Beyond the coffee-ring effect: Pattern formation by wetting and spreading of drops

Drying of colloidal drops on solid surfaces is the widely known method to form self-assembled patterns. The underlying principle of this method is the phenomenon known as the coffee-ring effect. Here, we report a phenomenon of pattern formation involving not drying but conversely wetting and spreading of drops on a solid surface containing a thin layer of dispersed particulates. Fascinating ringlike patterns are formed in a subsecond timescale by the interplay between the dynamics of spreading and imbibition. Occasionally, such patterns can be observed when, say, water spills on dusted floors and they are often misidentified as those formed by the coffee-ring effect. In the highly wetting scenario, we found that this pattern formation is independent of the liquid properties, however it is strongly dependent on the powder properties. Our findings have both fundamental and technological importance.