Spreading dynamics of water droplets on hydrophilic surfaces

In the early stage of droplet spreading, the inertia of the drop resists the capillary driven motion, and for liquids with low viscosity the spreading radius has been observed to grow with time as r(t)~t^1/2, independent of surface wettability. In the final stages, the effect of viscous forces acting in the neighbourhood of the three phase contact line become relevant and the competition between surface tension and viscous forces results in extremely slow spreading dynamics and follows what is called Tanner's law, r(t) ~ t^1/10. In this work, we will present results on the spreading behaviour of water droplets of varying sizes on a completely wetting surface investigated using fully atomistic molecular dynamic simulations. The spreading observed is characterized by a monolayer of molecular dimensions that moves ahead of the main bulk part of the droplet. Interestingly, the bulk part initially spreads over the monolayer with increasing radius until a characteristic time t^*; thereafter, the bulk radius shrinks maintaining a constant contact angle until it disappears altogether. We will show that a first principle model based on hydrodynamic theory describes the spreading data rather well in the regime where the low contact angle approximation holds.