# Is evaporation a pure diffusion process?

## Main Question or Discussion Point

Hey, I have a question for evaporation--What is the real driving reason for evaporation?
I usually believe that it is a pure diffusion process: the saturated pressure of liquid at the liquid vapor interface is higher than the partial pressure of vapor in the ambient, and this pressure difference makes the liquid evaporate.

However, I also know that evaporation absorbs heat. It looks like heat might be another driving force for evaporation. Think about this: although there is some pressure gradient, there is no heat supply to the liquid-vapor interface---then what will happen? The heat supply will limit the evaporation rate?

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Chestermiller
Mentor
The heat comes from the surrounding gas. It gets conducted to the interface. It can also come from the liquid.

Chet

The heat comes from the surrounding gas. It gets conducted to the interface. It can also come from the liquid.

Chet
Thanks for your reply. If the pressure gradient is very high, and the surrounding gas (also liquid) does not provide enough heat to the evaporating meniscus, then it could be limited by the heat supply or not?

Chestermiller
Mentor
Thanks for your reply. If the pressure gradient is very high, and the surrounding gas (also liquid) does not provide enough heat to the evaporating meniscus, then it could be limited by the heat supply or not?
Well there are certainly important practical applications where the heat transfer has to be considered.
Falling drop evaporation
Measurement of wet bulb temperature
Clothes dryers
Industrial dryers
Etc.

Chet

Baluncore
2019 Award
Temperature is the average kinetic energy of the molecules in the material. Evaporation is a loss of molecules from a liquid surface into the gas. The reverse process of condensation is also continuous. It is the statistical balance between evaporation and condensation that decides the net flow rate.

Molecules diffuse within the gas or liquid as you suggest. It will be the statistically faster molecules with higher energy that are more probable to cross the boundary. Collisions on both sides of the boundary share that energy and spread the statistical probability of transfer. How you model the interface is up to you. You can treat the boundary as a statistical diffusion layer between the solid and the liquid.

This is a bi-directional statistical process. You cannot model the process using pressure alone. For evaporation to exceed condensation you must provide additional energy. A net condensation will release energy and so without an external energy flow, an equilibrium will be reached.

You can have a liquid in high vacuum at low enough temperatures.
Surface tension.