Surface effects are crucial in several mesoscopic phenomena, especially those concerning biological entities. Herewe determine the eects of Van der Waals forces at relatively long range ( 80 nm) by optically trapping a probeparticle close to a large silica particle and modulating the spatial position of the probe employing oscillatingoptical tweezers. This method has greater signal-to-noise in the experimentally measured probe-response ascompare to that obtained from measurements of Brownian uctuations. We quantify the H-value experimentallyby analyzing the amplitude response of a single trapped particle in comparison to numerically expected resultsby employing chi-square tting, and obtain good agreement with the known H-value for the system.
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