Polar solvents are heated as their component molecules are forced to rotate with the field and lose energy in collisions.
This inhomogeneous energy dissipation means selective heating of different parts of the material is possible, and may lead to temperature gradients between them.
Nevertheless, the presence of zones with a higher temperature than others (called hot spots) must be subjected to the heat transfer processes between domains.
Where the rate of heat conduction is high between system domains, hot spots would have no long-term existence as the components rapidly reach thermal equilibrium.
However, the time within which thermal energy is repartitioned from such moieties is much shorter than the period of a microwave wave, thus precluding the presence of such 'molecular hot spots' under ordinary laboratory conditions.
In this case much higher heat transfer resistances are involved, and the possibility of the stationary presence of hot-spots should be contemplated.
A differentiation between two kinds of hot spots has been noted in the literature, although the distinction is considered by many to be arbitrary.
By these means it is possible to visualise thermal inhomogeneities within solid phases under microwave irradiation.
[16] Some theoretical and experimental approaches have been published towards the clarification of the hot spot effect in heterogeneous catalysts.