Brownian motion of optically trapped microspheres and suspended diaphragms




Xu, Yi, 1994-

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This dissertation details our experiments of measuring the mass of optically trapped microspheres and suspended diaphragms using their Brownian motion. The optically trapped microsphere is immersed in air and two different mass measurement methods are investigated based on fluctuations about thermal equilibrium. The first method is based on spectral analysis, which allows us to determine the relevant experimental parameters and also serves as a calibration step for the second method. The second method is based on the equipartition theorem and allows for rapid mass measurement. A new method for measuring the effective mass of suspended diaphragms is presented by combining laser Doppler vibrometry and Brownian motion power spectral density analysis. By analyzing the relation between the fundamental resonant frequencies, the mode shape and the total mass of the diaphragm can be determined without the need for prior knowledge of its mechanical properties. This technique provides a precise and efficient method for evaluating the mass of suspended diaphragms for real-life applications in a time-dependent way.



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