(628b) Unexpected Dynamics of Drop Coalescence

Drop coalescence is central to diverse processes involving dispersions of drops of one fluid in a second fluid.  Common practical occurrences of coalescence include demulsification of crude oil, phase separation in emulsion-based products including salad dressing, and sintering of ceramic powders.  In nature, drop coalescence is central to raindrop growth and, in biology, remarkable parallels exist between coalescence and fusion of nucleoli.  During coalescence, two drops first touch and then merge as the liquid neck connecting them grows from initially microscopic scales to a size comparable to the drop diameters.  The neck’s expansion is controlled by the Laplace pressure, which diverges when the curvature of the interface is infinite at the point where the drops first touch.  Thus the change in topology, as two drops become one, is inextricably linked to a singularity in the dynamics.  Conventionally, this process has been thought to have just two dynamical regimes:  a viscous and an inertial regime.  According to conventional wisdom, the initial regime is always the viscous one regardless of the value of the liquid’s viscosity, which is plausible given neck’s microscopic size.  Conventional wisdom further dictates that high viscosity liquids remain in the viscous regime but that low viscosity liquids transition from the viscous to the inertial regime as coalescence proceeds.  We use experiments and simulations to reveal that the regime that describes the initial dynamics of coalescence has been missed.  A new phase diagram of coalescence is presented.  The results have profound implications to diverse situations where hydrodynamic singularities arise.