Collective diffusion of nanoparticles and polyelectrolytes in confinement*

Transport through heterogeneous confined geometries is encountered in many processes and applications such as filtration, drug delivery, and enhanced oil recovery. To understand these complex phenomena, we have investigated dynamics of negatively-charged polystyrene particles and fluorescently labeled sodium polyacrylate in convex lens-induced confinement (CLiC). The confinement slit height was controlled from 0.085 µm to 3.6 mm by sandwiching the solutions between a glass coverslip and a convex lens using a homemade sample holder. Using differential dynamic microscopy and particle tracking, cooperative diffusive dynamics and interactions were determined. It was found that particle diffusion was more hindered as a function of increased confinement free energy, and electrostatic interactions were considerably reduced. Moreover, in high salt solutions, confinement-induced phase separation and adsorption were observed for both particles and polymers. These findings could lead to better understanding of separations and interactions in confining devices.