Water evaporation from solute-containing aerosol droplets: Effects of internal concentration and diffusivity profiles and onset of crust formation

Diffusive evaporation dynamics in polymer solutions is ubiquitous

Recent theory and experiments have shown how the buildup of a high-concentration polymer layer at a one-dimensional solvent-air interface can lead to an evaporation rate that scales with time as t-1/2 and that is insensitive to the ambient humidity. Using phase field modelling we show that this scaling law constitutes a naturally emerging robust regime, diffusion-limited evaporation (DLE). This regime dominates the dynamical state diagram of the system, which also contains regions of constant and arrested evaporation, confirming and extending understanding of recent experimental observations and theoretical predictions. We provide a theoretical argument to show that the scaling observed in the DLE regime occurs for a wide range of parameters, and our simulations predict that it can occur in two-dimensional geometries as well. Finally, we discuss possible extensions to more complex systems.

Evaporation of bacteria-laden surrogate respiratory fluid droplets: On a hydrophilic substrate vs contact-free environment confers differential bacterial infectivity

The transmission of viruses/bacteria causes infection predominantly via aerosols. The transmission mechanism of respiratory diseases is complex, which includes direct or indirect contact, large droplet, and airborne routes apart from close contact transmission. With this premise, two modes of droplet evaporation are investigated to understand its significance in airborne disease transmission; a droplet in a contact-free environment, which evaporates and forms droplet nuclei, and a droplet on a hydrophilic substrate (fomite). The study examines mass transport, the deposition pattern of bacteria in the precipitates, and their survival and virulence. The osmotic pressure increases with the salt concentration, inactivating the bacteria embedded in the precipitates with accelerated evaporation. Furthermore, the bacteria's degree of survival and enhanced pathogenicity are compared for both evaporation modes. The striking differences in pathogenicity are attributed to the evaporation rate, oxygen availability, and reactive oxygen species generation.

Evaporation of Concentrated Polymer Solutions Is Insensitive to Relative Humidity

A recent theory suggests that the evaporation kinetics of macromolecular solutions is insensitive to the ambient relative humidity (RH) due to the formation of a "polarization layer" of solutes at the air-solution interface. We confirm this insensitivity up to RH≈80% in the evaporation of polyvinyl alcohol solutions from open-ended capillaries. To explain the observed drop in evaporation rate at higher RH, we need to invoke compressive stresses due to interfacial polymer gelation. Moreover, RH-insensitive evaporation sets in earlier than theory predicts, suggesting a further role for a gelled "skin." We discuss the relevance of these observations for respiratory virus transmission via aerosols.

Airborne virus transmission: Increased spreading due to formation of hollow particles

The globally supported social distancing rules to prevent airborne transmission of COVID-19 assume small saliva droplets evaporate fast and large ones, which contain most viral copies, fall fast to the ground. However, during evaporation, solutes distribute non-uniformly within the droplets. We developed a numerical model to predict saliva droplet drying in different environments. We represent saliva droplets as a solution of NaCl mixed with water. In a hot and dry ambiance, the solutes form a shell on the droplets' surface, producing light, hollow particles. These hollow particles have a larger cross-sectional area compared to their solid counterparts and can float longer and travel farther in the air. We introduced the "hollowness factor," which serves as a measure of the ratio of the volume of a hollow particle and the volume of a solid residue formed during droplet drying. Through our investigations, we determined that under specific conditions, namely an ambient humidity level of 10% and a temperature of 40°C, the highest hollowness factor observed was 1.610. This finding indicates that in the case of hollow particle formation, the droplet nucleus expands by a factor of 1.610 compared to its original size.

The role of mucosal barriers in disease progression and transmission

Mucus is a biological hydrogel that coats and protects all non-keratinized wet epithelial surfaces. Mucins, the primary structural components of mucus, are critical components of the gel layer that protect against invading pathogens. For communicable diseases, pathogen-mucin interactions contribute to the pathogen's fate and the potential for disease progression in-host, as well as the potential for onward transmission. We begin by reviewing in-host mucus filtering mechanisms, including size filtering and interaction filtering, which regulate the permeability of mucus barriers to all molecules including pathogens. Next, we discuss the role of mucins in communicable diseases at the point of transmission (i.e. how the encapsulation of pathogens in emitted mucosal droplets externally to hosts may modulate pathogen infectivity and viability). Overall, mucosal barriers modulate both host susceptibility as well as the dynamics of population-level disease transmission. The study of mucins and their use in models and experimental systems are therefore crucial for understanding the mechanistic biophysical principles underlying disease transmission and the early stages of host infection.

Molecular dynamics study on evaporation of metal nitrate-containing nanodroplets in flame spray pyrolysis

Flame spray pyrolysis (FSP) provides an advantageous synthetic route for LiNi_1-x-yCo_xMn_yO₂ (NCM) materials, which are one of the most practical and promising cathode materials for Li-ion batteries. However, a detailed understanding of the NCM nanoparticle formation mechanisms through FSP is lacking. To shed light on the evaporation of NCM precursor droplets in FSP, in this work, we employ classical molecular dynamics (MD) simulations to explore the dynamic evaporation process of nanodroplets composed of metal nitrates (including LiNO₃, Ni(NO₃)₂, Co(NO₃)₂, and Mn(NO₃)₂ as solutes) and water (as solvent) from a microscopic point of view. Quantitative analysis on the evaporation process has been performed by tracking the temporal evolution of key features including the radial distribution of mass density, the radial distribution of number density of metal ions, droplet diameter, and coordination number (CN) of metal ions with oxygen atoms. Our MD simulation results show that during the evaporation of an MNO₃-containing (M = Li, Ni, Co, or Mn) nanodroplet, Ni²⁺, Co²⁺, and Mn²⁺ will precipitate on the droplet surface, forming a solvent-core-solute-shell structure; whereas the distribution of Li⁺ within the evaporating LiNO₃-containing droplet is more even due to the high diffusivity of Li⁺ compared with other metal ions. For the evaporation of a Ni(NO₃)₂- or Co(NO₃)₂-containing nanodroplet, the temporal evolution of the CN of M-OW (M = Ni or Co; OW represents O atoms from water) suggests a "free H₂O" evaporation stage, during which both CN of M-OW and CN of M-ON are unchanged with time. Evaporation rate constants at various conditions are extracted by making analogy to the classical D² law for droplet evaporation. Unlike Ni or Co, CN of Mn-OW keeps changing with time, yet the temporal evolution of the squared droplet diameter indicates the evaporation rate for a Ni(NO₃)₂-, Co(NO₃)₂-, or Mn(NO₃)₂-containing droplet is hardly affected by the different types of the metal ions.

Disposition of Aerosols of Isothiazolinone-Biocides: BIT, MIT and OIT

Biocides are widely used in everyday life, and accordingly, human exposure to them is inevitable. Especially, the inhalational exposure of humans to biocides and resultant respiratory toxicity are gaining public interest due to the recent catastrophe associated with humidifier disinfectants. Aerosolized chemicals are subject to gravitational deposition and chemical degradation. Therefore, the characterization of the disposition of aerosols is essential to estimate the inhalational exposure to biocides. Here, we compared the disposition of aerosols of one of the commonly used biocide classes, isothiazolinone-based biocides, BIT, MIT, and OIT. An acrylic chamber (40 cm × 40 cm × 50 cm) was created to simulate the indoor environment, and a vacuum pump was used to create airflow (1 LPM). Biocides were sprayed from a vertical nebulizer placed on the ceiling of the chamber, and the distribution of particle sizes and volume was measured using the Optical Particle Sizer (OPS) 3330 device. During and after the aerosol spraying, airborne biocides and those deposited on the surface of the chamber were sampled to measure the deposition using LC-MS/MS. As a result, the broad particle size distribution was observed ranging from 0.3 to 8 μm during the nebulization. The inhalable particle faction (>2 μm) of the isothiazolinones was 32−67.9% in number but 1.2 to 6.4% in volume. Most of the aerosolized biocides were deposited on the chamber’s surface while only a minimal portion was airborne (<1%) after the nebulization. More importantly, significant amounts of MIT and OIT were degraded during aerosolization, resulting in poor total recovery compared to BIT (31%, 71% vs. 97% BIT). This result suggests that some isothiazolinones may become unstable during nebulization, affecting their disposition and human exposure significantly.

Evaluating a transparent coating on a face shield for repelling airborne respiratory droplets

A face shield is an important personal protective equipment to avoid the airborne transmission of COVID-19. We assess a transparent coating on a face shield that repels airborne respiratory droplets to mitigate the spread of COVID-19. The surface of the available face shield is hydrophilic and exhibits high contact angle hysteresis. The impacting droplets stick on it, resulting in an enhanced risk of fomite transmission of the disease. Further, it may get wetted in the rain, and moisture may condense on it in the presence of large humidity, which may blur the user's vision. Therefore, the present study aims to improve the effectiveness of a face shield. Our measurements demonstrate that the face shield, coated by silica nanoparticles solution, becomes superhydrophobic and results in a nominal hysteresis to the underlying surface. We employ high-speed visualization to record the impact dynamics of microliter droplets with a varying impact velocity and angle of attack on coated and non-coated surfaces. While the droplet on non-coated surface sticks to it, in the coated surface the droplets bounce off and roll down the surface, for a wide range of Weber number. We develop an analytical model and present a regime map of the bouncing and non-bouncing events, parametrized with respect to the wettability, hysteresis of the surface, and the Weber number. The present measurements provide the fundamental insights of the bouncing droplet impact dynamics and show that the coated face shield is potentially more effective in suppressing the airborne and fomite transmission.