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.

  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.

    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

  11. Brett Keane says:

    pgs:. I think I saw that in the works of Thomson and/or Maxwell. A fascinating thing, and worth discussing in connection with the Gas Laws/ IGL. Because they underlie the GTE as opposed to warmist/wuwt-ista silliness as they wrongfully call it….

    I can picture how all gas molecules being owners of about 216 times their own volume, are possessed of enough freedom to all react near enough to equally when heated/energised. But how they initially distinguish from all other specie for purpose of phase change, that is a puzzle. Thinking molecularly seems to give some clues though…. Brett Keane

  12. Brett Keane says:

    Oops. Volume of space! Brett

  13. p.g.sharrow says:

    @Brent Keane; I would guess it would have something to do with Atomic Resonate Frequency. Atoms would more easily exchange energy with their own kind. So mater/energy density at the point of interface would determine the temperature point of phase change.

    There is an energy/mater density point where liquid Hydrogen ( H2) must become gas, There is a point where gas Hydrogen ( H2) must become atomic Hydrogen ( H). There is a point where Atomic Hydrogen becomes an apparent naked Proton or Plasma. There is also a point of Mater/energy density where a Hydrogen atom collapses to become a Neutron.

    All the result of Mater/energy density where the Electro-static pressure of energy from a proton encounters the pressure from the proton of it’s neighbor and establishes it’s Electronshell.

    This fundamental characteristic of the foundation of Mater, the Proton/Neutron, is the cause of all atomic process…pg

  14. tallbloke says:

    Good find OB. Experimental data which supports what I’ve been saying for years. Pressure is the primary variable affecting evaporation. This finding helps validate Nikolov and Zeller’s work too. A change in the max sea surface temperature over long periods as shown by proxy data indicates a significant change in atmospheric pressure.

  15. dai davies says:

    Boiling an egg at 800m asl for 3 minutes results in a very runny egg. I go for 4 or more. Not fully immersing them might be a factor, but steam should work as well. It dumps the heat of vaporisation on the food surface.

    Came across some interesting facts(?) researching my article on the Earth’s water thermostat. Water doesn’t turn straight to liquid above 0C but decreasingly maintains nanocrystaline structures up to just below 30C, and has structured surface layer 100 mol or so deep. Evaporation accelerates at 30C.

    Another was that oceans had a molecular thin layer of organic material on the surface. No idea how general that is but it would impede evaporation.

    snippet at

    And the experimental evidence:

    pochas94: yes, wind velocity is very important. Apparently, evaporation can increase with the 3rd or 4th power of wind speed.

  16. tallbloke says:

    Ned writes in email:

    It has been well-known for decades that evaporation primarily depends on the vapor-pressure deficit (VPD), which is the difference of vapor pressure between the surface and the ambient air outside of the surface boundary layer. This is part of the standard energy-balance equation, where the latent heat flux (evaporation in energy units) is proportional to VPD. I used this formulation in my ecosystem-atmosphere energy-exchange model, which I developed as part of my PhD Dissertation in the late 1990s.

    The dependence of evaporative cooling on VPD is what makes the convective heat exchange infinitely more efficient than radiative heat exchange. This is what I mean by that: The transfer of radiant heat between two bodies with temperatures T1 and T2 respectively, where T1 > T2, is proportional to the gradient (T1^4 – T2^4). Thus, radiant heat exchange cannot occur if T1 = T2. However, evaporation can transfer heat from body 1 to body 2 provided that the vapor pressure at the surface of body 1 is higher than the VP at the surface of body 2 (i.e. VPD > 0) even if even if T1 = T2 or in some cases even if T1 < T2. That's why solving the radiative transfer in the atmosphere without convective terms present in the matrix of equations (as is the case in ALL climate models) produces a mathematically wrong solution to the total heat transport that leads to violation of the Energy Conservation Law. Climate scientists call this erroneous result "CO2 climate sensitivity"!

  17. tallbloke says:

    Dai, the rapid increase in evaporation at 30C is interesting. That’s also the max ocean surface temp reached under Earth’s current sea level pressure. Ned has found paleoproxies showing tropical SST has reached 35C in the past. That suggests higher sea level pressure in the past.

  18. dai davies says:

    The green line in this is just below 30C. Sure looks like a thermostat in action, but I don’t understand how the data was constructed. Relative T values may be valid.Of course, the CO2 data has been dispited, but haven’t seen the T data disputed.
    There are a couple of bumps above the line.

  19. Ian MacCulloch says:

    The two blips occur when there is extreme volcanic activity – so breaching the ‘thermostat’ concept occurs only when there is an extremely anomalous event taking place. One is of course right on the P/T boundary while the other is related to significant volcanic events common in the Eocene which also occur about the same time as mineralising events for gold silver and other metals in many provinces around the world such as Nevada. The excess values would be ascribed to volcanic outgassing – a reasonable conclusion. A most interesting concept nonetheless also discussed by Roy Spencer though he limited his comments on a carbon dioxide budget to the last 100 years or so.