In this talk I will cover two recent works on droplets suspended in air: acoustic-driven vaporisation of droplets and the dynamics of respiratory saliva droplets. The latter addresses the timely concern of respiratory transmission of Covid-19 and is an ongoing work that I seek to extend by involving experiments.
Droplets in acoustic fields vaporise faster than in quiescent air. This has long been attributed to the oscillatory flow over the droplet, which enhances mass convection. However, an acoustic field is characterised by three organic features: pressure, velocity, and the phasing between these two oscillatory fields. In this work we demonstrated, both experimentally and using a simple model, how these features all contribute to the vaporisation, and suggested a generalisation to existing correlations for droplet vaporisation in acoustic fields.
The dynamics of saliva droplets exhaled from the human body is affected by gravity, friction with air, and entrainment to the air velocity. All these factors depend on the droplet size that changes as it vaporises, coupling the dynamics with thermodynamics of vaporisation. It is found that (possibly contagious) droplets can stay aloft for up to several hours before falling to the ground. An increase in relative humidity is found to shorten droplets’ lifetime, an unintuitive conclusion that can potentially impact our view of safe indoor environments.
Dr Avshalom Offner is a PDRA at the School of Mathematics, The University of Edinburgh. Trained as a mechanical engineer, he later completed his PhD in the interdisciplinary energy program (GTEP) at the Technion, Israel, studying “heat and mass transfer in acoustic energy conversion”. Dr Offner combines experimental work with mathematical modelling to study a wide range of fluid dynamics topics: thermoacoustic instability, porous media flows, droplet dynamics and vaporisation, and multiphase flows.