Asymmetric Diffusion

Diffusion rates through a membrane can be asymmetric, if the diffusing particles are spatially extended and the pores in the membrane have asymmetric structure. This phenomenon is demonstrated here via (i) deterministic simulations of a two-species hard-disk gas, (ii) simulations of two species in Brownian motion, diffusing through a membrane which is more permeable to one species than the other, and (iii) simulation of diffusion in a simple network model. In its extreme form, this effect will rapidly seal off flow in one direction through a membrane, while allowing free flow in the other direction. A single species of appropriately shaped particles will exhibit the same effect when diffusing through appropriately shaped pores. We hypothesize that purely geometric effects discussed here may play a role in common biological contexts such as the potassium ion channel.