Probing the Casimir force with optical tweezers

TL;DR

This paper explores using optical tweezers to measure and manipulate the Casimir force between microspheres in liquids, enabling studies beyond traditional approximations and potential applications in biology and physics.

Contribution

It introduces a novel method employing optical tweezers to probe Casimir interactions at larger aspect ratios and demonstrates femtonewton force measurements between microspheres.

Findings

Femtonewton forces measured between polystyrene microspheres at >400 nm.

Potential to tune Casimir forces from attraction to repulsion by salt concentration.

Method extends probing capabilities beyond proximity force approximation.

Abstract

We propose to use optical tweezers to probe the Casimir interaction between microspheres inside a liquid medium for geometric aspect ratios far beyond the validity of the widely employed proximity force approximation. This setup has the potential for revealing unprecedented features associated to the non-trivial role of the spherical curvatures. For a proof of concept, we measure femtonewton double layer forces between polystyrene microspheres at distances above $400$ nm by employing very soft optical tweezers, with stiffness of the order of fractions of a fN/nm. As a future application, we propose to tune the Casimir interaction between a metallic and a polystyrene microsphere in saline solution from attraction to repulsion by varying the salt concentration. With those materials, the screened Casimir interaction may have a larger magnitude than the unscreened one. This line of investigation has the potential for bringing together different fields including classical and quantum optics, statistical physics and colloid science, while paving the way for novel quantitative applications of optical tweezers in cell and molecular biology.