The physics of evaporation explained – pressure is the key factor

Not what some might have imagined perhaps. Researchers found that temperature difference between the surface and the liquid was less important than ‘the difference in pressure between the liquid surface and the ambient vapor’.

For the first time, MIT scientists have analyzed the evaporation process in detail at a molecular level and determined the physics of evaporation, reports Tech Explorist.

Evaporation is the process by which water changes from a liquid to a gas or vapor. The process is the primary path for water to move from the liquid state back to the water cycle as atmospheric water vapor.

Evaporation commonly occurs in everyday life. When you get out of the shower, the water on your body evaporates as you dry. If you leave a glass of water out, the water level will slowly decrease as the water evaporates.

For the first time, MIT scientists have analyzed the evaporation process in detail at a molecular level. For this, they used a new technique to control and detect temperatures at the surface of an evaporating liquid. Doing this, they were able to identify a set of universal characteristics involving time, pressure and temperature changes that determine the details of the evaporation process.

Mainly, they found, the key factor determining how fast the liquid could evaporate was not the temperature difference between the surface and the liquid, but rather the difference in pressure between the liquid surface and the ambient vapor.

Through this experiment, scientists also answered a rather simple question of how a liquid evaporates at a given temperature and pressure.

Pawel Keblinski, professor and head of Department of Materials Science and Engineering at Rensselaer Polytechnic Institute (RPI) said, “While theorists speculated for over a century, the experiment was of little help, as seeing the evaporating liquid-vapor interface and knowing the temperature and pressure near the interfaces is extremely challenging.”

The researchers’ success was partly the result of eliminating other factors that complicate the analysis. For example, evaporation of liquid into the air is strongly affected by the insulating properties of the air itself, so for these experiments, the process was observed in a chamber with only the liquid and vapor present, isolated from the surrounding air.

Then, in order to probe the effects right at the boundary between the liquid and the vapor, the researchers used a very thin membrane riddled with small pores to confine the water, heat it up, and measure its temperature.

MIT postdoc Zhengmao Lu, professor of mechanical engineering said, “That membrane, just 200 nanometers (billionths of a meter) thick, made of silicon nitride and coated with gold, carries water through its pores by capillary action, and is electrically heated to cause the water to evaporate. Then, we also use that membrane as the sensor, to sense the temperature of the evaporating surface in an accurate and noninvasive way.”

“The gold coating of the membrane is crucial. The electrical resistance of the gold varies directly as a function of the temperature, so by carefully calibrating the system before the experiment, they are able to get a direct reading of the temperature at the exact point where evaporation is taking place, moment by moment, simply by reading the membrane’s resistance.”

Wang said, “The data they gathered suggests that the actual driving force or driving potential in this process is not the difference in temperature, but actually the pressure difference. That’s what makes everything now aligned to this really nice curve, that matches well with what theory would predict.”

Full report here.