Session D26: Physics of Respiratory Droplets and Their Role in Disease Transmission

Diffusion of Sticky Nanoparticles in Unentangled Polymer Melts

Large-scale molecular dynamics simulations are used to study the diffusion of sticky nanoparticles in unentangled polymer melts, which models the transport of nano-scale objects in a sticky and viscous environment such as the polymer matrix in a polymer nanocomposite and complex fluids in a respiratory droplet. The simulated particles are larger than the average size of the polymers, and their diffusion coefficients follow the Stokes-Einstein relation.  In the athermal limit, the hydrodynamic radius r_H equals the bare particle radius r, and the particle-polymer boundary condition depends on polymer slippage over the particle surface. With increasing attractive interaction between the nanoparticles and the matrix (stickiness),  r_H remains equal to r, while polymer slippage is gradually suppressed. As the stickiness further increases, r_H is enlarged compared to r, with the particle-polymer boundary condition being no-slip. r_H increases because the nanoparticle moves together with longer sections of the adsorbed polymers. Eventually for sufficiently strong stickiness, r_H saturates at a value controlled by the sum of  r and the thickness of the entire adsorbed polymers. Sticky nanoparticles with r_H > r can be mapped to polymer-tethered nanoparticles.

    

Modeling Solution Drying by Moving a Liquid-Vapor Interface

A method of modeling the drying process of a soft matter solution by moving the liquid-vapor interface in an implicit solvent is applied to various solutions including films and droplets. For a solution of particle and polymer mixtures, drying outcomes similar to those found with an explicit solvent model are observed. For a bidisperse nanoparticle suspension droplet, a core-shell cluster of nanoparticles can be obtained via the "small-on-outside" stratification mechanism. Polymeric particles with various morphologies, including Janus particles, core-shell particles, onion-like structures, and patchy particles, can be produced from drying polymer solution droplets by combining fast drying with a controlled interaction between the polymers and the liquid-vapor interface. This moving interface method can be applied to the drying process of a wide range of systems including respiratory droplets. The limitation and possible extension of the method are also discussed.   

Stability of Respiratory-Like Droplets under Evaporation

Enveloped viruses contained in airborne respiratory droplets have been seen to lose infectability fastest at intermediate ambient relative humidities H_r. However, the precise physico-chemical mechanisms that generate such least-favorable conditions for the virus are not fully understood yet. Analyzing the evaporation dynamics of  respiratory-like droplets in air, we reveal that at high H_r, the salt dissolved in respiratory drops inhibits their evaporation indefinitely. Conversely, at low H_r the drop evaporates leaving a porous solid residue inside which virions may remain dormant for long times. The optimal H_r for the fastest reduction of the infectability lies in between, in line with the empirical evidence for the corona virus transmission probability. Finally, we use electron microscopy to examine the structure of the solid residue left upon the drop evaporates completely.   

Ballistic respiratory droplets impacting on facemasks: an overview

Facemask has become an integral part of our lives in protecting ourselves and others from CoVID-19. Proper use of facemasks blocks the ballistic respiratory droplets and liquid aerosols emanating from respiratory cavities and protects the user from inhaling the airborne droplets or aerosols. While N95 masks generally provide the best protection, single or multi-layered surgical masks, cloth masks, and other substitute masks have become popular during the pandemic. Naturally, their protection capability varies. In this talk, we will present an overview of what happens to ballistic large respiratory droplets when they impact single and multilayer masks. We will investigate both surgical masks and typical cloth masks. First, we will show that these droplets can penetrate the dry masks at certain conditions, generating many smaller atomized droplets. These smaller droplets can remain airborne for an extended period and hence, participate in disease transmission. Next, we will focus on wet masks. Indeed, long-term usage of masks leads to damp mask-matrix due to condensation of exhaled breaths. We will show how the wetness of the mask-matrix alters the penetration and atomization. Finally, we will present scaling analyses to explain the penetration dynamics for both dry and wet masks.   

Quantifying the effect of a mask on expiratory flows

Face masks are used widely to mitigate the spread of infectious diseases. While their main purpose is to filter pathogenic droplets, masks also represent a porous barrier to exhaled and inhaled air flow. In this study, we characterize the aerodynamic effect of the presence of a mask by tracking the air exhaled by a person through a mask, using both infrared imaging and particle image velocimetry performed on illuminated fog droplets surrounding a subject. We show how a mask confines the exhaled flows within tens of centimeters in front of a person breathing or speaking. In addition, we show that the tissue of common surgical face masks has a low permeability, which efficiently transforms the jetlike flows of exhalation produced during breathing or speaking into quasivertical buoyancy-driven flows. Therefore, wearing a mask offers a strong mitigation of direct transport of infectious material in addition to providing a filtering function. By comparing results on human subjects and model experiments, we propose a model to rationalize how a mask changes the air flow, and thus we provide quantitative insights that are useful for descriptions of disease transmission.   

Pathogen transport and air exchange during short conversations

The scenario where two unmasked individuals are in proximity having a face-to-face conversation is increasingly common and may contribute to the spread of the SARS-CoV-2 virus. We conduct flow visualization experiments and direct numerical simulations of colliding respiratory jets to study the airflow and exchange between two individuals during a short conversation. We observe that the vertical offset between the mouths is largely governed by the three-dimensional spread of the jets.  At low offsets, the jets collide and interact such that the susceptible speaker is temporarily protected from the pathogen-loaded saliva droplets in the jet, and is termed as the ‘blocking effect’. At large offsets, the interaction between the jets is minimal and the jet spreads axially without any hindrance. At intermediate offsets, the respiratory jets interact such that jet entrainment and inhaled breath, assist the pathogen-containing jets to propagate towards the susceptible speaker's mouth. Thus, the interaction of the respiratory jets permits air exchange to a varying degree depending upon the effectiveness of the blocking effect and jet entrainment.   

COVID-19 and other respiratory disease transmission during oxygen-therapy

We discuss effect of mitigation strategies on pathogen-laden aerosol transmissibility with experiments amenable to statistical analysis. For many patients with COVID-19, primary oxygen therapy is provided by either a nasal cannula (a tube with two prongs inserted into each nostril) or a simple O2 mask (a mask that fits over the lower half of a patients face). However, these devices produce exhalation jets and therefore can spread infection. Nonetheless, patients receiving oxygen by either of these methods rarely use further mitigation to prevent the spread of exhaled aerosols and droplets while being transported or treated in a healthcare facility. Our research team combined the expertise at the nonlinear physics research lab at Clark University with the knowledge and experience of respiratory therapists at Baystate Medical Center to produce visualizations of the exhalation patterns of a therapy-quality respiratory manikin with and without masks under a range of oxygen treatment conditions. Placing a surgical facemask loosely over the oxygenation device is demonstrated to deflect the direction of the exhalation jet downwards, although leaks can be observed from the bridge of the nose and from gaps between the face and mask sides. The primary method by which exposure is alleviated is by defecting the exhalation jets downward, away from the face of a clinician, and by preventing the exhalations from being launched directly over the patient and higher into the room. Less than 12% of the exhaled breath is observed to reach above the masked plane of a supine patient where a caregiver would be present, independent of oxygen flow rates. Our data can be used to calibrate models of respiratory disease spread including COVID-19, seasonal flu and tuberculosis.   

The abundance of speech-generated droplets and the role of masks in curbing airborne transmission of disease

Laser light scattering images have shown that droplets emitted during speech are abundant¹, can linger in the air for extended periods², and can contribute to airborne transmission of disease³. How long a droplet lingers in the air is inversely proportional to the square of its size, which shrinks by roughly a factor of three due to evaporation of its water content. Airborne transmission involves particle deposition along a circuitous path leading from the upper respiratory tract (URT) to alveoli in the lower respiratory tract (LRT)⁴, with deep penetration limited to aerosol particles smaller than around 5 microns. Since the number of virions that can be harbored in a particle is proportional to its volume, infections are far less likely to occur in the LRT than the URT. However, the consequences of infection originating in the LRT are typically far more severe and can lead to hospitalization and death. Since ordinary face masks capture larger, wet droplets more efficiently than smaller, dry particles, universal wearing of masks while in densely populated public spaces can reduce significantly the volume of aerosolized speech droplets emitted by loquacious asymptomatic carriers, and hence the risk of airborne transmission of this virus. Ongoing investigations of the number and size distribution of droplets emitted during speech aim to assess their fate as well as the relative effectiveness of mitigation strategies.