Experimental Investigation of Shear-Induced Generation of Respiratory Aerosol: Simultaneous Measurements of Particle Quantities and Wave Topology

Despite the high level of attention on infectious respiratory aerosol during the Covid19 pandemic, little is known about the processes how these particles form inside the respiratory system. Understanding the underlying fluid mechanical processes and their influencing factors would enable the development of drugs to suppress the generation of infectious aerosol. In the proposed work, we focus on the shear-induced mechanism of aerosol generation, which is supposed to occur mostly in the larger airways during coughing. In this process, high air velocities trigger Kelvin-Helmholtz waves in the mucus film, which lines the air vessels. Through a series of instabilities, particles detach from the crest of these waves. In the proposed work, we investigate the process of shear-induced aerosol generation in idealized experiments where we vary the air-flow characteristics and the properties of the mucus fluid. Our central aim here is to deduct an empirical model of the quantity and size distribution of generated particles depending on the mucus rheology and the local shear flow. Further, we observe the wave topology to better understand the coupling between the air flow and the waves. In our experimental setup, we measure the quantity of created particles and the emerging waves simultaneously. To ensure controllable conditions, we simplify the complex flow conditions in the airways. We use a rectangular channel with the bottom wall covered in a mucus mimetic. The mucus mimetic fluid is a synthetic hydrogel developed to recreate the viscoelastic properties and low surface tensions of the mucus. Filtered pressurized air is guided through the channel to trigger shear-induced aerosol generation. After passing the mucus mimetic, the air enters into a collection chamber from where particles are sampled continuously by an aerosol spectrometer. To measure wave topology, we use planar laser induced fluorescence. For this, we stain the mucus mimetic with fluorescent dyes and illuminate a line on the surface of the fluid film with a 532 nm laser. A high-resolution camera captures the resulting fluorescent glow of the mucus mimetic. Figure 1 presents exemplary wave topology results from the experiments, employing varying air flow volume rates, mucus mimetic gel properties, and different configurations of the laser and camera. The resulting wave topologies exhibit significant variation. For the conference, we will conduct parameter studies of the particle quantities and wave topology while varying the mucus mimetic properties and the flow rate of the air. Additionally, we will present grid projection-based techniques to extend the single-line wave measurements and asses the entire surface of the mucus film.