The present invention relates to droplet-based surface modification and washing. According to one embodiment, a method of providing a droplet in contact with a surface with a reduced concentration of a substance is provided, wherein the method includes: (a) providing a droplet microactuator comprising a surface in contact with a droplet comprising a starting concentration and starting quantity of the substance and having a starting volume; (b) conducting one or more droplet operations to merge a wash droplet with the droplet provided in step (a) to yield a combined droplet; and (c) conducting one or more droplet operations to divide the combined droplet to yield a set of droplets comprising: (i) a droplet in contact with the surface having a decreased concentration of the substance relative to the starting concentration; and (ii) a droplet which is separated from the surface.