New understanding of the liquid state of matter, viscosity and its lower bounds

Pr. Kostya Trachenko

School of Physics and Astronomy, Queen Mary University of London

Tue, Apr. 09th 2024, 11:00-12:01

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]. 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 experimental and theoretical results open a new way to understand liquid thermodynamics on the basis of collective modes (phonons) as is done in the solid state theory.

There are important differences between phonons in solids and liquids, and we have recently started to understand and quantify this difference.

- I will review collective modes in liquids including high-frequency solid-like transverse modes and will discuss how a gap in the reciprocal space emerges and develops in their spectrum [2]. This reduces the number of phonons with temperature, consistent with the experimental decrease of constant-volume specific heat with temperature [1].
- I will discuss the implication of the above theory for fundamental understanding of liquids. I will also mention how this picture can be extended above the critical point where the recently proposed Frenkel line [3] on the phase diagram separates liquid-like and gas-like states of supercritical dynamics [1].
- I will subsequently describe how this leads to the theory of minimal quantum viscosity in terms of fundamental physical constants and will compare this minimum to the holographic bound [4].
- Finally, I will note that the minimum of thermal diffusivity of liquids can be equally written as the same combination of fundamental constants, in agreement with the wide set of experimental data.

References:

- K. Trachenko and V. V. Brazhkin, Collective modes and thermodynamics of the liquid state, Reports on Progress in Physics 79, 016502 (2016)
- C. Yang, M. T. Dove, V. V. Brazhkin and K. Trachenko, Physical Review Letters 118, 215502 (2017)
- V. V. Brazhkin and K. Trachenko, Physics Today 65(11), 68 (2012)
- K. Trachenko and V Brazhkin, Minimal quantum viscosity from fundamental physical constants, Sci. Adv. 6, eaba3747 (2020)

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