Dynamics of nanoparticle tracers in supercooled nanoparticle matrices.

Abstract

We investigate the dynamics of tracer nanoparticles in bulk supercooled nanoparticle matrices using confocal microscopy. We mix fluorescent (tracer) and undyed (matrix) charged-stabilized polystyrene nanoparticles with tracer-to-matrix particle size ratios <i>δ</i> = 0.34, 0.36, 0.45, 0.71 at various matrix volume fractions <i>ϕ</i>. Single-particle and collective dynamics were obtained from particle-tracking algorithms and differential dynamic microscopy (DDM), respectively. The long-time behavior of the tracer mean-square displacement (MSD) and the shape of the distributions of particle displacements depend on <i>δ</i> and <i>ϕ</i>. At sufficiently large <i>ϕ</i>, small tracers (<i>δ</i> ≤ 0.36) remain mobile and subdiffusive but large tracers (<i>δ</i> ≥ 0.45) are dynamically arrested. The relaxation times determined from the intermediate scattering function (ISF) increase with <i>δ</i> and <i>ϕ</i>. Anomalous logarithmic decays in the ISF are observed for tracers of size <i>δ</i> ≤ 0.36 over a length scale of four to ten matrix particle diameters. These results provide insight into how penetrant size affects the transport of nanoparticles in porous media with soft interparticle interactions.