Effect of Surface Modification on Polymer Diffusion in Convex Lens-induced Confinement (CLiC)

In water purification processes, typical membrane foulants are flexible polymers such as dextran. Understanding how dextran diffuses in confinement can help reduce fouling of membranes and lead to better separation processes overall. Here, we have determined the diffusion, D, of dextran (500,000 g/mol) in Convex Lens-induced Confinement (CLiC) using differential dynamic microscopy (DDM). In this geometry, the confinement ranges continuously from 0.085-21.7 μm. The lens and coverslip was chemically modified with perfluorodecyltrichlorosilane (FDTS) via molecular vapor deposition (MVD). The solution was then stabilized with a non-ionic surfactant, Triton X-100, which segregates at the interface between the water phase and glass to promote wetting. The dextran diffusion was measured near these stabilized surfaces and compared to the diffusion on unmodified surfaces. The dextran diffusion remained consistent for both the modified and unmodified surfaces, however in strong confinement, while hydrophobic interactions slowed the dextran diffusion near a glass surface, no effect was observed near a hydrophic surface stabilized by a non-ionic surfactant. These findings indicate the influence surface chemistry has on the diffusion of flexible polymers in confinement and can lead to a better understanding of separation processes.