Abstract

When a fluid electrolyte moves relative to a solid, an electric field is generated that migrates ions and thus dissipates energy. This 'electrokinetic' dissipation is theoretically compared to the viscous shear dissipation for fluid flow generated by a fixed time harmonic pressure gradient in a planar quartz duct. It is shown that, regardless of frequency, it is safe to ignore the electrokinetic losses when the electrolyte molarity is on the order of 0.1 M or greater. For low molarity electrolytes (10 (super -3) M), however, the electrokinetic losses do become significant compared to the viscous losses for flow in sufficiently tight pores. The ratio of electrokinetic dissipation D E to viscous dissipation D V is always a maximum when the electrokinetic radius (the duct half-width divided by the Debye length) is nearly equal 1.5. The maximum value of D E /D V does not exceed 0.5 for the NaCl, KCl, and quartz systems considered.The generated electric field pushes on the excess ions in the duct in a direction opposite to the applied pressure gradient, thus giving rise to an apparent viscosity enhancement. This enhancement is < or =45 percent for the systems considered here, and can be directly obtained from the D E /D V ratio. Indeed, the central effect of a large D E /D V ratio is that the amount of relative fluid flow is reduced, and thus, the amount of wave attenuation is reduced.

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