Chasing drops: following escaper and pursuer drop couple system.

We study experimentally six different systems in which Marangoni flow is induced by two chemically different drops on a solid surface in air. In such systems one drop seems to chase away the other. We show that in all the systems studied, the Marangoni flow is induced at the solid-vapor interface as opposed to the air-liquid interface. This is true even for the case of water drop and alcohol drop on a glass surface (which corresponds to the "tears of wine" classical case). Thus we explain the drop motion as a result of an interfacial tension gradient which takes place primarily at the air-surface region and less, if at all, at the two other interfaces in the problem: the liquid-substrate or liquid-air interfaces. Then we follow the motion of drops on surfaces and find that it is discontinuous, i.e. characterized by stops and jumps as in a stick slip mechanism. We explain this behavior by an increase in the Laplace pressure that creates a higher anchoring pinning effect at the front edge of the moving drop. The understanding of this process has implications for passively separating mixed liquids.