Release a concentrated cluster of particles and watch them spread. Diffusion is the spontaneous tendency of matter to move from high to low concentration, driven not by force but by probability — the second law of thermodynamics in action.
Particles follow 2D Brownian motion. The concentration field C(x,t) obeys Fick's second law (∂C/∂t = D·∇²C), whose solution is a Gaussian that broadens as σ = √(2Dt). The simulation shows two species diffusing toward equilibrium — no net force is needed.
Use Temperature to speed diffusion (higher T → larger step size). Place a membrane with a small pore to see flux-driven flow. Toggle Concentration map to overlay the smoothed density, evolving toward uniform grey.
The scent of a single perfume molecule can trigger a human nose. Olfactory signals travel by diffusion across the ≈40 nm synapse cleft in under a millisecond. Fick's laws, derived in 1855, are now the foundation of drug delivery, semiconductor doping and food science.