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Title: Probing micro-dynamics with optical tweezers : pendula, cell-stretching, and thermodynamic fluctuations
Author: Richards, C. J.
ISNI:       0000 0004 7230 2323
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2018
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Optical tweezers are a powerful experimental tool, commonly utilised in microscopic experiments in a diverse cross-section of scientific fields, from biophysics to hydrodynamics of small systems, by enabling the exertion and measurement of forces that act in microscopic soft-matter systems. This thesis documents three experiments that probe the micro-dynamics of such systems. First the theory of the mechanical phenomenon of the Kapitza pendulum is described: the unusual behaviour of the equilibrium of a pendulum whose point of suspension undergoes high frequency vertical oscillations. To go beyond this case, a Kapitza pendulum in the presence of significant damping is considered. Theoretical calculations are presented, revealing new regimes and positions of dynamic equilibrium. These are supported by Brownian motion simulations. A microscopic optical analogue of the damped Kapitza pendulum is realised using optical tweezers and the results of the theory and simulations are confirmed experimentally. Next, the condition of diabetic retinopathy within sufferers of diabetes mellitus is discussed with reference to the deformability of red blood cells. A pilot study is conducted to assess the viability of investigating this correlation using optical tweezers with a healthy control group, a diabetic group, and a diabetic group with retinopathy. A dual optical tweezers set-up is utilised to stretch the cells and calculate the extent of their deformability. The deformability of red blood cells from the diabetic groups is found to be significantly lower than that of the healthy control group. Finally fluctuation theorems are considered: the asymmetry of the distributions of probabilities of observing forward and backward trajectories of micro-particles, and how this changes with the duration of the trajectories. Theoretical calculations are presented to find the fluctuation theorems for two methods of performing work on two hydrodynamically coupled particles. Simulations and trial experimental studies are performed to attempt to verify these calculations.
Supervisor: Jones, P. H. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available