Georgia Institute of Technology

Abstract:

A liquid droplet placed on a non-uniformly heated horizontal solid surface will develop internal flows driven by surface-tension and capillary-pressure gradients. when the tangential temperature variations in the solid are large enough, these thermocapillary motions will push or pull the contact line of the droplet in the direction of decreasing temperature. Under the right conditions, the entire droplet can be seen to migrate a constant velocity and without change in shape in the direction of decreasing temperature. We describe this motion for a fully three dimensional, but thin liquid droplet using a quasi-steady evolution equation for the droplet shape based on lubrication theory and a constitutive equation for the contact-line speed. Our simulations also show that when the average temperature of the solid surface is large enough compared to the temperature of the overlying gas, the thin droplet will exhibit an interfacial instability in which a small, nonaxisymetric dimple appears on the top of the droplet. The dimple is slightly longer in the direction of the imposed temperature gradient in the solid surface. We document this instability in terms of the underlying physics of surface-tension driven flows.