Self-assembly of colloidal nanoparticles from sessile evaporating drops

Abstract

Science and technology has advanced to such extent that the size of devices and machines used for making daily work easy has reduced greatly. As such, fabrication of devices on planar surfaces is catching more attention and traditional methods have become obsolete. Fabrication at small-scale has been made easier and inexpensive by the use of a relatively novel technique called self-assembly. The present work is divided into two parts: formation of fractal self-assembly patterns from colloidal sulphur in acidic medium, and from sodium carboxymethylcellulose (CMCNa) in alkaline medium. When droplets of colloidal suspensions are evaporated, a dense ring-like deposition is formed; a phenomenon known as coffee-ring effect. Since it takes place by the outward flow within the drop and an overall evaporative flux which causes circulation towards the centre of drop, a coffee-ring like structure is formed. According to theory, by varying the particle concentration and the alkali used, the structure morphology can be varied. In this work, formation of self-assembled structures from colloidal sulphur particles and polyelectrolyte chains of CMCNa by drying of microliter-sized droplets on glass surface has been investigated by optical microscopy. Results of this work show that tree-like branched structures are formed after evaporation of drops of the reaction mixture. Various factors influence the structure formation, such as particle concentration, alkali concentration, pH of the solution, drop volume, reaction and drying temperatures, and drying time. Different alkali and acid concentrations when used for the reaction bring about change in morphology of the structures in the reaction mixture.