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

The evaporation of multicomponent 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 particle tracking velocimetry. 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 hypothesise that pinning-induced motion mediated by suspended particles in the droplets (due to contamination or added on purpose) is the cause of this counter-intuitive behaviour. We also discuss an alternative interpretation, namely that the repulsion between the two droplets is caused by thermal Marangoni flow as is the case for a pair of pure droplets on an isothermal substrate (Malachtari and Karapetsas, J. Fluid Mech. vol. 978, 2024, p. A8), but give the arguments why that interpretation is not applicable in our case of binary droplets. To further support our interpretation, with the help of direct numerical simulations we explore the relative contributions of the replenishing flow and of the solutal and thermal Marangoni flows to the overall flow dynamics in one droplet. Finally, as further evidence, the azimuthal dependence of the radial velocity in the drop is compared with the evaporative flux and a perfect agreement is found.