Self-assembly of colloidal sulfur particles on flat surfaces from evaporating sessile drops

Abstract

The advancement of modern science and technology needs day by day smaller microprocessors, chips, sensors, machines, and devices, as a results, the existing methods of fabrication are slowly becoming obsolete. The fabrication of devices at small scale by the help of self-assembly is an inexpensive and promising technique. This work explores the formation of nano-microstructures using colloidal sulfur particles and the manipulation of their morphology using different acid salts, surfactants and substrates. Evaporation of sessile droplets containing colloidal particles induces outward flow within the drop, which producews the well-known “coffee-ring” effect or a dense ring-like deposition along the perimeter. In this work, the formation of self-assembled structure during drying of microliter 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 helpof optical microscopy. Our results show that the particles are self-assembled into a tree-like structure after the evaporation of liquid drops from the reaction mixture.he sulfur particles alone cannot form similar self-assembly structurein 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 evaporation flux inside the liquid drop, capillary, van der Waals attractive forces between the particles. The different parameters such as particle size, particle and salt concentrations, acid to thiosulphate ratio, drop volume, addition of surfactants strongly influences the structure formation. By varying the acids that are used for the reaction, the resultant salts are varied too and this brings about changes in the sulfur structure morphology. By using HCI the sulfur structure turns into a labyrinthine grid like structure with almost 90 0 branches growing from the main stems, which are also at 90 0 to each other. With h2so4 similar leaf like patterns as before were obtained, but they were somewhat more angular and rougher. Also by changing the substrate in which the self-assembly takes place, by coating glass slides with surfactants, or by using a polyester substrate the structure morphology can be influenced to a large extent.