Attraction and Repulsion between Liquid Droplets over a Liquid-Impregnated Surface.

We show that adjacent liquid droplets exhibit long-range attraction and repulsion on an immiscible liquid impregnating a surface when either the drop or the impregnating liquid is volatile. Remarkably, we find that at small times the interaction is attractive, analogous to the "Cheerios effect", but at large times the interaction becomes repulsive depicting the "reverse-Cheerios effect". Our study reveals that the interaction is underpinned by wetting and capillarity, buoyancy, and evaporation phenomena. We experimentally observe the interaction between a pair of droplets and provide a theoretical framework to quantitatively predict their transport behavior.