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 (U), contact width (w), contact length (L), advancing (θa), and receding contact angle (θr) 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 (η) ranging from 10⁻³ to 1 Pa⋅s can empirically be described by Ff(U)=F0+βwηU for a velocity range up to 0.7 ms⁻¹. The dimensionless friction coefficient (β) 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 θa and θr, for dynamic wetting the friction coefficient is additionally necessary.
