title: Surfactant-induced Marangoni eddies alter the coffee-rings of evaporating colloidal drops.
abstract: The influence of the small ionic surfactant sodium dodecyl sulfate (SDS) on the evaporation of drying colloidal droplets is quantitatively investigated. The addition of SDS leads to a significantly more uniform deposition of colloidal particles after evaporation (i.e., the so-called "coffee-ring effect" is dramatically altered). We understand this phenomenon in the context of circulating radial Marangoni flows induced by the variation of SDS concentration along the air-water interface. Video microscopy permits the direct visualization of the colloidal particles involved in these flows, revealing a surprisingly stable "Marangoni eddy" that prevents particle deposition at the drop perimeter.
authors.affiliation: Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States. timstill@seas.upenn.edu
authors.forename: Tim
authors.initials: T
authors.lastname: Still
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Yunker
authors.forename: Arjun G
authors.initials: AG
authors.lastname: Yodh