Pinning induced motion and internal flow in neighbouring evaporating multi-component drops

Abstract

The evaporation of multi-component sessile droplets is key in many physicochemical applications such as inkjet printing, spray cooling, and micro-fabrication. Past fundamental research has primarily concentrated on single drops, though in applications they are rarely isolated. Here, we experimentally explore the effect of neighbouring drops on the evaporation process, employing direct imaging, confocal microscopy, and PTV. Remarkably, the centres of the drops move away from each other rather than towards each other, as we would expect due to the shielding effect at the side of the neighbouring drop and the resulting reduced evaporation on that side. We find that pinning-induced motion mediated by suspended particles in the droplets is the cause of this counter-intuitive behaviour. Finally, the azimuthal dependence of the radial velocity in the drop is compared to the evaporative flux and a perfect agreement is found.