Hypotheses in phase transition theories: "What is " liquid?"

Theories predicting thermodynamic properties that describe liquid phase transitions and critical phenomena have resulted in the award of three Nobel prizes in physics: (i) "Continuity of Gaseous and Liquid States " hypothesis of van der Waals [1910], (ii) "Critical Point Universality " hypothesis embodied in the renormalization group (RG) theory of Wilson [1982], and (iii) "Topological Defect Melting " hypothesis that 2D-crystal-liquid states exhibit 'hexatic' phases in KTHNY theory [Kosterlitz et al. 2016]. All three hypotheses are invalidated by the reality of experimental results and raise a fundamental question first posed by Barker and Henderson in 1976: "What is liquid ". A single Gibbs phase, that includes triplepoint (Tt) liquid, extends over the whole fluid density range to temperatures above the Boyle temperature (TB). Below TB, above the critical temperature Tc, predominantly gas- and liquid-like states are bounded by a narrow colloidal 'supercritical mesophase' with constant rigidity (co = (dp/dp)T). The liquid phase also becomes colloidal at the onset of pre-freezing growth and percolation of crystallites in a narrow density range below freezing density for all T > Tt. Whereas the Boyle line (RT = p/p) defines a crystalline ground state, a rigidity line, RT = co, interpolates to an amorphous ground-state akin to random close packing (RCP) at T = 0. All states of gas, liquid, and crystals, are present in the stable 'liquid phase' and, are represented in thermodynamic p-T states all along the rigidity line. For 2D liquid-crystal coexistence in constrained computer models, the KTHNY theory describes a non-equilibrium fracture process. Hetero-phase fluctuations, leading to percolation transitions, have been misconstrued as "hexatic " in 2D, as also have 2-phase coexistence states, that are homogeneous in the absence of gravity. (c) 2023 Elsevier B.V. All rights reserved.