We propose to use optical tweezers to probe the Casimir interaction between micro-spheres 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.
Paulo A. Maia Neto, Luis Pires, Diney Ether, Ricardo S. Decca, Nathan B. Viana, Gert Ingold, Daniel Martinez, Yareni A. Ayala, Felipe Siqueira Rosa, Herch Moysés Nussenzveig, and Stefan Umrath, "Towards Casimir force measurements with optical tweezers (Conference Presentation)," Proc. SPIE 10120, Complex Light and Optical Forces XI, 1012016 (Presented at SPIE OPTO: February 02, 2017; Published: 28 April 2017); https://doi.org/10.1117/12.2255048.5397967379001.
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