Response to ”Comment on: ‘Thermodynamics of viscoelastic fluids: the temperature equation ’ ”

The thermodynamics of irreversible processes is based on the principle of local action, i.e. the principle of local and instantaneous equilibrium. This assumption means that, although the system as a whole is not in equilibrium, the material can be divided into small physical subelements, where each subelement forms a small homogeneous equilibrium system. All basic assumptions, including this one, in the theory of internal variables are discussed in detail by Kuiken (1994) (Chapter 3). We will only summarise his comments on the principle of local action in relation to the temperature here. Measurement of the temperature requires that an equilibrium situation between thermometer and fluid exists. In nonequilibrium situations, measurement of the temperature is only meaningful if the thermometer is at least surrounded by a small amount of fluid that is instantaneously in equilibrium with the thermometer. Here small means small compared to dimensions over which the temperature varies measurably, and large compared to the thermometer. Instantaneously means that the time scale of the temperature change in the fluid is large compared to the time scale of the thermometer. The principle of local action ensures that equilibrium quantities like temperature, pressure, and entropy are well defined in nonequilibrium situations. On the other hand, this principle, of course, restricts th