Discussion point 3: Modern theories of boiling And if we do assume that there is some degree of superheating in the first layer of water, and seek to say something about the effect of that superheating, we find that there is no theory that can be applied easily. The question cannot even be articulated in the standard physics discourse, because the theory there is based on the idealized assumption that superheating never occurs. The other main thing to note about the engineer’s boiling curve is that the main dependent variable is the rate of heat transfer. These engineers are mainly interested in boiling as a method of carrying heat away from hot places to colder places (one can easily imagine the consequences of not understanding this correctly, in trying to keep a nuclear reactor from overheating, for example). In that context, the temperature of the liquid water, especially well above the first layer, is distinctly of secondary interest, and is freely admitted to be quite variable depending of the situation. The engineering treatises on heat transfer give a detailed classification of boiling behavior, largely determined by the degree of surface superheat and the configuration of the boiling setup. A great deal of experimental work is also going on. See, for example, Incropera and DeWitt (1996), Hewitt et al. (1997), and Kandlikar (1999). Particularly pertinent for current purposes is the modern theory of nucleation (bubble-formation), which gives excellent and detailed explanations of the effect of vessel-surface quality on boiling behavior (shown in Experiment 2 and Experiment 4). Surface tension emerges as the basic reason why water is prone to superheating. In order for boiling to take place, vapour bubbles need to form within the body of the water, and grow sufficiently to be visible as they come up through the water. Now, the basic condition for a bubble to sustain itself is that the vapour pressure should match (or exceed) the extrernal pressure. (This is as specified in the pressure-balance theory of boiling from the 19th century.) However, there is a complication here becausse the water molecules which form the surface of the bubble attract each other. This attraction manifests itself in the form of surface tension, which tends to close up the bubble. Therefore an additional force of vapour is required to sustain the bubble, which means the water temperature has to be higher than the boiling point indicated by the simple pressure-balance theory. Standard theory says that the additional pressure created by surface tension is inversely proportional to the radius of the bubble (see Hewitt et al. 1999, 91). In other words, the additional pressure to be overcome becomes infinite when the radius is 0, which means that it would be impossible to grow a vapour bubble if there weren't a finite-sized space to begin with. Some other questions arising from my experiments are more difficult, and they are not satisfactorily resolved by an elementary knowledge of the modern engineering theory of boiling that I have so far acquired. First, it is difficult to understand the role of dissolved air in facilitating boiling; this may require some detailed molecular modelling, which the engineering theory does not provide. Second, the lowering of the boiling temperature below the thermodynamically defined boiling point is difficult to understand. The only explanation I can currently offer is that the "first layer" of water in those situations must be heated well beyond the normal boiling point although the main body of the water is much cooler, and that the bubbles rise to the surface before they have enough time to be collapsed in coming through the cooler water. |