Observations of nanoporous foam arising from impact and rapid solidification of molten Ni droplets

The authors report highly nanoporous surfaces on the underside of impacted, rapidly quenched Ni droplets (splats) removed from substrates. The formation mechanism is interpreted as nucleation of gas bubbles during rapid postimpact depressurization, and subsequent freezing to a porous solid. Good agreement is found between experimental observations and predictions from bubble nucleation theory. Results from experimental variations suggest the potential for control of pore size and distribution.