Dynamics of nanoparticles in glassy matrices of different tracer-matrix size ratios

The dynamics of particles in a dense suspension slow dramatically and become increasingly cooperative as the glass transition volume fraction Φ_g is approached. How these changes in the dynamics affect motion of penetrant particles remains incompletely understood. Using confocal microscopy, we investigate the dynamics of tracer nanospheres in glassy nanoparticle liquids at different tracer-to-particle size ratios δ. The dynamics of the tracer particles were calculated from trajectories obtained using particle-tracking algorithms, and differential dynamic microscopy was used to characterize the relaxation behavior of the matrix. Near Φ_g, the mean-square displacements of the tracer particles plateau on short time scales due to caging. For matrices with Φ < Φ_g, the cages relax on time scales and the tracers recover diffusive dynamics on long time scales. For matrices with Φ > Φ_g, however, tracers remain caged on all experimental time scales. For a given matrix Φ, the dynamics slow upon increasing δ. These results provide insight into how the ratio of tracer and matrix particle sizes affects the transport mechanism of particles confined in complex matrices, which can find applications in cell biology.