Self-assembly of colloidal sulfur particles influenced by sodium oxalate salt on glass surface from evaporating drops

Abstract

Evaporation of sessile droplets containing colloidal particles induces outward flow within the drop, which produces the well-known “coffee-ring” effect or a dense ring-like deposition along the perimeter. In this work, the formation of a self-assembled structure during drying of microlitre drops on the glass surface, containing colloidal sulfur particles (synthesized in situ by the reaction of oxalic acid and sodium thiosulphate), is investigated experimentally with help of optical microscopy. Our results show that the particles are self-assembled into a tree-like structure after the evaporation of liquid drops taken from the reaction mixture. The sulfur particles alone cannot form similar self-assembly structures in pure water, but the sodium oxalate salt present in the reaction mixture helps to form the tree-like structure. The structure formation is also influenced by evaporative flux inside the liquid drop, and capillary and van der Waals attractive forces between the particles. The different parameters such as particle size, particle and salt concentration, acid to thiosulphate ratio, drop volume and addition of surfactants strongly influences the structure formation.