Vapor-Induced Motion of Liquid Droplets on an Inert Substrate

Evaporating droplets are known to show complex motion that has conventionally been explained by the Marangoni effect (flow induced by the gradient of surface tension). Here, we show that the droplet motion can be induced even in the absence of the Marangoni effect due to the gradient of evaporation rate. We derive an equation for the velocity of a droplet subject to non-uniform evaporation rate and non-uniform surface tension placed on an inert substrate where the wettability is uniform and unchanged. The equation explains the previously observed attraction-repulsion- chasing behaviors of evaporating droplets.