Self-Assembly of Ordered Microparticle Monolayers from Drying a Droplet on a Liquid Substrate

Drying droplets on solid substrates has always formed a nonuniform and disordered "coffee ring" stain, which has a great negative effect on the application of inject printing and colloidal assembly. We obtain a macrouniform and micro-ordered pattern through evaporation of a colloidal droplet resting on a liquid substrate. The evaporative convection and the capillary forces were responsible for the formation of the ordered structures, which assembled into a monolayer pattern at the liquid-air interface under the action of the weak capillary flow and shrinkage of the triple line. The central bump deposits with disordered particle stacking on the liquid-liquid interface could be attributed to the fast meeting of the descending particles (gravitational sedimentation) and ascending liquid-liquid interface; they would scatter on the ordered monolayer structure and form the final uniform pattern.