Aerodynamic interactions of drops on parallel fibres

The wetting behaviour of drops on rigid and elastic fibres is important in many applications including textiles, fog collection, systems with absorbent fibres, and in natural settings such as spiderwebs. Yet, little is known about the behaviour of fibre-attached drops when exposed to a background air flow. Additionally, control of drop motion and coalescence with neighbouring drops remains a challenge. Here we show that in the presence of a uniform air flow, drops wetting parallel fibres can interact aerodynamically, both with their downstream and upstream neighbours. These interactions can lead to a variety of behaviours, including alignment, repulsion and coalescence. Using particle-image velocimetry, we visualize the wake patterns and explain the alignment and coalescence behaviours based on the interactions between the wakes and drops. We obtain a diagram of the different interaction regimes, depending on the distance between the fibres. Finally, we apply this knowledge to realize controlled drop coalescence.