Self-peeling of impacting droplets

Whether an impacting droplet(1) sticks or not to a solid surface has been conventionally controlled by functionalizing the target surface(2-8) or by using additives in the drop(9,10). Here we report on an unexpected self-peeling phenomenon that can happen even on smooth untreated surfaces by taking advantage of the solidification of the impacting drop and the thermal properties of the substrate. We control this phenomenon by tuning the coupling of the short-timescale fluid dynamics-leading to interfacial defects upon local freezing-and the longer-timescale thermo-mechanical stresses-leading to global deformation. We establish a regime map that predicts whether a molten metal drop impacting onto a colder substrate(11-14) will bounce, stick or self-peel. In many applications, avoiding adhesion of impacting droplets around designated target surfaces can be as crucial as bonding onto them to minimize waste or cleaning. These insights have broad applicability in processes ranging from thermal spraying(12,15) and additive manufacturing(16-19) to extreme ultraviolet lithography(20).