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Superhydrophobic textured surfaces are known to maintain a nonwetted state unless external stimuli are applied since they can withstand high wetting pressure. Herein, we report a new catego...

We report the surface-energy-dependent wetting transition characteristics of an evaporating water droplet on surface-energy-controlled microcavity structures with functional nanocoatings. The droplet wetting scenarios were categorized into four types depending on the synergistic effect of surface energy and pattern size. The silicon (Si) microcavity surfaces (γSi = 69.8 mJ/m2) and the polytetrafluoroethylene (PTFE)-coated microcavity surfaces (γPTFE = 15.0 mJ/m2) displayed stable Wenzel and Cassie wetting states, respectively, irrespective of time. In contrast, diamond-like carbon (DLC)-coated (γDLC = 55.5 mJ/m2) and fluorinated diamond-like carbon (FDLC)-coated (γFDLC = 36.2 mJ/m2) surfaces demonstrated a time-dependent transition of wetting states. In particular, the DLC-coated surface showed random filling of microcavities at the earlier time point, while the FDLC-coated surface displayed directional filling of microcavities at the late stage of drop evaporation. Such dynamic wetting scenarios based on surface energy, in particular, the random and directional wetting transitions related to surface energy of nanocoatings have not been explored previously. Furthermore, the microscopic role of nanocoating in the wetting scenarios was analyzed by monitoring the time-dependent deformation and movement of the air-water interface (AWI) at individual cavities using the fluorescence interference-contrast (FLIC) technique. A coating-dependent depinning mechanism of the AWI was responsible for variable filling of cavities leading to time-dependent wetting scenarios. A capillary wetting model was used to relate this depinning event to the evaporation-induced internal flow within the droplet. Interestingly, FLIC analysis revealed that a hydrophilic nanocoating can induce microscopic hydrophobicity near the cavity edges leading to delayed and variable cavity filling. The surface energy-dependent classification of the wetting scenarios may help the design of novel evaporation-assisted thermodynamic and mass-transfer processes.

A general relation between adhesion and friction was elusive. This was partly due to the limitations in experimentally configuring the wide variety of geometrical and chemical cues encountered at a sliding contact. We study the combined influence of capillary and van der Waals forces on the correlation between the adhesion (pull-off force) and friction of polymer patterns. We report the existence of master curves in plot of adhesion versus friction, spanning nearly two orders of magnitude, characteristic of the effective lateral contact stiffness of the contact determined by geometry and capillary forces. Further, we showed how nanocylindrical patterns, micropatterns and PTFE-coated PMMA flat surfaces subjected to varying capillary forces displayed similar sliding characteristics despite their large differences in contact characteristics.

Friction and adhesion becomes extremely important at nano and micro length scales, modulating the durability of several nano/micro electromechanical systems (NEMS/MEMS). Bio-mimicking the surface texture of different living organisms has helped improve the tribological performance of many such systems. In this study, we examined the friction and wetting behaviour of textured surfaces derived by mimicking the surface morphology of butterfly wing. Three different derivatives of mimicked structure with similar solid/air fraction but different contact aspect ratios were fabricated on Si wafer using photolithography and deep reactive ion-etching techniques. The textured surfaces patterns were subsequently coated with polytetrafluoroethylene (PTFE), diamond-like carbon (DLC), and fluorine incorporated diamond-like carbon (F-DLC) using plasma-enhanced chemical vapor deposition (PECVD) technique. Atomic force microscope was used to investigate the friction behaviour of the coated and un-coated samples at different applied normal load. Wetting behaviour of the textured and control surfaces was measured using sessile-drop method. Results showed that both wettability and friction were significantly influenced by the shape, orientation and surface chemistry of the textured structure. The PTFE and F-DLC coatings helped reduce the friction compared to Si control surface. The developed patterned displayed dual character with wetting and friction being function of the texture shape. The increase in aspect ratio of textured geometry enhanced directional wettability and friction. The wetting was controlled by the contact-line pinning phenomenon modulated by the texture geometry. The friction behaviour of the textured geometry varied in direct correlation with the contact area. Further, the edge-effect showed prominent influence leading to an increase in friction force in lateral direction. The effect of surface chemistry and texture geometry is explained on basis of intermolecular forces and contact mechanics. The directional friction and wetting characteristics of the developed surface would be of potential use for transport applications in different systems.

Nanoscale patterns have dimensions that are comparable to the length scales affected by intermolecular and surface forces. In this study, we systematically investigated the individual roles of curvature, surface energy, lateral stiffness, material, and pattern density in the adhesion and friction of nanopatterns. We fabricated cylindrical and mushroom-shaped polymer pattern geometries containing flat- and round-topped morphologies using capillary force lithography and nanodrawing techniques. We showed that the curvature, surface energy, and density of the patterns predominantly influenced the adhesive interactions, whereas lateral stiffness dominated friction by controlling the geometrical interaction between the indenter and pillar during sliding. Interestingly, in contrast to previous studies, cylindrical and mushroom-shaped pillars showed similar adhesion characteristics but very different frictional properties. Using fracture mechanics analysis, we showed that this phenomenon is due to a larger ratio of the mushroom flange thickness (t) to the radius of the pillar stem (ρ), and we proposed a design criterion for mushroom patterns to exhibit a geckolike effect. The most important result of our work is the discovery of a linear master curve in the graph of adhesion versus friction for pillars with similar lateral stiffness values that is independent of curvature, material, surface energy, and pattern density. These results will aid in the identification of simple pattern parameters that can be scaled to tune adhesion and friction and will help broaden the understanding of nanoscale topographical interactions.