Novel Marangoni Flows

Professor G. M. Homsy

Department of Mechanical Engineering, University of California, Santa Barbara

This talk describes three recent studies of novel Marangoni flows, i.e. flows that are driven by tangential stresses due to temperature, compositional, or electrical fields. These stresses can drive bulk flows that are vigorous and in many cases, counter-intuitive.

Our first two studies involve gradients of concentration of surfactants arising from variation in the rate of chemical reaction producing them. We study the effect of in-situ production of surfactants on viscous fingering instabilities.  We find that Maragoni stresses result in wider fingers, a larger fractal dimension of the pattern, and an increase in displacement efficiency.  We then describe a surprising phenomenon of spontaneous, self-sustained chemically driven oscillations at the tip of a drop suspended from a needle. We connect this phenomenon with the well-known tip-streaming in drops subjected to extensional flows.  A plausible physical mechanism related to tip-streaming is proposed for this second phenomena.

We then describe theory and experiment on internal circulations in drops that are driven by a combination of translation and tangential electrical stresses. Modulation of the electric field responsible for the latter then results in chaotic advection and good mixing within the drop.  Theory and experiment are found to be in good agreement.  Rates of heat and/or mass transfer from such chaotically mixed flows are computed and found to have a complex dependence on the frequency of driving of the electrical stresses.