INTERNAL DROPLET CIRCULATION INDUCED BY SURFACE-DRIVEN ROTATION

This paper presents a combined theoretical/experimental study of inter nal liquid circulation induced by droplet surface rotation. A numerica l model is presented first, examining the fluid transport within a sph erical liquid volume whose surface is subjected to rotation about a ce ntral axis. The model predicts that the steady-state motion establishe d from spatially nonuniform surface rotation has a helical character a nd bears little resemblance to the toroidal internal flows developed w ithin droplets under axisymmetric conditions. Similar internal flow pa tterns are predicted for temporally varying surface rotation occurring during droplet spin-up or spin-down. Planar laser-induced fluorescenc e is employed to provide high-resolution Images of fluid flow develope d within millimeter-sized suspended droplets that are exposed to stead y laminar air streams to induce repeatable surface rotation. The predi cted spiral flow patterns are corroborated by the pendant droplet visu alization experiments, and suggest that nonuniform rotation or transie nt spinning may significantly alter internal droplet dynamics.