Knudsen diffusion occurs predominantly in pores where the diameter is smaller than the mean free path of gas molecules.

Knudsen diffusion occurs predominantly in pores where the diameter is smaller than the mean free path of gas molecules.

• A. In pores with a diameter smaller than the mean free path of gas molecules.
• B. In pores with a diameter larger than the mean free path.
• C. In liquids rather than gases.
• D. When there is primarily convective mixing.

Answer: (A)

Knudsen diffusion occurs when molecule-wall collisions dominate over molecule–molecule collisions, which happens in very small pores where the pore diameter is smaller than the gas molecules’ mean free path. In this regime, the transport of gas species is controlled by how often and in what directions molecules collide with and bounce off the pore walls, rather than how they collide with each other. The diffusion behavior thus depends strongly on the pore size and the average molecular speed, and it becomes more pronounced as the pores get smaller and the gas becomes more rarefied. That's why the statement describing pores with a diameter smaller than the mean free path is the best fit. If the pores were larger than the mean free path, intermolecular collisions would govern diffusion more than wall collisions, leading to standard molecular (Fickian) diffusion. Knudsen diffusion is a gas-phase phenomenon and is not about liquids; convection implies bulk flow, which changes the transport mechanism away from pure diffusion.

Knudsen diffusion occurs when molecule-wall collisions dominate over molecule–molecule collisions, which happens in very small pores where the pore diameter is smaller than the gas molecules’ mean free path. In this regime, the transport of gas species is controlled by how often and in what directions molecules collide with and bounce off the pore walls, rather than how they collide with each other. The diffusion behavior thus depends strongly on the pore size and the average molecular speed, and it becomes more pronounced as the pores get smaller and the gas becomes more rarefied.

That's why the statement describing pores with a diameter smaller than the mean free path is the best fit. If the pores were larger than the mean free path, intermolecular collisions would govern diffusion more than wall collisions, leading to standard molecular (Fickian) diffusion. Knudsen diffusion is a gas-phase phenomenon and is not about liquids; convection implies bulk flow, which changes the transport mechanism away from pure diffusion.