COMMITTEE:            

Dr. Cheng Zhu, (BME & ME, Georgia Tech), Advisor

Dr. Evan Evans (BME, Boston University)

Dr. Susan N. Thomas (ME, Georgia Tech)

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

SUMMARY

 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.

 We first 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 creation of a bistable rotational potential. Surprisingly, the theory also predicts a second top-down symmetry breaking between the two bistable states as a consequence of radiation pressure.

 In the second part, we experimentally confirm the theoretical results. First, we quantify the top-down symmetry breaking by measuring the transition rates between the two rotational states. 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.