  Liquid wetting of a surface is omnipresent in nature and the advance of
micro-fabrication and assembly techniques in recent years offers increasing
ability to control this phenomenon. Here, we identify how surface roughness
influences the initial dynamic spreading of a partially wetting droplet by
studying the spreading on a solid substrate patterned with microstructures just
a few micrometers in size. We reveal that the roughness influence can be
quantified in terms of a line friction coefficient for the energy dissipation
rate at the contact line, and that this can be described in a simple formula in
terms of the geometrical parameters of the roughness and the line-friction
coefficient of the planar surface. We further identify a criterion to predict
if the spreading will be controlled by this surface roughness or by liquid
inertia. Our results point to the possibility of selectively controlling the
wetting behavior by engineering the surface structure.
