Spreading and bouncing of liquid alkane droplets upon impacting on a heated surface

This paper reports an experimental investigation on the impact dynamics of liquid normal alkane (n-heptane, n-decane and n-tetradecane) droplets on a stainless steel surface using high speed photography and long distance microscopic techniques. Particular interest is paid to comprehensively explore the effects of liquid viscosity and surface roughness on droplet spreading and bouncing dynamics at different thermal hydrodynamic impact regions. Specifically, firstly, high speed images identified four regimes (evaporation, nucleate boiling, transition boiling and film boiling regime) of physical phenomena that couple the droplet spreading hydrodynamics, heat transfer and phase change. Bubbles generation due to the heating of the surface with compression of air disk under the droplet was observed and this phenomenon is firstly promoted and then inhibited with the increase of the wall temperature until finally no bubbles were observed when wall temperature is beyond the Leidenfrost point (T_L). Rim disturbances during spreading were observed at relatively high Weber number with wall temperature higher than T_L. Increasing wall temperature reduces the rim disturbance. Secondly, the measured non-dimensional maximum spreading diameter β_max decreases with the increase of surface temperature until it becomes a constant when temperature is beyond T_L. Rough surface was found to have a lower T_L because of larger vapor pressure provided by more nucleation sites. Finally, for wall temperature beyond T_L, droplet bounces up after a certain period of residence time (τ_r). It takes more time for droplet to rebound at larger We because of larger β_max takes longer time to retract and rebound. Both surface roughness and liquid viscosity showed no influence on time to reach β_max (τ_max), but significantly increases τ_r by slowing the retracting process, which both should be considered in future model of τ_r.