Controlled Surface Roughness Research Articles

Lotus Effect and Friction: Does Nonsticky Mean Slippery?

Lotus-effect-based superhydrophobicity is one of the most celebrated applications of biomimetics in materials science. Due to a combination of controlled surface roughness (surface patterns) and low-surface energy coatings, superhydrophobic surfaces repel water and, to some extent, other liquids. However, many applications require surfaces which are water-repellent but provide high friction. An example would be highway or runway pavements, which should support high wheel–pavement traction. Despite a common perception that making a surface non-wet also makes it slippery, the correlation between non-wetting and low friction is not always direct. This is because friction and wetting involve many mechanisms and because adhesion cannot be characterized by a single factor. We review relevant adhesion mechanisms and parameters (the interfacial energy, contact angle, contact angle hysteresis, and specific fracture energy) and discuss the complex interrelation between friction and wetting, which is crucial for the design of biomimetic functional surfaces.

Hydromechanical properties of 3D printed fractures with controlled surface roughness: Insights into shear-permeability coupling processes

We explore the potentials of three-dimensional (3D) printing to explore hydromechanical processes in laboratory-scale fractures. 3D printing enables replication of designer fractures with quantified and repeatable roughness to examine the interdependencies between mechanical and hydraulic response including, for example, the Barton-Bandis model. The present study successfully probes subtle variations of shear strength and dilation behavior in 3D printed fractures controlled by surface roughness. For constant materials and basic frictional characteristics, shear strength increases with increasing the standard deviation of the surface height with dilation rate principally controlled by the average amplitude of the long period wavelength of the fracture surface and insensitive the secondary (minor amplitude) roughness. Importantly, these sensitivities are manifest in the permeability response. With increasing shear displacement, the fracture permeability first decreases and then increases with oscillations resulting from step changes in fracture-fracture contact architectures. As these distinctive behaviors are consistent with those of real rock fractures, we conclude that 3D printed fractures provide a useful analog to real rock fractures when constraining slip-permeability coupling during shear slip.

Dynamic self-assembly of silver nanoclusters into luminescent nanotubes with controlled surface roughness: Scaffold of superhydrophobic materials

Self-assembly of metal nanoclusters (NCs) have shown applicability, while the types of the structures of NCs aggregates are still limited. Proper control of the pathway complexity for supramolecular self-assembly process is the key to fabricate ideal supramolecular structures. Herein, the multi-pathway co-assembly of silver NCs [Ag6(mna)6] (NH4)6 (Ag6-NCs) with polyethyleneimine (PEI) into diverse nanostructures were systematically studied. Nanotubes with rough surfaces were obtained as following: kinetically trapped metastable nanoparticles formed firstly and transform into thermodynamically favored fibers. Subsequently, the fibers could breakup into nanotubes through “top-down” sculpture. These phenomena have rarely been reported in the field of nanoclusters self-assembly. Furthermore, the Ag6-NCs/PEI nanotubes were found to be a versatile material with aggregation-induce emission (AIE) in aqueous solution that the fluorescence of Ag6-NCs enhanced sharply with the formation of nanotubes owing to the formation of ordered nanotubes which suppress the ligand vibration and block energy loss of nonradiative decay. More importantly, the rough surface and anisotropic structure endow them potential application for fabrication of superhydrophobic surfaces and suppression of “coffee-ring” effects.

Influence of Surface State in Micro-Welding of Copper by Nd:YAG Laser

Laser welding of copper is characterized by low and unstable light absorption around 1000 nm wavelength. Combination of high thermal conductivity and low melting point makes it difficult to obtain good welding quality and leads to low energy utilization. To improve efficiency and welding quality, a technique to enhance process stability using 1064 nm wavelength Nd:YAG laser has been proposed, and absorption rate and molten volume in laser micro-welding were discussed. Since the surface state of specimen affects absorption phenomena, effects of surface shape and surface roughness were investigated. Absorption rate and molten volume were increased by creating appropriate concave holes and by controlled surface roughness. Stable micro-welding process with deep penetration and good surface quality was achieved for transitional processing condition between heat conduction and keyhole welding, by enhanced absorption rate.

Three-Dimensional Compatible Sacrificial Nanoimprint Lithography for Tuning the Wettability of Thermoplastic Materials

We report the tuning of surface wetting through sacrificial nanoimprint lithography (SNIL). In this process, grown ZnO nanomaterials are transferred by imprint into a metallic glass (MG) and an elastomeric material, and then etched to impart controlled surface roughness. This process increases the hydrophilicity and hydrophobicity of both surfaces, the Pt57.5Cu14.7Ni5.3P22.5 MG and thermoplastic elastomer (TPE), respectively. The growth conditions of the ZnO change the characteristic length scale of the roughness, which in turn alters the properties of the patterned surface. The novelty of this approach includes reusability of templates and that it is able to create superhydrophilic and superhydrophobic surfaces in a manner compatible with the fabrication of macroscopic three-dimensional (3D) parts. Because the wettability is achieved by only modifying topography, without using any chemical surface modifiers, the prepared surfaces are relatively more durable.

Vertically Aligned and Surface Roughed Pt Nanostructured Wire Array as High Performance Electrocatalysts for Methanol Oxidation

In this research, an enhanced electrocatalyst based on vertically aligned and surface roughened Pt nanostructured wire arrays (Pt NWAs) has been developed. Our results showed that Pt NWAs possessed extremely high electrochemical performance and efficiency toward methanol oxidation. The three-dimensional nanostructured wire array electrode was prepared with electrodeposition method using anodic aluminum oxide (AAO) membrane on silver substrate, after which the active array electrode was obtained by dissolving the AAO templates. Varying the electrodeposition current density and synthesis time led to the controlled length and surface roughness of the Pt nanostructured wires. Scanning electron microcopy, energy dispersive X-ray spectroscopy, and X-ray powder diffraction were employed to characterize the morphology, composition, and crystal structure of the nanostructured wires. Cyclic voltammetry (CV) and chronoamperometry were used to analyze the performance and endurance of the wire array electrode. Electrochemical impedance spectroscopy measurements provided insights in the electron transfer resistance between the electrode and analyte. Gas chromatography (GC) and UV–vis spectroscopy were carried out to identify the presence of the possible products and the relative concentration change of methanol vs the numbers of cyclic voltammetry scanning. The electrochemical measurements showed only one dominant forward anodic peak and the absence of backward anodic peak. GC results showed that methanol concentration decreased linearly with the CV scanning cycles, and a negligible amount of side products were detected, suggesting a complete methanol oxidation. This prepared electrode exhibited high electrochemical activity, high endurance, and low electron transfer resistance.

Roughness evolution in dewetted Ag and Pt nanoscale films

The surface roughness of nanoscale metal systems plays a key role in determining the systems properties and, therefore, the electrical, optical, etc. response of nanodevices based on them. In this work, we experimentally analyze the roughness evolution in dewetting Ag and Pt films deposited on SiO2 substrate. In particular, after depositing 15 nm-thick Ag or Pt films on the SiO2 substrate, standard annealing processes were performed below the melting temperatures of the metals so to induce the solid-state dewetting of the films. The surface morphology evolution of the Ag and Pt films was studied by means of Atomic Force Microscopy analysis as a function of the annealing temperature T and of the annealing time t. In particular, these analysis allowed to quantify the roughness σ of the Ag and Pt films versus the annealing temperature T and the annealing time t. The analysis of these plots allowed us to draw combined insights on the dewetting process characteristics, on the dewetting-induced roughening properties, and on the material-dependent parameters by the comparison of the results obtained for the Ag film and the Pt film. These analysis, in addition, open perspectives towards the development of a method to produce supported metal films with controlled surface roughness for designed applications.

Facile Synthesis and Self‐Cleaning Application of Bimetallic (CuSn, CuNi) Dendrites

AbstractBimetallic dendrites (Au, Ag, Pt, Pd) have received intense research interest due to their applications in catalysis and sensing. However, reports on rational synthesis of non‐noble bimetallic dendrites that finds self‐cleaning applications are scanty. Here, we demonstrate one step co‐reduction based galvanic replacement reaction (GRR) for synthesis of alloyed bimetallic dendrites (CuSn, CuNi) with controlled surface roughness and chemical composition. Probing their growth process reveals that i) for CuSn dendrites, co‐reduction of Cu2+ and Sn2+ is observed throughout GRR; ii) for CuNi dendrites, Cu nanoparticle, that deposits initially, augments the reduction of Ni2+ which leads to CuNi alloy phase formation during later stages of GRR. These dendrites, naturally having dual degree of surface roughness (micro/nano), have been exploited here for fabricating superhydrophobic surfaces (SHS) with excellent self‐cleaning abilities. They show enabling properties such as high contact angle, minimal contact angle hysteresis, and excellent Cassie state stability with no impalement. They maintain superhydrophobicity when exposed to different environmental conditions (low temperature, high temperature, exposure to corroding solutions and UV radiation). The present facile methodology for fabrication of bimetallic dendrites is beneficial for low‐cost production of functional nano/micro‐materials.

Controlled Hydrophobic Biosurface of Bacterial Cellulose Nanofibers through Self-Assembly of Natural Zein Protein.

A novel, highly biocompatible bacterial cellulose (BC)-zein composite nanofiber with a controlled hydrophobic biosurface was successfully developed through a simple and green solution impregnation method, followed by evaporation-induced self-assembly (EISA) of adsorbed zein protein. The surface hydrophobicity of the zein-modified BC nanofibers could be controlled by simply changing the zein concentration, which is able to tune the morphology of self-assembled zein structures after EISA, thus affecting the surface roughness of composite membranes. Zein self-assembly at low concentrations (5 mg/mL) resulted in the formation of hierarchical zein structures (spheres and bicontinuous sponges) on the BC surface, thus increasing the surface roughness and leading to high hydrophobicity (the water contact angle reached 110.5°). However, at high zein concentrations, these large zein spheres assembled into a flat zein film, which decreased the surface roughness and hydrophobicity of membranes. The homogeneous incorporation of zein structures on the BC surface by hydrogen bonding did not significantly change the internal structure and mechanical performance of BC nanofibers. In comparison with pure BC, the BC-zein nanofibers had a better biocompatibility, showing a significantly increased adhesion and proliferation of fibroblast cells. This is probably due to the rough surface structure of BC-zein nanofibers as well as the high biocompatibility of natural zein protein. The novel BC-zein composite nanofibers with controlled surface roughness and hydrophobicity could be of particular interest for the design of BC-based biomaterials and biodevices that require specific surface properties and adhesion.

Fabrication of rough colloids by heteroaggregation

Colloidal particles are normally considered to be topographically smooth and chemically uniform (down to the nanoscale). Nevertheless, surface heterogeneities do exist and have a strong impact on the particles’ behaviour, especially at interfaces. Surface properties for instance define the wetting of colloids, a process not only of fundamental interest, but also highly relevant for applications, e.g. in the Pickering stabilization of emulsions and foams. In particular, the effect of surface roughness has recently gained increasing interest due to the potential of rough particles to stabilize emulsions and foams more effectively, as well as from a more fundamental point of view in relation to the generation of capillary forces useful for particle assembly. In this work, we present a synthetic pathway for the fabrication of all-silica model rough particles with tuneable and controlled surface roughness and surface chemistry over a broad size range. Additionally, the fabrication of raspberry-like particles with soft asperities is proposed. These experiments have the aim to provide clear, robust and straightforward approaches to realise model systems for the investigation of particle surface roughness-related issues in soft matter.

Fluidization and wall slip of soft glassy materials by controlled surface roughness.

We present a comprehensive study of concentrated emulsions flowing in microfluidic channels, one wall of which is patterned with micron-size equally spaced grooves oriented perpendicularly to the flow direction. We find a scaling law describing the roughness-induced fluidization as a function of the density of the grooves, thus fluidization can be predicted and quantitatively regulated. This suggests common scenarios for droplet trapping and release, potentially applicable for other jammed systems as well. Numerical simulations confirm these views and provide a direct link between fluidization and the spatial distribution of plastic rearrangements.

Morphology-Controlled Coating of Colloidal Particles with Silica: Influence of Particle Surface Functionalization.

We present a general, convenient, and efficient synthetic concept for the coating of colloidal particles with a silica (SiO2) shell of well-defined and precisely controlled morphology and porosity. Monodisperse submicroscopic polystyrene (PS) particles were synthesized via two-stage emulsifier-free emulsion polymerization and subsequent swelling polymerization, enabling selective particle surface modification by the incorporation of ionic (methacrylic acid, MAA) or nonionic (hydroxyethyl methacrylate, HEMA or methacrylamide, MAAm) comonomers, which could be proven by zeta potential measurements as well as by determining the three-phase contact angle of the colloidal particles adsorbed at the air-water and n-decane-water interface. The functionalized particles could be directly coated with silica shells of variable thickness, porosity, and controlled surface roughness in a seeded sol-gel process from tetraethoxysilane (TEOS), leading to hybrid PS@silica particles with morphologies ranging from core-shell (CS) to raspberry-type architectures. The experimental results demonstrated that the silica coating could be precisely tailored by the type of surface functionalization, which strongly influences the surface properties of the colloidal particles and thus the morphology of the final silica shell. Furthermore, the PS cores could be easily removed by thermal treatment, yielding extremely uniform hollow silica particles, while maintaining their initial shell architecture. These particles are highly stable against irreversible aggregation and could be readily dried, purified, and redispersed in various solvents. Herein we show a first example of coating semiconducting CdSe/ZnS nanocrystals with smooth and spherical silica shells by applying the presented method that are expected to be suitable systems for applications as markers in biology and life science by using fluorescence microscopy methods, which are also briefly discussed.

Ellipsoidal Colloids with a Controlled Surface Roughness via Bioinspired Surface Engineering: Building Blocks for Liquid Marbles and Superhydrophobic Surfaces

Understanding the important role of the surface roughness of nano/colloidal particles and harnessing them for practical applications need novel strategies to control the particles' surface topology. Although there are many examples of spherical particles with a specific surface roughness, nonspherical ones with similar surface features are rare. The current work reports a one-step, straightforward, and bioinspired surface engineering strategy to prepare ellipsoidal particles with a controlled surface roughness. By manipulating the unique chemistry inherent to the oxidation-induced self-polymerization of dopamine into polydopamine (PDA), PDA coating of polymeric ellipsoids leads to a library of hybrid ellipsoidal particles (PS@PDA) with a surface that decorates with nanoscale PDA protrusions of various densities and sizes. Together with the advantages originated from the anisotropy of ellipsoids and rich chemistry of PDA, such a surface feature endows these particles with some unique properties. Evaporative drying of fluorinated PS@PDA particles produces a homogeneous coating with superhydrophobicity that arises from the two-scale hierarchal structure of microscale interparticle packing and nanoscale roughness of the constituent ellipsoids. Instead of water repelling that occurs for most of the lotus leaf-like superhydrophobic surfaces, such coating exhibits strong water adhesion that is observed with certain species of rose pedals. In addition, the as-prepared hybrid ellipsoids are very efficient in preparing liquid marble-isolated droplets covered with solid particles. Such liquid marbles can be placed onto many surfaces and might be useful for the controllable transport and manipulation of small volumes of liquids.

Microstructure and Optical Properties of SS/Mo/Al2O3 Spectrally Selective Solar Absorber Coating

Surface-textured Mo thin film is fabricated by magnetron sputtering through the adjustment of deposition parameters, which exhibits a high absorptance of 0.80 and a low emittance of 0.09. The single-layer Mo deposited on stainless steel (SS) is characterized by x-ray diffraction (XRD), ultra-high resolution scanning electron microscope, atomic force microscope and optical measurement. The controlled surface roughness combined with larger aspect ratio contributes much to the high absorptance and low emittance. Based on the SS/Mo coating, a spectrally selective coating (SS/Mo/Al2O3) is designed and fabricated. The coating shows an amorphous structure and exhibits an absorptance of 0.90 and an emittance of 0.08. Tauc-Lorentz and Drude free-electron models are used to modeling the optical properties of Al2O3 and Mo layers by phase-modulated spectroscopic ellipsometry.

Unraveling the nucleation and growth of spontaneous surface relief gratings

Nucleation and growth of spontaneous surface relief gratings (SSRGs) on a Disperse Red 1 (DR1) glass-forming derivative were investigated. No interference pattern is applied and surface patterning is induced using single-beam irradiation: the gratings are self-organized. Grating growth is assumed to initiate from an interference pattern formed between the incident light beam and waves scattered at grazing angle by surface defects. However, the mechanism is not yet fully understood and there is not a comprehensive explanation of the structure formation process. Herein, the grating formation procedure is studied by monitoring the surface topology of thin films exposed to one writing beam for various periods of time, under both linear and circular polarizations, using AFM. Even in the absence of surface defects on the initial film, irradiation produces light-induced surface defects due to the reorientation and mass movement of the azo molecules. These defects act as seeds for SSRG around which gratings gradually emerge and propagate throughout the sample. To consolidate this hypothesis, the formation of gratings was studied on samples with controlled surface roughness. Pore-shaped defects do not diffract light on top of the sample, and thus have no impact on SSRG growth, while for hill-shaped defects, growth rate decreases sharply with defect sizes larger than the writing beam wavelength. Two other analogous glass-forming azobenzene derivatives were studied, and in all cases, SSRG formation was correlated with the induction of birefringence in the early stages of the irradiation.

Optical and Geometric Properties of Free Silica Nanoparticles Studied by Small-Angle X-Ray Scattering

Abstract Elastic small-angle X-ray scattering (SAXS) of free silica (SiO2) nanoparticles is reported (d = 100–180 nm). The particles were prepared by a modified Stöber synthesis in narrow size distributions with controlled surface roughness and functionalization. Angle-resolved small-angle X-ray scattering patterns are shown to be sensitive to these changes in particle properties. It is reported that there is an exponential decrease in scattered X-ray intensity towards larger scattering angles as well as distinct oscillations, which is fully explained by Mie theory. Small-angle X-ray scattering of mesoporous nanoparticles with rough surfaces is compared to that of microporous nanoparticles with smooth surfaces, revealing distinct differences that are rationalized by diffuse scattering from nanoparticle pores in addition to the dominating contribution of Mie scattering. Furthermore, results from small-angle X-ray scattering experiments on functionalized silica nanoparticles are presented, where the incorporation of the dye fluorescein isothiocyanate is found to cause changes in the optical properties of the nanoparticles, as compared to non-functionalized samples. Small, but distinct deviations in particle size derived from electron microscopy and from small-angle X-ray scattering are observed. These are rationalized by particle shrinking occurring in electron microscopy as well as slight changes in optical properties of the nanoparticle samples.

Ratio of surface roughness to flow scale as additional parameter for shear-induced hemolysis.

In addition to the conventional knowledge that shear stress and its exposure time should have a large impact on hemolysis, it became obvious through Dr. Maruyama's study that surface roughness would be the additional factor for high shear-induced hemolysis. Concerning this new information, we hypothesized that the ratio of surface roughness to the flow scale should play a role as the additional factor for shear-induced hemolysis. The purpose of this study was to develop a constant shear generator as the method to provide a controlled shear flow field with the combination between the controlled surface roughness and the flow scale to the blood cells. Its preliminary application was to validate our hypothesis. We prototyped the constant shear stress generator with the cylindrical cone-cup structure made from the acrylic material. This chamber had 3 flow scales of 1.00, 1.25, and 1.5 mm according to the change of the inner stationary cone, at which the surface roughness was distributed into the several levels between 0.14 and 0.92 micrometers in arithmetic average roughness. Using this shear chamber, we examined what effect the flow scale and the surface roughness had on hemolysis. Our experimental data showed the tendency of a positive correlation between the ratio of surface roughness to the flow scale and the induced hemolysis levels, validating our hypothesis. The ratio of the surface roughness to the flow scale should be the additional parameter for shear-induced hemolysis.

RGD and BMP-2 mimetic peptide crosstalk enhances osteogenic commitment of human bone marrow stem cells

Human bone marrow mesenchymal stem cells (hBMSCs) commitment and differentiation are dictated by bioactive molecules sequestered within their Extra Cellular Matrix (ECM). One common approach to mimic the physiological environment is to functionalize biomaterial surfaces with ECM-derived peptides able to recruit stem cells and trigger their linage-specific differentiation. The objective of this work was to investigate the effect of RGD and BMP-2 ligands crosstalk and density on the extent of hBMSCs osteogenic commitment, without recourse to differentiation medium. RGD peptide promotes cell adhesion via cell transmembrane integrin receptors, while BMP-2 peptide, corresponding to residues 73–92 of Bone Morphogenetic Protein-2, was shown to induce hBMSCs osteoblast differentiation. The immobilization of peptides on aminated glass was ascertained by X-ray Photoelectron Spectroscopy (XPS), the density of grafted peptides was quantified by fluorescence microscopy and the surface roughness was evaluated using Atomic Force Microscopy (AFM). The osteogenic commitment of hBMSCs cultured on RGD and/or BMP-2 surfaces was characterized by immunohistochemistry using STRO-1 as specific stem cells marker and Runx-2 as an earlier osteogenic marker. Biological results showed that the osteogenic commitment of hBMSCs was enhanced on bifunctionalized surfaces as compared to surfaces containing BMP-2, while on RGD surfaces cells mainly preserved their stemness character. These results demonstrated that RGD and BMP-2 mimetic peptides act synergistically to enhance hBMSCs osteogenesis without supplementing the media with osteogenic factors. These findings contribute to the development of biomimetic materials, allowing a deeper understanding of signaling pathways that govern the transition of stem cells towards the osteoblastic lineage. Statement of SignificanceFor a long time, scientists thought that the differentiation of Mesenchymal Stem Cells (MSCs) into bone cells was dictated by growth factors. This manuscript shed light on other ligands that play a crucial role in regulating MSCs fate. In concrete terms, it was demonstrated that the osteoinductive effect of BMP-2 peptide is 2 folds improved in the presence of adhesive RGD peptide. Compared to previous works highlighting this synergistic cooperation between RGD and BMP-2 peptides, the main strength of this work lies to the use of primitive human cells (hMSCs) and well-defined biomimetic material surfaces (controlled surface roughness and peptide densities). This work provides valuable insights to develop custom-designed in vitro cell culture models, capable of targeting the desired cell response.

An advance Towards the Synthesis of Ag Nanorod Arrays with Controlled Surface Roughness for SERS Substrates

An innovative approach to produce silver nanorod (NRs) arrays with controlled morphological parameters and surface roughness is presented. The Ag NRs were obtained using a three-stage fabrication process based on the electron beam exposure of a metal-polymer nanocomposite resist on a transparent substrate and development, a post bake and then a series of non-electrochemical metallization steps. After each step the evolution of the Ag NRs was characterized by scanning electron microscopy (SEM) for morphology and optical transmittance (T) measurements for Localized Surface Plasmon Resonance (LSPR). The transmittance measurements were interpreted using models based on the Finite Element Method (FEM).

Shaping Nanoparticles with Hydrophilic Compositions and Hydrophobic Properties as Nanocarriers for Antibiotic Delivery.

Inspired by the lotus effect in nature, surface roughness engineering has led to novel materials and applications in many fields. Despite the rapid progress in superhydrophobic and superoleophobic materials, this concept of Mother Nature’s choice is yet to be applied in the design of advanced nanocarriers for drug delivery. Pioneering work has emerged in the development of nanoparticles with rough surfaces for gene delivery; however, the preparation of nanoparticles with hydrophilic compositions but with enhanced hydrophobic property at the nanoscale level employing surface topology engineering remains a challenge. Herein we report for the first time the unique properties of mesoporous hollow silica (MHS) nanospheres with controlled surface roughness. Compared to MHS with a smooth surface, rough mesoporous hollow silica (RMHS) nanoparticles with the same hydrophilic composition show unusual hydrophobicity, leading to higher adsorption of a range of hydrophobic molecules and controlled release of hydrophilic molecules. RMHS loaded with vancomycin exhibits an enhanced antibacterial effect. Our strategy provides a new pathway in the design of novel nanocarriers for diverse bioapplications.