Dynamics of composite beads in optical tweezers and their application to study of HIV cell entry

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Beranek, Vaclav
Zhu, Cheng
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In this thesis, we report a novel symmetry breaking system in single-beam optical trap. The breaking of symmetry is observed in Brownian dynamics of a linked pair of beads with substantially differing radii (500nm and 100nm). Such composite beads were originally conceived as a manipulation means to study of Brownian interactions between mesoscopic biological agents of the order of 100 – 200 nm (viruses or bacteria) with cell surfaces. During the initial testing of the composite bead system, we discovered that the system displayed thermally activated transitions and energetics of symmetry breaking. This thesis, while making a brief overview of the biological relevance of the composite bead system, focuses primarily on the analysis and experimentation that reveals the complex dynamics observed in the system. First, we theoretically analyze the origin of the observed symmetry breaking using electromagnetic theory under both Gaussian beam approximation and full Debye-type integral representation. The theory predicts that attachment of a small particle to a trapped microsphere results in creation of a bistable rotational potential with thermally activated transitions. The theoretical results are then verified using optical trapping experiments. We first quantify the top-down symmetry breaking based on measurement of the kinetic transition rates. The rotational potential is then explored using an experiment employing a novel algorithm to track rotational state of the composite bead. The results of the theory and experiments are compared with results of a Brownian dynamics simulation based on Smart Monte Carlo algorithm.
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