Measurements of Microscopic Droplet Surface Tensions and Investigations of the Potential Impacts of Aerosol Science and Decolonial Work on Wider Society

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Atmospheric aerosols play a pivotal role in the global radiative balance yet represent one of the largest uncertainties in radiative forcing. This work investigates essential microphysical characteristics of aerosols, such as surface tension, which influences both the direct and indirect impacts of aerosols on the atmosphere. Additionally, it explores viscosity, a factor capable of modifying mass and chemical reactivity through its influence on gas-particle partitioning.
The characterisation of these properties was conducted using various experimental techniques to analyse both aerosol droplets and bulk-phase solutions. The technique of the droplet oscillation method employed a microdispenser to eject particles of 30 – 50 μm diameter and capture their oscillation trajectory using precisely timed stroboscopic imaging. The surface tension was retrieved from the frequency of droplet oscillations at specific delay times. The validity of this technique was assessed with a non-dynamic system where the droplet and bulk data should be similar to one another. Following the validation of the method, more complex, dynamic systems were investigated. Notably, a novel data analysis method was employed to determine the dynamic surface tension of a surfactant droplet. These measurements can be used to provide early time points (within microseconds) of the dynamic surface tension profile before equilibrium is reached. The dynamic surface tension remains a crucial factor in exploring aerosol chemistry, surfactant dynamics and cloud droplet activation.
Additionally, this thesis reports the study of aerosol viscosity using the holographic optical tweezers for the trapping and manipulation of the levitated particles under different ambient conditions. Following on from previous studies, this work examined the effect of relative humidity on droplet viscosity and diffusion which are vital to understanding the kinetic limitations to particle growth. The principal components employed in this study were highly viscous organic proxies, organic acids, and inorganic salts, which were selected to mimic the mixtures present in the atmosphere. While real systems are more complex, this work forms part of a fundamental understanding from which complexity of the system can be increased.
The irradiation of a photoactive aerosol droplet was studied using the Electrodynamic Balance (EDB) and a broad wavelength UV light source. These preliminary experiments were conducted as part of the investigation into the properties that inhibit aerosol photochemistry, specifically, the effects of viscosity and the presence of surface-active molecules.
Research in aerosol science, as explored in this thesis, operates within a complex landscape shaped by social, economic, political, and cultural factors, all of which bear the historical imprint of colonialism in various forms. Since 2020, knowledge of colonialism's effects on wealth distribution, resources, and research funding have prompted universities to integrate decolonisation into their strategies to increase inclusivity and belonging. However, within STEM fields, there has been a lack of comprehensive frameworks to guide individuals towards achieving these goals. This thesis aimed to address this gap by pioneering a novel decolonial framework, developed by the Decolonising the Science Working Group from 2018 to 2022. The framework empowers universities to identify and confront colonial legacies within their scientific disciplines and research environments, thereby identifying meaningful steps towards decolonisation. Using this framework to analyse of the EPSRC Centre for Doctoral Training in Aerosol Science curriculum, revealed areas for enhancing diversity and adopting decolonial thinking within the field of aerosol science education. Herein, it is proposed that such efforts can increase social, cultural and self-awareness as well as foster a heightened global consciousness among university students, qualities highly valued by employers.
Date of Award7 May 2024
Original languageEnglish
Awarding Institution
  • University of Bristol
SupervisorAndrew J Orr-Ewing (Supervisor) & Bryan R Bzdek (Supervisor)

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