Thermal Shear Waves Induced in Mesoscopic Liquids at Low Frequency Mechanical Deformation

Abstract

We show that a confined viscous liquid emits a dynamic thermal response upon applying a low frequency (∼1 Hz) shear excitation. Hot and cold thermal waves are observed in situ at atmospheric pressure and room temperature, in a viscous liquid (polypropylene glycol) at various thicknesses ranging from 100 µm up to 340 µm, upon applying a mechanical oscillatory shear strain. The observed thermal effects, synchronous with the mechanical excitation, are inconsistent with a viscous behaviour. It indicates that mesoscopic liquids are able to (partly) convert mechanical shear energy in non-equilibrium thermodynamic states. This effect called thermo-elasticity is well known in solid materials. The observation of a thermal coupling to the mechanical shear deformation reinforces the assumption of elastically correlated liquid molecules. The amplitude of the thermo-elastic waves increases linearly by increasing the shear strain amplitude up to a transition to a non-linear thermal behavior, similar to a transition from an elastic to plastic regime. The thermo-elastic effects do not give rise to any change in stress measurements and thus the dynamic thermal analysis provides unique information about dynamic liquid properties.

Fundamental constants explain submillimeter liquid shear forces

“…Recent measurements show that at the submillimeter scale, viscosity is not universal but depends on how a liquid interacts with a substrate ( 2 , 5 ). This result echoes the Navier observation about water’s behavior on glass and copper ( 3 ).…”

“…When liquid/substrate wetting improves toward non-slip boundary conditions, the mesoscopic fluid response reveals a fundamentally different response ( 2 , 5 ). The fluid resists before flowing.…”

“… The image shows thermal traces left in a furrow in water illustrating long-range interactive forces in liquids ( 2 , 5 , 8 , 9 ). Credit: P. Baroni, M. Warburton, and L. Noirez.…”

“…The first microthermographic images of liquids such as glycerol, polymer melts, or water revealed that liquids emit a modulated thermal signal when subjected to a mechanical shear stress wave ( 1 , 5 ). Thermal waves propagate and split the liquid in hot and cold bands several tenths of a micron wide, varying synchronously with the applied stress wave.…”

Thermo-Mechanical and Mechano-Thermal Effects in Liquids Explained by Means of the Dual Model of Liquids

“…Typical examples are the Soret, the Dufour, the Peltier, the Seebeck, the Thomson effects, etc. We will deal with in this paper mainly two of such coupled effects, one is the Soret effect [4,5] (with reference also to the Dufour effect), and the other is an "unexpected" mechano-thermal effect recently revealed in confined liquids under shear geometry [6][7][8][9][10][11].…”