This invention pertains to a new microfluidic device and the method of using it to sort droplets. The method comprises (a) providing a plurality of droplets flowing in a microfluidic channel, wherein the plurality of droplets comprise desired droplets and undesired droplet (b) identifying desired droplets in the plurality of droplets, (c) changing volume of the desired droplets relative to volume of the undesired droplets such that at least some of the desired droplets have a different volume than the undesired droplets, and (d) passively sorting the desired droplets having the different volume from the undesired droplets.