(620a) Diffusiophoretic Effects on Coffee Ring Patterns

Coffee rings are a natural phenomena ubiquitous in the evaporative drying of colloidal suspensions. They are thought to be associated with only evaporation-induced capillary convection. In applications related to spray painting, coating and ink-jet printing, non-equilibrium convection-driven particle aggregation causes peripheral ring structures. The ring structure can be suppressed using higher aspect ratio particles or Marangoni convection. On the other hand, various applications require formation of strong coffee rings, while other applications want to avoid the ring structure altogether. Despite several technological advances to control the coffee ring effect, modulation from well-defined ring to completely dispersed has not been achieved successfully. Here we show both experimentally and theoretically that diffusiophoresis occurs during the evaporation of saline droplets and is efficient to concentrate or disperse the coffee rings according to the salt generated intrinsic electric fields. The coffee ring structure can be controlled by choosing various salts, whose ions have different relative diffusivities, by adjusting the particle zeta potential, or by changing the rate of evaporation. In this talk the essential physics of the process is shown, an application to particle separation is shown, and the incorporation of active colloidal motors in the coffee ring structures is explored.