Enhancement of translational diffusion coefficient of a probe in a rotationally anisotropic fluid

Enhancement of the translational diffusion coefficient of a probe molecule in a supercooled liquid over the value predicted by the traditional Stokes-Einstein equation has been interpreted as arising from dynamic heterogeneity. We show that the dynamic coupling of the translation to rotation can also lead to a strong enhancement in a rotationally anisotropic molecular fluid. For a dynamically heterogeneous fluid, the probe size may play an important role. We show that if the diffusion probe is large so as to encompass several regions of high and low mobility, the rotation-translation coupling parameter needs to be modified to reflect the averaging heterogeneity effect. We show that, while the averaging effect arising from dynamic heterogeneity may alter the final enhancement, the coupling of translation to rotational degrees of freedom must be taken into account to properly account for the experimental result.