Steady DiffusionA mass transfer process where the concentration profile and molar flux within a diffusion medium do not change with time, resulting in zero accumulation inside any differential control volume. In the thin film model, this means the rate of solute entering any layer equals the rate leaving it.
Species 1The specific chemical solute whose transport and diffusion behavior is being tracked and analyzed as it moves from a high-concentration region across a film or membrane into a dilute solution.
Thin FilmA narrow physical barrier or layer of thickness 'l' through which a solute diffuses from one well-mixed solution to another, serving as the standard geometric model for evaluating steady-state Fickian diffusion.
Concentration GradientThe spatial rate of change of concentration over distance 'z', representing the driving force that propels solute molecules from regions of higher concentration to lower concentration according to Fick's Law.
Fick's Law of DiffusionThe fundamental constitutive equation stating that the molar flux of a diffusing species is proportional to its concentration gradient, dictating that mass flows down the concentration slope.
Boundary ConditionsMathematical constraints applied at specific spatial coordinates—such as $z = 0$ ($c_1 = c_{10}$) and $z = l$ ($c_1 = c_{1l}$)—that enable the integration and exact solution of differential equations governing mass transfer.
Solute AccumulationThe net rate of mass buildup within a differential control volume, calculated as the difference between the rate of diffusion into the layer and the rate of diffusion out of the layer.
MembraneA selective barrier separating two well-stirred solutions that is chemically distinct from the solutions it divides, requiring phase equilibrium considerations at its interfaces during mass transfer analysis.
Well-Mixed SolutionA fluid phase characterized by uniform composition throughout due to active stirring or agitation, ensuring that concentration gradients are restricted entirely to the diffusion film or membrane.
Molar FluxThe amount of substance (in moles) passing through a unit cross-sectional area per unit time, representing the primary output parameter calculated when solving steady-state diffusion problems.
