A microfluidic device for electrically activating a passive capillary stop valve, an apparatus and method are provided. The microfluidic device includes a first channel for containing a first fluid, and an output channel, wherein the first channel comprises a first interface with the output channel, and the first interface comprises a capillary stop valve characterised in that the microfluidic device also comprises a second channel for containing a second fluid, wherein the second channel comprises a second interface with the output channel, and the first channel and the second channel are electrically isolated from each other, and the first interface and the second interface are arranged relative to each other thereby being configured to activate fluid flow from the first channel into the output channel when a first fluid and a second fluid are present, and an electrical potential difference is applied between the first fluid and the second fluid.