  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.
