http://simulations.narod.ru/
The diffusion is simulated as set of points with random step each frame of time. Looks like brownian motion.
Step of point can be estimated from diffusion equation:
dn/dt=D*(d2n/dx2+d2n/dx2)
then approximately:
1/dt=D/dx^2
dx=sqrt(D*dt)
dt - time step
D - diffusion coefficient
dx - step of point