This work reports on coalescence-induced selfpropulsion as a driving mechanism to actuate microfluidic droplet assays. We demonstrate multi-droplet translation and assay-type sensing on a digital microfluidics platform by use of surface tension gradients alone. These gradients arise during the coalescence of two droplets of liquid having different compositions and therefore surface tensions. We demonstrate a chemiluminescence blood-detection reaction based on the mixing of two carrier droplets containing solutions of synthetic blood and luminol sensor solution. Presence of iron in the blood solution is recorded using digital imaging and analyzed via offline image processing. The results demonstrate the capability of the propulsion mechanism to propel droplets over several millimeters, thus enabling one to design a new family of chip-based biochemical sensor assays.