ABSTRACT
The systems that have been considered in previous chapters are by and large at equilibrium and reversible. The movement of molecules from one position to another by processes such as diffusion, electrophoresis, and sedimentation is irreversible and at a distance from equilibrium. However, these transport processes have an interesting feature in that a system not in equilibrium proceeds to equilibrium. Moving toward equilibrium requires flow. Forces acting on the molecules drive the flow, and the forces result from the gradients (rate of change with distance) of potential energy (i.e., temperature/heat, pressure, mass/concentration, or external forces imposed on the system). When the gradients of potentials become uniform throughout the system, flow will stop. In other words, flow continues until these gradients disappear. The flow of mass is our focus here (e.g., the movement of smoke from chimney in air).
