Events before droplet splashing on a solid surface

Shreyas Mandre, Harvard University
October 28th, 2009 at 4PM–5PM in 939 Evans Hall [Map]

A high velocity impact between a liquid droplet and a solid surface produces a splash. Classical observations traced the origin of this splash to a thin sheet of fluid ejected near the impact point, though the fluid mechanical mechanism leading to the sheet is not known. Mechanisms of sheet formation have heretofore relied on initial contact of the droplet and the surface. In this paper, we theoretically and numerically study the events preceding typical impacts of droplets within approximately 1 microsecond of contact. The droplet initially tries to contact the substrate by either draining gas out of a thin layer or compressing it, with the local behavior described by a self similar solution of the governing equations. This similarity solution is not asymptotically consistent: forces that were initially negligible become relevant and dramatically change the behavior. Depending on the radius and impact velocity of the droplet, we show that the solution is overtaken by either the surface tension of the liquid–gas interface or inertia of the drop. At low impact velocities surface tension stops the droplet from impacting the surface, whereas at higher velocities inertial effects may cause the droplet interface to overturn, signifying the possibility of a splash. This inertial effect causing splashing is currently under investigation.