Structural anisotropy and orientation-induced Casimir repulsion in fluids

TL;DR

This paper theoretically investigates how the microstructure and orientation of nanowire arrays influence Casimir forces in fluids, revealing anisotropy-induced repulsion and limitations of traditional approximation methods.

Contribution

It introduces an effective-medium theory approach to accurately predict Casimir forces between nanowire arrays, including orientation-dependent repulsion, beyond the Proximity Force Approximation.

Findings

Effective-medium theory accurately predicts Casimir forces at sub-half-period separations.

Orientation of nanowire arrays can induce Casimir force sign reversal.

Microstructure effects lead to deviations from traditional PFA predictions.

Abstract

In this work we theoretically consider the Casimir force between two periodic arrays of nanowires (both in vacuum, and on a substrate separated by a fluid) at separations comparable to the period. Specifically, we compute the dependence of the exact Casimir force between the arrays under both lateral translations and rotations. Although typically the force between such structures is well-characterized by the Proximity Force Approximation (PFA), we find that in the present case the microstructure modulates the force in a way qualitatively inconsistent with PFA. We find instead that effective-medium theory, in which the slabs are treated as homogeneous, anisotropic dielectrics, gives a surprisingly accurate picture of the force, down to separations of half the period. This includes a situation for identical, fluid-separated slabs in which the exact force changes sign with the orientation of the wire arrays, whereas PFA predicts attraction. We discuss the possibility of detecting these effects in experiments, concluding that this effect is strong enough to make detection possible in the near future.