  Would a raindrop impacting on a coarse beach behave differently from that
impacting on a desert of fine sand? We study this question by a series of model
experiments, where the packing density of the granular target, the wettability
of individual grains, the grain size, the impacting liquid, and the impact
speed are varied. We find that by increasing the grain size and/or the
wettability of individual grains the maximum droplet spreading undergoes a
transition from a capillary regime towards a viscous regime, and splashing is
suppressed. The liquid-grain mixing is discovered to be the underlying
mechanism. An effective viscosity is defined accordingly to quantitatively
explain the observations.
