Fluctuating diffusivity · Binary gas
Diffusivity Fluctuates Even in a Simple Gas
Main claim
A light minority molecule in a binary hard-sphere gas shows fluctuating diffusivity when its velocity direction relaxes much faster than its speed. A molecule can quickly forget where it was moving while retaining how fast it was moving, making its apparent mobility change with time.
Problem
Fluctuating diffusivity is usually associated with heterogeneous environments or molecules with slow internal conformations. A dilute binary gas has neither feature, so it appears to be one of the simplest systems in which diffusivity should remain constant. We asked whether a large contrast in molecular mass and concentration could overturn that expectation.
Approach
Using collision statistics for a dilute hard-sphere gas, we built a kinetic Monte Carlo simulation for a minority molecule over a wide range of mass ratios. We examined trajectories, van Hove displacement distributions, ergodicity-breaking measures, and the separate relaxation of velocity direction and speed.
What we found
For a sufficiently light minority species, spatial patches of fast and slow motion appear and the displacement distribution becomes non-Gaussian at intermediate times. The velocity direction relaxes quickly through collisions, whereas the speed remains correlated much longer. During that separation of timescales, the speed acts as a slowly changing diffusivity; ordinary Gaussian diffusion returns after speed relaxation.
Why it matters
This identifies a microscopic source of fluctuating diffusivity that requires neither structural heterogeneity nor internal molecular freedom. The mechanism is simple enough to analyze from gas kinetics and broadens the range of systems in which Brownian-yet-non-Gaussian transport should be expected.
Keywords
fluctuating diffusivity, binary gas, non-Gaussian diffusion, kinetic Monte Carlo, timescale separation.
Paper
, “Fluctuating diffusivity emerges even in binary gas mixtures,” Physical Review E 107, 014605 (2023).