Absence of enhanced diffusion in the dynamics of a thick needle through three-dimensional fixed spherical scatterers.

The existence of three regimes in the dynamics of a thin needle-like particle diffusing through a two-dimensional random array of scatterers as the needle length is varied relative to the scatterer density was previously seen in a series of simulations. The first regime occurs at low density when the needle's diffusion follows the expected Enskog behavior. An intermediate regime gives rise to enhanced diffusion after a critical density of scatterers is reached, a manifestation of the suppression of librational motion as the needle is confined to effectively thinner but longer tubes. The third regime occurs at high scatterer density with the particle dynamics characteristic of a glass. In this article, we investigate whether the tubes seen in the second regime persist in a three-dimensional array of scatterers. The fact that the enhanced regime is not observed in a three-dimensional random array of scatterers suggests that the effective tubes formed by the moving needle are fragile structures highly dependent on dimensionality.