  Superhydrophobic surfaces can exhibit icephobicity in many ways due to their
large contact angles and small rolling angles. The melting process of frozen
droplets on superhydrophobic surfaces is still unclear, hindering the
understanding of surface icephobicity. In this experimental study of the
melting process of frozen sessile droplets on superhydrophobic surfaces, we
find two types of melting morphologies with opposite vortex directions on a
single-scale nano-structured (SN) superhydrophobic substrate and a
hierarchical-scale micro-nano-structured (HMN) superhydrophobic substrate.
Melting pattern visualizations and flow field measurements showwed Marangoni
convection and natural convection occuring in the melting sessile droplets. For
the HMN superhydrophobic substrate, the internal flow was found to be dominated
by Marangoni convection due to the temperature gradient along the surface of
the droplet. For the SN superhydrophobic substrate, Marangoni convection was
inhibited by the superhydrophobic particles at the surface of the droplet,
which were shed from the fragile superhydrophobic substrate during the
freezing--melting process, as confirmed by surface characterizations of the
substrate and flow measurements of a water pool. These results will help
researchers better understand the melting process of frozen droplets and in
designing novel icephobic surfaces for numerous applications.
