begin quote <<Modern theories of boiling In the traditional theory of boiling in physics and physical chemistry, the boundary between the liquid state and the gas state is sharply defined as the line on which the vapor pressure (function of temperature) reaches the level of the external pressure. This is as shown in a typical phase diagram on the right. The assumption of the sharpness of the liquid-gas boundary theoretically precludes the variability of boiling point under fixed external pressure; this means that there is no obvious way of accommodating the observed variations within the traditional physical theory.
In modern treatises on boiling in mechanical and chemical engineering, we do not find the standard thermodynamic phase diagrams. Instead, the engineer’s paradigmatic representation of boiling is the “boiling curve”, which plots the rate of heat transfer against the degree of the “surface superheat” or the "excess temperature". The figure on the left shows a typical boiling curve, taken from Incropera and DeWitt (1996), p. 540. (Click on the picture to see a larger version with readable text.) The boiling curve shows a couple of important things about the incommensurability between the physicist’s and the engineer’s understanding of boiling. The main independent variable in the engineering discourse is how much the temperature of the heating element exceeds the "normal" boiling point. I assume that the water in immediate contact with the heating element (what De Luc called the "first layer" of water) is also heated beyond the normal boiling point. By how much, we cannot really say -- it would be extemely difficult to measure such a thing and, presumably, the engineers are more interested in variables that they can measure and control, like the temperature of the heating element. Therefore, in the best modern theory of boiling we have, the temperature of the water itself has no role to play! >> end quote more... http://www.hps.cam.ac.uk/people/chang/boiling/discussion3.htm
