Selective droplet splitting using single layer microfluidic valves

•Selective droplet splitting is achieved in a microfluidic system.•A microvalve is used to change the channel geometry at the entrance to a loop of the main channel.•The system is operated close to the transition regime between non-splitting and splitting.•Control over the pressure of inlets and valve, dictates the size of the split droplets.•The system can be used to conduct permutations of reactions on chip.