An application of supercritical extraction is the removal of solutes from particles of porous natural materials. Such applications include extraction of caffeine from coffee beans and extraction of ginger oil from ginger root. When CO₂ is used as the solvent, the rate of extraction is found to be independent of flow rate of CO₂ past the particles, but dependent upon the particle size. Develop a mathematical model for the rate of extraction consistent with these observations. What model parameter would have to be determined by experiment?