Vapor-Mediated versus Substrate-Mediated Interactions between Volatile Droplets

Understanding the behavior of sessile drops can be rewarding in many applications while also fostering progress in the rapidly evolving field of capillarity and wetting. Our experiments reveal that two evaporating sessile drops on a solid substrate do attract even if, unlike the binary-liquid drops recently studied in the literature [Cira et al., Nature, 2015], they are made of the same pure liquid. Several perfectly wetting liquids of different volatilities are tested to unveil and quantify the mechanisms enabling droplets to communicate. While all recent works focusing on the topic consider vapor-mediated interactions only, we here identify not less than three substrate-mediated forces, important for not too heat-conducting substrates (e.g. glass) and driven by the thermal Marangoni effect (favoring droplet motion toward colder regions) and by evaporation-induced variations of the apparent contact angles (acting similarly to a wettability gradient). In addition to an attractive mechanism and a (generally weaker) repelling one, the third effect acts on each droplet individually due to the self-centering cold spot it induces in the substrate. Interestingly, in the force balance used to rationalize our results, this "cold-trap resistance" enters as an effective drag force opposing any motion, like the viscous drag does. The interaction mechanisms described here could hopefully open new directions of research about thermal effects as a mean of self-organizing evaporating/condensing liquid entities on substrates of various shapes and thermal properties.