Coalescence of sessile drops

We present an experimental and theoretical description of the kinetics of coalescence of two water drops on a plane solid surface. The case of partial wetting is considered. The drops are in an atmosphere of nitrogen saturated with water where they grow by condensation and eventually touch each other and coalesce. A new convex composite drop is rapidly formed that then exponentially and slowly relaxes to an equilibrium hemispherical cap. The characteristic relaxation time is proportional to the drop radius R* at final equilibrium. This relaxation time appears to be nearly 10(7) times larger than the bulk capillary relaxation time t(b) = R*eta/sigma, where sigma is the gas-liquid surface tension and eta is the liquid shear viscosity. In order to explain this extremely large relaxation time, we consider a model that involves an Arrhenius kinetic factor resulting from a liquid-vapour phase change in the vicinity of the contact line. The model results in a large relaxation time of order t(b) exp(L/RT) where L is the molar latent heat of vaporization, 9 is the gas constant and T is the temperature. We model the late time relaxation for a near spherical cap and find an exponential relaxation whose typical time scale agrees reasonably well with the experiment.