Nanoparticle Dynamics near Polyacrylamide Gel Interfaces.

To determine the impact of the interface material properties on hydrodynamic interactions and transport, we have investigated how "soft" gel surfaces influence the local diffusion of polystyrene nanoparticles in deionized water in convex lens-induced confinement (CLiC). The gel coatings are created by polymerizing polyacrylamide onto glass surfaces, resulting in surfaces with varying moduli of 60, 1300, 2620, and 8400 Pa that are 30-100 nm in height. We analyze the diffusion using differential dynamic microscopy (DDM) as a function of proximity to the surface. We find that diffusion depends on the material properties of the surface. The gel layers are too thin to impact the hydrodynamic interactions experienced on the surface, mirroring the contact angle measurements. However, near softer, more hydrated layers, the nanoparticles can permeate into or between the gel surfaces. As the modulus increases, the partition into the gel is lower, and we observe absorption of the particles into the gel, but no discernible motion in the gel layers. No additional effects are observed as a function of the height of the surface coatings within the experimental range. We postulate that these findings contribute to understanding polymer dynamics at complex interfaces and can potentially lead to a transformative understanding of biofouling and polymer-based separations.