The Investigation of the Exhalation and Deposition of Biologically Relevant Aerosol

Abstract

Respiratory pathogens can be transmitted through the generation and exhalation of respiratory aerosol. Determining the relative risks of exhalatory activities, as well as the fate of exhaled respiratory aerosol, is crucial in mitigating disease transmission. While previous studies quantified the exhalation of respiratory aerosol, the results were limited by small sample sizes and large inter-subject variability, preventing robust conclusions of the mechanism of generation and the relative emissions of different activities. Additionally, studies of binary droplet collisions and single droplets impacting on a surface have largely focused on microlitre droplets, the results of which may not be applicable to respiratory aerosol which are picolitres in volume. This thesis presents experimental results from exhalations of respiratory aerosol from large cohorts of participants, binary collisions of picolitre droplets, and the impact and evaporation of single picolitre droplets on a surface.
To assess the risk of musical instrument playing compared to other exhalatory activities such as breathing, speaking, and singing, measurements of aerosol number and mass concentrations were conducted for a total of 13 instruments across 9 participants. The findings indicated that both aerosol size distributions and concentrations correlate best with the sizes and concentrations generated by the participant while breathing, rather than while speaking and singing. Additionally, aerosol number and mass fluxes were determined by combining aerosol concentration measurements with respiratory airflow measurements. Aerosol number and mass exhalation rates were estimated for large cohorts of 33 adults and 18 children, and the variability between the cohorts was explored. Comparisons of the number and mass exhalation rates for children and adults when breathing, speaking, and singing indicate that child and adult cohorts generate similar amounts of aerosol when performing the same activity.
To examine the applicability of existing models predicting binary droplet outcome boundaries, collisions between pairs of picolitre pure water droplets and picolitre aqueous-sucrose droplets were investigated. In general, the regime boundaries were found to agree with existing models for both systems, giving more confidence that the models tested are appropriate for predicting collision outcomes for droplets of this size and these viscosities.
A high frame rate technique was developed to observe single picolitre droplets impacting, oscillating, spreading, and evaporating on a surface. The validity of the technique was assessed using droplets of mixtures of water and ethanol. The frequency of droplet oscillations was used to infer the droplet surface tension. The surface tension of high surface tension, high contact angle droplets could be accurately retrieved, due to a reduction in the spreading-induced damping of oscillations observed for droplets of this size. Additionally, the rate of droplet spreading was assessed and compared with a widely used model. While all droplets typically spread at the same rate predicted by the model, a delay in spreading was observed for high surface tension, high contact angle droplets, due to the oscillations of the droplet prior to spreading. Finally, the drying mode and kinetics of surrogate respiratory droplets, including aqueous salts and artificial saliva, were investigated on Teflon, as well as the spatial distribution of particles on the surface upon drying. Aqueous sodium chloride sessile droplets on Teflon supported by glass were shown to dry isothermally, in contrast to aqueous sodium chloride free droplets. The artificial saliva droplets dried by a pinned contact line mechanism, likely due to the presence of low solubility salts, and gave rise to circular residue deposits on the surface.

Date of Award	5 Dec 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Jonathan P Reid (Supervisor)