Wetting behavior and nanotribological properties of silicon nanopatterns combined with diamond-like carbon and perfluoropolyether films

Comparison between thermophysical and tribological properties of two engine lubricant additives: electrochemically exfoliated graphene and molybdenum disulfide nanoplatelets

Recently graphene and other 2D materials were suggested as nano additives to enhance the performance of nanolubricants and reducing friction and wear-related failures in moving mechanical parts. Nevertheless, to our knowledge there are no previous studies on electrochemical exfoliated nanomaterials as lubricant additives. In this work, engine oil-based nanolubricants were developed via two-steps method using two different 2D nanomaterials: a carbon-based nano additive, graphene nanoplatelets (GNP) and a sulphide nanomaterial, molybdenum disulfide (MoS₂) nanoplatelets (MSNP). The influence of these nano additives on the thermophysical properties of the nanolubricants, such as viscosity index, density and wettability, was investigated. The unique features of the electrochemical exfoliated GNP and MSNP allow the formulation of nanolubricant with unusual thermophysical properties. Both the viscosity and density of the nanolubricants decreased by increasing the nanoplatelets loading. The effect of the nano additives loading and temperature on the tribological properties of nanolubricants was investigated using two different test configurations: reciprocating ball-on-plate and rotational ball-on-three-pins. The tribological specimens were analysed by scanning electron microscopy (SEM) and 3D profiler in order to evaluate the wear. The results showed significant improvement in the antifriction and anti-wear properties, for the 2D-materials-based nanolubricants as compared with the engine oil, using different contact conditions. For the reciprocal friction tests, maximum friction and worn area reductions of 20% and 22% were achieved for the concentrations of 0.10 wt% and 0.20 wt% GNP, respectively. Besides, the best anti-wear performance was found for the nanolubricant containing 0.05 wt% MSNP in rotational configuration test, with reductions of 42% and 60% in the scar width and depth, respectively, with respect to the engine oil.

Toward a Better Understanding of Hemiwicking: A Simple Model to Comprehensive Prediction

The hemiwicking state has attracted much interest because of numerous important potential applications in inking, printing, boiling heat transfer, and condensation. However, the mechanism of the emergence of hemiwicking has not been well understood, especially the effects of geometry of patterned surfaces on the hemiwicking state has not been systematically investigated. Here, we presented a new method to study the critical conditions for hemiwicking on patterned surfaces. By minimizing the variation of the free energy, we obtain the corresponding stable height of the hemiwicking film and find that it is easier for a droplet to be in the hemiwicking state if the pillar surface has small spacing, large radius and height, and a small intrinsic contact angle. Our established model is applied to a flat-topped cylindrical pillar-patterned surface, and the modeling results are in well agreement with experiments and other existing theories. Besides, our model is also applied to other kinds of patterned surfaces including hemispherical-topped cylindrical and conical pillars, about which the other existing theories are deficient. Our theoretical results not only are in well agreement with the experimental observations but also provide some important predictions, which implies that the established model could be applicable to understanding the basic physical mechanism of the hemiwicking state and be useful in guiding the design and fabrication of hemiwicking surfaces.

Improving frictional properties of DLC films by surface energy manipulation

Surface energy is essential to the friction properties of materials, but until now the investigating scope for DLC films has still been narrow. In this paper, we try to expand the surface energy scope of DLC films to their limits by surface modification and study their influence on friction properties. In this case, we not only control the surface energy of DLC films but also manipulate that of the counter balls, by using piranha etching and octadecyltrichlorosilane (OTS) modification. The surface compositions, wettabilities and friction properties of DLC films and counter balls were investigated. The results indicate that the surface energies of DLC films and counter balls can be adjusted successfully in the ranges of 31.2 to 73.73 mJ m⁻² and 15.69 to 72.93 mJ m⁻², respectively. The frictional tests show that all the as-modified DLC films retain relatively stable friction curves, which derive from their good load-carrying and wear-resistance capabilities. Specifically, the DLC-OH covered with vast oxygen-containing groups shows poor frictional properties, owing to its high surface energy and strong adhesion. In contrast, the DLC-OTS exhibits amazing friction reduction properties, due to its ultra-low surface energy and special film structure.

Surface energy is essential to the friction properties of materials, but until now the investigating scope for DLC films has still been narrow.

AFM-based tribological study of nanopatterned surfaces: the influence of contact area instabilities

Although the importance of morphology on the tribological properties of surfaces has long been proved, an exhaustive understanding of nanopatterning effects is still lacking due to the difficulty in both fabricating 'really nano-' structures and detecting their tribological properties. In the present work we show how the probe-surface contact area can be a critical parameter due to its remarkable local variability, making a correct interpretation of the data very difficult in the case of extremely small nanofeatures. Regular arrays of parallel 1D straight nanoprotrusions were fabricated by means of a low-dose focused ion beam, taking advantage of the amorphization-related swelling effect. The tribological properties of the patterns were detected in the presence of air and in vacuum (dry ambient) by atomic force microscopy. We have introduced a novel procedure and data analysis to reduce the uncertainties related to contact instabilities. The real time estimation of the radius of curvature of the contacting asperity enables us to study the dependence of the tribological properties of the patterns from their geometrical characteristics. The effect of the patterns on both adhesion and the coefficient of friction strongly depends on the contact area, which is linked to the local radius of curvature of the probe. However, a detectable hydrophobic character induced on the hydrophilic native SiO2 has been observed as well. The results suggest a scenario for capillary formation on the patterns.

Nanotribological and wetting performance of hierarchical patterns

Surface modification is a promising method to solve the tribological problems in microsystems. To modify the surface, we fabricated hierarchical patterns with different pitches of nano-scale features and different surface chemistries. Micro- and nano-patterns with similar geometrical configurations were also fabricated for comparison. The nano-tribological behavior of the patterns was investigated using an atomic force microscope at different relative humidity levels (5% to 80%) and applied normal loads (40 nN to 120 nN) under a constant sliding velocity. The results showed significant enhancement in the de-wetting and tribological performance of the hierarchical patterns compared with those of flat and micro- and nano-patterned surfaces. The PTFE-coated hierarchical patterns showed similar dynamic contact angles (advancing and receding) to those of the real lotus leaf. The influence of relative humidity on adhesion and friction behavior was found to be significant for all the tested surfaces. The tribological performance was improved as the pitch of the nano-scale geometry of the hierarchical pattern increased, even though the wetting property was not influenced significantly. A model was proposed based on the role of intermolecular force to explain the effect of the pitch of the hierarchical patterns on the adhesion and friction behavior. According to the model based on the molecular force, the contact between a ball and the patterned surface was a multi-asperity contact, contrary to the single-asperity contact predicted by the Johnson-Kendall-Roberts (JKR) and Maugis-Dugdale (MD) models. The strong intermolecular forces, which are activated in the confined spaces between the adjacent nano-pillars and the ball, contributed to the contact area and hence the adhesion and friction forces.

Role of Viscous Dissipative Processes on the Wetting of Textured Surfaces

We investigate the role of viscous forces on the wetting of hydrophobic, semi-hydrophobic, and hydrophilic textured surfaces as second-order effects. We show that during the initial contact, the transition from inertia- to viscous-dominant regime occurs regardless of their surface topography and chemistry. Furthermore, we demonstrate the effect of viscosity on the apparent contact angle under quasi-static conditions by modulating the ratio of a water/glycerol mixture and show the effect of viscosity, especially on the semi-hydrophobic and hydrophobic textured substrates. The reason why the viscous force does not affect the apparent contact angle of the hydrophilic surface is explained based on the relationship between the disjoining pressure and surface chemistry. We further propose a wetting model that can predict the apparent contact angle of a liquid drop on a textured substrate by incorporating a viscous force component in the force balance equation. This model can predict apparent contact angles on semi-hydrophobic and hydrophobic textured surfaces exhibiting Wenzel state more accurately than the Wenzel model, indicating the importance of viscous forces in determining the apparent contact angle. The modified model can be applied for estimating the wetting properties of arbitrary engineered surfaces.

Excellent friction-reducing performance of superhydrophobic steel surface in dry sliding

Superhydrophobic micro/nano-engineered steel surfaces with friction-reducing properties were fabricated by the combination of chemical etching and stearic acid coating.

Electrophoretic coating of amphiphilic chitosan colloids on regulating cellular behaviour

In this communication, we report a facile nanotopographical control over a stainless steel surface via an electrophoretic deposition of colloidal amphiphilic chitosan for preferential growth, proliferation or migration of vascular smooth muscle cells (VSMCs) and human umbilical vein endothelial cells (HUVECs). Atomic force microscopy revealed that the colloidal surface exhibited a deposition time-dependent nanotopographical evolution, wherein two different nanotopographic textures indexed by 'kurtosis' (Rkur) value were easily designed, which were termed as 'sharp' (i.e. high peak-to-valley texture) surface and 'flat' (i.e. low peak-to-valley texture) surface. Cellular behaviour of VSMCs and HUVECs on both surfaces demonstrated topographically dependent morphogenesis, adherent responses and biochemical properties in comparison with bare stainless steel. The formation of a biofunctionalized surface upon a facile colloidal chitosan deposition envisions the potential application towards numerous biomedical devices, and this is especially promising for cardiovascular stents wherein a new surface with optimized texture can be designed and is expected to create an advantageous environment to stimulate HUVEC growth for improved healing performance.

Origin of hydrophobicity in FIB-nanostructured Si surfaces

Surface morphology has been demonstrated to influence the tribological properties at different scales, but the phenomena which occur at the nanoscale have not been completely understood. The present study reports on the effect of focused ion beam nanopatterning on coefficient of friction (CoF) and adhesion of Si(001) surface covered by native oxide. Regular arrays of nanogrooves reduce both CoF and adhesion, related to a hydrophobic character of the patterned surface, but this effect disappears as soon as the separation among the nanostructures approaches the microscopic scale. The dependence of this hydrophobic effect on the pitch is not linked to the corresponding contact area. It has been found that each nanostructure is surrounded by a low-friction region which extends some hundreds of nanometers from it. For pitches of 125 and 250 nm these low-friction regions completely overlap, generating a consistent decrease in CoF and adhesion, while for pitches of 500 and 1000 nm their effect is negligible. The low-friction regions were not observed in humidity-free ambient, indicating that they are the origin of patterns of hydrophobicity.