Flat-cupped transition in freezing drop impacts

We present an experimental study on the freezing of alkane drops impacted on a liquid bath. More specifically, for drops of hexadecane and tetradecane on brine, we found a morphological transition of the solid between a flat disk and a cupped shape. We show that this transition depends mainly on melting temperatures and thermal shock, and varies weakly with impact velocity. We observed that the impact dynamics do not depend on the thermal shock before the drop starts to solidify, which allows a rationalization of the solid size by models established for impact without phase change. Finally, we show that the relevant timescale setting the onset of solidification is associated with the formation of a thin solid layer between the drop and the bath, a timescale much shorter than the total solidification time. These findings offer the possibility of collapsing the data for both liquids in a single-phase diagram.