active transport ammonia anomalous diffusion atomic diffusion bohm diffusion brownian motion carbon dioxide cell biology collective diffusion convection cotton diffusion mri eddy diffusion effusion electronics entropy facilitated diffusion gaseous diffusion heat equation isotope separation knudsen diffusion lung mammal mass transfer momentum diffusion osmosis oxygen permeation phagocytosis photon diffusion pinocytosis random walk reverse diffusion rotational diffusion second law of thermodynamics self-diffusion statistical mechanics surface diffusion thermodynamics transport phenomena viscosity wiktionary wool