Upon impact, the contact-line advances with contact angle θ a up to a maximum radius r = r max before it retracts with contact angle θ r until r = 0, and the droplet lifts off the surface. The friction forces per unit length during the advancing and retracting processes are given by f a = γ(cos θcos θ a ) and f r = γ(cos θ rcos θ), where θ is the static contact angle. The energy loss E f due to contact-line pinning or friction is therefore given by