Fast-freezing kinetics inside a droplet impacting on a cold surface

Significance From critically affecting the performance of an aircraft to droplet-based additive manufacturing, the solidification of impacting droplets influences a wide range of industrial applications; thus, a deeper fundamental understanding of the solidification of an impacting droplet is necessary. In this work, we reveal and rationalize a peculiar freezing morphology originating from the complex interplay between the droplet-scale hydrodynamics and phase-transition effects at sufficiently high substrate undercooling. The direct visualization of different freezing morphologies only became possible as we adopt an optical technique (TIR) in the context of freezing. This technique can be employed in more complex situations, such as solidification of an impacting droplet on liquid-infused or patterned surfaces, which potentially can influence many industrial processes.