Kinetic drop friction.

Liquid drops sliding on tilted surfaces is an everyday phenomenon and is important for many industrial applications. Still, it is impossible to predict the drop's sliding velocity. To make a step forward in quantitative understanding, we measured the velocity [Formula: see text], contact width [Formula: see text], contact length [Formula: see text], advancing [Formula: see text], and receding contact angle [Formula: see text] of liquid drops sliding down inclined flat surfaces made of different materials. We find the friction force acting on sliding drops of polar and non-polar liquids with viscosities ([Formula: see text]) ranging from 10 to 1 [Formula: see text] can empirically be described by [Formula: see text] for a velocity range up to 0.7 ms. The dimensionless friction coefficient [Formula: see text] defined here varies from 20 to 200. It is a material parameter, specific for a liquid/surface combination. While static wetting is fully described by [Formula: see text] and [Formula: see text], for dynamic wetting the friction coefficient is additionally necessary.