Transport processes in fluids are described macroscopically using continuum theory, which approximates fluid properties (temperature, concentration, velocity) as continuously varying, single-valued fields defined within a specific length scale strictly between molecular dimensions and macroscopic system sizes. This framework relies on the derivation of partial differential equations based on conservation laws for mass, momentum, and energy coupled with constitutive relations that link fluxes to local gradients. The theory distinguishes fluids from solids by defining stress as proportional to strain rate rather than displacement, enabling the analysis of diffusion mechanisms driven by molecular thermal motion in both gases and liquids where mean free paths differ significantly but governing equations remain structurally identical due to variable transport coefficients.
Transport processes in fluids are described macroscopically using continuum theory, which approximates fluid properties (temperature, concentration, velocity) as continuously varying, single-valued f…