, Volume 26, Issue 6, pp 1743-1757

Dynamics of Drop Coalescence on a Surface: The Role of Initial Conditions and Surface Properties

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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* values are smaller by many orders of magnitude than the bulk hydrodynamic velocity (U = σ /η, with σ the gas–liquid surface tension and η 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.

Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.