Abstract
Differential dynamic microscopy (DDM) was used to investigate the diffusive dynamics of nanoparticles
of diameter 200–400 nm that were strongly confined in a periodic square array of cylindrical nanoposts.
The minimum distance between posts was 1.3–5 times the diameter of the nanoparticles. The image
structure functions obtained from the DDM analysis were isotropic and could be fit by a stretched exponential
function. The relaxation time scaled diffusively across the range of wave vectors studied, and the corresponding
scalar diffusivities decreased monotonically with increased confinement. The decrease in diffusivity could be
described by models for hindered diffusion that accounted for steric restrictions and hydrodynamic interactions.
The stretching exponent decreased linearly as the nanoparticles were increasingly confined by the posts.
Together, these results are consistent with a picture in which strongly confined nanoparticles experience a
heterogeneous spatial environment arising from hydrodynamics and volume exclusion on time scales comparable
to cage escape, leading to multiple relaxation processes and Fickian but non-Gaussian diffusive dynamics.
