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New understanding of liquid thermodynamics, viscosity and its lower bounds

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Understanding most basic thermodynamic properties of the liquid state such as energy and heat capacity turned out to be a long-standing problem in physics [1,2]. Landau&Lifshitz textbook states that no general formulas can be derived for liquid thermodynamic functions because the interactions are both strong and system-specific. Phrased differently, liquids have no small parameter. Recent theoretical results open a new way to understand liquid thermodynamics on the basis of collective excitations (phonons) as is done in the solid state theory. Differently from solids, the phase space for these excitations reduces with temperature [2,3], explaining the universal decrease of liquid constant-volume specific heat. I will discuss the implication of the above theory for fundamental understanding of liquids. I will also explain how this picture extends above the critical point where the Frenkel line separates two physically distinct states on the supercritical phase diagram [4]. I will subsequently describe how this leads to the theory of minimal quantum viscosity in terms of fundamental physical constants [5]. This answers the long-standing question discussed by Purcell and Weisskopf of why viscosity never drops below a certain value [6] and implies that water and life and well attuned to the degree of quantumness of the physical world [6,7].

1. K. Trachenko, Theory of liquids: from excitations to thermodynamics (Cambridge University Press, 2023) 2. K Trachenko and V Brazhkin, Reports on Progress in Physics 79, 016502 (2016) 3. M Baggioli, M Vasin, V Brazhkin and K Trachenko, Physics Reports 865, 1 (2020) 4. C Cockrell, V Brazhkin and K Trachenko, Physics Reports 941, 1 (2021) 5. K Trachenko and V Brazhkin, Minimal quantum viscosity from fundamental physical constants, Science Adv. 6, eaba3747 (2020) 6. K Trachenko and V Brazhkin, Physics Today 74, 12, 66 (2021) 7. K. Trachenko, Properties of condensed matter from fundamental physical constants, Advances in Physics 70,469 (2023)

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