Probing the screening of the Casimir interaction with optical tweezers

TL;DR

This study measures colloidal interactions between silica microspheres in water using optical tweezers, revealing the dominance of unscreened magnetic Casimir forces at short distances and highlighting discrepancies with standard theoretical models.

Contribution

It provides experimental evidence of the unscreened magnetic Casimir contribution in electrolyte solutions and compares it with theoretical predictions, revealing a longer-range Casimir attraction than previously thought.

Findings

Unscreened transverse magnetic Casimir contribution dominates at short distances.

Standard models underestimate the Casimir attraction by about an order of magnitude.

Casimir attraction in polar liquids extends over longer ranges than predicted.

Abstract

We measure the colloidal interaction between two silica microspheres in aqueous solution in the distance range from $0.2\,\mu$m to $0.5\,\mu$m with the help of optical tweezers. When employing a sample with a low salt concentration, the resulting interaction is dominated by the repulsive double-layer interaction which is fully characterized. The double-layer interaction is suppressed when adding $0.22\,$M of salt to our sample, thus leading to a purely attractive Casimir signal. When analyzing the experimental data for the potential energy and force, we find good agreement with theoretical results based on the scattering approach. At the distance range probed experimentally, the interaction arises mainly from the unscreened transverse magnetic contribution in the zero-frequency limit, with nonzero Matsubara frequencies providing a negligible contribution. In contrast, such unscreened contribution is not included by the standard theoretical model of the Casimir interaction in electrolyte solutions, in which the zero-frequency term is treated separately as an electrostatic fluctuational effect. As a consequence, the resulting attraction is too weak in this standard model, by approximately one order of magnitude, to explain the experimental data. Overall, our experimental results shed light on the nature of the thermal zero-frequency contribution and indicate that the Casimir attraction across polar liquids has a longer range than previously predicted.