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.

中文翻译：

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介观液体在低频机械变形下的热剪切波

我们表明，受限的粘性液体在施加低频（~1 Hz）剪切激发时会发出动态热响应。观察到热和冷的热波原位在大气压和室温下，在粘度从 100 µm 到 340 µm 不等的粘性液体（聚丙二醇）中，施加机械振荡剪切应变。观察到的与机械激发同步的热效应与粘性行为不一致。这表明介观液体能够（部分）在非平衡热力学状态下转换机械剪切能。这种称为热弹性的效应在固体材料中是众所周知的。对机械剪切变形的热耦合的观察强化了弹性相关液体分子的假设。热弹性波的振幅随着剪切应变振幅的增加而线性增加，直至转变为非线性热行为，类似于从弹性到塑性的过渡。热弹性效应不会引起应力测量的任何变化，因此动态热分析提供了有关动态液体特性的独特信息。