Dynamics of Nanoparticle Dispersions in Model Confined Media

Understanding the dynamics of nanoparticle dispersions, especially in confined media, have significant technological and environmental implications. In this study, we investigated the dynamics of nanoparticle dispersions in micro-patterned channels that serve as a model for confined media using fluorescence differential dynamic microscopy (f-DDM). We first confirmed that the free-diffusion dynamics of dilute aqueous dispersions of fluorescently labeled polystyrene nanoparticles, ranging in diameter from 100 to 400 nm, measured using f-DDM in unconfined media are consistent with those obtained using dynamic light scattering. We then examined the dynamics of nanoparticles in micro-patterned channels, which contained rectangular arrays of cylindrical posts of diameter 500 nm separated by a spacing ranging from 400 nm to 10 microns. Using f-DDM we showed that the diffusion coefficients of the nanoparticles decrease as the particles are increasingly confined by the cylindrical posts and surprisingly exhibit a much stronger dependence on the post volume fraction than that expected by simple tortuosity arguments.