  We report experiments on the rapid contact line motion present in the early
stages of capillary driven spreading of drops on dry solid substrates. The
spreading data fails to follow a conventional viscous or inertial scaling. By
integrating experiments and simulations, we quantify a contact line friction
($\mu_f$), which is seen to limit the speed of the rapid dynamic wetting. A
scaling based on this contact line friction is shown to yield a universal curve
for the evolution of the contact line radius as a function of time, for a range
of fluid viscosities, drop sizes and surface wettabilities.
