Mechanisms of the Assembly of Nano- and Microparticle Two-Dimensional Structures in a Wedge Film

When a drop of colloidal suspension is deposited on a smooth solid surface, the colloidal spheres self-assemble into two-dimensional ordered structures in the three-phase contact region (i.e., wedge film) at the edge of the evaporating fluid. We used advanced microinterferometric techniques to study the trajectories of polystyrene microspheres in water inside the wedge film. We also monitored the temperature gradient over the drying droplet surface using a sensitive infrared technique. Experimental investigations show that colloidal spheres are forced into the wedge region by the Marangoni-driven flow caused by the temperature gradient for small wedge angles with a pinned contact line, as discussed by Hu and Larson (Langmuir 2005, 21, 3963). The role of the Marangoni flow was verified by adding a small amount of surfactant to the evaporating fluid and observing recirculating flow (i.e., Marangoni−Benard convection cell induced by the surface tension gradients). In addition, the results of our experiments on particle dynamics show that the dominant mechanism for the self-structuring of the particles in the confines of the wedge surfaces is the stagnation point flow; it is not the capillary attraction due to the menisci formed between the particles.