massatransport

Mass transport, or mass transfer, is the movement of matter from regions of higher concentration to lower, and more generally the transport of species due to gradients in concentration, pressure, temperature, or electric potential. It includes diffusion, convection (bulk fluid flow), and migration under electric fields, with applications across engineering, environmental science, biology, and energy.

Diffusion describes net movement down a concentration gradient. Fick's first law gives the molar flux J equal to minus D times the gradient of concentration, with D the diffusion coefficient. Fick's second law states that the change of concentration with time equals D times the second spatial derivative of concentration. Diffusion can be molecular or enhanced by turbulence.

Convection is transport by bulk fluid motion. When diffusion and convection act together, the transport is described by the convection-diffusion equation: the time rate of change of concentration plus the convective term equals the diffusive term. In porous media, Darcy's law relates fluid flux to pressure gradients.

Migration is movement of charged species under electric fields, important in electrochemical cells and membranes. Mass transfer is described by a driving force such as the change in concentration, pressure, or chemical potential, and a mass transfer coefficient, with dimensionless groups such as Peclet (Pe) and Sherwood (Sh) numbers used in design.

Typical applications include separation processes such as distillation and absorption, pollutant transport in soils, gas exchange in biological systems, and ion transport in batteries and fuel cells. Analysis uses analytical solutions for simple cases and numerical methods (finite difference or finite element) for complex geometries.