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Title: Colloidal particles for confocal microscopy and optical tweezing
Author: Liu, Yanyan
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2017
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A novel colloidal model system is developed for the purpose of achieving simultaneous three-dimensional (3D) confocal imaging and optical tweezing of impurity probe particles embedded in a dense material of colloidal host particles. First, the colloidal host particles are developed from 3-trimethoxysilyl-propylmethacrylate (TPM), and the solvent mixture is tuned to match the refractive index and the density of the particles. A sedimentation-diffusion equilibrium profile of the TPM particles was imaged in 3D to establish suitability of the system for 3D confocal laser scanning microscopy, and to study its phase behaviour and particle dynamics. Then, core-shell particles, which consist of a high refractive index core and a TPM shell, are synthesised to be used as the impurities. The versatility of the two-step coating procedure with TPM has been demonstrated on several core materials, and the optical properties of the core-shell particle are established using digital holographic microscopy and their optical trapping strengths. Together with the TPM host particles, the core-shell particles are shown to be suitable impurity probes in dense colloidal materials, as they can be manipulated using optical tweezers in all three-dimensions, whilst the structure and dynamics of the surrounding host particles can be imaged simultaneously using fast confocal laser scanning microscopy. Subsequently, to demonstrate the capability of the TPM-based colloidal model system, the depletion potential of a pair of core-shell probe particles embedded in a sea of TPM host particles has been measured using optical tweezing. This is derived from comparing the direct pair potential between the core-shell particles in a TPM refractive index matching solvent, and the potential of mean force for the core-shell particles embedded in a dense fluid region of the TPM host particles. Direct visualisation of the liquid structures of the TPM host particles in the binary system around the probe particles has been linked to the form of the depletion potential, and its oscillatory nature as a function of the particle separation. Lastly, the formation of colloidal dumbbell particles is discussed. The dumbbell formation has been rationalised in terms of the total surface energy of droplet formation on spherical surfaces and the calculations are compared with experimental results from coating various seed particles with TPM. Using optically anisotropic dumbbell particles and tuning the refractive index of the dispersion medium, a preliminary two-trap experiment has been conducted which shows unusual trapping behaviour when a time-delayed feedback trapping trajectory has been applied.
Supervisor: Dullens, Roel Sponsor: European Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Physical Chemistry ; Soft condensed matter ; Confocal Microscopy ; Optical Tweezing ; Colloids