Spatiotemporal Evolution of Diffusiophoresis in a 2D Drying of mixed-sized Colloidal Droplet

Particle segregation during drying of mixed-sized colloidal suspension has been attributed to diffusiophoresis that large particles migrate in the concentration gradient of the small particles. However, diffusiophoresis has not been observed in colloidal mixtures to rule out other effects such as convection or sedimentation which could also give rise to particle segregation. To eliminate sedimentation and convection, an experiment was designed by using an H2O/D2O mixture to match the mass-density of the polystyrene particles and the aqueous solvents. Experiments show the mass-density matching reduced both sedimentation and convection when drying took place in a drop of colloidal suspension confined between two glass slides with a gap of 150 um. Upon drying, a concentration gradient of small fluorescent particles (the majority species) and motion of the different color-labeled large particles (at very low concentration) were measured by time-lapse confocal microscope. The drift speed of large particles was found to depend on both the concentration gradient of the small particles and the salt concentration of the suspension. The ratio of the drift velocity and the concentration gradient, so called diffusiophoretic mobility, was found to decrease with the salt concentration and non-linear with the concentration. These experiments show diffusiophoresis followed only qualitatively with the models developed by Sear and Warren's theories. Sharp drops of the mobility occurred at large density gradient where the absolute densities were also high suggesting the diffusiophoresis motions were hindered by crowding. By eliminating convection and sedimentation, this study quantifies diffusiophoresis in the drying of mixed-sized colloids, and crowding induced hinderance limits the degree of segregation by diffusiophoresis.