Evaporation dynamics of a surrogate respiratory droplet in a vortical environment.

HYPOTHESIS: Vortex droplet interaction is crucial for understanding the route of disease transmission through expiratory jet where several such embedded droplets continuously interact with vortical structures of different strengths and sizes. EXPERIMENTS: A train of vortex rings with different vortex strength, quantified with vortex Reynolds number (Re'=0,53,221,297) are made to interact with an isolated levitated droplet, and the evolution dynamics is captured using shadowgraphy, particle image velocimetry (PIV), and backlight imaging technique. NaCl-DI water solution of 0, 1, 10 and 20 wt% concentrations are used as test fluids for the droplet. FINDINGS: The results show the dependence of evaporation characteristics on vortex strength, while the crystallization dynamics was found to be independent of it. A reduction of 12.23% and 14.6% in evaporation time was seen in case of de-ionized (DI) water and 1% wt NaCl solution respectively in presence of vortex ring train at Re'=221. In contrast to this, a minimal reduction in evaporation time (0.6% and 0.9% for DI water and 1% wt NaCl solution, respectively) is observed when Re' is increased from 221 to 297. The mechanisms for evaporation time reduction due to enhancement of convective heat and mass transfer from the droplet and shearing away of vapor layer by vortex ring interaction are discussed in this work.

An opinion on the multiscale nature of Covid-19 type disease spread.

Recognizing the multiscale, interdisciplinary nature of the Covid-19 transmission dynamics, we discuss some recent developments concerning an attempt to construct a disease spread model from the flow physics of infectious droplets and aerosols and the frequency of contact between susceptible individuals with the infectious aerosol cloud. Such an approach begins with the exhalation event-specific, respiratory droplet size distribution (both airborne/aerosolized and ballistic droplets), followed by tracking its evolution in the exhaled air to estimate the probability of infection and the rate constants of the disease spread model. The basic formulations and structure of submodels, experiments involved to validate those submodels, are discussed. Finally, in the context of preventive measures, respiratory droplet-face mask interactions are described.

Precipitation dynamics of surrogate respiratory sessile droplets leading to possible fomites.

HYPOTHESIS: The droplets ejected from an infected host during expiratory events can get deposited as fomites on everyday use surfaces. Recognizing that these fomites can be a secondary route for disease transmission, exploring the deposition pattern of such sessile respiratory droplets on daily-use substrates thus becomes crucial. EXPERIMENTS: The used surrogate respiratory fluid is composed of a water-based salt-protein solution, and its precipitation dynamics is studied on four different substrates (glass, ceramic, steel, and PET). For tracking the final deposition of viruses in these droplets, 100 nm virus emulating particles (VEP) are used and their distribution in dried-out patterns is identified using fluorescence and SEM imaging techniques. FINDINGS: The final precipitation pattern and VEP deposition strongly depend on the interfacial transport processes, edge evaporation, and crystallization dynamics. A constant contact radius mode of evaporation with a mixture of capillary and Marangoni flows results in spatio-temporally varying edge deposits. Dendritic and cruciform-shaped crystals are majorly seen in all substrates except on steel, where regular cubical crystals are formed. The VEP deposition is higher near the three-phase contact line and crystal surfaces. The results showed the role of interfacial processes in determining the initiation of fomite-type infection pathways in the context of COVID-19.

On secondary atomization and blockage of surrogate cough droplets in single- and multilayer face masks.

Face masks prevent transmission of infectious respiratory diseases by blocking large droplets and aerosols during exhalation or inhalation. While three-layer masks are generally advised, many commonly available or makeshift masks contain single or double layers. Using carefully designed experiments involving high-speed imaging along with physics-based analysis, we show that high-momentum, large-sized (>250 micrometer) surrogate cough droplets can penetrate single- or double-layer mask material to a significant extent. The penetrated droplets can atomize into numerous much smaller (<100 micrometer) droplets, which could remain airborne for a significant time. The possibility of secondary atomization of high-momentum cough droplets by hydrodynamic focusing and extrusion through the microscale pores in the fibrous network of the single/double-layer mask material needs to be considered in determining mask efficacy. Three-layer masks can effectively block these droplets and thus could be ubiquitously used as a key tool against COVID-19 or similar respiratory diseases.

Insights on drying and precipitation dynamics of respiratory droplets from the perspective of COVID-19.

We isolate a nano-colloidal droplet of surrogate mucosalivary fluid to gain fundamental insights into airborne nuclei's infectivity and viral load distribution during the COVID-19 pandemic. The salt-water solution containing particles at reported viral loads is acoustically trapped in a contactless environment to emulate the drying, flow, and precipitation dynamics of real airborne droplets. Similar experiments validate observations with the surrogate fluid with samples of human saliva samples from a healthy subject. A unique feature emerges regarding the final crystallite dimension; it is always 20%-30% of the initial droplet diameter for different sizes and ambient conditions. Airborne-precipitates nearly enclose the viral load within its bulk while the substrate precipitates exhibit a high percentage (∼80-90%) of exposed virions (depending on the surface). This work demonstrates the leveraging of an inert nano-colloidal system to gain insights into an equivalent biological system.

Analyzing the dominant SARS-CoV-2 transmission routes toward an ab initio disease spread model.

Identifying the relative importance of the different transmission routes of the SARS-CoV-2 virus is an urgent research priority. To that end, the different transmission routes and their role in determining the evolution of the Covid-19 pandemic are analyzed in this work. The probability of infection caused by inhaling virus-laden droplets (initial ejection diameters between 0.5 µm and 750 µm, therefore including both airborne and ballistic droplets) and the corresponding desiccated nuclei that mostly encapsulate the virions post droplet evaporation are individually calculated. At typical, air-conditioned yet quiescent indoor space, for average viral loading, cough droplets of initial diameter between 10 µm and 50 µm are found to have the highest infection probability. However, by the time they are inhaled, the diameters reduce to about 1/6th of their initial diameters. While the initially near unity infection probability due to droplets rapidly decays within the first 25 s, the small yet persistent infection probability of desiccated nuclei decays appreciably only by O ( 1000 s ) , assuming that the virus sustains equally well within the dried droplet nuclei as in the droplets. Combined with molecular collision theory adapted to calculate the frequency of contact between the susceptible population and the droplet/nuclei cloud, infection rate constants are derived ab initio, leading to a susceptible-exposed-infectious-recovered-deceased model applicable for any respiratory event-vector combination. The viral load, minimum infectious dose, sensitivity of the virus half-life to the phase of its vector, and dilution of the respiratory jet/puff by the entraining air are shown to mechanistically determine specific physical modes of transmission and variation in the basic reproduction number R 0 from first-principles calculations.

Modeling the role of respiratory droplets in Covid-19 type pandemics.

In this paper, we develop a first principles model that connects respiratory droplet physics with the evolution of a pandemic such as the ongoing Covid-19. The model has two parts. First, we model the growth rate of the infected population based on a reaction mechanism. The advantage of modeling the pandemic using the reaction mechanism is that the rate constants have sound physical interpretation. The infection rate constant is derived using collision rate theory and shown to be a function of the respiratory droplet lifetime. In the second part, we have emulated the respiratory droplets responsible for disease transmission as salt solution droplets and computed their evaporation time, accounting for droplet cooling, heat and mass transfer, and finally, crystallization of the dissolved salt. The model output favourably compares with the experimentally obtained evaporation characteristics of levitated droplets of pure water and salt solution, respectively, ensuring fidelity of the model. The droplet evaporation/desiccation time is, indeed, dependent on ambient temperature and is also a strong function of relative humidity. The multi-scale model thus developed and the firm theoretical underpinning that connects the two scales-macro-scale pandemic dynamics and micro-scale droplet physics-thus could emerge as a powerful tool in elucidating the role of environmental factors on infection spread through respiratory droplets.