Nonequilibrium Casimir pressures in liquids under shear

Area of Science:

• Physics
• Fluid Dynamics
• Soft Matter

Background:

• Non-equilibrium states exhibit coupling between hydrodynamic modes, leading to long-ranged thermal fluctuations and non-equilibrium Casimir pressures.
• Previous studies have explored Casimir pressures induced by temperature or concentration gradients.

Purpose of the Study:

• To investigate non-equilibrium Casimir pressures induced specifically by a velocity gradient (shear) in liquids.
• To derive explicit expressions for shear-induced pressure enhancements and correct existing literature.
• To clarify the physical origins of observed phenomena in simulations and experiments.

Main Methods:

• Derivation of explicit analytical expressions for shear-induced pressure enhancements in a liquid layer between plates.
• Comparison and correction of previously published results.
• Analysis of the role of short-range fluctuations and molecular correlations.

Main Results:

• Explicit expressions for shear-induced pressure enhancements were obtained, completing and correcting prior literature.
• Unlike other non-equilibrium gradients, short-range fluctuations are significant in shear-induced non-equilibrium.
• Experimental shear-induced pressure enhancements are primarily due to viscous heating, not thermal velocity fluctuations.

Conclusions:

• Current computer simulations of shear in model fluids do not probe the relevant shear-induced pressures from hydrodynamic modes.
• Isothermal computer simulations are not suitable for interpreting experimental shear-induced pressure enhancements.
• Viscous heating is the dominant mechanism for shear-induced pressure enhancements in experimental conditions.