Coalescence-induced transition between unidirectional and bidirectional propagation of droplets

We report a coalescence mechanism between the droplet and precursor liquid that is responsible for the transition between unidirectional and bidirectional propagation of droplets on heterogeneous surfaces. Through visualizing the liquid propagation dynamics at a microscopic scale, we show how the coalescence between the droplet and precursor film/filament devastates the pinning effect induced by heterogeneous topography, and in turn, results in the macroscopic breakdown of unidirectional droplet propagation. We further show the manipulation of liquid propagation dynamics with surface wettability and topography. This work advances the fundamental understanding of navigating tiny liquid droplets on-demand that finds applications in various fields, oil-water separation, water harvesting, microfluidics, printing, coatings and microfabrication, to name a few.