A method of forming a droplet interface in an electro-wetting device, the electro-wetting device comprising: an array of actuation electrodes; an insulator layer covering the actuation electrodes and having an outermost hydrophobic surface; disposed on the hydrophobic surface of the insulator layer, a fluid medium and two droplets comprising liquid in the fluid medium, one of the liquid and the fluid medium being polar, and the other of the liquid and the fluid medium being apolar, whereby the actuation electrodes are capable of electro-wetting the droplets when actuation signals are applied thereto; and a second substrate facing the hydrophobic surface of the insulator layer, wherein the second substrate is coated by a hydrophobic material forming a further hydrophobic surface facing the hydrophobic surface of the insulator layer, the droplets being disposed on the further hydrophobic surface of the hydrophobic layer as well as the hydrophobic surface of the insulator layer, wherein the second substrate supports sensor electrodes that make an electrical connection with the droplets between which a droplet interface is formed, the method comprising: applying actuation signals to selected actuation electrodes to place one or both of the two droplets in an energised state in which the shape of said one or both droplets is modified compared to when in a lower energy state and to bring the two droplets into proximity with a gap therebetween, the gap being chosen such that the two droplets do not contact each other when one or both are in the energised state and contact each other to form a droplet interface when in the lower energy state; changing the actuation signals applied to the actuation electrodes to lower the energy of said one or both droplets into the lower energy state so that said one or both droplets relax into the gap and the two droplets contact each other thereby forming a droplet interface; and taking electrical measurements between the droplets across the droplet interface, wherein the electrical measurements are made using the sensor electrodes. an array of actuation electrodes; an insulator layer covering the actuation electrodes and having an outermost hydrophobic surface; disposed on the hydrophobic surface of the insulator layer, a fluid medium and two droplets comprising liquid in the fluid medium, one of the liquid and the fluid medium being polar, and the other of the liquid and the fluid medium being apolar, whereby the actuation electrodes are capable of electro-wetting the droplets when actuation signals are applied thereto; and a second substrate facing the hydrophobic surface of the insulator layer, wherein the second substrate is coated by a hydrophobic material forming a further hydrophobic surface facing the hydrophobic surface of the insulator layer, the droplets being disposed on the further hydrophobic surface of the hydrophobic layer as well as the hydrophobic surface of the insulator layer, wherein the second substrate supports sensor electrodes that make an electrical connection with the droplets between which a droplet interface is formed, the method comprising: applying actuation signals to selected actuation electrodes to place one or both of the two droplets in an energised state in which the shape of said one or both droplets is modified compared to when in a lower energy state and to bring the two droplets into proximity with a gap therebetween, the gap being chosen such that the two droplets do not contact each other when one or both are in the energised state and contact each other to form a droplet interface when in the lower energy state; changing the actuation signals applied to the actuation electrodes to lower the energy of said one or both droplets into the lower energy state so that said one or both droplets relax into the gap and the two droplets contact each other thereby forming a droplet interface; and taking electrical measurements between the droplets across the droplet interface, wherein the electrical measurements are made using the sensor electrodes. applying actuation signals to selected actuation electrodes to place one or both of the two droplets in an energised state in which the shape of said one or both droplets is modified compared to when in a lower energy state and to bring the two droplets into proximity with a gap therebetween, the gap being chosen such that the two droplets do not contact each other when one or both are in the energised state and contact each other to form a droplet interface when in the lower energy state; changing the actuation signals applied to the actuation electrodes to lower the energy of said one or both droplets into the lower energy state so that said one or both droplets relax into the gap and the two droplets contact each other thereby forming a droplet interface; and taking electrical measurements between the droplets across the droplet interface, wherein the electrical measurements are made using the sensor electrodes.