Controllable dried patterns of colloidal drops

Hypothesis: When an aqueous colloidal drop dries on a solid substrate, the final pattern of the dried deposit can be manipulated through controlling the internal flow states of the drop. Experiments: We report a strategy to control the dried patterns of aqueous colloidal drop by controlling the drop configurations and relative humidity. For this purpose, both sessile and pendant drops are studied. Finding: The capillary flow, which is responsible for coffee-ring, is suppressed by increasing the relative humidity. Then, surprisingly, the internal convection in the pendant drop is significantly stronger than that in the sessile drop. This phenomenon leads to the formation of the disc-like and spot-like dried patterns in the sessile and pendant drop, respectively, which are the results of different interactions between the Marangoni and (buoyancy-induced) natural convections in the sessile and pendant drops. In the sessile drop, the Marangoni and natural convections mutually restrain each other due to their opposite flow directions. In contrast, in the pendant drop, the two convections mutually enhance each other, due to their same flow directions. This new strategy offers a foreign-material-free and external-force-free means to control the dried patterns of the drop.