Case Study: High-Speed Camera Provides Deeper Understanding of COVID-19

Among its many consequences, the COVID-19 pandemic has resulted in more research into the nature and spread of airborne pathogens. Until recently, however, studies on exactly how virus-containing aerosol particles form in the upper airways did not exist.

Typically, processes that result in the formation of respiratory droplets have been observed and studied in the bronchioles, the laryngeal region, and in the oral cavity. However, viruses like COVID-19 inhabit the upper respiratory tract, including the nose and throat, during their most infectious periods. This fact prompted a research team from the Technical University Bergakademie Freiberg (TUBAF) in Saxony, Germany to study how droplets form in this region.

“When it comes to the airborne transmission of viruses like COVID-19, aerosol droplets made from respiratory liquids are of fundamental importance,” says TUBAF researcher Katrin Bauer. “While their transmission route through the air has been intensely researched, especially in the last two years, few studies deal with the actual formation of these droplets.”

According to the research team, these virus-laden droplets are generated by upper airway activities like talking. It was for these reasons that the TUBAF team focused its research efforts on the atomization process of respiratory liquid attached to the vocal folds.

Atomization is the process of breaking a liquid into smaller droplets.

The Experimental Setup

Bauer and her team constructed a physiologically accurate, motor-driven model of vocal folds using two elastic cylinders. Made of silicone, these cylinders had a length and diameter of 20 millimeters (mm) and 4 mm, respectively. They attached the vocal folds to the end of a model trachea; humidified air entered the trachea at a constant flow rate of 10, 20 and 30 liters per minute ( l/min) to mimic what transpires during talking, and then exited through the vocal folds. The vocal folds were continuously fed by a model respiratory liquid with saliva-like properties.

To visualize the atomization of the respiratory liquid into the air stream, the TUBAF team used a Phantom VEO 710 high-speed camera. As the vocal folds moved, the team recorded the formation of liquid bubbles and bridges across the glottis, along with the breakup of these bubbles and bridges into smaller droplets, at a rate of 5,000 frames per second (fps) at full resolution (1280 × 800). Due to the high airflow speed and particle velocities, the team used an exposure time of 60 microseconds (μs), minimizing the motion blur.

“The Phantom camera allowed us to qualitatively observe the different mechanisms by which the droplets form over the course of one oscillation period, starting with a closed glottis,” Bauer explains. According to her, the 100-Hertz (Hz) oscillation frequency and 5,000-fps frame rate meant that one complete oscillation period consisted of 50 time steps. For their experiments, the researchers recorded the atomization process over multiple oscillation periods.

Afterward, they observed the mechanisms by which the respiratory droplets formed and then burst. These mechanisms included:

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The Phantom VEO 710

The Phantom VEO 710 camera is perfect for the TUBAF team’s experiments, which involved the generation and movement of small aerosol particles through the vocal fold model. It features high frame rates, short exposure times and excellent light sensitivity. The system can capture images at 7,400 fps at full resolution and up to 1,000,000 fps at reduced resolutions with the export-controlled FAST option. While the camera’s minimum exposure time (standard) is 1 µs, it can achieve exposure times as low as 300 nanoseconds (ns) with the FAST option. For these experiments, the TUBAF team used an exposure time of 60 µs, which was sufficient to image the atomization of aerosol particles. In addition, the camera’s large pixel size — 20-micrometer (µm) pixel size — coupled with the custom 35-millimeter, 1-megapixel CMOS sensor, achieves high light sensitivity at fast recording speeds.

Other notable features of the VEO 710 include: 

In addition to these high-speed visualizations, Bauer and her team measured the size of the resulting mucus droplets downstream of the vocal folds. They also analyzed the influence of the oscillation frequency, amplitude and airflow rate on droplet size. “We found that an increase in both frequency and amplitude led to smaller particles while raising the airflow rate resulted in a higher proportion of larger particles,” Bauer says.

Dispelling Previous Assumptions

The Phantom VEO 710 camera enabled the TUBAF team to visualize different but reproducible aerosol generation mechanisms. “We found that the breakup of large, thin bubbles in the glottal gap dominates the aerosol formation process. We can also predict particle sizes — and their variation with flow rate and frequency changes — based on the bubble sizes.”

These findings differ from previous assumptions that the filament breakup mechanism was the dominant aerosol-generating mechanism at play during talking. “It seems to be playing only a minor role here,” she says.

These findings were published in Physics of Fluids in December, 2022.

To learn more, please visit: www.phantomhighspeed.com.

References

L. Fritzsche, R. Schwarze, F. Junghans, K. Bauer; Toward unraveling the mechanisms of aerosol generation during phonation. Physics of Fluids 1 December 2022; 34 (12): 121904. https://doi.org/10.1063/5.0124944