ABSTRACT
We begin our deliberations by introducing the reader to the basic rate laws that govern the transport of mass. In choosing this topic as our starting point, we follow the pattern established in previous treatments of the subject, but depart from it in some important ways. We start, as do other texts, with an introduction to Fick’s law of diffusion, but treat it as a component of a broader class of processes, which is termed gradient-driven transport. This category includes the laws governing transport by molecular motion, Fourier’s law of conduction and Newton’s viscosity law, as well as Poiseuille’s law for viscous flow through a cylindrical pipe and D’Arcy’s law for viscous flow through a porous medium, both of which involve the bulk movement of fluids. In other words, we use as common ground the form of the rate law, rather than the underlying physics of the system. This treatment is a departure from the usual pedagogical norm and is designed to reinforce the notion that transport of different types can be drawn together and viewed as driven by a potential gradient (concentration, temperature, velocity, pressure), which diminishes in the direction of flow.
