Size-Dependent Tracer Diffusion in Colloidal Gels*

Colloid diffusion in porous media is ubiquitous in industrial and biophysical systems, such as the extracellular matrix hydrogels of cell tissues and the nucleoid region inside prokaryotic cells, where specific pore morphologies enable selective filtering. Obtaining a relation between pore size and colloid diffusion in porous media will lead to the development of models that will elucidate how cells regulate biomolecule transport. In this study, we seek a fundamental understanding of how the diffusion of tracer colloids of various sizes are hindered by a porous medium. We present our results from simulations of tracer colloids undergoing Brownian diffusion in the voids of a colloidal gel. Detailed characterization of tracer diffusion reveals that all tracers experience sub-diffusive motion over short to intermediate length scales. Small tracers eventually recover the diffusive regime over longer length scales while large tracers remain in a local cavity. We additionally present algorithms to identify the characteristic length scale that marks the onset of the recovery to normal diffusion and the cutoff tracer size that determines whether a tracer is too large to break through the void network.