Analogy between coalescence and spreading for water*

A water drop gently placed on a flat solid surface rapidly spreads, while on a flat water surface, the drop rapidly merges into the water surface. Here we utilize high-speed X-ray microscopy to explore the initial coalescence of a water drop into a flat water surface: there clearly exists analogy in initial dynamics of coalescence and spreading. By comparing experimental and numerical results taken by Lattice Boltzmann simulations, we attribute the analogy to the hydrodynamic nature of water. The coalescence-spreading analogy for water would be important with respect to the universality of coalescence and spreading between liquids and solids (S.J.L. and E.G.K. equally contributed).