Study of Rotational Dynamics of Trapped Asymmetric Particles in Optical Tweezers

Mondal, Argha (2014) Study of Rotational Dynamics of Trapped Asymmetric Particles in Optical Tweezers. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

We design an annular potential that facilitates spontaneous revolution of asymmetric micro swimmers in a spherically aberrated optical trap. The rate of revolution can be controlled from a few Hz to tens of Hz by changing the intensity of the trapping light which can be achieved either by modifying the laser power or the annular trap diameter. Theoretical simulations performed using Finite-difference time-domain method in Lumerical verify the experimental observations. The electric field distribution confirms that the particle revolution is the effect of asymmetrical scattering by the particle in the annular potential that gives rise to a tangential force. In the second half of this thesis we investigate the reason behind spinning of deformed Red Blood Cells trapped inside an optical tweezers. Previous notion of rotation due to modification of birefringence is nullified with a detailed Mueller imaging performed both theoretically and verified experimentally. Numerical simulations of the scattered field profile was performed in order to and the numerical scattering field profile simulation was done in order to confirm that scattering force generates a non-zero torque due to asymmetric scattering from the deformed RBC, which led to spinning. A manifestation of shape anisotropy is found in the coupling between translation and rotation Brownian motion. It was found that a proper study of crosscorrelation function between translational and rotational Brownian motion reveals the degree of anisotropy of a shape-asymmetric object.

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