This is a diffusion-limited aggregation (DLA) model — the same random-walk process Witten & Sander used in 1981 to explain why crystals, mineral dendrites and electrodeposits grow branching, fractal shapes instead of smooth blobs. Each tick, new "vapor" particles are released on a ring around the growing cluster and take independent random walks (an 8-direction lattice walk simulating thermal diffusion). When a walker becomes adjacent to the cluster it sticks with the sticking probability you set; if it fails to stick it keeps wandering.
- Low stickiness — walkers bounce off many times before attaching, so they penetrate deep into concave gaps before sticking, producing dense, compact growth.
- High stickiness — a walker attaches on its very first contact, so tips that reach outward shield the gaps behind them from ever being filled — the classic thin, branching dendrite look.
- Seeds sets how many nucleation sites the crystal starts from; diffusion speed just controls how many lattice steps each walker takes per frame (visual speed, not the sticking physics).