Weather-related changes in the dehydration of respiratory droplets on surfaces bolster bacterial endurance.

HYPOTHESIS: The study shows for the first time a fivefold difference in the survivability of the bacterium Pseudomonas Aeruginosa (PA) in a realistic respiratory fluid droplet on fomites undergoing drying at different environmental conditions. For instance, in 2023, the annual average outdoor relative humidity (RH) and temperature in London (UK) is 71 % and 11 °C, whereas in New Delhi (India), it is 45 % and 26 °C, showing that disease spread from fomites could have a demographic dependence. Respiratory fluid droplet ejections containing pathogens on inanimate surfaces are crucial in disease spread, especially in nosocomial settings. However, the interplay between evaporation dynamics, internal fluid flow and precipitation and their collective influence on the distribution and survivability of pathogens at different environmental conditions are less known. EXPERIMENTS: Shadowgraphy imaging is employed to study evaporation, and optical microscopy imaging is used for precipitation dynamics. Micro-particle image velocimetry (MicroPIV) measurements reveal the internal flow dynamics. Confocal imaging of fluorescently labelled PA elucidates the bacterial distribution within the deposits. FINDINGS: The study finds that the evaporation rate is drastically impeded during drying at elevated solutal concentrations, particularly at high RH and low temperature conditions. MicroPIV shows reduced internal flow under high RH and low temperature (low evaporation rate) conditions. Evaporation rate influences crystal growth, with delayed efflorescence and extending crystallization times. PA forms denser peripheral arrangements under high evaporation rates and shows a fivefold increase in survivability under low evaporation rates. These findings highlight the critical impact of environmental conditions on pathogen persistence and disease spread from inanimate surfaces.

Future research perspective on the interfacial physics of non-invasive glaucoma testing in pathogen transmission from the eyes.

Non-contact tonometry (NCT) is a non-invasive ophthalmologic technique to measure intraocular pressure (IOP) using an air puff for routine glaucoma testing. Although IOP measurement using NCT has been perfected over many years, various phenomenological aspects of interfacial physics, fluid structure interaction, waves on corneal surface, and pathogen transmission routes to name a few are inherently unexplored. Research investigating the interdisciplinary physics of the ocular biointerface and of the NCT procedure is sparse and hence remains to be explored in sufficient depth. In this perspective piece, we introduce NCT and propose future research prospects that can be undertaken for a better understanding of the various hydrodynamic processes that occur during NCT from a pathogen transmission viewpoint. In particular, the research directions include the characterization and measurement of the incoming air puff, understanding the complex fluid-solid interactions occurring between the air puff and the human eye for measuring IOP, investigating the various waves that form and travel; tear film breakup and subsequent droplet formation mechanisms at various spatiotemporal length scales. Further, from an ocular disease transmission perspective, the disintegration of the tear film into droplets and aerosols poses a potential pathogen transmission route during NCT for pathogens residing in nasolacrimal and nasopharynx pathways. Adequate precautions by opthalmologist and medical practioners are therefore necessary to conduct the IOP measurements in a clinically safer way to prevent the risk associated with pathogen transmission from ocular diseases like conjunctivitis, keratitis, and COVID-19 during the NCT procedure.

Observations on phenomenological changes in Klebsiella Pneumoniae under fluidic stresses.

In the present work, experiments are conducted to understand the consequence of stresses generated by flowing fluid on the bacterial morphology and virulence in microfluidic channels. We consider Klebsiella pneumoniae (KP, a clinical isolate), an ESKAPE pathogen, to be the model bacteria responsible for blood stream infections, bacteremia, including pneumonia, urinary tract infections and more. Four different stress conditions are generated by changing the flow rate and channel geometry subsequently altering the shear rate and stressing time (τ). We observe significant changes in the structural aspects of the stressed bacteria. With an increase in stressing parameters, the viability of the bacterial sample deteriorated. Most importantly, these stressed samples proliferate much more than unstressed samples inside the RAW264.7 murine macrophages. The results shed light on the complex relationship between flow stresses and bacterial virulence. Furthermore, the bacterial samples are challenged with ciprofloxacin to see how they behave under different stress conditions. The observations presented in the present study can be extended to model deadly diseases including bacteremia using organ-on-a-chip technology and to understand bacterial pathogenicity under realistic environments.

phoP maintains the environmental persistence and virulence of pathogenic bacteria in mechanically stressed desiccated droplets.

Despite extensive studies on kinematic features of impacting drops, the effect of mechanical stress on desiccated bacteria-laden droplets remains unexplored. In the present study, we unveiled the consequences of the impaction of bacteria-laden droplets on solid surfaces and their subsequent desiccation on the virulence of an enteropathogen Salmonella typhimurium (STM). The methodology elucidated the deformation, cell-cell interactions, adhesion energy, and roughness in bacteria induced by impact velocity and low moisture because of evaporation. Salmonella retrieved from the dried droplets were used to understand fomite-mediated pathogenesis. The impact velocity-induced mechanical stress deteriorated the in vitro viability of Salmonella. Of interest, an uninterrupted bacterial proliferation was observed in macrophages at higher mechanical stress. Wild-type Salmonella under mechanical stress induced the expression of phoP whereas infecting macrophages. The inability of STM ΔphoP to grow in nutrient-rich dried droplets signifies the role of phoP in sensing the mechanical stress and maintaining the virulence of Salmonella.

On the specific heat capacity of HITEC-salt nanocomposites for concentrated solar power applications.

High specific heat capacity or C P of molten salt is crucial for concentrated solar power plants as it will enhance the energy density of thermal energy storage. It can be achieved by doping nanoparticles into molten salts. However, reported results show inconsistency in C P enhancement (positive and negative). Since the results are based on Differential Scanning Calorimeter (DSC) measurements of small batches (<10 mg), the average C P obtained from these results may not represent the bulk-C P of the nanocomposite, which is an important parameter from an application viewpoint. Moreover, the methods of salt-nanoparticle composite production lack industrial scalability. In this work, we examined a potentially scalable method based on mechanical shear mixing. The molten-salt of choice was HITEC due to its lower melting point, while inexpensive alumina and silica nanoparticles were used as dopants. To compare and contrast variability in C P enhancement, the sample selection was made by random sampling; DSC measurement was performed on small-sized batches (<10 mg), and the T-history method was applied on large-sized batches (20 g). While DSC tests indicated a mean decrease in C P for alumina (-43%) and an increase in C P for silica nanocomposite (+15%), T-history tests indicated a mean decrement in the bulk-C P for both alumina (-49%) and silica nanocomposites (-3%). This anomalous behavior in C P values was further compared using a nonparametric statistical test, the Mann-Whitney U test, which revealed that the C P of small-sized batches is statistically different from that of large-sized batches. Given their industrial scale of usage, the C P of the nanocomposite must be measured using both DSC and T-history methods to ascertain the effect of nanoparticles.

Physics of self-assembly and morpho-topological changes of Klebsiella pneumoniae in desiccating sessile droplets.

HYPOTHESIS: The bacteria suspended in pure water self-assemble into unique patterns depending on bacteria-bacteria, bacteria-substrate and bacteria-liquid interactions. The physical forces acting on bacteria vary based on their respective spatial location inside the droplet cause an assorted magnitude of physical stress. The shear and dehydration induced stress on pathogens(bacteria) in drying bio-fluid droplets alters the viability and infectivity. EXPERIMENTS: We have investigated the flow and desiccation-driven self-assembly of Klebsiella pneumoniae in the naturally evaporating sessile droplets. Klebsiella pneumoniae exhibits extensive changes in its morphology and forms unique patterns as the droplet dries, revealing hitherto unexplored rich physics governing its survival and infection strategies. Self-assembly of bacteria at the droplet contact line is characterized by order-to-disorder packing transitions with high packing densities and excessive deformations (analysed using scanning electron microscopy and atomic force microscopy). In contrast, thin-film instability-led hole formation at the center of the droplet engenders spatial packing of bacteria analogous to honeycomb weathering. FINDINGS: Self-assembly favors the bacteria at the rim of the droplet, leading to enhanced viability and pathogenesis on the famously known "coffee ring" of the droplet compared to the bacteria present at the center of the droplet residue. Mechanistic insights gained via our study can have far-reaching implications for bacterial infection through droplets, e.g., through open wounds.

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.

Malleable Patterns from the Evaporation of a Colloidal Liquid Bridge: Coffee Ring to the Scallop Shell.

The present article highlights an approach to generating contrasting patterns from drying colloidal droplets in a liquid bridge configuration, different from well-known coffee rings. Reduction of the confinement distance (the gap between the solid surfaces) leads to systematized nanoparticle agglomeration yielding spoke-like patterns similar to those found on scallop shells instead of circumferential edge deposition. Alteration of the confinement distance modulates the curvature that entails variations in the evaporation flux across the liquid-vapor interface. Consequently, flow inside different liquid bridges (LBs) varies significantly for different confinement distance. Small confinement distance results in the stick-slip motion of squeezed liquid bridges. On the contrary, the stretched LBs exhibit pinned contact lines. The confinement distance determines the characteristic length scales of the thin film formed near the contact line, and its theoretical estimations are validated against the experimental observations using reflection interferometry, further exhibiting good agreement (in order of magnitude). We decipher a proposition that a drying liquid thin film (height ∼ O(10-7)m) present during dewetting near the three-phase contact line is responsible for the aligned deposition of particles. The coupled interplay of contact line dynamics, evaporation induced advection, and dewetting of the thin film at a three-phase interface contributes to the differences in deposition patterns.

Multiscale vapor-mediateddendritic pattern formation and bacterial aggregation in complex respiratory biofluid droplets.

HYPOTHESIS: Deposits of biofluid droplets on surfaces (such as respiratory droplets formed during an expiratory) are composed of water-based salt-protein solution that may also contain an infection (bacterial/viral). The final patterns of the deposit formed and bacterial aggregation on the deposits are dictated by the fluid composition and flow dynamics within the droplet. EXPERIMENTS: This work reports the spatio-temporal, topological regulation of deposits of respiratory fluid droplets and control of bacterial aggregation by tweaking flow inside droplets using non-contact vapor-mediated interactions. Desiccated respiratory droplets form deposits with haphazard multiscale dendritic, cruciform-shaped precipitates when evaporated on a glass substrate. However, we showcase that short and long-range vapor-mediated interaction between the droplets can be used as a tool to control these deposits at nano-micro-millimeter scales. We morphologically control hierarchial dendrite size, orientation and subsequently suppress cruciform-shaped crystals by placing a droplet of ethanol in the vicinity of the biofluid droplet. Active living matter in respiratory fluids like bacteria is preferentially segregated and agglomerated without its viability and pathogenesis attenuation. FINDINGS: The nucleation sites can be controlled via preferential transfer of solutes in the droplets; thus, achieving control over crystal occurrence, growth dynamics, and the final topology of the deposit. For the first time, we have experimentally presented a proof-of-concept to control the aggregation of live active matter like bacteria without any direct contact. The methodology can have ramifications in biomedical applications like disease detection and bacterial segregation.

Two-dimensional mathematical framework for evaporation dynamics of respiratory droplets.

In majority of pandemics in human history, respiratory bio-aerosol is the most common route of transmission of diseases. These tiny droplets ejected through mouth and nose from an infected person during exhalation process like coughing, sneezing, speaking, and breathing consist of pathogens and a complex mixture of volatile and nonvolatile substances. A cloud of droplets ejected in such an event gets transmitted in the air, causing a series of coupled thermo-physical processes. Contemplating an individual airborne droplet in the cloud, boundary layers and wakes develop due to relative motion between the droplet and the ambient air. The complex phenomenon of the droplet's dynamics, such as shear-driven internal circulation of the liquid phase and Stefan flow due to vaporization or condensation, comes into effect. In this study, we present a mathematical description of the coupled subprocesses, including droplet aerodynamics, heat, and mass transfer, which were identified and subsequently solved. The presented two-dimensional model gives a complete analysis encompassing the gas phase coupled with the liquid phase responsible for the airborne droplet kinetics in the ambient environment. The transient inhomogeneity of temperature and concentration distribution in the liquid phase caused due to the convective and diffusive transports are captured in the 2D model. The evaporation time and distance traveled by droplets prior to nuclei or aerosol formation are computed for major geographical locations around the globe for nominal-windy conditions. The model presented can be used for determining the evaporation timescale of any viral or bacterial laden respiratory droplets across any geographical location.

Spatiotemporal evaporating droplet dynamics on fomites enhances long term bacterial pathogenesis.

Naturally drying bacterial droplets on inanimate surfaces representing fomites are the most consequential mode for transmitting infection through oro-fecal route. We provide a multiscale holistic approach to understand flow dynamics induced bacterial pattern formation on fomites leading to pathogenesis. The most virulent gut pathogen, Salmonella Typhimurium (STM), typically found in contaminated food and water, is used as model system in the current study. Evaporation-induced flow in sessile droplets facilitates the transport of STM, forming spatio-temporally varying bacterial deposition patterns based on droplet medium's nutrient scale. Mechanical and low moisture stress in the drying process reduced bacterial viability but interestingly induced hyper-proliferation of STM in macrophages, thereby augmenting virulence in fomites. In vivo studies of fomites in mice confirm that STM maintains enhanced virulence. This work demonstrates that stressed bacterial deposit morphologies formed over small timescale (minutes) on organic and inorganic surfaces, plays a significant role in enhancing fomite's pathogenesis over hours and days.

Risk Assessment of Aerosol Generation During Vitreoretinal Surgery Using High Speed Imaging Amidst the COVID-19 Pandemic.

Purpose: The purpose of this study was to discuss the propensity of aerosol and droplet generation during vitreoretinal surgery using high speed imaging amidst the coronavirus disease 2019 (COVID-19) pandemic. Methods: In an experimental set up, various steps of vitreoretinal surgery were performed on enucleated goat eyes. The main outcome measures were visualization, quantification of size, and calculation of aerosol spread. Results: During intravitreal injection, insertion of cannulas, lensectomy, and vitrectomy with both 23 and 25-gauge instruments, with either valved or nonvalved cannulas, aerosols were not visualized which was confirmed on imaging. Although there was no aerosol generation during active fluid air exchange (FAE), there was bubbling and aerosol generation at the exit port of the handle during passive FAE. Under higher air pressure, with reused valved and fresh nonvalved cannulas, aerosol generation showed a trajectory 0.4 to 0.67 m with droplet size of 200 microns. Whereas removing cannulas or suturing under active air infusion (35 mm Hg and above) aerosols were noted. Conclusions: Based on the above experiments, we can formulate guidelines for safe vitrectomy during COVID-19. Some recommendations include the use of valved cannulas, avoiding passive FAE or to direct the exit port away from the surgeon and assistant, and to maintain the air pressure less than or equal to 30 mm Hg. Translational Relevance: In the setting of the COVID-19 pandemic, the risk from virus laden aerosols, as determined using an experimental setup, appears to be low for commonly performed vitreoretinal surgical procedures.

Fluid dynamics of droplet generation from corneal tear film during non-contact tonometry in the context of pathogen transmission.

Noninvasive ocular diagnostics demonstrate a propensity for droplet generation and present a potential pathway of distribution for pathogens such as the severe acute respiratory syndrome coronavirus 2. High-speed images of the eye subjected to air puff tonometry (glaucoma detection) reveal three-dimensional, spatiotemporal interaction between the puff and tear film. The interaction finally leads to the rupture and breakup of the tear film culminating into sub-millimeter sized droplet projectiles traveling at speeds of 0.2 m/s. The calculated droplet spread radius ( ∼ 0.5 m) confirms the likelihood of the procedure to generate droplets that may disperse in air as well as splash on instruments, raising the potential of infection. We provide a detailed physical exposition of the entire procedure using high fidelity experiments and theoretical modeling. We conclude that air puff induced corneal deformation and subsequent capillary waves lead to flow instabilities (Rayleigh-Taylor, Rayleigh-Plateau) that lead to tear film ejection, expansion, stretching, and subsequent droplet formation.

Efficacy of homemade face masks against human coughs: Insights on penetration, atomization, and aerosolization of cough droplets.

Ever since the emergence of the ongoing COVID-19 pandemic, the usage of makeshift facemasks is generally advised by policymakers as a possible substitute for commercially available surgical or N95 face masks. Although such endorsements could be economical and easily accessible in various low per-capita countries, the experimental evidence on the effectiveness of such recommendations is still lacking. In this regard, we carried out a detailed experimental investigation to study the fate of a large-sized surrogate cough droplet impingement at different velocities (corresponding to mild to severe coughs) on various locally procured cloth fabrics. Observation shows that larger ejected droplets (droplets that would normally settle as fomites in general) during a coughing event have enough momentum to penetrate single-layer cloth masks; the penetrated volume atomize into smaller daughter droplets that fall within aerosol range, thereby increasing infection potential. Theoretically, two essential criteria based on the balances of viscous dissipation-kinetic energy and surface tension-kinetic energy effects have been suggested for the droplet penetration through mask layers. Furthermore, a new parameter called η (the number density of pores for a fabric) is developed to characterize the volume penetration potential and subsequent daughter droplet size. Finally, the effect of mask washing frequency is analyzed. The outcomes from the current study can be used as a guide in selecting cloth fabrics for stitching multi-layered.

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.

Spatio-temporal modulation of self-assembled central aggregates of buoyant colloids in sessile droplets using vapor mediated interactions.

A functional sessile droplet containing buoyant colloids (ubiquitous in applications like chemical sensors, drug delivery systems, and nanoreactors) forms self-assembled aggregates. The particles initially dispersed over the entire drop-flocculates at the center. We attribute the formation of such aggregates to the finite radius of curvature of the drop and the buoyant nature of particles. Initially, larger particles rise to the top of the droplet (due to higher buoyancy force), and later the smaller particles join the league, leading to the graded size distribution of the central aggregate. This can be used to segregate polydisperse hollow spheres based on size. The proposed scaling analysis unveils insights into the distinctive particle transport during evaporation. However, the formation of prominent aggregates can be detrimental in applications like spray painting, sprinkling of pesticides, washing, coating, lubrication, etc. One way to avoid the central aggregate is to spread the droplets completely (contact angle ~ 00), thus theoretically creating an infinite radius of curvature leading to uniform deposition of buoyant particles. Practically, this requires a highly hydrophilic surface, and even a small inhomogeneity on the surface would pin the droplet giving it a finite radius of curvature. Here, we demonstrate using non-intrusive vapor mediated Marangoni convection (Velocity scale ~ O(103) higher than the evaporation-driven convection) can be vital to an efficient and on-demand manipulation of the suspended micro-objects. The interplay of surface tension and buoyancy force results in the transformation of flow inside the droplet leads to spatiotemporal disbanding of agglomeration at the center of the droplet.

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.

Dissolution dynamics of a vertically confined sessile droplet.

We experimentally investigate the dissolution of microscale sessile alcohol droplets in water under the influence of impermeable vertical confinement. The introduction of confinement suppresses the mass transport from the droplet to bulk medium in comparison with the nonconfined counterpart. Along with a buoyant plume, flow visualization reveals that the dissolution of a confined droplet is hindered by a mechanism called levitated toroidal vortex. The morphological changes in the flow due to the vortex-induced impediment alters the dissolution rate, resulting in enhancement of droplet lifetime. Further, we have proposed a modification in the key nondimensional parameters [Rayleigh number Ra^{'} (signifying buoyancy) and Sherwood number Sh^{'} (signifying mass flux)] and droplet lifetime τ_{c}^{'}, based on the hypothesis of linearly stratified droplet surroundings (with revised concentration difference ΔC^{'}), taking into account the geometry of the confinements. We show that experimental results on droplet dissolution under vertical confinement corroborate scaling relations Sh^{'}∼Ra^{'}^{1/4} and τ_{c}^{'}∼ΔC^{'}^{-5/4}. We also draw attention to the fact that the revised scaling law incorporating the geometry of confinements proposed in the present work can be extended to other known configurations such as droplet dissolution inside a range of channel dimensions, as encountered in a gamut of applications such as microfluidic technology and biomedical engineering.