The leidenfrost phenomenon on structured surfaces

Abstract

The lifetime of a droplet released on a hot plate decreases when the temperature of the plate increases. But above some critical value of the temperature, the lifetime suddenly increases. This is due to the formation of a thin layer of vapour between the droplet and the substrate. This layer plays a double role: first it thermally isolates the droplet from the plate and second it allows the droplet to “levitate.” This effect was discovered by Leidenfrost in 1756, but remains an active field of research nowadays, motivated by a wide range of applications. The Leidenfrost point is affected by the roughness or microstructure of the surface. In this work, a silicon surface with different micro-structured regions of square-pillars was prepared such that there is a sharp transition (boundary) between areas of different pillar spacing. The Leidenfrost point was identified in experiments using water drops ranging in average size from 8 μL to 24 μL and the behaviour of the droplets was recorded using a high-speed digital camera. We show that the Leidenfrost point can vary by up to 120 °C for pillar spacings varying from 10 microns to 100 microns. If the drop is placed on the boundary between structured sections, the drop becomes asymmetric. Drop motion may also be observed and some occurrences of drop spinning have been seen. In this paper we present experimental data on Leidenfrost behaviour of drops placed structured surfaces and on the boundary between surfaces with different micro-structures.