Internal droplet circulation induced by surface-driven rotation

Abstract

This paper presents a combined theoretical/experimental study of internal liquid circulation induced by droplet surface rotation. A numerical model is presented first, examining the fluid transport within a spherical liquid volume whose surface is subjected to rotation about a central axis. The model predicts that the steady-state motion established from spatially nonuniform surface rotation has a helical character and bears little resemblance to the toroidal internal flows developed within droplets under axisymmetric conditions. Similar internal flow patterns are predicted for temporally varying surface rotation occuring during droplet spin-up or spin-down. Planar laser-induced fluorescence is employed to provide high-resolution images of fluid flow developed within millimeter-sized suspended droplets that are exposed to steady laminar air streams to induce repeatable surface rotation. The predicted spiral flow patterns are corroborated by the pendant droplet visualization experiments, and suggest that nonuniform rotation or transient spinning may significantly alter interenal droplet dynamics.