Oscillatory dynamics of colloidal particles by surfactant in an evaporating droplet

Abstract

Inside an evaporation droplet, a radially outer capillary flow, known as a coffee ring effect, is created to push the particles toward the contact line. This effect is inhibited by surfactant-induced Marangoni effect, resulting in a local vortex called a Marangoni eddy. Due to the competition between the Marangoni and coffee ring effects, an oscillatory flow in the droplet is sometimes generated, which is observed through the behavior of the particles along the flow in the droplet. In the present study, we investigate transport and deposition processes of colloidal particles in a drying droplet containing surfactant using a coarse-grained lattice model. We find that the temporal evolution of the oscillatory flow varies with the sorption rate and its characteristics are identified with the Marangoni eddy size and the duration of the oscillatory motion of particles. We also find that these temporal patterns of particles are attributed to the interplay between the coffee ring and the Marangoni effects in different spatio-temporal regimes. Finally, we devise a simple formula to qualitatively predict the occurrence of the oscillatory regimes. The formula can explain the origin of the oscillations and correctly correlate the dependence of key quantities such as the sorption rate on the deposition dynamics of colloidal particles.