Session A01: Mini-Symposium: Fluid Mechanics of Infectious Diseases

Live

Exhalations are impulsive emissions of a gas phase - air from the lungs - laden with a payload of liquid that originates from the respiratory tract. This mixture is emitted in the form of a turbulent droplet-laden puff cloud. Here we consider emissions in a quiescent environment, in which the expansion rate of the cloud is known, associated with entrainment of ambient air. It is commonly assumed that the evaporation physics of such respiratory droplets is governed by the {\it d-squared} law describing the fate of a single drop evaporating in a quiescent environment, rooted in the assumption of homogeneity of the multiphase cloud. Yet, our direct experimental observations and in situ measurements of such clouds, show that this law does not in fact govern the evaporation of the droplets in exhaled multiphase clouds. Combining these observations with analog experiments and theory, we derive insights and predictions of the lifetime of the droplets and discuss how the coupling between the saturated vapor field and its heterogeneity in the turbulent cloud has substantial implications for the lifetime of the droplets within it. We discuss the implications in the context of respiratory disease transmission relevant to human exhalations like breathing, sneezing or coughing.   

Live

COVID-19 pandemic dramatically affects our daily life and our research activities. It also stimulates diverse efforts and contributions from the academic community to help mitigating the sanitary crisis. Fluid mechanics are involved in a wide range of questions from the design of respiratory devices to the understanding of the air flows involved in contagion. As part of the Princeton Open Ventilation Monitor Collaboration (\underline {http://ovm.princeton.edu}), we designed an inexpensive multi-patient respiratory monitoring system. We will describe the Fluid mechanics behind the design of a flowmeter able to measure and monitor the respiratory signals from a patient. Moreover, such device can also be used in combination with optical techniques of flow visualizations to track and understand the flow around us while breathing. Those observations can provide insights of practical interest to understand and quantify the key features involved in social distancing policies and improving ventilation strategies.   

Live

Much attention has focused on the role of droplets generated by coughing and sneezing for transmitting infectious disease through the air. The relative importance of these expiratory activities to airborne transmission, however, has never been definitively established. Here, we discuss recent experimental evidence implicating two less-considered but potentially significant mechanisms for airborne disease transmission. (1) In people, we demonstrate that the number of micron-scale expiratory particles emitted during vocalization, such as speaking or singing, increases dramatically with loudness, and can greatly exceed those generated by coughing. Theoretical calculations suggest that vocalizing less often and more quietly yields substantial decreases in transmission probability. (2) In guinea pig experiments, we establish that influenza is transmitted via ``aerosolized fomites,'' which are virus-contaminated dust particulates released from the fur and cage environment of the animals, not from their expiration. We further establish that aerosolized fomites can be emitted from sources widely used by people, such as paper tissues. Our results suggest that researchers should expand their focus beyond coughing and sneezing as the presumed mechanism for airborne disease transmission.   

Live

It is now well established that droplets exhaled during respiratory events are carriers of SARS-CoV-2 virus which is responsible for Covid-19 pandemic. To gain fundamental insights into the infectivity of air borne nuclei during such pandemic, we present a study of an isolated nano-colloidal droplet of surrogate mucosalivary fluid. Saltwater solutions containing nanoparticles at reported viral loads are acoustically trapped in contactless environment to emulate the drying, flow and precipitation dynamics of real airborne respiratory droplets. Observations with the surrogate fluid are validated by similar experiments with actual samples from a healthy human subject. A unique feature emerges in the final crystallite dimension; it is always 20-30 {\%} of the initial droplet diameter for different sizes and ambient conditions. Precipitates formed in air trap approximately 15 {\%} of the virions on the substrate while if the same droplet dries on a surface, the fraction of exposed virions increases to approximately 85-90 {\%} (depending on the surface). The letter demonstrates the leveraging of an inert nano-colloidal system to gain insights into an equivalent biological system.   

Live

Transmission of highly infectious respiratory diseases, including SARS-CoV-2 are facilitated by the transport of tiny droplets and aerosols (harboring viruses, bacteria, etc.) that are breathed out by individuals and can remain suspended in air for extended periods of time in confined environments. A passenger car cabin represents one such situation in which there exists an elevated risk of pathogen transmission. Here we present results from numerical simulations of the potential routes of airborne transmission within a model car geometry, for a variety of ventilation configurations representing different combinations of open and closed windows. We estimate relative concentrations and residence times of a non-interacting, passive scalar – a proxy for infectious pathogenic particles – that are advected and diffused by the turbulent airflows inside the cabin. Our findings reveal that creating an airflow pattern that travels across the cabin, entering and existing farthest from the occupants can potentially reduce the transmission.   

Live

The post-pandemic revival of the world's economy is being predicated on the social distancing required by the Six-Foot Rule, a guideline that offers little protection from pathogen-bearing droplets sufficiently small to be continuously mixed through an indoor space. The importance of indoor, airborne transmission of COVID-19 is now widely recognized by the epidemiology community; nevertheless, no measures have been proposed to protect against it. We build upon models of airborne disease transmission in order to derive a quantitative safety guideline that would impose an upper bound on the ``cumulative exposure time", the product of the number of occupants and their time in an enclosed space. We demonstrate the manner in which this bound depends on the ventilation rate and dimensions of the room; the breathing rate, respiratory activity and face covering of its occupants; and the infectiousness of the respiratory aerosols, a disease-specific parameter that we estimate from epidemiological data. Specific case studies are considered, and caveats enumerated.