Spontaneous Chaos in a Droplet: Marangoni-Driven Flows in an Evaporating Binary Drop

Sessile binary droplets can often develop such a strong solutal Marangoni flow than it can overcome the capillary flow typically leading to the familiar coffee-stain effect. One paradigmatic case is that of glycerol-water, which results in a stable axisymmetric toroidal flow. The other paradigmatic case is that of ethanol-water, which instead develops highly unstable non-axisymmetric flows [Gelderblom et al. Soft Matter 18.45 (2022): 8535-8553].

The spontaneous evaporation-driven flow in an ethanol-water droplet advects tracer particles in a complex three-dimensional and non-axisymmetric flow, with tracer particles jumping from unstable orbits oriented in all directions (even for thin droplets), featuring strong intermittencies and with particle velocities than can reach up to several droplet diameters per second.

Up to now, such a complex flow has been only studied using standard two-dimensional particle image velocimetry, which is insufficient to capture the flow's temporal and spatial complexity; and can easily suffer from depth of correlation issues [Rossi et al. Experiments in fluids 52 (2012): 1063-1075.]

In this work, using the open source method DefocusTracker, a 3D particle tracking technique developed by Barnkob & Rossi [Journal of Open Research Software, 9(1), 22, (2021).], we characterize the stability of the flow by carefully analyzing the three-dimensional trajectories with high spatial and time resolution in the context of chaotic advection.

The results are compared with the flows obtained by numerical simulations of Duarte Rocha, Detlef Lohse and Christian Diddens [oral contribution in this conference].