Self-assembly of colloidal sulfur particles influenced by sodium oxalate salt on glass surface from evaporating drops† Naveen Noah Jason, Rajib Ghosh Chaudhuri and Santanu Paria * Received 8th November 2011, Accepted 25th January 2012 DOI: 10.1039/c2sm07136c 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. 1. Introduction Self-assembly refers to the process by which any kind of colloidal or nano-particles, or macromolecules, follow an organized arrangement driven by some favorable forces. The process of self-assembly involves mobile components that orient into a structure that is both predictable and organized. The mobile components must be in an environment (certain conditions) that will induce the desired interaction(s). Once components are introduced into a self-assembly environment, they will organize through attractive or repulsive forces between the components. When the structure is stable there must be net attractive forces, and this fabrication process is an example of a bottom-up approach. Self-assembly formation of nano or colloidal particles can be broadly classified into ‘‘dry methods’’ 1–3 and ‘‘liquid phase methods’’. 4,5 Formation in dry conditions include, chemical vapour deposition (CVD), 1–3 laser ablation etc. Liquid phase formation of self-assembly can again be classified into (i) inside the bulk media 6–14 and (ii) onto the solid surface. 15–20 Self- assembly onto the solid surface may occur because of the adsorption of the particles on the surface 21–29 or during the evaporation of a liquid drop. 15,30–39 Till now many researchers have reported the self-assembly of colloidal particles 15,21–29,31,33–40 as well as microbes 41,42 from evaporating droplets. The study by Deegan et al. 15 explained the reason for ring-like stain formations when a spilled drop of coffee containing colloidal particles dries on a solid surface. They have explained the mechanism in terms of the capillary flow and mentioned that self-assembly can be controlled without knowing the chemical nature of the liquid, solute or substrate. The theo- retical study along with the experiment shows the role of DLVO interaction on three types of commonly observed self-assembly, such as a peripheral ring, 38,43 a small central bump, 30 or a uniform layer. 39 The direction of Marangoni convection inside an evap- orating drop (depends on the ratio between the substrate and droplet thermal conductivities) has a direct effect on the depo- sition pattern. 44 Marangoni induced flow is also able to show the formation of evaporative lithographic patterning in nonaqueous colloidal films. 45,46 However, the outward flow of fluid and entrained particles is reversed in freely evaporating drops composed of nonaqueous colloidal suspensions because of Marangoni stresses. 47 Kralchevsky and co-workers have repor- ted the formation of 2-D array 37,48–50 of colloidal particles. Since the self-assembly is formed because of convective flow in the wetting film, that is driven by the evaporation, they used a specific term called convective self-assembly. When a mixture of particles is present, the larger particles gather in the center and become surrounded by small particles. This is known as size dependent separation. Other recent works also try to control or suppress the coffee- ring effect by various methods. The use of a mixture of spherical and anisotropic or only anisotropic (ellipsoid) particles induce strong long-ranged interparticle interactions and thereby suppress the coffee-ring effect. 51 Electrowetting, where electric Department of Chemical Engineering, National Institute of Technology, Rourkela 769 008, Orissa, India. E-mail: santanuparia@yahoo.com; sparia@nitrkl.ac.in; Tel: +91 661 246 2262 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c2sm07136c This journal is ª The Royal Society of Chemistry 2012 Soft Matter , 2012, 8, 3771–3780 | 3771 Dynamic Article Links C < Soft Matter Cite this: Soft Matter , 2012, 8, 3771 www.rsc.org/softmatter PAPER