Diffusiophoresis: size segregation during drying of mixed-sized colloidal suspension confined between a thin gap

Particle size-segregation during drying of mixed-sized colloidal suspension has been attributed to diffusiophoresis – migration of large particles in the concentration gradient of the small particles - however, individual particle migrations could also be due to convection or sedimentation unless conditions causing these effects can be ruled out and direct observation of particle migration can be made. To minimize the effects due to sedimentation and convection, experiments are made by 1) matching the particle-solvent mass density is made by using H2O/D2O mixtures as solvents for colloidal polystyrene particles and 2) controlling the thickness of the gap in which the drying takes place. Time-lapse fluorescence confocal microscopy is used to observe motions of both the large and small particles. Upon drying, a concentration gradient of the majority small fluorescent particles and the motion of the different color-labeled large tracer particles are measured. Relative motions of the large, tracer particles in the concentration gradient of the small particles are clearly observed. Drift speed of large particles as a function of the concentration gradient of the small particles for colloidal mixtures with different salt concentration is measured. Diffusiophoresis mobility, - the ratio of the drift velocity and the concentration gradient - decreases with increasing concentration gradient, and at a fixed gradient, also decreases with the salt concentration. The experimental results are compared against existing theories, computer simulations, and experiments by others.