Patterning of colloids into spirals via confined drying
Drying of complex fluids is a fascinating subject of interest to several growing fields, for example, forensic science, lithography, printing and coating technologies. In this article, we report that the drying of charged stabilized colloidal dispersion of fixed volume confined between two parallel plates is a route to intriguing self-assembly patterns. We show that when the dispersions are dried in parallel plate confinement, particles deposit as spiral patterns after the complete evaporation of the solvent irrespective of the confinement spacing. The formation of such patterns are understood by analyzing the underlying three phase contact line dynamics during drying. Compared to usual discrete stick-slip motion of the the contact line, typically observed in several drying configurations, in parallel plate drying configuration, the contact line is found to exhibit continuous stick-slip motion. The de-pinning of the contact line is found to occur only locally and is observed to propagate in both clockwise and anticlockwise direction leading to the patterning of colloids as spirals. Furthermore, we show that while the number of turns in the spiral deposit is influenced by dispersion volume and particle concentration, the spiral patterns form irrespective of the shape of particles in the dispersion.