Thermal convection: hot bottom, cool top. Particles trace convection rolls.
Bénard convection occurs when a fluid is heated from below (Ra > Ra_c ≈ 1708). Buoyancy overcomes viscosity and diffusion, forming organized convection rolls. Hot fluid rises (red→orange), cool fluid sinks (blue). The number of rolls depends on the aspect ratio. Above Ra ≈ 10⁴ the flow becomes chaotic.