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While seagrass meadows are perceived to be pertinent blue carbon reservoirs, they also potentially release methane (CH4) into the atmosphere. Seasonal and diurnal variations in CH4 emissions from a subtropical hypersaline lagoon dominated by Halodule wrightii in southern Texas, USA, on the northwest coast of the Gulf of Mexico were investigated. Dissolved CH4 concentrations decreased in the daytime and increased overnight during the diel observation period, which could be explained by photosynthesis and respiration of seagrasses. Photosynthetic oxygen was found to significantly reduce CH4 emissions from seagrass sediment. Diffusive transport contributed slightly to the release of CH4 from the sediment to the water column, while plant mediation might be the primary mechanism. The diffusive CH4 flux at the sea-air interface was 12.3–816.2 µmol/m2 d, over the range of the sea-air fluxes previously reported from other seagrass meadows. This was related to relatively higher dissolved CH4 concentrations (11.6–258.2 nmol/L) in a mostly closed lagoon with restricted water exchange. This study emphasizes seagrass meadows in the subtropical hypersaline lagoon as a source of atmospheric CH4, providing insights into the interactions between seagrass ecosystems and methane dynamics, with potential implications for seagrass meadow management and conservation efforts. [ABSTRACT FROM AUTHOR]

The high-intensity partial pressure of CO2 (pCO2) in karst underground rivers rapidly releases in open-flow sections. This is an important process affecting the global karst carbon cycle. This study focuses on the diurnal variation patterns and driving factors of CO2 exchange flux at the water–air interface in the open-flow sections of typical karst underground rivers in southwestern China. The inorganic carbon in water and water–air interface exchange fluxes are observed. Three representative survey stations, i.e., the outlet of the underground river (Q1), the river sections without submerged plants (H1), and the river sections with submerged plants (H2), are selected to study the CO2 exchange process and its influencing factors. The results show that the CO2 release flux at Q1 exhibits high pressure in the daytime and low pressure in the nighttime, while H1 and H2 exhibit the opposite pattern. The photosynthesis of submerged plants significantly inhibits the carbon release flux of the river, and in the river sections where submerged plants are distributed, their biological effects have inhibited approximately 0.131 Tg C/yr of carbon emissions. This study emphasizes the significant contribution of submerged plants in restraining the release of CO2, thereby promoting carbon sequestration and storage in karst water systems. [ABSTRACT FROM AUTHOR]

As a volatile organic compound, nitrobenzene has high vapor pressure and low boiling point, and it is very volatile when it enters the water body and enters the air. The mass transfer of VOCs at the water-air interface is a complex process of transboundary transport. In this paper, the effects of water temperature, interface turbulence, surfactant concentration, and humic acid concentration on the volatilization of nitrobenzene at the water-air interface were investigated. Under the influence of temperature, the volatilization of nitrobenzene accorded with the first-order kinetic equation. When the temperature increased from 5 ℃ to 25 ℃, the volatilization rate of nitrobenzene increased by 2.03 times. Temperature for volatilization rate constant was in accordance with the Arrhenius equation. The water-gas distribution of volatile organic compounds was in accordance with the Boltzman equation. Under the same temperature conditions, when the agitating intensity increased from 0 r/min to 250 r/min, the volatilization rate constant of nitrobenzene increased by 1.51 times. When the surfactant is greater than the critical micelle concentration, the volatilization rate constant of nitrobenzene decreases with the increase of surfactant. When the concentration of humic acid increased from 100 mg/L to 500 mg/L, the half-life increased by 1.14 h, and the volatilization rate decreased by 1.14 h, reduced by 17%. The results showed that the increase of temperature and the intensification of stirring had a significant promoting effect on the volatilization of nitrobenzene, while the surfactant and humic acid both played an inhibitory effect on the volatilization of nitrobenzene. [ABSTRACT FROM AUTHOR]

The significance of CO2 emissions at the water–air interface from inland water bodies in the global carbon cycle has been recognized and is being studied more and more. Although it is important to accurately assess CO2 emission flux in a catchment, little research has been carried out to investigate the spatio-temporal variations in CO2 emissions in view of a water continuum. Here, we systematically compared the differences and control factors of CO2 degassing across the water–air interface of a spring–river–lake continuum in the discharge area of Baotuquan Spring in July 2017, which is a typical temperate karst spring area in Jinan city, northern China, using hydrogeochemical parameters, stable carbon isotope values, and CO2 degassing flux. Affected by the pCO2 concentration gradient between the water and ambient air, the spring water showed a high CO2 degassing flux (166.19 ± 91.91 mmol/(m2 d)). After the spring outlet, the CO2 degassing flux in the spring-fed river showed a slight increase (181.05 ± 155.61 mmol/(m2 d)) due to river flow rate disturbance. The river flow rate was significantly reduced by the "blockage" of the lake, which promoted the survival and reproduction of phytoplankton and provided favorable conditions for aquatic plant photosynthesis, increasing the plankton biomass in the lake to 3383.79 × 104/L. In addition, the significant decrease in the dissolved inorganic carbon (DIC) concentration and the increase in the δ13CDIC values in the lake also indicated that the photosynthesis of the lake's aquatic plants resulted in a significant decrease in the pCO2 concentration, thus limiting the amount of CO2 off-gassing (90.56 ± 55.03 mmol/(m2 d)). [ABSTRACT FROM AUTHOR]

Extreme impedance mismatch causes sound insulation at water–air interfaces, limiting numerous cross‐media applications such as ocean‐air wireless acoustic communication. Although quarter‐wave impedance transformers can improve transmission, they are not readily available for acoustics and are restricted by the fixed phase shift at full transmission. Here, this limitation is broken through impedance‐matched hybrid metasurfaces assisted by topology optimization. Sound transmission enhancement and phase modulation across the water–air interface are achieved independently. Compared to the bare water–air interface, it is experimentally observed that the average transmitted amplitude through an impedance‐matched metasurface at the peak frequency is enhanced by ≈25.9 dB, close to the limit of the perfect transmission 30 dB. And nearly 42 dB amplitude enhancement is measured by the hybrid metasurfaces with axial focusing function. Various customized vortex beams are experimentally realized to promote applications in ocean‐air communication. The physical mechanisms of sound transmission enhancement for broadband and wide‐angle incidences are revealed. The proposed concept has potential applications in efficient transmission and free communication across dissimilar media. [ABSTRACT FROM AUTHOR]

The temporal and spectral characteristics of nanosecond laser-induced underwater acoustic shockwave for varying input laser energies are obtained. The underwater acoustic shockwave results in reflection and transmission at the water-air interface due to enormous acoustic impedance mismatching between the water and air. With increasing input laser energies, the peak-to-peak overpressures of the reflected and transmitted signals increase linearly. The relationship between the reflected and transmitted acoustic energy coefficients across the water-air interface and the incident laser energy is studied. The advantage of power spectral density in classifying reflected and transmitted signals is based on their peak frequency and full width at half maximum (FWHM). With increasing incident laser energy, the FWHM decreases, and the area under the curve increases for reflected and transmitted signals. In addition to the experimental investigation, finite element analysis (FEA) was used to visualize underwater acoustic signal reflection and transmission at the interface. Experimental data is used as input for FEA in visualizing the acoustic signal in the spatio-temporal domain at the water-air interface. These studies could also be useful for understanding the blast wave evolution of underwater explosions and their interactions at the interfaces. [ABSTRACT FROM AUTHOR]

This paper presents the results of detailed studies of the processes of droplet formation and its characteristics under conditions of artificially induction of a bag-breakup fragmentation event. A shadow imaging method was used in combination with the high-speed video filming of the side-view fragmentation process. Trajectories and ejection velocity characteristics of the formed droplets are determined by identifying particles in consecutive frames with combined use of Particle Imaging Velocimetry (PIV) and Particle Tracking Velocimetry (PTV). Based on the results of trajectory processing, the distributions of droplet velocities for the selected regions are obtained, and estimates of the ejection velocities at various heights are proposed. [ABSTRACT FROM AUTHOR]

The design of grouting engineering in practice is either based on conventional soil mechanics or empirical procedures ignoring the effect of degree of saturation (water content). In this study, a series of laboratory-pressurized grouting tests were conducted on unsaturated sand to reveal the influence of soil water content on the grouting characteristics. With combination of direct shear tests at constant water content, water retention tests as well as microscopy observations, the mechanisms that controlling the strength and in turn the grouting characteristics in unsaturated sand were interpreted from the perspective of water–air interface. It was found that the non-monotonic phenomena of grouting characteristics (injectability and diffusion characteristics) with increasing water content were strongly dependent on the shear strength, which is influenced by the apparent cohesion induced by capillary mechanisms relating to the water–air interface. The threshold value of the injectability and diffusion pattern is corresponding to the boundary of the two transition zones (two different desaturation mechanisms) in the water retention curve. In the primary transition zone, the water phase is interconnected with air bulbs entrapped. With the drainage of bulk water in the large pores, the amount of water menisci increases, generating larger and larger surface tension force between particles. Therefore, less and less grout was injected as the bearing capacity and shear strength increase. However, in the second transition zone, with the drainage of menisci water, the menisci area of each pores decreases, inducing less and less surface tension force. Thus, more and more grout was injected as the bearing capacity and shear strength decrease. It is hoped that the work in this study will facilitate researching the grouting mechanisms in unsaturated soil, thus optimizing the grouting parameters in engineering practice. [ABSTRACT FROM AUTHOR]

In this study, a pressurized, water-filled impedance tube (WFIT) was developed to measure the reflection coefficients of sound-absorbing materials under various hydrostatic pressures. The developed WFIT was calibrated using a two-microphone, three-parameter calibration method (3PCM). The accuracy and repeatability of the measured reflection coefficients for the water–air interface in the WFIT were determined by comparing these coefficients with corresponding theoretical reflection coefficients. The WFIT was then used to measure the acoustic reflection coefficient of a porous rubber specimen on three dates, and the corresponding measurement results exhibited satisfactory repeatability. The aforementioned impedance tube was also used to measure the reflection coefficient of a porous rubber specimen under a hydrostatic pressure of 4 Patm three times on the same day, and one time each on three days, using the same experimental setup and measurement procedure. The results obtained in the aforementioned tests also exhibited satisfactory repeatability. Finally, the WFIT was used to measure the reflection coefficients of porous rubber specimens with various thicknesses under different hydrostatic pressures. The results of this study indicate that the developed WFIT calibrated with the 3PCM can achieve suitable repeatability in the measurement of the reflection coefficients of sound-absorbing materials under various hydrostatic pressures. [ABSTRACT FROM AUTHOR]

Using a similar approach as Korteweg and de Vries, [19], we obtain periodic solutions expressed in terms of the Jacobi elliptic function cn, [3], for the self-focusing and defocusing one-dimensional cubic nonlinear Schrödinger equations. We will show that solitary wave solutions are recovered through a limiting process after the elliptic modulus of the Jacobi elliptic function cn that describes the periodic solutions for the self-focusing nonlinear Schrödinger model. [ABSTRACT FROM AUTHOR]

The white colour morph of the black bear (Ursus americanus kermodei) occurring on islands on the coast of British Columbia, western Canada, captures more salmon (Oncorhynchus spp.) than does the black morph and is hypothesized to have reduced contrast against the sky from the visual perspective of the salmon. We tested this hypothesis in a natural salmon stream by recording the number and proximity of chum salmon (Oncorhynchus keta) approaches (N = 1617 fish, 91 trials) towards life-size bear models differing in body and leg coloration under a mixed forest-sky canopy. Although salmon approached the white models at a much higher rate than black models, consistent with camouflage, we found greater abrupt evasions to the black models, largely independent of their contrast against the above-surface or below-surface backgrounds. Upward-facing sub-surface video-imaging through the rippled water-air interface indicated major visual fragmentation of the model's integrity. We suggest that increased evasiveness to black models reflects an evolutionary response due to 3+ million years of trophic interaction between salmon and bears, and that the major differences between calm vs. rippled conditions through the optical cone (Snell's window) at the water-air interface remains a largely unexplored theme in assessing foraging preferences and adaptive coloration within and among species using the water-air interface. [ABSTRACT FROM AUTHOR]

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While animals generally move through the aquatic environment by propelling themselves through the water or walking on submerged substrates, some have evolved the unique capacity to move along the water–air interface. This is because cohesive forces between water molecules cause the surface to be in tension, providing a physical substrate that can be used for support and pushed against for horizontal movement. Herein, we report on the use of the underside of the water's surface for locomotion in an unidentified Australian beetle (likely family: Hydrophilidae). Field observations revealed that the beetle was able to rest and move along the surface of an ephemeral pool inverted without penetrating the surface above. The beetle possessed a layer of trapped air along its abdomen and moved forward by walking, with deformations of the water's surface apparent each time a leg made contact. This appears to be the first visual evidence illustrating the ability of aquatic beetles to directly rest and walk on the underside of the water's surface. We propose that the air bubbles located on the abdomen and/or legs may be providing the upward force necessary for the beetle to be pushed against the underside of the water's surface. The water's surface seems to be acting as a support for the beetle, with the force needed for forward movement generated by the beetle's legs as each transition between the stance and swing phases of walking. Our work adds to the small number of observations of animals using the underside of the water's surface to move and highlights the likely importance of surface tension for locomotion on either side of the water–air interface. [ABSTRACT FROM AUTHOR]

It was previously shown that hydrogen peroxide (H2O2) is spontaneously produced in micrometer-sized water droplets (microdroplets), which are generated by atomizing bulk water using nebulization without the application of an external electric field. Here we report that H2O2 is spontaneously produced in water microdroplets formed by dropwise condensation of water vapor on low-temperature substrates. Because peroxide formation is induced by a strong electric field formed at the water-air interface of microdroplets, no catalysts or external electrical bias, as well as precursor chemicals, are necessary. Time-course observations of the H2O2 production in condensate microdroplets showed that H2O2 was generated from microdroplets with sizes typically less than ∼10 μm. The spontaneous production of H2O2 was commonly observed on various different substrates, including silicon, plastic, glass, and metal. Studies with substrates with different surface conditions showed that the nucleation and the growth processes of condensate water microdroplets govern H2O2 generation. We also found that the H2O2 production yield strongly depends on environmental conditions, including relative humidity and substrate temperature. These results show that the production of H2O2 occurs in water microdroplets formed by not only atomizing bulk water but also condensing water vapor, suggesting that spontaneous water oxidation to form H2O2 from water microdroplets is a general phenomenon. These findings provide innovative opportunities for green chemistry at heterogeneous interfaces, self-cleaning of surfaces, and safe and effective disinfection. They also may have important implications for prebiotic chemistry. [ABSTRACT FROM AUTHOR]

Researchers have a great interest in determining the characteristics of shock wave propagation by an underwater explosion. But it involves complex physical processes near boundaries during the underwater explosion. The major issues are explosions, fluid interactions, fluid–structure interactions, and shock wave propagation. In general, the underwater explosion leads to shock wave propagation and it moves toward two types of boundaries. The first one is the free surface of the water, and the second is fluid–structure interaction. The underwater explosion also deals with bulk cavitation behind shock waves. This paper deals with the shock wave propagation and bulk cavitation because of the underwater explosion near the free surface. The computation and analytical results of the underwater explosion were discussed and compared. Also, polynomial and Mie–Gruneisen (shock) equation of state (EOS) of water was implemented to determine the incompressible nature of water and to get realistic results. The results of both the EOS of water were compared against each other and validated against analytical results. These results show that the equation of state of water and water–air interface has a significant influence on the characteristics of the shock wave propagation and unsteady cavitation near the free surface. [ABSTRACT FROM AUTHOR]

The effect of anomalous transparency on the passage of sound from water to air is considered. A mathematical model is used to calculate the coefficient of pressure transmission through the water–air interface taking into account the influence of inhomogeneous plane waves. The features of the acoustic field in air are shown for an underwater source located at a distance from the media interface smaller than the wavelength. The theoretical dependences of the pressure transmission coefficient on the wave distance from the source to the interface are given. The conditions and methods for measuring the coefficient of transmission of spherical waves through a planar interface are described. The experimental dependences of the pressure transmission coefficient on the radiation frequency and depth of the source are presented. Oscillograms of acoustic signals transmitted from water to air are shown. [ABSTRACT FROM AUTHOR]

A storage tunnel system is the critical infrastructure of urban drainage systems, in which the rapid filling of water and release of trapped air can lead to the "geyser" phenomenon. This may cause serious damage, threatening both system operation and personal safety. In this paper, a detailed experimental study was carried out based on synchronous recorded video images and digital image processing technology. According to experimental observations, gas-flow geysers and surge-type geysers were analyzed deeply. The former is caused by high-speed gas flow and is accompanied by a pressure drop; the latter is caused by surge pressure and is accompanied by a pressure increase. The free surface velocity of the gas-flow geyser is mainly affected by the external pressure, the air volume, the diameter of the shaft, and the height of the water column, and the geyser phenomenon cannot occur when the air column is mainly dominated by buoyancy. Based on dimensional analysis and data fitting, this paper presents the empirical formula for the free surface velocity and the interface net velocity and puts forward the critical occurrence conditions for the gas-flow geyser. [ABSTRACT FROM AUTHOR]

We have shown that, unlike bulk water, tiny water droplets (microdroplets) cause reduction of gold ions (1) as well as a number of organic compounds (2). Evidence has been presented that the source of electrons arises from hydroxyl anions (OH-) at or near the surface of the microdroplet (2). We report the formation of hydrogen peroxide (H2O2) in aqueous microdroplets and suggest that the observed H2O2 results from the recombination of hydroxyl radicals (OH) at or near the air- water interface of aqueous microdroplets sprayed into roomtemperature air. Hydrogen peroxide is a commodity chemical that has many different applications, such as chemical synthesis or as a disinfectant, in mining and metal processing, as well as pulp and textile bleaching (3). H2O2 has often been touted as a green oxidant because, upon decomposition, it generates oxygen and water (4). However, the most common industrial method (~95% worldwide) for 2O2 synthesis (5), the 2-step anthraquinone process, cannot be considered green (6) because organic wastes are generated from inefficient oxidation of the anthraquinone. Some advances in H2O2 synthesis have focused on catalytically combining 2 and 2 (7, 8). Other methods electrochemically generate H2O2 by electrolysis of O2 at the anode (9, 10), or photocatalytically generate reactive superoxo radicals (11). Recently, H2O2 was formed from a reaction between plasma and a water surface (12). However, these direct synthesis methods of H2O2 have limitations, including the use of precious metal catalysts, low yields, required 2 supply, and high energy consumption (13, 14). In what follows, we report the direct, spontaneous generation of H22 from aqueous microdroplets in the absence of applied voltage, catalyst, or any other added chemicals. We alsospeculate about the nature of the mechanism responsible for these observations. [ABSTRACT FROM AUTHOR]

The measurements of the surface tension of aqueous solutions of 10 monoterpene alcohols, MAs: (−)-β-citronellol, geraniol, tetrahydrolinalool, linalool, (−)-menthol, (−)-terpinen-4-ol, p-cymene-8-ol, α-terpineol, isoborneol and (−)-borneol were made at T = 293 K. On the basis of the obtained results the values of the adsorption constant, the excluded area per molecule at the interface and the interaction parameter were determined by fitting the experimental data to various adsorption models. Next the adsorption free energy of studied MAs was determined and discussed in terms of the preferred orientation of studied MAs at the water–air interface and molecular structure of MAs. [ABSTRACT FROM AUTHOR]

This paper presents an improved annular water-air jet pump concept design through integrating a self-induced oscillation mixing chamber with the conventional annular jet pump (AJP). The internal flow characteristics for both conventional and improved AJP were numerically investigated and compared by a validated computational fluid dynamics model. The numerical comparison demonstrated an approximately 10% pumping performance increase compared with the conventional pump, which is mostly attributed to the improved mass and energy transfer along the oscillating phase interface. Furthermore, transient flow analysis was conducted to resolve the unsteady self-introduced oscillation. The results revealed the self-introduced oscillation induces a continuous break-up and formation of fresh water-air interfaces, which exhibits a periodic feature with a dominant frequency of 147 Hz for the current design under given operational conditions. This study contributes toward a better understanding of the internal annular water-air jet pump flow patterns, and also demonstrates the feasibility of incorporating self-introduced oscillation chamber into AJP design. [ABSTRACT FROM AUTHOR]

Tracy et al. (2015, doi: ) assumed in their recent paper that water velocity at the water-air interface is zero in their pore-scale simulations of water flow in 3-D soil images acquired using X-ray computed tomography. We comment that such a treatment is physically wrong, and explain that it is the water-velocity gradient in the direction normal to the water-air interface, rather than the water velocity, that should be assumed to be zero at the water-air interface if one needs to decouple the water flow and the air flow. We analyze the potential errors caused by incorrectly taking water velocity at the water-air interface zero based on two simple examples, and conclude that it is not physically sound to make such a presumption because its associated errors are unpredictable. [ABSTRACT FROM AUTHOR]

A mathematical model for describing the temperature distribution in the near-surface layer at the water-air interface is proposed. The model is composed based on the theory of contrast structures. Using numerical calculations, the temperature distribution in a 10-cm wide near-boundary layer has been obtained. The calculation results coincide well with the experimental data. [ABSTRACT FROM AUTHOR]

Water interfaces provide the platform for many important biological, chemical, and physical processes. The water-air interface is the most common and simple aqueous interface and serves as a model system for water at a hydrophobic surface. Unveiling the microscopic (<1 nm) structure and dynamics of interfacial water at the water-vapor interface is essential for understanding the processes occurring on the water surface. At the water interface the network of very strong intermolecular interactions, hydrogen-bonds, is interrupted and the density of water is reduced. A central question regarding water at interfaces is the extent to which the structure and dynamics of water molecules are influenced by the interruption of the hydrogen-bonded network and thus differ from those of bulk water. Herein, we discuss recent advances in the study of interfacial water at the water-air interface using laser-based surface-specific vibrational spectroscopy. [ABSTRACT FROM AUTHOR]

Problems of acoustic wave propagation near the interface between media are discussed. The processes that occur at a liquid-gas interface are considered, and their characteristic features related to the effect of inhomogeneous waves are pointed out. The transmission factors of spherical and cylindrical waves incident on the aforementioned interface are calculated with allowance for the contribution of the inhomogeneous component. Experimental dependences of the transmission factor on frequency and source depth measured with respect to the interface are presented. It is shown that the lower the frequency generated by the source and the closer the source to the water-air interface, the higher the acoustic transparency of the interface. [ABSTRACT FROM AUTHOR]

Measurements of the surface tension of aqueous solutions of ternary and binary surfactant mixtures of p-(1,1,3,3-tetramethylbutyl) phenoxypoly(ethylene glycols), i.e., ethoxylated tert-octylphenol with 8, 10 and 16 ethylene oxide groups, respectively called TOP8, TOP10 and TOP16, were carried out at 293 K. Based on the surface tension isotherms, the Gibbs surface excess of surfactants concentration at the water-air interface was determined and then recalculated for the Guggenheim-Adam surface excess concentration of these substrates. Next the monomer mole fractions of surfactants in the surface layer were determined and compared with those calculated on the basis of both Rosen's approach and the composition in the bulk phase. Taking into account these different values of composition of the mixed monolayer at the water-air interface, the molecular interaction parameters of surfactants, their activity coefficients and the free enthalpy of their mixing were also calculated. As follows from the measurements and calculations, among other things, by the addition of the differences between the values of the mole fraction of surfactants calculated on the basis of Rosen's approach and those determined on the basis of surface excess concentrations, the mole fraction of the solvent at the water-air interface can be determined. Graphical Abstract: Values of the monomer mole fractions of surfactants in the mixed monolayer calculated on the basis of the Rosen's approach and surface excess concentration determined from the Gibbs ( X( G)) and Guggenheim-Adam ( X( G − A)) equations.[Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

The role of the polar properties of water in separation methods are presented in two parts: Part I: Properties of Water General principles are given of the three non-covalent energy types acting upon non-polar as well as polar entities when these are immersed in water: Lifshitz-van der Waals (LW), Lewis Acid-Base (AB) and Electrostatic (EL) energies. The dominant one of these three is the Lewis acid-base (AB) interaction, which in biochemical and other organic interactions are about an order of magnitude stronger than the van der Waals and electrostatic energies combined. Included are the most important equations, describing these interactions when occurring in water. Part II: Forces Originating in Water and their Significance in Separation Methods • Hydrophobic attraction between particles. • Hydrophobic interaction between solute molecules immersed in liquid water is caused by the hydrogen-bonding free energy of cohesion between the water molecules which surround these particles or solute molecules. • The solubility equation is given, which links the free energy of interaction, ΔGiwi, between the solute molecules (i) immersed in water (w), and the natural logarithm of the aqueous solubility (s) of i, expressed in mol fractions. Insolubility and partial solubility in water are due to strong mutual attraction and moderate attraction among molecules, respectively. Total solubility in water is due to mutual repulsion between solute molecules. • Discusses conditions of stability versus instability (flocculation) of aqueous particle suspensions. • Discusses the hyper-hydrophobicity of the water-air interface and its interactions with: A) small water-soluble molecules (it repels them) and: (B) amphiphilic molecules such as surfactants, alcohols, proteins, etc. (it strongly attracts them). • Treats adhesion and adsorption energies between two different entities, immersed in water, such as antigens and antibodies. These energies are commonly expressed as affinity constants (Kaff), the value of which is strongly influenced by the time spent during the initial adhesion or adsorption step, where the Kaff value steadily increases the longer that initial adsorption step lasts. This phenomenon is called hysteresis, or binding hysteresis. However the hysteresis effect can be avoided by reducing the initial exposure episode to time (t) approaching t = 0, to obtain Kafft→0. • Two different aqueous phases can be formed by dissolving two different water-soluble polymers, e.g., poly-(ethylene oxide) and dextran, which repel one another in aqueous solution. Such aqueous two-phase systems can be used cause the preferential migration of two different solutes to two different phases and thus to effect their separation and purification. Results of a study of multiple sub-phases forming within the middle phase of micro-emulsions are also discussed. • Advancing freezing fronts are discussed in this Section, i.e., the rejection or engulfment of certain solutes or cells by a vertical ice column which is made to advance further vertically by continuing to cool it from below. • The size, or rather the radius of curvature (R) of blood cells, blood proteins, bacteria and viruses, can play an important role in, e.g., the in vivo phagocytic engulfment of bacteria by leucocytes, the transformation of platelets to become 'sticky', the attachment of viruses to cells and of bacteriophages to bacteria. [ABSTRACT FROM AUTHOR]

The air side of the water‐air interface is the most hydrophobic surface known. In quantitative terms the water‐air interface is about 30% more hydrophobic than the surfaces of nonpolar condensed‐phase compounds or materials such as octane or Teflon. The hyperhydrophobicity of the air side of the water‐air interface is the main cause of the large increase in contact angle of drops of water deposited upon rough surfaces of apolar materials, as compared with the water contact angle on smooth surfaces of the same materials. A water drop supported on a very porous fractal surface, encountering only about 1% solid support and 99% air, can reach a contact angle of 174°, which is exceedingly close to the (albeit unattainable) maximum of 180°. The water‐air interface hydrophobically attracts completely apolar molecules, as well as the apolar side of amphiphilic molecules (such as surfactants). Thus, for instance, dissolved surfactant molecules aggregate at a high concentration at the water‐air interface when dissolved in water. On the other hand, the water‐air interface repels dissolved hydrophilic (or near‐hydrophilic) solutes, such as sugars and polysaccharides, mainly via net repulsive van der Waals forces. Thus, the water‐air interface is depleted of such hydrophilic (or near‐hydrophilic) solutes, leaving a significantly higher concentration of these solutes in the bulk of the aqueous medium than at its air interface. As both of these contrasting phenomena result in strongly anisotropic concentration distributions in liquid drops and as contact angle determinations depend on a known and homogeneous free energy of cohesion of the liquid throughout the drop, one should never measure contact angles on solid surfaces for the purpose of measuring their surface thermodynamic properties by using aqueous solutions, mixtures, or solutions in or mixtures of other polar or partly polar liquids. Finally, the peculiar properties of the water‐air interface give rise to what at first sight appears to be paradoxical behavior of air bubbles in water: in pure deionized water, air bubbles attract one another and coalesce. On the other hand, upon the addition of salt (e.g., NaCl), air bubbles repel each other and thus do not coalesce, all in apparent contradiction of the classical rules governing the stability or instability of colloidal suspensions in water. [ABSTRACT FROM AUTHOR]

Redox molecules, i.e., ferrocene (Fc), ferrocene carboxylic acid (Fc-COOH), and 3-3', 5-5'-tetramethylbenzidine (TMB), are good mediators for typical coenzymes of several dehydrogenases. Such mediators were immobilized in monolayers of dipalmitoylphosphatidic acid (DPPA) by means of the cospreading technique at the water-air interface. TMB was found to interact more strongly than Fc and Fc-COOH with the DPPA matrix; moreover, we observed that DPPA/TMB monolayers could be transferred on different solid supports with the Langmuir-Blodgett (LB) technique. LB films, containing TMB immobilized by means of cospreading and incubation procedure, were characterized recording UV-Vis absorption spectra. Such investigation evidenced the presence of TMB with different oxidation states. Hybrid films containing the mediators were prepared combining two techniques: LB and self-assembly of octadecylmercaptane and octylmercaptane on gold. The resulting structures were characterized by means of ellipsometry and contact angle measurements. Information on the influence of the multilayer structure on the electrode currents and on the redox activity of the immobilized mediators was obtained by cyclic voltammetry. A significant electrochemical signal was observed for TMB included in LB films. The sets of results independently evidenced that TMB can be quantitatively embedded in the ultrathin films of a lipid matrix maintaining its electroactive behavior. [ABSTRACT FROM PUBLISHER]

Mixed monolayers of two polypeptides, poly- γ-methyl-L-glutamate and poly-L-alanine in the same α-helix surface conformation, were studied at the water/air interface. From a comparison of the experimental π-A isotherms with Huggins' theory and from the examination of the MIR spectra for the two components and their mixtures, it was possible to deduce that poly- γ-methyl-L-glutamate and poly-L-alanine were totally miscible at the surface with prevalently attractive interactions and that the mixtures maintained the conformation of the two components. The low values of the interaction parameters, deduced by applying Joos' theory to the collapse pressures of the mixtures, demonstrated that the interactions were prevalently hydrophobic [ABSTRACT FROM AUTHOR]

This paper investigates the physical processes involved in the water filling and air expelling process of a pipe with multiple air valves under water slow filling condition, and develops a fully coupledwater–air two-phase stratified numerical model for simulating the process. In this model, the Saint-Venant equations and the Vertical Average Navier–Stokes equations (VANS) are respectively applied to describe the water and air in pipe, and the air valve model is introduced into the VANS equations of air as the source term. The finite-volume method and implicit dual time-stepping method (IDTS) with two-order accuracy are simultaneously used to solve this numerical model to realize the full coupling between water and air movement. Then, the model is validated by using the experimental data of the pressure evolution in pipe and the air velocity evolution of air valves, which respectively characterize the water filling and air expelling process. The results show that the model performs well in capturing the physical processes, and a reasonable agreement is obtained between numerical and experimental results. This agreement demonstrates that the proposed model in this paper offers a practical method for simulating water filling and air expelling process in a pipe with multiple air valves under water slow filling condition. [ABSTRACT FROM AUTHOR]

Pore-scale simulation and tomography have been used in a combination over the past decade to study some fundamental physical and biogeochemical processes in soil that are difficult or impossible to measure, one of which is water flow in unsaturated soil. Considering that an important application of pore-scale simulation in soil is to elucidate how convective water flow distributes soluble nutrients through the heterogeneous pore space when water flow is in steady state, we propose an alternative model in this paper to simulate unsaturated water flow at pore scale. The model is quasi-static, consisting of two steps. The first one is to determine water distribution in the pore geometry using a morphological model; once the water distribution is known under a pressure, the water-air interface is fixed in space. The second step is to simulate water flow numerically by solving the Stokes equations in which the water-air interface is treated as a boundary. We test the water flow model against analytical solution of thin water film flow over a non-slip solid wall, and then combine it with the morphological model to simulate water flow in 3D soil structures acquired using X-ray tomography at resolution of 10 µm. As a validation, we compare the permeability calculated by the model under different saturations with those estimated by the van Genuchten formula using the saturated permeability and water retention parameters obtained from the morphological model. We also discuss the pros and cons of the model and its implications in unsaturated soil modelling. [ABSTRACT FROM AUTHOR]