problem that hinders maritime industries. Various microscale surface structures inspired by marine biological species have been explored for their anti‐fouling properties. However, systematic studies of anti‐marine‐fouling performance on surface architectures with characteristic length‐scales spanning from below 100 nm to greater than 10 µm are generally lacking. Herein, a study on the rational design and fabrication of ZnO/Al2O3 core–shell nanowire architectures with tunable geometries (length, spacing, and branching) and surface chemistry is presented. The ability of the nanowires to significantly delay or prevent marine biofouling is demonstrated. Compared to planar surfaces, hydrophilic nanowires can reduce fouling coverage by up to ≈60% after 20 days. The fouling reduction mechanism is mainly due to two geometric effects: reduced effective settlement area and mechanical cell penetration. Additionally, superhydrophobic nanowires can completely prevent marine biofouling for up to 22 days. The nanowire surfaces are transparent across the visible spectrum, making them applicable to windows and oceanographic sensors. Through the rational control of surface nano‐architectures, the coupled relationships between wettability, transparency, and anti‐biofouling performance are identified. It is envisioned that the insights gained from the work can be used to systematically design surfaces that reduce marine biofouling in various industrial settings.

Microscale structure designing of nanoparticle-based photocatalyst films for enhanced functionalities and efficient energy utilization

Photocatalysts have increasingly become important materials to utilize renewable solar energy for decomposing pollutants, producing clean water, achieving self-cleaning surface, and so on. Therefore, it is desirable to expand the applications of photocatalysts by enhancing the functionalities and allowing ample design flexibility. Here the potential of nanoparticle-based photocatalyst films with microscale surface structures was investigated in order to enhance their useful functionalities. Photocatalyst films with microscale surface structures were prepared using the suspension of titanium dioxide (TiO2) nanoparticles. The micro-scale structural features of prepared films were evaluated using a scanning electron microscope. Also, an atomic force microscope (AFM) was used. The AFM analysis of a film consisting of nanoparticle spherical aggregates revealed a surface profile of subwavelength surface structure. Then the optical and wetting characteristics were investigated. It was found that the visible-light transmittance increased due to subwavelength surface structures and that the microstructured TiO2 films exhibited the contact angles below 10 degrees, i.e., superhydrophilic behavior, without ultraviolet-light illumination. On the basis of the presented results, it was suggested that the microscale surface structures of photocatalyst film can be designed to achieve enhanced functionalities. It is expected to effectively utilize the energy of sunlight in many applications using functional photocatalyst films with optimized design.

The effect of sub-facet scale surface structure on wall brightness temperatures at multiple scales

Wall surface temperatures are important components of urban climates but are under-sampled by satellite and airborne remote sensing and at the microscale are under-sampled in observational studies. In urban canopy models, they are represented with simplistic geometries. This study examines the effect of microscale (sub-facet) surface structure geometries on wall surface brightness temperature distributions at micro- to neighbourhood scales using mobile sampling traverses of two suburban neighbourhoods with different sub-facet geometries. Visible and thermal imagery were recorded simultaneously and combined and classified to create a database of temperatures with associated geographic and thermal attributes. This study investigates (1) if sub-facet scale geometries affect temperature distributions, (2) if these cause canyon scale biases, and (3) if there are therefore inter-neighbourhood biases. It is shown that sub-facet geometries modify wall surface temperatures predominantly by cooling due to self-shading. Surface-sun geometry thus leads to intra- and inter-neighbourhood temperature differences of several degrees Celsius. The observed effects have important implications for modelling of urban surface temperatures, where simplified geometries may overestimate wall surface temperatures.

Geometric modulation of microscale surface structures for defined stimulation of osteogenic differentiation on implant materials

AbstractMetal implants for orthopedic applications have been in use for a long time. Improvements of the osseointegration behaviour by surface‐changing processes, such as sand blasting or etching, were so far only obtained in empirical studies. It is not yet completely understood which surface chemistry and structure can support the initial cell‐implant response. Electron beam material processing in vacuum has been used successfully for many decades. The method has not been used for the targeted micro‐structuring of implant material surfaces yet. Especially the material processing in vacuum reveals advantages, due to the high affinity of titanium for oxygen. The electron beam structuring process is based on the Surfi‐Sculpt process developed by TWI ‐ The Welding Institute. A repetitive beam overflow establishes in several steps defined microstructures. This study presents the results of the investigation of the Surfi‐Sculpt process of the implant material TiAl6 V4, in particular the influence of different process parameters on the formation of structure height and geometry. Particularly the local alloy composition of the micro‐structured surface is investigated.

Bioinspired Functional Surfaces Enabled by Multiscale Stereolithography

AbstractAdditive manufacturing has many advantages in creating highly complex customized structures. In this study, a low‐cost multiscale stereolithography technology that can print a macroscale object with microscale surface structures with high throughput is demonstrated. The developed multiscale stereolithography is realized by dynamic switching of laser spot size and adaptively sliced layer thickness. An optical filter based on subwavelength resonance grating is used to modify laser spot size for lasers with different wavelengths and achieves a maximum resolution of 37 µm. The multiscale stereolithography process has 4.4× throughput improvement compared with the traditional stereolithography process with a single laser spot. For proof‐of‐concept testing, artificial shark skins with microriblet features are designed and 3D printed. In pipe flow experiments, the 3D printed shark skin demonstrates almost 10% average fluid drag reduction. Artificial lotus leaf surfaces are also 3D printed to demonstrate superhydrophobic property. This new process has the potential to serve as a powerful tool that can bring bioinspired structures into real‐life applications.

Free-Standing Metal Films Prepared via Electroless Plating at Liquid-Liquid Interfaces.

We report a simple preparation of free-standing metal films via electroless plating (ELP) at the liquid-liquid (L-L) interface between an aqueous electroless plating solution and an organic solvent. The use of ELP does not require any external energy in the form of heating and stirring. We find that the affinity of the organic solvent for the palladium nanoparticles (PdNPs) as catalysts and the vertical position of the organic and aqueous phases in the biphasic system are important considerations for synthesizing a robust copper film. Specifically, 1,2-dichloroethane which has an appropriate affinity for PdNPs and a higher density than water was found to be a good candidate for use as the organic phase in this system. However, a poor-quality copper film was obtained in the system with 1-hexanol as the organic phase. We also controlled the microscale surface structure of the copper films by using different concentrations of the injected PdNP dispersion. A high density of PdNPs caused smaller regions of metal growth, which contributed to the formation of smoother metal films. Moreover, under the optimal synthesis condition, we confirmed the electrical conductivity of the obtained copper film to be 1.16 × 10-7 Ωm. We believe that this metal film preparation represents a promising way to produce a range of metal film structures through the use of flexible L-L interfaces as templates.

Surface modification of bulk titanium substrates for biomedical applications via low-temperature microwave hydrothermal oxidation.

Micro-to-nanoscale surface topographies of orthopaedic and dental implants can affect fluid wetting and biological response. Nanoscale features can be superimposed on microscale roughness of titanium (Ti) surfaces at high temperatures, resulting in increased osteoblast differentiation. However, high temperatures can compromise mechanical properties of the bulk material. Here, we have developed a novel low-temperature microwave hydrothermal (MWHT) oxidation process for nanomodification of microrough (SLA) Ti surfaces. Nanoscale protuberances (20 -100 nm average diameter) were generated on SLA surfaces via MWHT treatment at 200°C in H2 O, or in aqueous solutions of H2 O2 or NH4 OH, for times ranging from 1 to 40 h. The size, shape, and crystalline content of the nanoprotuberances varied with the solution used and treatment time. The hydrophilicity of all MWHT-modified surfaces was dramatically enhanced. MG63 and normal human osteoblasts (NHOsts) were cultured on MWHT-treated SLA surfaces. While most responses to MWHT-modified surfaces were comparable to those seen on SLA controls, the MWHT-generated nanotopography reduced osteocalcin production by NHOst cells, suggesting that specific nanotopographic characteristics differentially mediate osteoblast phenotypic expression. MWHT processing provides a scalable, low-temperature route for tailoring nanoscale topographies on microroughened titanium implant surfaces with significantly enhanced wetting by water, without degrading the microscale surface structure of such implants. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 782-796, 2018.

Wettability modification of polydimethylsiloxane surface by fabricating micropillar and microhole arrays

Fabrication of microscale surface structures is a promising method for controlling the surface wettability of materials. Herein, the micropillar and microhole arrays of polydimethylsiloxane (PDMS) were fabricated through a soft lithography process, and the modified wettability was investigated. Using PDMS samples with different degrees of crosslinking, a PDMS–PDMS pattern transfer was achieved, and the microhole and micropillar arrays were obtained from a single master substrate. The surface geometries of the prepared PDMS samples were characterized via scanning electron microscopy and laser scanning microscopy. It was demonstrated that the PDMS micropillar and microhole arrays were effective in changing the contact angles of liquids with different surface tensions (water, ethylene glycol, olive oil, and hexadecane). The measured contact angles of the liquids on a PDMS micropillar array were quantitatively interpreted in both the wetting and repellent regimes based on the Cassie-Baxter model.

On-Machine Surface Texture Measuring System Using Laser Speckle Pattern Analysis

Functional surfaces are expected technologies in various industries. However, the efficient generation of microscale surface structures is difficult because the process relies on precision surface texture assessments, which require long measuring times. An on-machine measuring system may solve this issue. In this study, a new on-machine surface texture measuring system based on laser speckle pattern analysis is proposed. The proposed system efficiently assesses various qualities of the surface texture, such as surface roughness, undulation of microscale surface structures, and anisotropy, from a laser speckle pattern obtained from the precision-machined surface. For precise measurements, disturbances caused by the machining system itself and the environment must be avoided. The laser speckle detection unit in the proposed system is supported by non-contact active vibration-isolation units, which reduce the transmission of ground vibrations. From the experimental results, the system indicated high repeatability in measurements and robustness against disturbances.

Surface nano-texturing of silicon by picosecond laser irradiation through TiO2 nanotube arrays

This article presents, nano-texturing of crystalline silicon by irradiating picosecond laser with variable spatial intensity, caused by optically non-linear TiO2 nanotube arrays (TNTA). Along with micro-scale surface structure, highly ordered laser-induced periodic surface structures (LIPSS) was observed at nano-scale. The periodicity (Λ) of the LIPSS generated was near to the laser wavelength (532nm). Surface morphology at micro-level was characterized by optical microscopy (OM) and white light interferometer (WLI) and at the nano-scale by scanning electron microscope (SEM) and atomic force microscope (AFM). The results highlight the potential use of TNTA as a single step process to produce micro/nanostructures without any gas/liquid medium under ambient condition.

Metallic Light Absorbers Produced by Femtosecond Laser Pulses

Using high-intensity femtosecond laser pulses for surface structuring, technologically important metallic light absorbers (dark Au, W, and Ti alloy) with absorption of about 85–95% over a broad wavelength range from ultraviolet to infrared were produced. It was found that the enhanced absorption of the dark metals is caused by a rich variety of nano-/microscale surface structures. The dark metals produced in this study may find a variety of applications in the fields of renewable energy and energy efficiency, such as thermophotovoltaics, solar energy absorbers, thermal radiation sources, and radiative heat transfer devices.

Preparation of iron oxides using ammonium iron citrate precursor: Thin films and nanoparticles

Ammonium iron citrate (C 6H 8O 7 · nFe · nH 3N) was used as a precursor for preparing both iron-oxide thin films and nanoparticles. Thin films of iron oxides were fabricated on silicon (111) substrate using a successive-ionic-layer-adsorption-and-reaction (SILAR) method and subsequent hydrothermal or furnace annealing. Atomic force microscopy (AFM) images of the iron-oxide films obtained under various annealing conditions show the changes of the micro-scale surface structures and the magnetic properties. Homogenous Fe 3O 4 nanoparticles around 4 nm in diameter were synthesized by hydrothermal reduction method at low temperature and investigated using transmission electron microscopy (TEM).

Micro-manufacturing of micro-scale porous surface structures for enhanced heat transfer applications: an experimental process optimization study

Integrated and compact products necessitate the use of advanced thermal management systems with reduced footprint and cost as well as increased efficiency. Micro-scale, porous and modulated (i.e. channels, pyramids, etc) surfaces offer increased surface area for a given volume and lead to two-phase heat transfer conditions with efficiency enhancements up to 300%. Such surfaces made of copper powders were demonstrated to be quite effective by several researchers after they were produced in controlled lab environments. Similar surfaces made of high temperature resistant materials such as stainless steel, nickel and titanium can also be used in fuel processor, SOFC and PEM fuel cell applications as bipolar/interconnect plates. However, their fabrication under mass-production conditions for marketable and cost-effective products requires well-established process parameters. In this study, warm compaction of copper powders onto thin copper solid substrates was experimented with under different compaction pressure (15–50 MPa), temperature (350–500 °C) and surface geometry (flat, large and small channeled) parameters using a design of experiment (DOE) approach to determine the proper process conditions. Porosity and bonding strength of compacted samples were measured to characterize their feasibility for compact and/or micro-scale heat/mass transfer applications. Results showed that a minimum 350 °C temperature and 15 MPa pressure level is necessary to obtain sound porous and micro-channeled surface layers. It was also found that at higher pressure levels (50 MPa), fabrication of micro-scale surface structures is highly repeatable with enhanced bonding strength characteristics. DOE findings will be used to establish proper process conditions to produce such porous surfaces using a continuous roll compaction process in the future.