Publication: Nonequilibrium Casimir pressures in liquids under shear
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2019-08
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Springer
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Abstract
In stationary nonequilibrium states coupling between hydrodynamic modes causes thermal fluctuations to become long ranged inducing nonequilibrium Casimir pressures. Here we consider nonequilibrium Casimir pressures induced in liquids by a velocity gradient. Specifically, we have obtained explicit expressions for the magnitude of the shear-induced pressure enhancements in a liquid layer between two horizontal plates that complete and correct results previously presented in the literature. In contrast to nonequilibrium Casimir pressures induced by a temperature or concentration gradient, we find that in shear nonequilibrium contributions from short-range fluctuations are no longer negligible. In addition, it is noted that currently available computer simulations of model fluids in shear observe effects from molecular correlations at nanoscales that have a different physical origin and do not probe shear-induced pressures resulting from coupling of long-wavelength hydrodynamic modes. Even more importantly, we find that in actual experimental conditions, shear-induced pressure enhancements are caused by viscous heating and not by thermal velocity fluctuations. Hence, isothermal computer simulations are irrelevant for the interpretation of experimental shear-induced pressure enhancements.
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© EDP Sciences / Società Italiana di Fisica / Springer-Verlag GmbH Germany, part of Springer Nature 2019. We thank J.R. Dorfman for valuable discussions and R. Monchaux for some comments regarding Couette-flow experiments. We are indebted to R. A. Perkins for providing us with the relevant thermophysical-property information for liquid water and liquid argon. The research at the Complutense University was supported by grant ESP2017-83544-C3-2-P of the Spanish Agencia Estatal de Investigación. The research at the University of Maryland was supported by the US National Science Foundation under Grant No. DMR-1401449.