Liquid ring formation from contacting, nonmiscible sessile drops

When two pure and nonmiscible liquid drops at rest on a rigid substrate come into close contact with a quasi-zero spreading velocity, one of them may be sucked around the second into a liquid ring, leading in some cases to the complete engulfment of the latter. We here show that the conditions for this amazing and unusual capillary effect to develop are defined by two sets of criteria: the "reciprocal" spreading of one drop with respect to the other and a "geometrical-wetting" criterion related to the opening of the groovelike channels along the base of the attracting drop. Despite the exceeding simplicity and roughness of liquid drops as compared to living cells, the phenomenon strangely recalls, at least in its mechanistic aspect, the fundamental biological process of phagocytosis. Besides these fundamental aspects, this effect may also have interesting implications for microstructuring techniques.