The physics of evaporation explained – pressure is the key factor

Posted: June 11, 2019 by oldbrew in physics, research, Temperature


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.

Comments
  1. ivan says:

    Interesting, they worked out how and why a pressure cooker works. Now how are the climatology modellers going to work this into their models to give another scare for the disbelieving public.

    This does support the theory that it is pressure that gives the surface temperature.

  2. gseine says:

    I’m not a scientist but this article still does not explain to my satisfaction energy a water molecule needs to absorb in order to change state and the process by which it does so at a low temperature. We know and continually work with adding energy to bulk water and turning it into vapour, steam, in power-plants with some 1800 calories of heat needed to change the state from liquid to vapour. We also know that the conversion can be controlled with containing the pressure, flashing the water to steam by releasing that pressure.
    It seems to me that the energy added to a system is not distributed evenly but rather is collected by individual molecules that then change state when energetic enough. I’ve never seen the process explain in a simple enough manner for my understanding.

  3. oldmanK says:

    Quote “–carries water through its pores by capillary action, and is electrically heated to cause the water to evaporate–“. Not sure whether this highlights the process or obfuscates it. Many who worked with power boilers and the design of drum water level control know the phenomenon of ‘water hideout’.

  4. Kip Hansen says:

    Evaporation has a huge energy element as well — energy is necessary to change the water from its liquid state to a gaseous state. The usual effect is that the energy flows from the liquid water (liquid water cools). Then, when water vapor condenses back into liquid water, more energy is transferred. This all has to be considered in the crazy calculations in climate models.

  5. Schrodinger's Cat says:

    Molecules of liquid bounce around because of their kinetic energy. There is a distribution of energies and the more energetic molecules manage to break free of the surface by overcoming the attractive forces of surface tension. The latter probably includes hydrogen bonding in the case of water. It is a bit like achieving escape velocity. Because we always think of bulk liquids and single temperatures and pressures we forget that at the molecular level there is a distribution of kinetic energies and molecular velocities.

    The gas pressure above the liquid is effectively forcing the gas molecules to re-enter the liquid, so the higher the gas pressure the more the evaporation is suppressed. Equilibrium is reached if the temperature, pressure and container volume are held constant.

    I think I was taught all this at school a long time ago.

  6. Schrodinger's Cat says:

    Just to complete my earlier comment, the pressure when a liquid and its own gas are in equilibrium (rate of evaporation equals rate of condensation) is the vapour pressure for that liquid at the given temperature. The energy used to overcome the attractive forces that hold molecules loosley together in a liquid is the latent heat.

  7. oldbrew says:

    Wikipedia says:
    As for other substances, water vapour pressure is a function of temperature and can be determined with the Clausius–Clapeyron relation.

    https://en.wikipedia.org/wiki/Vapour_pressure_of_water

    They also show ‘Graphical pressure dependency on temperature’ (two graphs). Might have to re-visit that page…

    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.”

  8. See my related article via the link above.
    The weight of an atmosphere determines the amount of energy required to achieve the phase change from liquid to vapour and thus determines the maximum anount of energy that the oceans can contain at a given level of insolation.
    These ‘scientists’ are just beginning to catch on.

  9. pochas94 says:

    If you’re well below boiling, wind velocity is very important.

  10. p.g.sharrow says:

    WOW!, Collage boys discover Gas Pressure Laws.

    Very cool test setup to learn something that has been known for nearly 200 years. The pressure of a gas sets the temperature of evaporation of it’s associated liquid. The combined pressure of Oxygen O2 and Hydrogen H2 sets the evaporation/temperature of Water H2O. The pressures of Nitrogen N2 and others have no effect on Water’s Evaporation/temperature….pg

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