In ESEM (environmental SEM), when we switch from rotary pump to turbo pump (which means pressure suddenly drops), water condensation on the sample is observed. Such condensed water should come from the left moisture in the vacuum and will soon evaporate.
Why would this water condensation happen?
because if you lower the pressure in your chamber you also lower the boiling point of the water inside - if there is too much water or the pressure is reduced too fast, the steam of the water takes too much heat away and the temperature of the small water droplets decreases significantly - hence the condensation. If you would have even more water inside your chamber, it would even freeze over before the heat conduction of your surfaces increases the temperature of the water again, so that it quickly evaporates.
When vacuum drying porous materials that are not hot enough you even get ice crystals. So to avoid condensation the sample and/or chamber should be heated and that way the amount of water will also be reduced.
Many thanks to both of your advices! You mentioned that the decreased temperature is the major reason. However, there is no liquid water in the system (so no temperature decrease because of no phase change). As I mentioned, when I change the pump from rotary pump to turbo pump, condensation happens. I am thinking that condensation is due to the suddenly reduced pressure.
the water is mostly in your chamber in the form of very small droplets (which you cannot see with the bare eyes), rather than in the form of real vapor (of course, there is some real vapor too) - just a drop in pressure won't yield a phase transition from gaseous to liquid, as you can see in the phase diagram of water.
Do you use water vapor as a gaseous environment for ESEM observation? You may want to check if you have water accumulation somewhere in your feeding system.
Yinxiao,
Water vapor cools as it expands adiabatically in vacuum as characterized by the Joule-Thomson (JT) effect. For an ideal gas this temperature will remain constant as an ideal gas is neither absorbing any heat nor is it doing any work when expanding freely in vacuum. However, these approximations do not hold true for a real gas where a temperature change is noted on adiabatic expansion. The throttling principle of JT effect is exploited in refrigeration systems. However, some gases like H2 and He undergo a negative JT effect where a temperature increase is noted when the gases are throttled starting at room temperature.
Further reading on JT effect
1) Principles of physical chemistry - Hans Kuhn, Horst-Dieter Försterling, David H. Waldeck
2) Atkins' Physical Chemistry - Peter Atkins, Julio de Paula
3) http://teacher.buet.ac.bd/nusairh/res/default/refrigr.pdf
https://books.google.com/books?id=E5SBRSKR53EC&pg=PA646&dq=joule+thomson+effect+physical+chemistry&hl=en&sa=X&ved=0ahUKEwiF35DJ4fTRAhVCs1QKHWnIAdMQ6AEIITAB#v=onepage&q=joule%20thomson%20effect%20physical%20chemistry&f=false
https://books.google.com/books?id=BV6cAQAAQBAJ&pg=PA81&dq=joule+thomson+effect+physical+chemistry&hl=en&sa=X&ved=0ahUKEwiF35DJ4fTRAhVCs1QKHWnIAdMQ6AEILjAD#v=onepage&q=joule%20thomson%20effect%20physical%20chemistry&f=false
As pressure is suddenly reduced, the steam also lost appropriate huge temperature, this result in condensation
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