Question

In a diffusion process, if the concentration of a species is higher at the surface of a material, the diffusion flux will:

• A. Increase with time
• B. Remain constant
• C. Decrease with time
• D. Be zero

Hint:

In non-steady-state diffusion, flux is not constant—it can increase with time if the concentration gradient increases or remains steep.

Answer 1

The Correct Option is A

In diffusion, the flux is driven by a concentration gradient. According to Fick’s First Law: \[ J = -D \frac{dC}{dx} \] where:
\( J \) = diffusion flux
\( D \) = diffusion coefficient
\( \frac{dC}{dx} \) = concentration gradient
When the surface concentration of a species is high, a steep concentration gradient develops across the material. Over time, as diffusion continues, this gradient can persist or even increase, especially if the surface continues to supply the diffusing species.
This results in an increasing diffusion flux over time, particularly in non-steady-state diffusion scenarios. Hence, the correct answer is: **Increase with time**.