Stokes-Einstein: D = kT / (6πηr)
Diffusion is the net movement of molecules from a region of higher concentration to lower concentration. The Stokes-Einstein equation D = kT/(6πηr) relates the diffusion coefficient to temperature (T), solvent viscosity (η), and particle radius (r), where k is Boltzmann's constant. This equation works well for spherical particles much larger than solvent molecules. Typical values: small molecules in water ≈ 10⁻⁹ m²/s, proteins ≈ 10⁻¹¹ m²/s, gases in air ≈ 10⁻⁵ m²/s. Diffusion governs many processes: drug delivery, membrane transport, corrosion, crystal growth, and biological signaling. Fick's first law J = −D(dC/dx) uses D to calculate the flux of molecules across a concentration gradient.