Time evolution of colloidal stratification in a drying droplet

In a droplet drying, uniformly mixed colloidal particles with two differ sizes could separate either due to the difference in particle diffusion or the convection flow induced by difference in buoyancy. The role for each mechanism causing the stratification is unclear if we only watch the final distribution of particles after drying. To distinguish how each mechanism stratifies the particles, we need to control diffusiophoresis and convection by varying size ratio of big to small particles and the degree of density mismatching respectively. We record time evolution of distributions for two-sized particles to understand how particles are stratified by each mechanism. The experiments are designed into a droplet drying process confined between two parallel plates where fluorescent particles are tracked by a confocal fluorescence microscopy and analyzed by MATLAB imaging processing. The results show the two-type particles accumulate at different locations when the direction of the convection flow changes. This talk will present the spatiotemporal evolution of the concentration for small and big particles and show the stratification can be controlled by adjusting the convection and diffusiophoresis.