Coalescence control of droplets moving on surfaces using a homogeneous electric field

Controlling coalescence is crucial for reliable manipulation of droplets in microfluidic applications. Here, we present a method that allows to suppress the coalescence of droplets moving on a liquid-infused surface in an efficient and predictable manner. The method relies on the mutual electrostatic interaction between the droplets induced by a homogeneous external electric field normal to the substrate. In addition to experiments in which droplet trajectories are recorded, detailed numerical computations of the repulsive electrostatic force are performed. The experiments and numerical computations show excellent quantitative agreement. Furthermore, we propose a semi-analytical model based on interacting dipoles, which predicts the relation between repulsion force, applied electric field strength, droplet volumes and separation distance with decent accuracy. Besides coalescence control, our setup allows for taking minute samples from droplet arrays if the electric field exceeds a certain threshold such that tip-streaming occurs.