Stefan’s law (Stefan diffusion) in gas-phase mass transfer Stefan’s law primarily describes mass transfer under which mechanism/condition?

Introduction / Context:
When a volatile species evaporates into a stagnant or slowly moving gas, the observed flux includes the effect of diffusion and, in some cases, a compensating bulk flow. Stefan’s law gives a convenient closed-form expression for steady diffusion of a species through a stagnant gas film, widely used for evaporation and drying calculations.

Given Data / Assumptions:

• Binary gas mixture with species A diffusing through stagnant B.
• Steady state, isothermal, ideal gas behavior near the interface.
• No homogeneous chemical reaction in the gas phase.

Concept / Approach:
Stefan’s diffusion law derives from a simplified Stefan–Maxwell framework for the special case of A diffusing through non-diffusing B. It yields a logarithmic form for flux that accounts for the build-up of A in the film. Despite the presence of a compensating molar flux of A (bulk flow term appears in the derivation), the law is fundamentally a diffusion model in a stagnant film and is classed under diffusion-controlled mass transfer correlations.

Step-by-Step Solution:

Verification / Alternative check:
Drying and evaporation models use Stefan’s law for gas-side control, validating its classification as a diffusion relation in a stagnant film.

Why Other Options Are Wrong:

• Bulk flow alone ignores the essential concentration-gradient-driven diffusion.
• “Both equally” is imprecise; the formulation is a diffusion law including the stagnant-gas effect.
• “Neither” contradicts the derivation and application.

Common Pitfalls:
Confusing Stefan’s law with equimolar counterdiffusion; its hallmark is diffusion of one species through a non-diffusing second species, not equal counterflows.

Final Answer:
diffusion