Particle monolayer assembly in evaporating salty colloidal droplets

Evaporating a sessile colloidal droplet containing small amounts of salt results in the formation of a ring-shaped stain, resembling the classical coffee-stain effect. However, this ring shape is the only similarity: The structure is actually formed by a single monolayer of particles at the liquid-air interface of the droplet and driven by a solutal Marangoni flow. An experimental study, particle-per-particle, of the formation of this monolayer is presented and numerical simulations are performed to understand the particle aggregation mechanism.