• This simulation from PhET shows two-gas diffusion with adjustable parameters.

https://phet.colorado.edu/en/simulation/diffusion

 

Andrew Duffy of BU also has two simulations on this topic

• Brownian Motion, with variable temperature

http://physics.bu.edu/~duffy/HTML5/brownian_motion.html

• One or two-gas diffusion, also with variable temperature

http://physics.bu.edu/~duffy/HTML5/diffusion.html

 

• This simulation by B. Surendranath traces the motion of a massive dust particle undergoing Broanian motion; the number and mass of the air molecules and their temperature can be adjusted.

https://www.surendranath.org/GPA/Heat/BrownianMotion/BrownianMotion.html

 

• On a related topic, this additional app by Andrew Duffy simulates diffusion-limited aggregation with variable attraction.

http://physics.bu.edu/~duffy/HTML5/DLA4c.html