Browsing by Author "Pires, L. B."

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    Double-layer force suppression between charged microspheres
    (APS, 2018-02) Ether, D. S.; Rosa, F. S. S.; Tibaduiza, D. M.; Pires, L. B.; Decca, Ricardo S.; Maia Neto, P. A.; Physics, School of Science
    In this paper we propose a protocol to suppress double-layer forces between two microspheres immersed in a dielectric medium, being one microsphere metallic at a controlled potential ψ M and the other a charged one either metallic or dielectric. The approach is valid for a wide range of distances between them. We show that, for a given distance between the two microspheres, the double-layer force can be totally suppressed by simply tuning ψ M up to values dictated by the linearized Poisson-Boltzmann equation. Our key finding is that such values can be substantially different from the ones predicted by the commonly used proximity force approximation, also known as the Derjaguin approximation, even in situations where the latter is expected to be accurate. The proposed procedure can be used to suppress the double-layer interaction in force spectroscopy experiments, thus paving the way for measurements of other surface interactions, such as Casimir dispersion forces.
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    Probing the screening of the Casimir interaction with optical tweezers
    (American Physical Society, 2021) Pires, L. B.; Ether, D. S.; Spreng, B.; Araújo, G. R. S.; Decca, R. S.; Dutra, R. S.; Borges, M.; Rosa, F. S. S.; Ingold, G.-L.; Moura, M. J. B.; Frases, S.; Pontes, B.; Nussenzveig, H. M.; Reynaud, S.; Viana, N. B.; Maia Neto, P. A.; Physics, School of Science
    We measure the colloidal interaction between two silica microspheres in an aqueous solution in the distance range from 0.2 to 0.5 μm 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.