Impact of a suspension drop onto a hot substrate: diminution of splash and prevention of film boiling.

In the present study, the effect of graphite lubricant additives on the dynamics of a single drop impact onto a heated surface has been investigated in the nucleate boiling and thermal atomization regimes. In the nucleate boiling regime the drop impact is accompanied by the nucleation and expansion of multiple vapor bubbles. The drop residence time at the substrate is determined by the time of its mass loss due to splash and evaporation. At higher temperatures, above the Leidenfrost point, impact may lead to drop rebound. In this experimental and theoretical study the effect of additives on the outcome of drop impact, in particular, the addition of solid graphite particles, is investigated. The residence time of the drop has been measured for various initial drop temperatures and suspension concentrations. The addition of the particles leads to some increase of the residence time, while its dependence on the substrate temperature follows the scaling relation obtained in the theory. Moreover, the presence of the particles in the drop leads to suppression of splash and a significant increase of the drop rebound temperature, which is often associated with the Leidenfrost point. These effects are caused by the properties of the deposited layer, and pinning of the contact line of the entire drop and of each vapor bubble, preventing bubble coalescence and drop rebound. The phenomena are also explained by a significant increase of the liquid viscosity caused by the evaporation of the bulk liquid at high wall temperatures.