Near-Field Hydrodynamics of Optically Trapped Colloids in Simple and Complex Fluids

The thermal fluctuations of colloidal particles change drastically near a symmetry-breaking interface due to long-range hydrodynamic interactions. In this thesis, I investigate this phenomenon through both experiments and simulations. Using Optical Tweezers (OTs), I measured the correlated thermal fluctuations between two micron-sized particles held at controlled distances from a solid interface, in water and in complex fluids. These results are further complemented by Molecular Dynamics simulations. I show that hydrodynamic interactions between two trapped colloids encode detailed information about the surrounding flow field and its modulation by nearby boundaries. A rapid cross-correlation analysis of particle trajectories reveals a time-independent delay in the influence of one particle’s motion on the other, mediated by the solvent between them. Close to a solid wall, I observe a surprising reduction in the cross-correlation local minimum — interpreted as a “shielding” or secondary flow deflection effect — arising from the wall’s alteration of the fluid flow. I conclude that the wall effectively reduces hydrodynamic coupling by limiting the volume of fluid participating in the interaction. The simulations, based on Langevin dynamics and Dissipative Particle Dynamics (DPD), provide additional insights into behaviours not accessible in experiments. We implement a detailed simulation framework including raspberry-modelled colloids, explicit solvent particles, and tunable wall boundary conditions. Each component is carefully validated — such as friction coefficient of the solvent and the autocorrelation behaviour of the embedded colloid particle — before analysing particle interactions the same way as in experiment. This simulation platform reproduces key experimental features and sets the foundation for further studies such as local convective current and particle interactions. Beyond simple fluids, I extend the study to a temperature-sensitive block copolymer system: Pluronic F127. Using Diffusing Wave Spectroscopy (DWS), we construct a detailed phase diagram based on temperature- and concentration-dependent microrheology. The viscoelasticity behaviour is quantitatively analysed using complex G modulus, where we observe F127’s resemblance of a Maxwellian fluid. A theoretically expected re-entrant liquid behaviour is present in our measurement, owing to F127’s high EO/PO (arm/core) ratio. Together, this work provides a comprehensive view of near-field hydrodynamics in both simple and complex fluids, and highlights the rich interplay between confinement, interfacial effects, and fluid microstructure.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Tang, Xiaoying

Advisors dc:contributor.advisor

• Eiser, Erika
• Fusco, Diana

• Microrheology
• Optical Tweezers
• Dissipative Particle Dynamics

Licence

• https://www.repository.cam.ac.uk/bitstreams/1d710def-d655-4518-928a-72da52adfe0a/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.122628

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/391515