Thermally induced atomization during droplet impingement on superheated hydrophobic and superhydrophobic surfaces

Abstract

This paper presents on thermally induced atomization dynamics during liquid water droplet impingement on superheated hydrophobic and superhydrophobic surfaces. Circular micropillars 4 μm in height with varying center-to-center distances (pitch) of 8 μm, 12 μm and 16 μm, are used to create the superhydrophobic surfaces. The range of surface temperatures explored is 110°C<Ts<337°C. An image processing algorithm was developed to quantify the temporal behavior of thermally induced atomization captured via high-speed photography. Results show thermally induced atomization is initially absent (<1 ms following impact) on all surfaces. The amount of ejected droplets later during droplet spreading is determined as a function of surface structuring and temperature. Atomization ceases when the surface has cooled sufficiently (2.5–4 ms depending on the surface). The maximum amount of atomization for a given scenario is highly dependent on surface temperature and surface microstructure characteristics. At low surface temperatures, atomization increases with increasing surface temperature; however at high surface temperatures atomization decreases with increasing surface temperature. This rise and fall behavior is tantamount to the classical relationship between heat flux and surface temperature for pool boiling. Both droplet impingement and pool boiling depend on vapor bubble formation dynamics and the stability of the so-called Leidenfrost vapor film, which is impacted by surface wettability. Results also show that small surface pitch causes a high atomization intensity, indicating that not all superhydrophobic surfaces sustain Leidenfrost-like behavior at all excess temperatures.