Contact line deposits in an evaporating drop

TLDR

Solids dispersed in a drying drop migrate to the edge, forming a solid ring, a phenomenon observed across many surfaces, solvents, and solutes. The study aims to demonstrate that particle migration to the drop edge is driven by an outward flow caused by evaporation loss and geometric constraints maintaining an equilibrium shape. We develop a theory that predicts the flow velocity, ring growth rate, and solute distribution within the drop. These predictions are compared with experimental results.

Abstract

Solids dispersed in a drying drop will migrate to the edge of the drop and form a solid ring. This phenomenon produces ringlike stains and occurs for a wide range of surfaces, solvents, and solutes. Here we show that the migration is caused by an outward flow within the drop that is driven by the loss of solvent by evaporation and geometrical constraint that the drop maintain an equilibrium droplet shape with a fixed boundary. We describe a theory that predicts the flow velocity, the rate of growth of the ring, and the distribution of solute within the drop. These predictions are compared with our experimental results.