Thermocapillary migration characteristics of self-rewetting drop

Abstract

The thermocapillary migration characteristics of a self-wetting drop on the non-uniformly heated, horizontal, solid substrate are investigagted by numerical simulation. Based on the lubrication theory, an evolution equation for the height of the two-dimensional drop is established. The substrate underlying the drop is subjected to a temperature gradient which induces surface tension gradient-driven drop deformation and migration. The self-rewetting fluid has non-monotonic dependence of the surface tension on temperature with a well-defined minimum, and the position of the minimum corresponding to the temperature on the substrate is called the critical point. The effect of the relationship between the critical point and the drop position on drop dynamics is analyzed. With the temperature sensitivity coefficient of three interfaces under the same condition, the substrate is illustrated with constant wettability. The direction of drop migration will alter as the initial drop location moves to the left relative to the critical point position, resulting from the variation of the interplay among thermocapillary, gravity, and capillarity forces within the drop. But the drop always migrates toward the high interfacial tension region due to the thermocapillary force. In the presence of substrate wettability variations, the drop migrates toward the low temperature region no matter where the drop is placed relative to the critical point. This is due to the fact that the deterioration of substrate wettability on the right side of the drop prevents the drop from migrating toward the hot region. Under the critical point being on the left or within the drop, as the initial drop location moves to the left relative to the critical point position, the enhancement of the thermocapillary force toward the left leads to increased moving speed of the left contact line and increased spreading area. When the critical point is positioned on the outer right side of the drop, the speed of the left contact line sharply decreases at t=6103, caused by the suddenly deteriorating substrate wettability. Hence, it is effective to manipulate the self-wetting drop movement by regulating the relationship between the critical point and the initial drop location. To inhibit the migration of the drop toward the cold region, the drop should be placed on the right side of the critical point.