  We investigate a subfreezing droplet impact scenario in a low-humidity
environment, where the target is a cold granular monolayer. When the
undercooling degree of targets passes a threshold, such a granular layer
effectively postpones the bulk freezing time of the droplet in comparison with
the impact on the bare substrate underneath. In this case, the retraction of
the droplet after impact reduces the contact area with the cold substrate, even
though both the grains and the substrate are wettable to the liquid. We find
that the significant changes in the dynamic behavior are triggered by freezing
the liquid that wets the pores. Owing to the small dimension of the pores, the
freezing process is rapid enough to match the dynamics over the droplet
dimension. In certain circumstances, the rapid freezing may even stop liquid
penetration and shed icing from the underneath surface.
