Oscillation dynamics of colloidal particles caused by surfactant in an evaporating droplet

Abstract

Surfactant-induced Marangoni flows can alter the typical outward capillary flows in a drying droplet. Previous studies show that intricate time-dependent periodic motions of colloidal particles, such as Marangoni eddies and back-and-forth oscillatory motions, can be developed due to the coupled dynamics of surfactant and evaporation. Here, the temporal motions are investigated using a coarse-grained lattice model. We show that during the evaporation, aggregation and depletion zones of colloidal particles emerge close to the contact line when the capillary and Marangoni flows are comparable in the opposite directions. It is found that the type of periodic motions is determined by the initial surfactant concentration. When the strength of the Marangoni flows is sufficiently large at the early stage of evaporation swirling motions rather than oscillatory motions are developed. The periodic motions of particles persist until the surfactant concentration on the surface is saturated. Also, the onset and terminate times of the motions decrease with the initial surfactant concentration.