Self-Assembly and Sorting of Graphene Nanoplatelets and Spherical Colloidal Particles via Sessile Droplet Evaporation in Saturated Alcohol Vapor

We investigated the dried deposit patterns of quasi-two-dimensional graphene nanoplatelets (GNP) on hydrophilic glass via aqueous sessile droplet evaporation in a saturated alcohol vapor atmosphere. In addition, we studied both the size- and shape-dependent self-assembly and self-sorting of colloids by employing droplets containing bidispersed suspensions comprising GNP and bulk three-dimensional colloidal particles, e.g., polystyrene microspheres (PS). We report distinct self-sorting regimes as the PS diameter D p varies between 0.1 and 6.4 μm. Under ambient air, the droplet evaporation does not yield discernible self-sorting between GNP and PS. Under saturated alcohol vapor, a critical D p = 1.1 μm exists, for which the self-sorting does not occur. For D p < 1.1 μm, the PS sort out from the bidispersed suspension to form the outermost ring of the deposit. The deposit pattern reverses for D p > 1.1 μm, and GNP deposit on the outermost edge. We corroborate experimental observations with a theoretical model based on the size- and shape-dependent interfacial forces between the colloids.