title: 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.
authors.affiliation: Department of Physics, School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. bmweon@hotmail.com
authors.forename: Byung Mook
authors.initials: BM
authors.lastname: Weon
authors.forename: Jung Ho
authors.initials: JH
authors.lastname: Je