Numerical Modeling of the Bead Mobility Technique
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
According to the Intergovernmental Panel on Climate Change, aerosols have been the most important atmospheric cooling component in the industrial period. How- ever, the predictions of aerosol impacts are the largest individual source of uncertainty in climate models [1, 2]. Accurately representing organic aerosols (OA) in atmospheric models is one of the key requirements for understanding the climate effects of aerosols. The viscosity of OA’s has been shown to range from liquid to solid/semi-solid across the range of atmospheric relative humidity . The viscosity of these atmospheric particles is currently a topic of interest due to its role in the profound effect OAs have on the environment. A method known as the ’bead-mobility technique’ has been developed by Renbaum-Wolff et al  which is able to quantify the viscosity of an atmospheric particle over a range of atmospherically relevant humidities by studying the motion of fluid inside small droplets placed in a flow cell. This project uses numerical simulations, in conjunction with an analytical solution, to provide validation of the technique and to investigate potential sources of error that could be contributing to the large uncertainties currently present in experimental results. As presented by Renbaum-Wolff et al, the relationship between the droplet viscosity and the droplet fluid velocity was found to fit a single term power law function of the form u = aµb, where a and b represent constants with b ≈ −1. The effect of variations in the droplet surface tension, size, contact angle and the effect of droplet drafting were analyzed over ranges relevant to the bead mobility technique. It was found that the largest contributors of error were due to the range of sizes and contact angles of droplets studied, producing variations in droplet velocity by factors of approximately 2.8 and 1.7 respectively. In light of these results, an alteration to the current technique has been proposed that reduces the errors present due to variations between droplets.