Mechanisms for heating of water by microwaves

[bcrowell]

There are four qualitatively different effects by which microwaves can heat matter:

1. dielectric heating
2. ionic conductivity
3. electronic conductivity
4. hysteresis

In most foods, 1 and 2 are of approximately equal strength. 3 can occur in soot particles formed when food burns. In pure water and ice, only 1 is significant. Effect 1 is not a resonant effect, contrary to what many people believe.

I think what I've said so far is noncontroversial. What I'd like to hear more about is the mechanisms of dielectric heating. WP http://en.wikipedia.org/wiki/Dielectric_heating describes it as a phenomenon in which molecules with a permanent dipole moment oscillate rotationally in response to the electric field. Since collisions are frequent, the molecule doesn't rotate freely. The collisions dump the energy out of the rotation and thermalize it almost as fast as the energy comes in. (Quantum-mechanically, I think this corresponds to the fact that discrete rotational band structure does not exist in solids of liquids.) This description seems to do a good job of explaining certain observations. It explains why water heats faster than ice in a microwave oven (because $W=\tau\Delta\theta$, and in a solid, you can't get much of a rotation angle). It explains why water, which has a big dipole moment, is heated more efficiently than lipids.

When I asserted the above picture on an online forum, I got the response that I was totally wrong, and that the correct answer is as follows:

The unusual thing is the really high absorption of microwaves by bulk water, whereas the ice behaves more normal like most solids and liquids. In liquid water we have an effect of relaxation of orientational polarisation. The polarisation is achieved not by rotation (not possible in liquid water) but by shift of hydrogen atoms along the hydrogen bonds. This is a kind of Kohlrausch conduction mechanism. This process is extremely fast, so polarisation of water is one of the fastest processes in liquids. There is debate, whether tunneling plays a role to enhance the shift of the protons. The same mechanism is responsible for the extraordinary (about tenfold) mobilities of H+ Ions in water.

-- http://physics.stackexchange.com/questions/12657/ice-in-a-microwave

Gee whiz, he sounds so impressive, but I can't find any support for this explanation anywhere on the web. He also seems to contradict himself by saying that there is orientational polarization but no rotation (how do you change orientation without rotation through some angle?). [Edited] I'm not sure if the hydrogen bonds he refers to are intermolecular or intramolecular. The latter seems unlikely, since then he'd be describing vibrational modes, which cause absorption in the infrared region.

Opinions? Information? Web searching seems to turn up mostly either popularizations, which may be presenting an over-simplified picture, or empirical engineering data, which doesn't necessarily imply any specific mechanism.

[EDIT] This may be helpful:
http://www.btinternet.com/~martin.chaplin/vibrat.html -- mostly about visible and infrared
http://www.btinternet.com/~martin.chaplin/microwave.html -- microwave

 

[Future Bruno]

http://www.princeton.edu/~asmits/Bicycle/microwave.html -- microwaveThe consensus seems to be that dielectric heating is caused by the molecular dipole moment oscillating in response to the electric field, and that this does not involve a resonant effect. However, the precise mechanism of how the energy of the oscillations is thermalized is still under debate, with some people suggesting a Kohlrausch conduction mechanism involving hydrogen bond shifts, while others suggest that this is not necessary.