Abstract
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 10-3 to 1 [Formula: see text] can empirically be described by [Formula: see text] for a velocity range up to 0.7 ms-1. 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.