Migration of liquid drops in a vertical temperature gradient—interaction effects near a horizontal surface

Abstract

Results are reported from experiments conducted on drops migrating in a vertical direction normal to a horizontal solid plane surface. Three situations were investigated—migration driven by gravity, migration driven predominantly by thermocapillarity, and migration when the gravitational and thermocapillary driving forces are comparable in magnitude, but act in opposite directions. In all three cases, the experimentally measured drop velocities were found to be in agreement with theoretical predictions. Drops are influenced by a solid plane surface when they are as far away as 12 drop radii when motion is driven by gravity. In contrast, for motion driven principally by thermocapillarity, drops are affected appreciably only when they are within 3 drop radii from the surface. The increased resistance to motion offered by the solid surface leads to reduced drop velocities when motion is driven purely by gravity or predominantly by thermocapillarity. However, when gravity and thermocapillarity are comparable in their influence, but act in opposite directions, drops which are sufficiently far from the surface are found to move more rapidly in the direction of the temperature gradient than they would if isolated. This observation is consistent with expectation, and an explanation is given.