Executive Summary : | From fundamental and application viewpoints, wetting and evaporation of a droplet on a deformable and liquid-infused surface is necessary. The former surface is a soft surface, such as a PDMS layer deposited on a solid surface, and can be deformed by surface tension forces. This phenomenon is known as elastocapillarity. After the deposition of the droplet, these surfaces exhibit a ridge-like profile near the contact line due to deformation by surface tension forces. The liquid-infused surface contains a lubricant film on a micro-textured surface (inspired by the surface of a pitcher plant). Such surfaces are potentially helpful for designing adhesives, lubricating surfaces, stretchable synthetic materials, and surfaces with properties such as self-cleaning, ice-repellent, low hysteresis, anti-fouling, and conducive to droplet transport, etc. A primary goal of this project is to understand the droplet wetting on these surfaces in complex scenarios and configurations. The proposed research will record time-varying droplet shapes by high-speed visualization and corresponding thermal maps by the infrared camera. Fluorescent microparticles in the droplet will be tracked to capture the internal microfluidic flow, especially near the contact lines, where deformations occur. The proposed research will address several open questions in this arena. While it has been shown that gradient in substrate stiffness can be engineered to control the wettability gradient, the role of thermal or solutal Marangoni stress along the liquid-gas interface of a sessile droplet resting on a deformable surface has not been explored yet. In the case of a liquid-infused surface, the complexity is even higher since the variation of the stress may also occur in the lubricant film in the case of an imposed thermal gradient. The effect of droplet size around elastocapillary length on wetting remains unclear. This project will address the sliding of the droplet on a liquid-infused surface, which has potential applications in designing droplet transport for microfluidic applications. Binary droplets, such as a mixture of water and alcohol will also be tested on the proposed functional surfaces. The wetting could be a function of the spatial gradient of surface tension along the liquid-gas interface due to preferential evaporation of alcohol near the contact line. A secondary goal of this project is to understand the deposition of colloidal particles present in an evaporating droplet or capillary bridge on a deformable surface. The Marangoni-stress mediated flow can potentially modify the dried colloidal deposits, thereby suppressing the typical coffee-ring effect. Overall, the present proposal will address several fundamental research questions in these upcoming areas of Colloids and Interfaces and is believed to be highly transformative to developing technologies such as the design of low hysteresis surfaces. |