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Title: Probing the dynamics of nanoparticles at oil-water interfaces using high-speed total-internal-reflection dark-field microscopy
Author: Schumacher, Anne
ISNI:       0000 0005 0292 1142
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2020
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Particles at fluid-fluid interfaces have a wide range of powerful uses in industry, technology and science. There is therefore a strong interest in exploring the dynamics of these systems to understand and gain more control in applications, as well as for fundamental investigations in condensed matter and statistical physics. Questions remain in particular about what governs adsorption dynamics and inter-particle interactions, and how mixtures of different-sized particles as well as particles and surfactants behave. This thesis probes the dynamics of nanoparticles at oil-water interfaces using high-speed total-internal-reflection dark-field (TIR-DF) microscopy, comparing gold particles of diameters 20, 40 and 80 nm, and investigating the effect of an added surfactant (n-dodecyl-β-D-maltoside (DDM)). We built a TIR-DF microscope with a spatiotemporal resolution of up to 1 nm and 20 μs, and used home-written tracking and analysis software to characterise the particle motion. The diffusive motion within the interfacial plane is measured directly; out-of-plane motion is inferred from light scattering and in-plane motion. The addition of DDM was found to qualitatively alter the interaction between the particles and the interface, with adsorptions becoming reversible and an inversion of the size trend in diffusivity (with larger particles diffusing faster than smaller ones, contrary to expectations). This, in combination with a paucity of adsorption events in larger particles, leads us to propose size-dependent barriers to adsorption to DDM interfaces. In addition, at DDM interfaces, many particles' diffusive behaviour changed over the course of a trajectory, indicating several different immersion states that are stable on the experimental timescale (~1 s). This study shows that there remain qualitatively new dynamics of nanoparticles at oil-water interfaces to be explored. These dynamics can only be resolved at a high spatial resolution, and many of them only at high imaging speed, underlining the importance of novel imaging approaches probing the barriers of achievable spatiotemporal resolution.
Supervisor: Kukura, Philipp Sponsor: European Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: single-particle tracking ; interfaces ; optical microscopy with high spatiotemporal resolution