Journal
INTERNATIONAL JOURNAL OF THERMOPHYSICS
Volume 26, Issue 6, Pages 1743-1757Publisher
SPRINGER/PLENUM PUBLISHERS
DOI: 10.1007/s10765-005-8593-4
Keywords
capillary; coalescence; contact angle; contact line velocity; wetting
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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 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.
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