  Droplet impacts are common in many applications such as coating, spraying, or
printing; understanding how droplets spread after impact is thus of utmost
importance. Such impacts may occur with different velocities on a variety of
substrates. The fluids may also be non-Newtonian and thus possess different
rheological properties. How the different properties such as surface roughness
and wettability, droplet viscosity and rheology as well as interfacial
properties affect the spreading dynamics of the droplets and the eventual drop
size after impact are unresolved questions. Most recent work focuses on the
maximum spreading diameter after impact and uses scaling laws to predict this.
In this paper we show that a proper rescaling of the spreading dynamics with
the maximum radius attained by the drop, and the impact velocity leads to a
unique single and thus universal curve for the variation of diameter versus
time. The validity of this universal functional shape is validated for
different liquids with different rheological properties as well as substrates
with different wettabilities. This universal function agrees with a recent
model that proposes a closed set of differential equations for the spreading
dynamics of droplets.
