Which Physical Quantity Deserves the Name “Quantity of Heat”?

“What is heat?” was the title of a 1954 article by Freeman J. Dyson, published in Scientific American. Apparently, it was appropriate to ask this question at that time. The answer is given in the very first sentence of the article: heat is disordered energy. We will ask the same question again, but with a different expectation for its answer. Let us imagine that all the thermodynamic knowledge is already available: both the theory of phenomenological thermodynamics and that of statistical thermodynamics, including quantum statistics, but that the term “heat” has not yet been attributed to any of the variables of the theory. With the question “What is heat?” we now mean: which of the physical quantities deserves this name? There are several candidates: the quantities Q, H, Etherm and S. We can then formulate a desideratum, or a profile: What properties should such a measure of the quantity or amount of heat ideally have? Then, we evaluate all the candidates for their suitability. It turns out that the winner is the quantity S, which we know by the name of entropy. In the second part of the paper, we examine why entropy has not succeeded in establishing itself as a measure for the amount of heat, and we show that there is a real chance today to make up for what was missed.

Numerical simulation of free expansion of an ideal gas

Classical statistical thermodynamics was developed in the second half of the 19th century. The implied fact is that since then all processes and effects that fall into the domain of classical statistical thermodynamics have been discovered and explained. However, analyzing the outputs from our numerical model of an ideal gas, we noticed that the atoms separate during the expansion of the gas into empty space. This separation occurs as a consequence of the distribution of molecules by velocities. Namely, faster molecules will move further away from their initial position than slower ones. The objective of this paper is to describe the existence of this process and to present some basic physical consequences of this effect.

Contact guidance as a consequence of coupled morphological evolution and motility of adherent cells

Adherent cells seeded on substrates spread and evolve their morphology while simultaneously displaying motility. Phenomena such as contact guidance viz. the alignment of cells on patterned substrates, are strongly linked to the coupling of morphological evolution with motility. Here we employ a recently developed statistical thermodynamics framework for modelling the non-thermal fluctuating response of the cells to probe this coupling. This thermodynamic framework is first extended to predict temporal responses via a Langevin style model. The Langevin model is then shown to not only predict the different experimentally observed temporal scales for morphological observables such as cell area and elongation but also the interplay of morphology with motility that ultimately leads to contact guidance.