High-speed scattered-light imaging for sizing respiratory droplets

• The COVID-19 pandemic has illuminated the lack of knowledge regarding the airborne transmission pathway of disease. The pathway consists of pathogens contained in small particles ejected when speaking and coughing. A crucial characteristic of these particles is their size that is connected to the their suspension longevity in the air as well as the location of generation and deposition within a subject’s respiratory system. Sizing of particles launched from the respiratory system is a challenge for a number of reasons: (1) the size of ejected particles varies over a wide range, between sub- to several hundreds of microns, (2) particles are ejected at various speeds in different directions, (3) each single event is unique where for example the number of particles can vary greatly between two occurrences of the same event and subject, (4) the size of the particles vary significantly as they evaporate over time. To overcome these challenges and categorize full coughing and speaking events, new measuring methods are needed. In this work, we present, in detail, high-speed scattered light imaging to size liquid particles (droplets) in unique respiratory events. A high-speed camera records scattered laser light at 16 000 frames per second in a semi-forward scattering direction. The illumination is close to the ejection source which means that the particles are sized before evaporation. The measurement can size stationary droplets from 3.4 to 44 µm and moving droplets from 4 to 80 µm resolved in both time and space. To get a reliable estimation, careful calibration of scattering angles and calibration uncertainty has been performed showing a general uncertainty of 8%. Thus, the approach proposed in this article can provide valuable and accurate data to improve the understanding of the airborne transmission pathway.

Ämnesord

NATURVETENSKAP  -- Fysik -- Annan fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Other Physics Topics (hsv//eng)

Nyckelord

Lorenz–Mie scattering

Respiratory droplets

Particle sizing

High-speed imaging

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