Saliva Droplets Can Travel More Than Two Metres In Light Winds

According to a University of Nicosia paper published in Physics of Fluids, a mild cough occurring in low wind speeds of 4-15 km/h can propel saliva droplets for up to 6 metres. Important input in reviewing social distancing guidelines in windy conditions.

Airborne transmission of viruses, like COVID-19, is not well understood, but a good baseline for study is a deeper understanding of how particles travel through the air when people cough.

In a paper published this week in Physics of Fluids, by AIP Publishing (On coughing and airborne droplet transmission to humans), Dr. Talib Dbouk and Professor Dimitris Drikakis of the University of Nicosia (UNIC) discovered that with even a slight breeze of 4 km/h, saliva travels 6 metres in 5 seconds.

“The droplet cloud will affect both adults and children of different heights”, noted Dimitris Drikakis, a Professor in both the School of Sciences and Engineering and the Medical School of the University of Nicosia. “Shorter adults and children could be at higher risk if they are located within the trajectory of the traveling saliva droplets”, he continued.

Saliva is a complex fluid, and it travels suspended in a bulk of surrounding air released by a cough. Many factors affect how saliva droplets travel, including the size and number of droplets, how they interact with one another and the surrounding air as they disperse and evaporate, how heat and mass are transferred, and the humidity and temperature of the surrounding air.

To study how saliva moves through air, the researchers created a computational fluid dynamics simulation that examines the state of every saliva droplet moving through the air in front of a coughing person. Their simulation considered the effects of humidity, dispersion force, interactions of molecules of saliva and air, and how the droplets change from liquid to vapor and evaporate. The computational domain in the simulation is a grid representing the space in front of a coughing person. The analysis involved running partial differential equations on 1,008 saliva droplets and solving approximately 3.7 million equations in total.

“Each cell holds information about variables like pressure, fluid velocity, temperature, droplet mass, droplet position, etc.”, explained Dr. Dbouk, a Senior Researcher at the University of Nicosia. “The purpose of the mathematical modeling and simulation is to take into account all the real coupling or interaction mechanisms that may take place between the main bulk fluid flow and the saliva droplets, and between the saliva droplets themselves”, he added.

Further studies are needed to understand the droplet evaporation more deeply, especially at different environmental conditions. The violent cough of patients with respiratory diseases will also affect droplet generation. This factor needs to be further quantified. Moreover, the dosage and time needed for infection are not yet understood and may vary from one person to another. Further research and closer interaction between biomedicine and engineering fluid physics are necessary to understand the conditions under which patients are being infected.

Commenting on the broader significance of the results, for policy-makers and citizens alike, especially at this juncture, Professor Drikakis highlighted that: “This work is vital, because it concerns health and safety distance guidelines, advances the understanding of spreading and transmission of airborne diseases, and helps form precautionary measures based on scientific results”.

Those interested can find out more about Dbouk and Drikakis’s research findings, as well as access the full paper published in Physics of Fluids via the University of Nicosia website: www.unic.ac.cy/coronavirus/airborne-transmission/.

The AIP article is also featured on the University’s dedicated Coronavirus Health and Research Portal, along with resources focused on the novel coronavirus, including an online COVID-19 diagnostic tool, healthy living tips, and ongoing UNIC faculty research and analysis. Many UNIC faculty are actively involved in helping respond to the unprecedented public health and societal challenge underway and welcome collaborations with other universities or stakeholders. Please contact the University of Nicosia at [email protected] if there is interest in collaborating on COVID-19 related research.

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ABOUT THE JOURNAL

Physics of Fluids is devoted to the publication of original theoretical, computational, and experimental contributions to the dynamics of gases, liquids, and complex or multiphase fluids. Please see https://aip.scitation.org/journal/phf