L. NOIREZ “Hidden Mesoscopic Liquids & Melts properties : from “static” shear elasticity to thermo-elasticity”

On the basis of a Maxwell gas model (1867), it has long been assumed that liquids and polymer melts exhibit (shear) elasticity at high solicitation frequencies (MHz or GHz) only. Recent experimental improvements indicate that liquid shear elasticity can be also found at very low frequency ( Hz) ; i.e. a “static” shear elasticity [1]. The low frequency shear elasticity is identified at sub-millimeter scale, on simple liquids and complex fluids (polymer melts, molecular glass formers, Van der Waals liquids, ionic liquids, H-bond liquids) pointing out a probable generic character. This low scale property means that liquids flow over a scaledependent elastic threshold below which they are solid-like. While early experimental evidence of the solidlike property by Derjardin [2] was met with skepticism, liquids confined to sub-millimeter scales remain poorly understood. Emerging disciplines such as microfluidics point out continuously the limitations of a continuum from macroscale properties down to the nanoscale. Recently, a new atomistic theoretical framework have confirmed the experimental findings that liquids indeed possess a finite low-frequency shear modulus G’[3,4]. It will be shown on some examples how the elastic response and the usual viscous or viscoelastic behaviors are related. Low frequency shear elasticity has profound implications on flow, surface instabilities, thermodynamics, fluidic transport mechanisms and make possible the identification of new liquid properties like thermo-elasticity [5 ], solid-liquid phonon coupling or spectacular optical conversions in strain-driven optical harmonic oscillators. [1] L. Noirez, H. Mendil-Jakani, P. Baroni, P. Identification of finite shear-elasticity in the liquid state of molecular and polymeric glass-formers. Philosophical Magazine (2011), 91, 1977–1986.
[2] B.V. Derjaguin, U .Bazaron, K. Zandanova, O. Budaev, O. The complex shear modulus of polymeric and smallmolecule liquids. Polymer (1989), 30, 97 – 103.
[3] Zaccone, A. ; Trachenko, K. Explaining the low-frequency shear elasticity of confined liquids. PNAS (2020), 117, 19653–19655.
[4] Alessio Zaccone and Laurence Noirez, Universal G′ ∼ L–3 Law for the Low-Frequency Shear Modulus of Confined Liquids, J. Phys. Chem. Lett. (2021), 12, 1, 650–657.
[5] E. Kume, P. Baroni & L. Noirez, Strain-induced violation of temperature uniformity in mesoscale liquids. Scientific Reports (2020) 10, 13340 doi : 10.1038/s41598-020-69404-1.