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The evolution of the particle‑induced hemispherical bubble bursting process is invistigated using high‑speed camera filming. The edge regeneration and convection after bubble formation, the probability of bubble bursting when subjected to particle penetration, and the bubble bursting behavior after hole formation are observed and analyzed. Through dimensional analysis and experimental research, it is found that the probability of bubble breaking probability is a function of Reynolds number and Weber number. The probability is given by P = 1. 08 (Re0. 5We0. 25)0. 664, and the probability is reaches to 100% when Re0. 5We0. 25>1 020. The number of ligaments formed by bubble bursting is a function of the Oh number. The rim receding velocity of the bubble liquid film increases with the concentration of the solution increases and the film thickness decreases with the concentration increases. [ABSTRACT FROM AUTHOR]

The bursting phenomenon consists in the switch of a laminar separation bubble from a short to a long configuration. In the former case, reduced effects on profile pressure distribution are typically observed with respect to the attached condition. On the contrary, long bubbles provoke significant variations in the loading coefficient upstream of the separation position, with increased risk of stall of the lifting surfaces. The present work presents an experimental database describing separated boundary layers evolving under different Reynolds numbers, adverse pressure gradients and free-stream turbulence levels. Overall, more than 80 flow conditions were tested concerning short and long bubbles for the characterization of separated flows under turbine-like conditions. Measurements were performed on a flat plate geometry using a fast-response Particle Image Velocimetry (PIV) system. For each flow case, two sets of 6000 flow records were acquired with an acquisition frequency equal to 300 and 1000 Hz. Based on existing criteria for the identification of the bursting phenomenon, the flow cases were clustered in terms of short and long bubble states. Additionally, the kind of instability (i.e., convective or absolute) developing into the separated boundary layer was identified based on flow statistics. The present data captures the existing link between the bursting of a laminar separation bubble and the onset of the absolute instability of the separated shear layer, with stationary vortices forming in the dead air region. [ABSTRACT FROM AUTHOR]

Experimental results are reported on the airborne release, under fire conditions, of hazardous materials dissolved in a mixture of organic solvents [tributylphosphate (TBP) and hydrogenated tetrapropylene (HTP)] representative of the nuclear fuel recycling process. Cerium and ruthenium have been considered, respectively, as stable and volatile fission products that eventually could be released as airborne particles during thermal degradation of contaminated and inflammable liquids. Airborne release fractions (ARFs) and their experimental uncertainties have been determined. Considering fire involving contaminated organic solvents, higher ARFs are reported for ruthenium Ru(+III) (0.99 ± 1.20%) in comparison with cerium [0.22 ± 0.31% and 0.20 ± 0.28% for Ce(+III) and Ce(+IV), respectively]. This discrepancy is partially due to the volatility of ruthenium formed under these conditions. Considering configurations involving an aqueous nitric acid phase placed below contaminated solvents, boiling of this phase enhances the release of contaminant materials: 1.78 ± 1.06% and 1.01 ± 1.31% for Ce(+III) and Ce(+IV), respectively, and 12.41 ± 29.45% for Ru(+III). Analysis of the size distribution, morphology, and chemical composition of the released particles and droplets emitted during HTP/TBP bubble collapse are reported, highlighting the contribution of bubble bursting at the solvent surface to airborne release. [ABSTRACT FROM AUTHOR]

The bursting phenomenon consists in the switch of a laminar separation bubble from a short to a long configuration. In the former case, reduced effects on profile pressure distribution are typically observed with respect to the attached condition. On the contrary, long bubbles provoke significant variations in the loading coefficient upstream of the separation position, with increased risk of stall of the lifting surfaces. The present work presents an experimental database describing separated boundary layers evolving under different Reynolds numbers, adverse pressure gradients and free-stream turbulence levels. Overall, more than 80 flow conditions were tested concerning short and long bubbles for the characterization of separated flows under turbine-like conditions. Measurements were performed on a flat plate geometry using a fast-response Particle Image Velocimetry (PIV) system. For each flow case, two sets of 6000 flow records were acquired with an acquisition frequency equal to 300 and 1000 Hz. Based on existing criteria for the identification of the bursting phenomenon, the flow cases were clustered in terms of short and long bubble states. Additionally, the kind of instability (i.e., convective or absolute) developing into the separated boundary layer was identified based on flow statistics. The present data captures the existing link between the bursting of a laminar separation bubble and the onset of the absolute instability of the separated shear layer, with stationary vortices forming in the dead air region.

Abstract Bubbles bursting at the ocean surface are an important source of ocean‐spray aerosol, with implications on radiative and cloud processes. Yet, very large uncertainties exist on the role of key physical controlling parameters, including wind speed, sea state and water temperature. We propose a mechanistic sea spray generation function that is based on the physics of bubble bursting. The number and mean droplet radius of jet and film drops is described by scaling laws derived from individual bubble bursting laboratory and numerical experiments, as a function of the bubble radius and the water physico‐chemical properties (viscosity, density and surface tension, all functions of temperature), with drops radii at production from 0.1 to 500 µm. Next, we integrate over the bubble size distribution entrained by breaking waves. Finally, the sea spray generation function is obtained by considering the volume flux of entrained bubbles due to breaking waves in the field constrained by the third moment of the breaking distribution (akin to the whitecap coverage). This mechanistic approach naturally integrates the role of wind and waves via the breaking distribution and entrained air flux, and a sensitivity to temperature via individual bubble bursting mechanisms. The resulting sea spray generation function has not been tuned or adjusted to match any existing data sets, in terms of magnitude of sea salt emissions and recently observed temperature dependencies. The remarkable coherence between the model and observations of sea salt emissions therefore strongly supports the mechanistic approach and the resulting sea spray generation function.

Abstract Basaltic magma becomes more viscous, solid-like (elastic), and non-Newtonian (shear-thinning, non-zero yield stress) as its crystal content increases. However, the rheological effects on bubble bursting and airwave excitation are poorly understood. Here we conduct laboratory experiments to investigate these effects by injecting a bubble of volume V into a refractive index-matched suspension consisting of non-Brownian particles (volumetric fraction $$\phi$$ ϕ ) and a Newtonian liquid. We show that depending on $$\phi$$ ϕ and V, airwaves with diverse waveforms are excited, covering a frequency band of $$f = {\mathcal {O}}(10-10^4)$$ f = O ( 10 - 10 4 ) Hz. In a suspension of $$\phi \le 0.3$$ ϕ ≤ 0.3 or in a suspension of $$\phi = 0.4$$ ϕ = 0.4 with a V smaller than critical, the bubble bursts after it forms a hemispherical cap at the surface and excites a high-frequency (HF) wave ( $$f \sim 1-2 \times 10^4$$ f ∼ 1 - 2 × 10 4 Hz) with an irregular waveform, which likely originates from film vibration. However, in a suspension of $$\phi = 0.4$$ ϕ = 0.4 and with a V larger than critical, the bubble bursts as soon as it protrudes above the surface, and its aperture opens slowly, exciting Helmholtz resonance with $$f = {\mathcal {O}}(10^3)$$ f = O ( 10 3 ) Hz. Superimposed on the waveform are an HF wave component excited upon bursting and a low-frequency ( $$f = {\mathcal {O}}(10)$$ f = O ( 10 ) Hz) air flow vented from the deflating bubble, which becomes dominant at a large V. We interpret this transition as a result of the bubble film of a solid-like $$\phi = 0.4$$ ϕ = 0.4 suspension, being stretched faster than the critical strain rate such that it bursts by brittle failure. When the Helmholtz resonance is excited by a bursting bubble in a suspension whose surface level is further below the conduit rim, an air column (length L) resonance is triggered. For L larger than critical, the air column resonance continues longer than the Helmholtz resonance because the decay rate of the former becomes less than that of the latter. The experiments suggest that a bubble bursting at basaltic volcanoes commonly excites HF wave by film vibration. The Helmholtz resonance is likely to be excited under a limited condition, but if detected, it may be used to track the change of magma rheology or bubble V, where the V can be estimated from its frequency and decay rate.

The oceans represent a significant global source of atmospheric aerosols. Sea spray aerosol (SSA) particles comprise sea salts and organic species in varying proportions. In addition to size, the overall composition of SSA particles determines how effectively they can form cloud droplets and ice crystals. Thus, understanding the factors controlling SSA composition is critical to predicting aerosol impacts on clouds and climate. It is often assumed that submicrometer SSAs are mainly formed by film drops produced from bursting bubble-cap films, which become enriched with hydrophobic organic species contained within the sea surface microlayer. In contrast, jet drops formed from the base of bursting bubbles are postulated to mainly produce larger supermicrometer particles from bulk seawater, which comprises largely salts and water-soluble organic species. However, here we demonstrate that jet drops produce up to 43% of total submicrometer SSA number concentrations, and that the fraction of SSA produced by jet drops can be modulated by marine biological activity. We show that the chemical composition, organic volume fraction, and ice nucleating ability of submicrometer particles from jet drops differ from those formed from film drops. Thus, the chemical composition of a substantial fraction of submicrometer particles will not be controlled by the composition of the sea surface microlayer, a major assumption in previous studies. This finding has significant ramifications for understanding the factors controlling the mixing state of submicrometer SSA particles and must be taken into consideration when predicting SSA impacts on clouds and climate.

This work addresses the production of aerosol particles from bursting of air bubbles at the water-air interface. Experiments were performed in a laboratory system designed to minimize bubble interactions. Air bubbles of an equivalent spherical radius of ∼3 mm were generated in both real and artificial seawater at temperatures of 0 and 19 °C respectively. Particle concentrations were measured and used to derive particle production per bursting bubble. The particle production in surface seawater from the Bay of Aarhus showed remarkably strong sensitivity to temperature, with ∼40 particles per bursting bubble at 19 °C compared to ∼2300 particles per bubble at 0 °C. A similar effect was observed for bubbles bursting in NaCl solutions. In contrast, the effect of temperature on particle production from artificial seawater was minimal. Further experiments including exclusion of selected inorganic components from artificial seawater point to magnesium and calcium ions as key role players on the effect of temperature. Experiments adding varying amounts of the weak surfactant succinic acid to sodium chloride solutions showed that the influence of temperature on particle production can also be modulated by organic molecules. A complex interplay between inorganic and organic constituents seems to determine the response of particle production to temperature in real seawater. Our study demonstrates that temperature can have a very large (orders of magnitude) effect on the production of particles formed from bubbles bursting at the liquid/air interface, and that chemical composition of the liquid is a controlling parameter for the magnitude of this effect.

The different flow regimes of two‐phase gas‐in‐liquid flow—as it occurs in the shallow conduit of basaltic volcanoes—are characterized by distinct frequency‐size distributions of the liquid and gaseous slugs. Assuming that the ascent of gaseous bubbles is indicated by seismic events, we explore the possibility of inferring the flow regime from the frequency distributions of magnitudes and interevent times. Our data set consists of 20,000 volcanic seismic events recorded in early March 2012 at Villarrica Volcano (Chile), which are commonly attributed to Strombolian activity. One crucial factor is the completeness of the catalog in terms of detectable amplitudes, which we assess using a stochastic simulation of the network output based on statistical properties of the ambient seismicity. Magnitudes, at which we consider the catalog complete, show an exponential occurrence. Yet, the simulation approach indicates that low magnitude events occur indeed more sparsely than expected for an exponential distribution, and that the magnitude distribution does not obey the Gutenberg‐Richter law. Interevent times are log‐normally distributed, which implies a preferred recurrence interval. The distributions are consistent with a slug flow regime. They correlate weakly with the preceding magnitude, suggesting that slugs coalesce while ascending. Plain Language Summary: A common type of volcanic seismic events at Villarrica is thought to result from gas bubbles, which ascend through the liquid magma in the conduit and produce mild volcanic explosions at the surface. We investigated the statistical occurrence of the time between these events and of their size. In contrast to normal earthquakes, the events recur more periodically and the number of smaller events does not increase exponentially, hence confirming their different nature. Laboratory experiments showed that bubbles flowing in a liquid develop characteristic patterns of size and distance between them, which depend on the gas supply rate and the flow ability of the liquid. We qualitatively compared the statistical distributions of event sizes to bubble sizes and the time between the events to the distance between gas bubbles. The result suggests a flow pattern where the gas bubbles span the entire width of the conduit and are several times longer than wide. Such batches of gas are known to produce the typical volcanic activity of Villarrica. Key Points: Seismic transient events are commonly seen as ascending gas slugs but other mechanisms, for example, brittle failure, are conceivableAmplitudes and time between events are approximately log‐normally distributed and thus different from tectonic earthquakesComparison with slug size distributions in gas‐in‐liquid flow suggests a sustained slug flow regime [ABSTRACT FROM AUTHOR]

Basaltic magma becomes more viscous, solid-like (elastic), and non-Newtonian (shear-thinning, non-zero yield stress) as its crystal content increases. However, the rheological effects on bubble bursting and airwave excitation are poorly understood. Here we conduct laboratory experiments to investigate these effects by injecting a bubble of volume V into a refractive index-matched suspension consisting of non-Brownian particles (volumetric fraction ϕ ) and a Newtonian liquid. We show that depending on ϕ and V, airwaves with diverse waveforms are excited, covering a frequency band of f = O (10 - 10 4) Hz. In a suspension of ϕ ≤ 0.3 or in a suspension of ϕ = 0.4 with a V smaller than critical, the bubble bursts after it forms a hemispherical cap at the surface and excites a high-frequency (HF) wave ( f ∼ 1 - 2 × 10 4 Hz) with an irregular waveform, which likely originates from film vibration. However, in a suspension of ϕ = 0.4 and with a V larger than critical, the bubble bursts as soon as it protrudes above the surface, and its aperture opens slowly, exciting Helmholtz resonance with f = O (10 3) Hz. Superimposed on the waveform are an HF wave component excited upon bursting and a low-frequency ( f = O (10) Hz) air flow vented from the deflating bubble, which becomes dominant at a large V. We interpret this transition as a result of the bubble film of a solid-like ϕ = 0.4 suspension, being stretched faster than the critical strain rate such that it bursts by brittle failure. When the Helmholtz resonance is excited by a bursting bubble in a suspension whose surface level is further below the conduit rim, an air column (length L) resonance is triggered. For L larger than critical, the air column resonance continues longer than the Helmholtz resonance because the decay rate of the former becomes less than that of the latter. The experiments suggest that a bubble bursting at basaltic volcanoes commonly excites HF wave by film vibration. The Helmholtz resonance is likely to be excited under a limited condition, but if detected, it may be used to track the change of magma rheology or bubble V, where the V can be estimated from its frequency and decay rate. [ABSTRACT FROM AUTHOR]

We study the bursting of a bubble covered with a surfactant experimentally. We conclude that the bubble bursting takes longer than that of a surfactant-free bubble with the same equilibrium surface tension due to the interfacial elasticity. A tiny bubble is formed at the cavity bottom right before the free surface reversal due to the Marangoni stress. This stress also drives the liquid flow that makes the jet escape from the end-pinching mechanism for a certain surfactant concentration interval. A diminutive liquid droplet manages to escape from the jet for a sufficiently large surfactant concentration. This droplet is much smaller than its surfactant-free counterpart but moves at a similar speed. The surfactant molecules are convected toward the jet tip so that the monolayer covering the first-emitted droplet is practically saturated. • The bubble bursting of a bubble covered with an insoluble surfactant takes longer than that of a surfactant-free bubble. • The jet escape from the end-pinching mechanism for a certain surfactant concentration interval. • The surfactant molecules are convected toward the jet tip so that the monolayer covering the first-emitted droplet is practically saturated. [ABSTRACT FROM AUTHOR]

This work addresses the production of aerosol particles from bursting of air bubbles at the water-air interface. Experiments were performed in a laboratory system designed to minimize bubble interactions. Air bubbles of an equivalent spherical radius of ~3mm were generated in both real and artificial seawater at temperatures of 0 and 19 °C respectively. Particle concentrations were measured and used to derive particle production per bursting bubble. The particle production in surface seawater from the Bay of Aarhus showed remarkably strong sensitivity to temperature, with ~40 particles per bursting bubble at 19 °C compared to ~2300 particles per bubble at 0°C. A similar effect was observed for bubbles bursting in NaCl solutions. In contrast, the effect of temperature on particle production from artificial seawater was minimal. Further experiments including exclusion of selected inorganic components from artificial seawater point to magnesium and calcium ions as key role players on the effect of temperature. Experiments adding varying amounts of the weak surfactant succinic acid to sodium chloride solutions showed that the influence of temperature on particle production can also be modulated by organic molecules. A complex interplay between inorganic and organic constituents seems to determine the response of particle production to temperature in real seawater. Our study demonstrates that temperature can have a very large (orders of magnitude) effect on the production of particles formed from bubbles bursting at the liquid/air interface, and that chemical composition of the liquid is a controlling parameter for the magnitude of this effect. [ABSTRACT FROM AUTHOR]

Bubble bursting is a common phenomenon in many industrial and natural processes, plays an important role in mediating mass transfer across the water-air interface. But the interplay between bubbles and pathogens remains unclear and the mechanisms of virus aerosolization by the bubble properties have not been well studied. The main objective of this study was to evaluate the water-to-air transfer of viruses by bubbles of different sizes. Unlike the dominant view of smaller bubbles less bioaerosols, it was found that the smaller bubbles could generate significantly more viral aerosols regardless of the virus species (Phi6, MS2, PhiX174, and T7), when the Sauter mean bubble diameters were between 0.56 and 1.65 mm under constant aeration flow rate. The mechanism studies denied the possibilities of more aerosols or better dispersion of viruses in the aerosols generated by the smaller bubbles. However, deeper bubbling could transfer more viruses to the air for MS2, PhiX174, and T7. Their concentrations in aerosols were linearly related to the bubbling depth for these non-enveloped viruses, which demonstrates the bubble-scavenging effect as a main mechanism except for the enveloped virus Phi6. Whereas, unlike these three non-enveloped viruses, Phi6 could survive relatively better in the aerosols generated from the smaller bubbles, though the enhancement of aerosolization by the smaller bubbles was much larger than the improvement of survival. Other mechanisms still remain unknown for this enveloped virus. This study suggests that the attempt of decreasing the bubble size in aeration tank of the wastewater treatment plant might significantly increase the solubility of oxygen as well as the risk of viral aerosols. [Display omitted] • Release of viral aerosols is linearly correlated with virus concentration in bulk. • Small bubbles produce viral aerosols more efficiently than large bubbles. • Bubble scavenging involves in bubble-mediated viral aerosol production. • Flotation kinetics partially explain the effect of bubble on viral aerosol release. [ABSTRACT FROM AUTHOR]

Airborne dispersal of microorganisms influences their biogeography, gene flow, atmospheric processes, human health and transmission of pathogens that affect humans, plants and animals. The extent of their impact depends essentially on cell-survival rates during the process of aerosolization. A central factor for cell-survival is water availability prior to and upon aerosolization. Also, the ability of cells to successfully cope with stress induced by drying determines their chances of survival. In this study, we used the ice-nucleation active, plant pathogenic Pseudomonas syringae strain R10.79 as a model organism to investigate the effect of drying on cell survival. Two forms of drying were simulated: drying of cells in small droplets aerosolized from a wet environment by bubble bursting and drying of cells in large droplets deposited on a surface. For drying of cells both in aerosol and surface droplets, the relative humidity (RH) was varied in the range between 10 and 90%. The fraction of surviving cells was determined by live/dead staining followed by flow cytometry. We also evaluated the effect of salt concentration in the water droplets on the survival of drying cells by varying the ionic strength between 0 and 700 mM using NaCl and sea salt. For both aerosol and surface drying, cell survival increased with decreasing RH (p < 0.01), and for surface drying, survival was correlated with increasing salt concentration (p < 0.001). Imaging cells with TEM showed shrunk cytoplasm and cell wall damage for a large fraction of aerosolized cells. Ultimately, we observed a 10-fold higher fraction of surviving cells when dried as aerosol compared to when dried on a surface. We conclude that the conditions, under which cells dry, significantly affect their survival and thus their success to spread through the atmosphere and colonize new environments as well as their ability to affect atmospheric processes.

Bubble bursting is a primary source of marine aerosols, yet little is known about particle emissions due to the bubble bursting in slicks containing oil‐dispersant mixtures. In this study, bubbles with mode sizes of 86 μm (denoted as small), 178 μm (medium), and 595 μm (large) are injected into a seawater column covered by slicks of crude oil, pure dispersant, and dispersant premixed with oil at a ratio of 1:25. The aerosol size distributions are monitored in the 0.5‐ to 20‐μm and 10‐ to 380‐nm ranges both in clean and ambient air environments. In ambient air, a tenfold increase in submicron particle concentration occurs when large bubbles burst on slicks of 500‐μm dispersant premixed with oil at a ratio of 1:25 oil or 50‐μm pure dispersant. Yet, in multiple tests performed at different ambient particle concentrations, the elevated size distributions persistently maintain the same shape as that of the ambient air. In contrast, smaller bubbles and tests not involving dispersants do not cause such an increase. Nanodroplets are also generated by large bubbles in particle‐free air, but their concentrations are much lower. All plumes generate micron‐sized aerosols, but trends vary. For the same contaminant, the microdroplet concentration decreases with increasing slick thickness. Particularly striking is a reduction of 2 orders of magnitude in the microdroplet concentration when medium and small bubbles burst on 500‐μm crude oil slicks. Chemical analysis of air and particulates collected from filters sampling the particles confirms the presence of airborne oil above the slicks. Key Points: Bursting of bubbles in slicks containing crude oil and dispersant mixtures aerosolizes the oil, generating microdroplets and nanodropletsIn ambient air, a tenfold increase in submicron particle concentration occurs when large bubbles burst on slicks containing dispersantsMicrodroplets are generated for all bubble plumes and slick types, but their concentration decreases with increasing slick thickness [ABSTRACT FROM AUTHOR]

Gas–liquid phase separation under microgravity conditions or in small‐scale fluidic systems represents a challenge for two‐phase liquid‐continuous systems. In this study, capillary channels formed by 3‐mm diameter stretched stainless‐steel springs coated with a commercial superhydrophobic coating are used to remove air bubbles from water. A single channel is capable of absorbing a stream of 3.7‐mm diameter bubbles impinging on a small area of the channel at a rate of over 50 bubbles/s. High‐permeability walls lead to fast individual absorption events (4 ms for 2.5‐mm bubbles) where bubble collapse time is limited by the inertia of the surrounding liquid. A horizontal three‐channel array has been shown capable of absorbing impinging bubbles from a sparger at superficial gas velocities of 0.03 m/s. The ultimate capacity of the 3‐mm diameter channel is predicted to be much higher than what could be measured with the existing apparatus. © 2018 American Institute of Chemical Engineers AIChE J, 65: 1340–1354, 2019 [ABSTRACT FROM AUTHOR]

High-fidelity large eddy simulation is carried out for the flow field around a NACA-0012 aerofoil at Reynolds number of 9 × 104, Mach number of 0.4, and various angles of attack around the onset of stall. The laminar separation bubble is formed on the suction surface of the aerofoil and is constituted by the reattached shear layer. At these conditions, the laminar separation bubble is unstable and switches between a short bubble and an open bubble. The instability of the laminar separation bubble triggers a low-frequency flow oscillation. The aerodynamic coefficients oscillate accordingly at a low frequency. The lift and the drag coefficients compare very well to recent high-accuracy experimental data, and the lift leads the drag by a phase shift of π=2. The mean lift coefficient peaks at the angle of attack of 10:25 °, in total agreement with the experimental data. The spectra of the lift coefficient does not show a significant low-frequency peak at angles of attack lower than or equal the stall angle of attack (10:25 °). At higher angles of attack, the spectra show two low-frequency peaks and the low-frequency flow oscillation is fully developed at the angle of attack of 11:0 °. The behaviour of the flow-field and changes in the turbulent kinetic energy over one low-frequency flow oscillation cycle are described qualitatively. [ABSTRACT FROM AUTHOR]

Microplastics are globally ubiquitous in marine environments, and their concentration is expected to continue rising at significant rates as a result of human activity. They present a major ecological problem with well-documented environmental harm. Sea spray from bubble bursting can transport salt and biological material from the ocean into the atmosphere, and there is a need to quantify the amount of microplastic that can be emitted from the ocean by this mechanism. We present a mechanistic study of bursting bubbles transporting microplastics. We demonstrate and quantify that jet drops are efficient at emitting microplastics up to 280 μ m in diameter and are thus expected to dominate the emitted mass of microplastic. The results are integrated to provide a global microplastic emission model which depends on bubble scavenging and bursting physics; local wind and sea state; and oceanic microplastic concentration. We test multiple possible microplastic concentration maps to find annual emissions ranging from 0.02 to 7.4-with a best guess of 0.1-mega metric tons per year and demonstrate that while we significantly reduce the uncertainty associated with the bursting physics, the limited knowledge and measurements on the mass concentration and size distribution of microplastic at the ocean surface leaves large uncertainties on the amount of microplastic ejected., (© The Author(s) 2023. Published by Oxford University Press on behalf of National Academy of Sciences.)

Natural aquatic environments such as oceans, lakes, and rivers are home to a tremendous diversity of microorganisms. Some may cross the air-water interface within droplets and become airborne, with the potential to impact the Earth's radiation budget, precipitation processes, and spread of disease. Larger droplets are likely to return to the water or adjacent land, but smaller droplets may be suspended in the atmosphere for transport over long distances. Here, we report on a series of controlled laboratory experiments to quantify wind-driven droplet production from a freshwater source for low wind speeds. The rate of droplet production increased quadratically with wind speed above a critical value (10-m equivalent 5.7 m/s) where droplet production initiated. Droplet diameter and ejection speeds were fit by a gamma distribution. The droplet mass flux and momentum flux increased with wind speed. Two mechanisms of droplet production, bubble bursting and fragmentation, yielded different distributions for diameter, speed, and angle. At a wind speed of about 3.5 m/s, aqueous suspensions of the ice-nucleating bacterium Pseudomonas syringae were collected at rates of 283 cells m-2 s-1 at 5 cm above the water surface, and at 14 cells m-2 s-1 at 10 cm above the water surface. At a wind speed of about 4.0 m/s, aqueous suspensions of P. syringae were collected at rates of 509 cells m-2 s-1 at 5 cm above the water surface, and at 81 cells m-2 s-1 at 10 cm above the water surface. The potential for microbial flux into the atmosphere from aquatic environments was calculated using known concentrations of bacteria in natural freshwater systems. Up to 3.1 × 104 cells m-2 s-1 of water surface were estimated to leave the water in potentially suspended droplets (diameters <100 mm). Understanding the sources and mechanisms for bacteria to aerosolize from freshwater aquatic sources may aid in designing management strategies for pathogenic bacteria, and could shed light on how bacteria are involved in mesoscale atmospheric processes. [ABSTRACT FROM AUTHOR]

The oceans represent a significant global source of atmospheric aerosols. Sea spray aerosol (SSA) particles comprise sea salts and organic species in varying proportions. In addition to size, the overall composition of SSA particles determines how effectively they can form cloud droplets and ice crystals. Thus, understanding the factors controlling SSA composition is critical to predicting aerosol impacts on clouds and climate. It is often assumed that submicrometer SSAs are mainly formed by film drops produced from bursting bubble-cap films, which become enriched with hydrophobic organic species contained within the sea surface microlayer. In contrast, jet drops formed from the base of bursting bubbles are postulated to mainly produce larger supermicrometer particles from bulk seawater, which comprises largely salts and water-soluble organic species. However, here we demonstrate that jet drops produce up to 43% of total submicrometer SSA number concentrations and that the fraction of SSA produced by jet drops can be modulated by marine biological activity. We show that the chemical composition, organic volume fraction and ice nucleating ability of submicrometer particles from jet drops differ from those formed from film drops. Thus, the chemical composition of a substantial fraction of submicrometer particles will not be controlled by the composition of the sea surface microlayer, a major assumption in previous studies. This finding has significant ramifications for understanding the factors controlling the mixing state of submicrometer SSA particles and must be taken into consideration when predicting SSA impacts on clouds and climate. [ABSTRACT FROM AUTHOR]

In the present work, large eddy simulations of the flow field around a NACA-0012 aerofoil near stall conditions are performed at a Reynolds number of 5 × 104, Mach number of 0.4, and at various angles of attack. The results show the following: at relatively low angles of attack, the bubble is present and intact; at moderate angles of attack, the laminar separation bubble bursts and generates a global low-frequency flow oscillation; and at relatively high angles of attack, the laminar separation bubble becomes an open bubble that leads the aerofoil into a full stall. Time histories of the aerodynamic coefficients showed that the low-frequency oscillation phenomenon and its associated physics are indeed captured in the simulations. The aerodynamic coefficients compared to previous and recent experimental data with acceptable accuracy. Spectral analysis identified a dominant low-frequency mode featuring the periodic separation and reattachment of the flow field. At angles of attack α ≤ 9.3°, the low-frequency mode featured bubble shedding rather than bubble bursting and reformation. The underlying mechanism behind the quasi-periodic self-sustained low-frequency flow oscillation is discussed in detail. [ABSTRACT FROM PUBLISHER]

Cell damage, one of critical issues in the bioreactor design for animal cell culture, is caused mainly from the bubble bursting at the free surface subjected to strong extensional flows. In this work, extensive computational studies are performed to investigate bubble bursting process in great details. Extensive numerical simulations are performed for a wide range of bubble diameters (from 0.5 to 6 mm) and the surface tension values (from 0.03 to 0.072 N/m), with which effects of the bubble size and surfactant (PF68) concentration on the hydrodynamic stress are investigated. For all the cases, the maximum extensional stress appears at the instance when receding films impact each other at the bottom of the bubble. A model equation based on numerical simulations is presented to predict the maximum extensional stress as a function of the bubble diameter and the surface tension. The bubble diameter has turned out to contribute significantly the maximum hydrodynamic extensional stress. In addition, the bubble collapsed time and the affected volume around a bubble subjected to the critical extensional stress are investigated. The extensional stress estimation is reported as a function of the bubble size and the surface tension. The influence of the bubble size on the maximum stress dominates and extensional stress reaches up to the order of 10 Pa for bubble size of 0.5 mm. [ABSTRACT FROM AUTHOR]

Directly-irradiated fluidized bed solar reactors are very promising in the context of solar chemistry and concentrated solar power (CSP) applications. With proper choice of the bed solids, fluidized bed reactors can be operated at fairly high process temperatures that enable thermochemical storage with high energy density and production of solar chemicals and fuels. Bed surface overheating upon irradiation is one key to the efficiency of the fluidized bed as thermal receiver and may be responsible for sintering and/or degradation of the fluidized particles. Tailoring the hydrodynamics of the bed close to the region where the incident power is concentrated may disclose effective measures to improve the interaction between the incident radiative flux and the bed and mitigate bed surface overheating. In the present study radiative heat transfer from a concentrated simulated solar radiation source to a fluidized bed is investigated by time-resolved infrared mapping of the bed surface temperature. A fluidized bed of silicon carbide particles (0.127 mm), whose cross-sectional area is 0.78 × 0.78 m, was directly irradiated by highly concentrated simulated solar radiation, emitted by a 4 kW el short-arc Xe lamp coupled with an elliptical reflector. The experimental apparatus is also equipped with a movable nozzle coupled with a bubble generation system located coaxially to the concentrated simulated solar beam. The interaction of the concentrated radiative flux with the fluidized particles moving under the action of bubble bursting was assessed by characterizing the time-resolved bed surface temperature as the fluidization gas velocity was varied. The effect of localized generation of bubbles was also investigated by injecting chains of multiple bubbles from the nozzle located at variable distance from the bed surface. [ABSTRACT FROM AUTHOR]

Boundary element method has been used to simulate the fluid motion generated by a bubble near the free surface. When its internal pressure is high relative to the ambient pressure and submergence is relatively small, the bubble will burst through the free surface. A numerical model has been introduced for such a process. A methodology has been developed to deal with the sharp corner formed when the bubble surface merges with the free surface after it breaks up. Detailed simulations are made to include the initial motion and migration of the bubble, its bursting through the free surface, rising of the free surface in the form of a long and thin jet with a much higher second peak. Discussions of physics for these observations have been given. The effects of various physical parameters such as, the ratio of internal pressure to ambient pressure, the ratio of the submergence of the bubble to its initial radius and Froude number are investigated. [ABSTRACT FROM AUTHOR]

The fundamental aspects of rising argon bubbles in molten metal for a laminar flow were investigated by numerical simulations. The Volume-of-Fluid (VOF) model was used to track the interface between argon and liquid metal. The process of a bubble rising in the molten metal includes two steps, one is the bubble rising inside the liquid, and the other one is the bubble rising across the liquid surface. The bubbling dynamics inside the liquid phase was studied in terms of the bubble's trajectory, shape and terminal velocity over a wide range of bubble diameters. It shows that ≈3-10mm bubbles rise in a spiral way with strong instabilities by changing their instantaneous shapes, while ≈10-20mm bubbles rise rectilinearly and their shapes are kept almost steady. All these bubbles' terminal velocities are around 0.3 m s−1, which are in accordance with literature data. For a bubble with a specific size, small metal droplets can be formed due to the bubble bursting on the free surface. In a situation when the top surface of the bubble is ruptured, the remains of the bubble will collapse and jet droplets can be formed. Simulations of jet droplets were qualitatively analyzed. It shows that when the surface tension is 1.4 N m−1, the critical bubble size is 9.3 mm. Also, the ejection is found to increase with an increased surface tension value, unless a critical bubble size is reached. [ABSTRACT FROM AUTHOR]

• Microplastics can be transferred by bubble bursting at the water–air interface. • First study on plastic-gel enhancement of microplastic transfer from water to air. • High salinity, gel concentration, and viscosity induced microplastic transfer to air. • Plastic-gel formation via cation-linking bridges modulates microplastic transfer capacity. The pervasiveness of microplastics (MPs) in global oceans is raising concerns about their adverse impacts on ecosystems. The mechanistic understanding of MP transport is critical for evaluating its fate, flux, and ecological risks specifically. Currently, bubble bursting is believed to represent an important route for MP transfer from sea surfaces to the atmosphere. However, the detailed mechanisms of the complex physico-chemical interactions between MPs, water composition, and gel particles in the air–sea interface remain unknown. Our results suggested three steps for MP transfer between air–sea phases: (1) MPs incorporating into gel aggregates in the water column; (2) further accumulation of plastic-gel aggregate in the surface layer phase; finally (3) ejection of aggregates from the sea when bubbles of trapped air rise to the surface and burst. The water composition (e.g., high salinity, gel concentration and viscosity) can modulate plastic-gel aggregation and subsequent transport from water to the atmosphere. The possible mechanism may be closely tied to the formation of plastic-gel via cation-linking bridges, thereby enhancing plastic-gel ejection into air. Collectively, this work offers unique insights into the role of marine plastic-gels in determining MP fate and transport, especially at air–sea interfaces. The data also provide a better understanding of the corresponding mechanism that may explain the fates of missing plastics in the ocean. [Display omitted] [ABSTRACT FROM AUTHOR]

Shear layer development over a thick flat plate with a semi-circular leading edge is investigated for a range of angles of attack under different pressure gradients for a Reynolds number of 2.44×105 (based on chord and free-stream velocity). The characteristics of the separated shear layer are very well documented through a combination of surface pressure measurement and smoke flow visualization. The instability of the separated layer occurs because of enhanced receptivity of perturbations leading to the development of significant unsteadiness and three-dimensional motions in the second-half of the bubble. The onset of separation, transition and the point of reattachment are identified for varying angles of attack and pressure gradients imposed by tail flap deflections. The data concerning bubble length, laminar portion and reattachment points agree well with the literature. [ABSTRACT FROM AUTHOR]

Ocean spray aerosol formed by bubble bursting are at the core of a broad range of atmospheric processes: they are efficient cloud condensation nuclei and carry a variety of chemical, biological, and biomass material from the surface of the ocean to the atmosphere. The origin and composition of these aerosols is sensibly controlled by the detailed fluid mechanics of bubble bursting. This perspective summarizes our present-day knowledge on how bursting bubbles at the surface of a liquid pool contribute to its fragmentation, namely to the formation of droplets stripped from the pool, and associated mechanisms. In particular, we describe bounds and yields for each distinct mechanism, and the way they are sensitive to the bubble production and environmental conditions. We also underline the consequences of each mechanism on some of the many air-sea interactions phenomena identified to date. Attention is specifically payed at delimiting the known from the unknown and the certitudes from the speculations., (© The Author(s) 2022. Published by Oxford University Press on behalf of the National Academy of Sciences.)