Characterizing single airborne droplets using photoacoustic sensing and acoustic levitation

The inhalation of viral airborne droplets is classified as the primary mode of COVID-19 transmission. However, there is a notable gap in current research with respect to real-time detection of viral droplets in open air. Previously, it was shown that photoacoustics can be used to distinguish between airborne particulates sprayed into the open air based on absorber concentration. It was determined that the peak photoacoustic amplitudes from water were highest, and as the absorber concentration within water increased, the signal decreased, opposite to expectations; the detected photoacoustic signals were suspected to be attributed to the spherical geometry of the droplets. To understand the impact of droplet geometry on the signal intensity in isolation, an open-air photoacoustic system was developed using a 500 kHz ultrasound transducer and 475nm nanosecond pulsed laser. An acoustic levitation system was constructed to examine light interactions with single droplets while maintaining the spherical geometry of the sample. Single droplets of acridine orange dye were analyzed at various concentrations and diameters. The transducer was positioned 5 cm away from the levitation setup to detect the generated photoacoustic signals, which were then analyzed to determine the characteristics of the droplets based on diameters ranging from 300 micron to 2.5 mm. The signals increased proportional to the absorber concentration of the dye droplet; as they evaporated, the signal increased with each subsequent pulse before the droplet vaporized. Current experiments aim to categorize the photoacoustic response based on droplet and laser parameters, with potential applications in sensing airborne viral material.