The minimum fluidisation velocity $U_{mf}$ — the superficial liquid velocity needed to just suspend settled particles — depends on: (1) density difference between solid and liquid ($\rho_p - \rho_L$), (2) particle diameter $d_p$, and (3) liquid viscosity $\mu_L$. The Ergun equation or its simplified forms (e.g. Wen and Yu) are used to calculate $U_{mf}$. Fluidised beds are used for immobilised enzyme and cell reactors.