The advancing front of a spreading liquid

Abstract

A WETTING liquid makes a zero contact angle to a solid, and a drop will spread completely over the surface. There are two stages in the spreading process¹². Initially, the advancing front is pushed from behind by the hydrostatic head of the liquid. As the drop thins out this becomes unimportant and the front is pulled along by the unbalanced surface forces near the perimeter. The shape of the advancing front is not easy to observe without special lighting. In addition, for most liquids it cannot be seen unless precautions are taken to minimise evaporation. I report here microscopic observations which show that the advancing front has a remarkable structure. The moving front does not retain the shape of the original drop. Instead, depending on the liquid, it either assumes a scalloped periodic structure or else moves by a series of random advances, resembling the locomotion of an amoeba, but still with some approximate periodicity.