Cyclic rupture of oil droplets

The classic “oil drop experiment” by Benjamin Franklin suggests that a drop of oil will spread to a monolayer film on the surface of a large body of water. However, we observed an unusual cyclic rupture behavior when fish oil is deposited onto a lake. This phenomenon occurs only under specific conditions: 1) a surfactant must be present in either the oil or aqueous phase, 2) the system must exhibit pseudo-partial wetting, 3) surfactant or oil molecules adsorbed at the air-water interface must be actively or passively depleted, 4) the adsorption of either or both oil and surfactant molecules to the air-water interface must occur slowly. We demonstrate this behavior in three distinct oil-surfactant systems, each governed by different mechanisms that drive Marangoni flow. We hypothesize that the resulting toroidal shape arises from Marangoni stresses caused by differential adsorption rates of surfactants across neighboring interfaces. Additionally, we measure and provide theoretical arguments for the hole-opening rate and the number of “petals” surrounding the opening hole. Finally, we describe droplet interactions and autonomous self-organization emerging from this phenomenon.