Transition in Droplet State on Heated Structured Hydrophobic Substrates

Abstract Interaction of a droplet on a heated substrate is a subject of extensive research due to its application in spray cooling of surfaces, thermal management of micro-scale devices, water harvesting etc. Depending on the surface temperature the droplet shows nucleate, transition boiling and Leidenfrost state. The presence of surface textures gives rise to various hydrodynamic outcomes such as droplet atomization, interfacial oscillations, and lift off. When a drop is deposited on a textured surface, it typically exists in two states called Cassie-Baxter and Wenzel depending on the substrate wettability and liquid-solid contact area. In the present work we explore a unique lift off mechanism at temperature range of (140–170°C) attributed to the excessive vapor force generated due to droplet impalement into the gap between the textures followed by evaporation of the imbibed liquid. The presence of microtextures (solid fraction) offers flow resistance to the escaping vapor which leads to increase in vapor pressure underneath the drop. When the vapor force exceeds the pinning force and weight of the drop, an explosive lift off occurs. We develop a force-based model to estimate the droplet volume corresponding to lift off on substrates with different morphologies and temperatures.