There is broad agreement that at low U I , energy loss is dominated by contact-line pinning. This explains why there can only be rebound (e > 0) for impact velocity above a critical speed (U I > U crit ): the droplet must have sufficient kinetic energy to overcome pinning. Different models have been proposed to relate e to surface wetting properties (in particular to θ and ∆ cos θ), but they have not been rigorously tested (in particular, ∆ cos θ for the surface was not carefully quantified ). At high U I , contact-line pinning is no longer important and different groups have proposed different physical explanations (inertialcapillary 5 vs. viscous effects ) for observed drop of e with increasing U I .