Dynamics of drop coalescence on a surface: The role of initial conditions and surface properties

An investigation of the coalescence of two water drops on a surface is presented and compared with drop spreading. The associated capillary numbers are very low (< 10(-5)). The drops relax exponentially towards equilibrium. The typical relaxation time t(c) decreases with contact angle. t(c) is proportional to the drop size R, thus defining a characteristic velocity U = R/t(c). The corresponding U star values are smaller by many orders of magnitude than the bulk hydrodynamic velocity (U = sigma/eta, with sigma the gas-liquid surface tension and sigma the viscosity). The dynamics of receding (coalescence) and spreading motion is found to be of the same order when coalescence or spreading is induced by a syringe. The dynamics of coalescence induced with the syringe deposition is systematically faster by an order of magnitude than condensation-induced coalescence. This disparity is explained by the coupling of the contact line motion with the oscillation of the drop observed for syringe deposition but absent for condensation-induced coalescence.