Microscopic insight into the nanocoalescence of a water droplet on a water bath
1 Institut de Physique de Rennes, UMR CNRS 6251, Université Rennes 1 - 263 avenue du Général Leclerc, 35042 Rennes, France
2 Institut de Chimie de Clermont-Ferrand, ICCF, UMR CNRS 6296 - BP 10448, F-63000 Clermont-Ferrand, France
Received: 19 October 2013
Accepted: 24 November 2013
Coalescence of a millimetric water droplet on a water bath has been extensively investigated by experiments, theory and continuum approaches. While the hydrodynamic processes have been largely studied the underlying microscopic mechanisms are much less understood. Thanks to the recent advances in mesoscopic modelling the physics occurring at the nanometric scale can be captured. By using the coarse- and fine-grained simulations we investigate the mechanism of the coalescence of a water nanodroplet on a water bath. In contrast to the millimetric drop we show the absence of a coalescence cascade. The two most probable mechanisms of coalescence largely discussed in the literature are i) the drainage of an air cushion between the water droplet and the bath and ii) the rupture of both water drop and reservoir interfacial layers. From the time-evolution of the 3D water density profile we show that the mechanism begins by the formation of a coalescence bridge during the first picosecond, followed by a temporary noncoalescence where the shape of the drop is modified. The life time of the temporary noncoalescence is ruled by the dissipation of the interfacial layers of the droplet and of the water bath. During this phase the droplet expels water downward through the bridge leading to a thinning of the interfacial layers and to their dissipation that corresponds to the beginning of the drop coalescence into the water bath. In this work we show that the microscopic mechanism of nanocoalescence is partially in line with the millimetric process. Indeed, in contrast with the macroscopic droplet a deformation of the interface was evidenced at the nanometric scale while at the millimetric scale the interfaces suddenly coalesce after the interfacial layers sufficiently dissipated and the drop merges into the bulk.
PACS: 68.05.Cf – Liquid-liquid interface structure: measurements and simulations / 68.15.+e – Liquid thin films / 47.55.nk – Liquid bridges
© EPLA, 2013