Sorting of Colloids Through Evaporating and Spreading Droplets

We examine the spreading, evaporation, and deposition patterns of aqueous sessile droplets containing colloidal particles in a saturated alcohol vapor environment. As the alcohol vapor diffuses toward the droplet's curved interface, it adsorbs onto the surface, causing a non-uniform reduction in surface tension along the liquid-vapor boundary. This generates solutal Marangoni flow, leading to spontaneous spreading and intricate internal fluid dynamics. The flow features are confirmed by flow-visualization and preliminary particle image velocimetry (PIV) measurements. Our findings highlight a strong dependence of colloidal deposition on particle size: smaller particles form multiple rings, whereas larger particles produce single rings and patch-like deposits. The reduction in surface tension and its non-uniform variation along the liquid-gas interface are supported by theoretical models. Additionally, we demonstrate the size-based sorting of polystyrene microspheres and polystyrene microspheres mixed with graphene nanoplatelets, suspended in evaporating droplets within a saturated alcohol vapor environment. The experimental observations are validated by a theoretical model that incorporates size- and shape-dependent interfacial forces between the colloids. These findings have significant implications for the design of advanced biosensors and coating applications.