ABSTRACT
In simple flow systems in nature, the fundamental law of flow is linear, i.e., the flow rate increases in direct proportion to the ‘‘driving force’’ for the flow, where the driving ‘‘force’’ is the gradient (rate of change with distance) of some so-called ‘‘potential.’’ For example, the flow of heat obeys Fourier’s law, with the heat flux proportional to the gradient of temperature, T. (T is sometimes called the thermodynamic potential). For the flow of liquid water in permeable materials, it fell to the French hydraulics engineer, Henry Philibert Gaspard Darcy (1800-1858) to show that the flow obeys a similar law (‘‘Darcy’s law’’), with the flow rate proportional to the gradient of the hydraulic potential. More generally, we now know that Darcy’s law applies to flow of most simple liquids in any porous (and permeable) medium.
