Capillary force repels coffee-ring effect

Abstract

When a coffee drop dries on a solid surface, it leaves a ringlike deposit along the edge and this is known as the "coffee-ring effect." We find a different motion of particles repelling the coffee-ring effect in drying droplets; the motion of particles that is initially toward the edge by the coffee-ring effect is reversed toward the center by a capillary force. The reversal takes place when the capillary force prevails over the outward coffee-ring flow. We discuss the geometric constraints for the capillary force and the reverse motion. Our findings of reversal phenomena would be important in many scenarios of drying colloidal fluids.