Evaporating polygonal shape of multi-component droplets in a confined space suppresses coffee-ring effects

Abstract

To control and suppress coffee-ring effects are crucial to achieve uniform patterns from a droplet evaporation. To date, a non-uniform evaporative flux along the radial direction is mainly studied, which is the case of a constant radius of curvature of the droplet. It is reported that if the contact area shape has a non-uniform curvature along the azimuthal direction, the coffee-ring flow accelerates toward the vertex. Then, the coffee-ring patterns become much more serious at the vertex. In this study, we showed that if a multi-component droplet evaporated inside a confined space, the coffee-ring patterns completely disappeared. Not only a circular droplet but also non-spherical asymmetric droplets, e.g., triangle, square, and hexagon, were investigated. From PIV experiments, we observed that the confinement geometry induces a radially-inward circulating solutal Marangoni flow by capturing the evaporated vapors regardless of the droplet shape. Finally, we obtained the coffee-ring-less dried patterns after a complete evaporation in the confined space. During the talk, we will provide scaling arguments and theoretical model to explain the main mechanism. We expect that it can be widely utilized in a QLED color filter with various shapes of the patterns in a display field.