Colloidal Nanoparticle Interaction Transition during Solvent Evaporation Investigated by in-Situ Small-Angle X-ray Scattering.

In-situ scanning small-angle X-ray scattering (SAXS) experiments have been performed to probe the drying of a single suspended droplet of silica colloids. It has been demonstrated that the formation of a nanoparticle shell during drying can be confirmed just by measuring the temporal evolution of the spatial transmission profile across the drying droplet. The shrinkage of the droplet stops once the shell is formed. The temporal dependence of the shell thickness and droplet radius has been estimated by quantitative analysis of the functionality of the transmission profiles. It is revealed that the position of the correlation peak originating from interactions between silica nanoparticles evolves linearly during the initial stage of drying and exhibits sigmoidal growth behavior in later stages. The interaction between colloidal particles in different drying stages has been investigated. We provide experimental confirmation of the transition from repulsive interaction to a capillary-driven short-range attraction during shell formation. The present work demonstrates that in-situ scanning SAXS on a suspended droplet is an invaluable technique for monitoring the dynamic self-organization of colloids as it probes the drying of complex fluids without the interference of a substrate.