Rates of Shrinkage and Growth of Air Bubbles Entrained in Wheat Flour Dough

The rates of shrinkage at constant temperature, and growth under a temperature rise below 100°C, of bubbles entrained in wheat flour dough were analyzed and compared with those of a bubble in water. The rate of shrinkage of bubbles in flour dough was controlled by the diffusion of dissolved air from the surface of bubbles to the bulk of flour dough. The apparent diffusion coefficient of the dissolved air in wheat flour dough with the water fraction of 0.49 calculated from the shrinkage of bubbles, was (3.2 ± 1.5) × 10^1?1 m²/sec (19°C), and (6.4 ± 2.0) × 10^?11 m²/sec (42°C). However, the growth behavior of bubbles in flour dough under a temperature rise was very different from that predicted from the diffusion theory. The critical radius of bubbles to grow was larger than that estimated from the diffusion theory. The mechanism of growth of bubbles in wheat flour dough, which was different from that of a bubble in water, is a subject that needs to be clarified.     

Hydraulic Conductivity Recovery versus Water Pressure in Xylem of Acer saccharum

Experiments were conducted to determine the influence of stem diameter, xylem pressure potential, and temperature on the rate of recovery of hydraulic conductivity in embolized stems of Acer saccharum Marsh. Recovery of conductivity was accompanied by an increase in stem water content as water replaced air bubbles and bubbles dissolved from vessels into the surrounding water. The time required for stems to go from less than 3 to 100% hydraulic conductivity increased approximately with the square of the stem diameter and increased with decreasing xylem pressure potential. Recovery was halted when xylem pressure potential decreased below −6 kPa. Increasing xylem pressure from 13 to 150 kPa reduced the time for recovery by a factor of 4. Temperature had little influence on the rate of recovery of hydraulic conductivity. All of these results are in accord with a theory of bubble dissolution in which it is assumed that: (a) the rate of bubble dissolution is rate limited by diffusion of air from the bubbles to the outer surface of the stems, (b) the equilibrium concentration of gases in liquid in stems is determined by Henry's law at all air-water interfaces, (c) the equilibrium solubility concentration is determined only by the partial pressure of the gas in the gas phase and not directly by the liquid-phase pressure, and (d) the gas pressure of an entrapped air bubble in the lumen of a cell can never be less than atmospheric pressure at equilibrium.     

Phenomena at the advancing ice-liquid interface: solutes, particles and biological cells

Ice formation in aqueous solutions and suspensions involves a number of significant changes and processes in the residual liquid. The resulting effects were described concerning the redistribution of dissolved salts, the behaviour of gaseous solutes and bubble formation, the rejection and entrapment of second-phase particles. This set of conditions is also experienced by biological cells subjected to freezing. The influences of ice formation in that respect and their relevance for cryopreservation were considered as well. A model of transient heat conduction and solute diffusion with a planar ice front, propagating through a system of finite length was found to be in good agreement with measured salt concentration profiles. The spacing of the subsequently developing columnar solidification pattern was of the same order of magnitude as the pertubation wavelengths predicted from the stability criterion. Non-planar solidification of binary salt solutions was described by a pure heat transfer model under the assumption of local thermodynamic equilibrium. The rejection of gaseous solutes and the resulting gas concentration profile ahead of a planar ice front has been estimated by means of a test bubble method, yielding a distribution coefficient of 0.05 for oxygen. The nucleation of gas bubbles has been observed to occur at slightly less than 20-fold supersaturation. The subsequent radial growth of the bubbles obeys a square-root time dependence as expected from a diffusion controlled model until the still expanding bubbles become engulfed by the advancing ice-liquid interface. The maximum bubble radii decrease for increasing ice front velocities. The transition between repulsion and entrapment of spherical latex particles by an advancing planar ice-front has been characterized by a critical value of the velocity of the solidification interface. The critical velocity is inversely proportional to the particle radius as suggested by models assuming an undisturbed ice front. The increase of the critical velocity for increasing thermal gradients shows good agreement with a theoretically predicted square-root type of dependence. Critical velocities have also been measured for yeast and red blood cells. The effect of freezing on biological cells has been analyzed for human lymphocytes and erythrocytes. The reduction of cell volume observed during non-planar freezing agrees reasonably well with shrinkage curves calculated from a water transport model. The probability of intracellular ice formation has been characterized by threshold cooling rates above which the amount of water remaining within the cell is sufficient for crystallization.(ABSTRACT TRUNCATED AT 400 WORDS)     

Molecular dynamics simulation of proteins under high pressure: Structure,function and thermodynamics

BackgroundMolecular dynamics (MD) simulation is well-recognized as a powerful tool to investigate protein structure, function, and thermodynamics. MD simulation is also used to investigate high pressure effects on proteins. For conducting better MD simulation under high pressure, the main issues to be addressed are: (i) protein force fields and water models were originally developed to reproduce experimental properties obtained at ambient pressure; and (ii) the timescale to observe the pressure effect is often much longer than that of conventional MD simulations.Scope of reviewFirst, we describe recent developments in MD simulation methodologies for studying the high-pressure structure and dynamics of protein molecules. These developments include force fields for proteins and water molecules, and enhanced simulation techniques. Then, we summarize recent studies of MD simulations of proteins in water under high pressure.Major conclusionsRecent MD simulations of proteins in solution under pressure have reproduced various phenomena identified by experiments using high pressure, such as hydration, water penetration, conformational change, helix stabilization, and molecular stiffening.General significanceMD simulations demonstrate differences in the properties of proteins and water molecules between ambient and high-pressure conditions. Comparing the results obtained by MD calculations with those obtained experimentally could reveal the mechanism by which biological molecular machines work well in collaboration with water molecules.     

A Novel Transformation to S-Methyl Phosphorodichloridothiolate and the Fungitoxicity of Diaryl S-Methyl Phosphorothiolates

Prior to an analysis of the shrinkage and growth of air bubbles entrained in wheat flour dough, the shrinkage and growth under a temperature rise of a small bubble in water was analysed for comparison. The rates of shrinkage and growth of the bubble were respectively controlled by the diffusion of under- and over-saturated dissolved air from and into the bubble. The diffusion coefficient of the dissolved air in water calculated from the shrinkage of the bubble was 2.10 × 10^_9m²/sec (17°C), which agrees with the literature value. On the other hand, at below 100°C, the effects on the bubble growth of the expansion of air due to the temperature rise and the increase in the saturation vapor pressure of water were negligibly small. The accompanying air entrained in flour particles suspended in water was much more stable than a free bubble in water. However, the growth under a temperature-rise of a bubble evolved from wheat flour particles was the same as the growth of a bubble in water, if many bubbles did not coexist.     

Differences in the way potassium chloride and sucrose solutions effect osmotic potential of significance to stomata aperture modulation

Guard cell solution osmotic potential changes resulting in the opening and closing of stomata apertures follow an initial influx of potassium ions, their substitution with sucrose molecules and the subsequent reduction of the latter. To provide an insight into the osmotic mechanism of the changes, the new equation for calculating osmotic pressure, which equates the difference between the energy of pure water across a semi-permeable membrane interface with that of solution water, was used to compare the osmotic properties of KCl and sucrose. For sucrose solutions, the effect of the sucrose molecules in increasing the spacing of the solution water was mainly responsible for osmotic potential; this contrasted with K⁺ + Cl^? ions where their spacing effect was only a little higher to that of water held to those ions. At solute concentrations giving an osmotic potential level of ?3.0 MPa near that of turgid guard cells, the spacing effect on the potential of the unattached solution water molecules caused by sucrose, but in its theoretical absence, was estimated as ?2.203 MPa compared with ?1.431 MPa for KCl. In contrast, the potential attributed to water molecules firmly held to the K⁺ + Cl^? ions was ?1.212 MPa versus zero for sucrose. The potential to keep the sucrose molecules in solution was ?0.797 MPa compared with ?0.357 MPa for KCl. The findings illustrate that the way KCl effects osmotic pressure is very different to that of sucrose. It is concluded that stomata aperture modulation is closely linked to the osmotic properties of its guard cell solution solutes.     

Analysis of freeze-thaw embolism in conifers. The interaction between cavitation pressure and tracheid size

Ice formation in the xylem sap produces air bubbles that under negative xylem pressures may expand and cause embolism in the xylem conduits. We used the centrifuge method to evaluate the relationship between freeze-thaw embolism and conduit diameter across a range of xylem pressures (Px) in the conifers Pinus contorta and Juniperus scopulorum. Vulnerability curves showing loss of conductivity (embolism) with Px down to -8 MPa were generated with versus without superimposing a freeze-thaw treatment. In both species, the freeze-thaw plus water-stress treatment caused more embolism than water stress alone. We estimated the critical conduit diameter (Df) above which a tracheid will embolize due to freezing and thawing and found that it decreased from 35 microm at a Px of -0.5 MPa to 6 microm at -8 MPa. Further analysis showed that the proportionality between diameter of the air bubble nucleating the cavitation and the diameter of the conduit (kL) declined with increasingly negative Px. This suggests that the bubbles causing cavitation are smaller in proportion to tracheid diameter in narrow tracheids than in wider ones. A possible reason for this is that the rate of dissolving increases with bubble pressure, which is inversely proportional to bubble diameter (La Place's law). Hence, smaller bubbles shrink faster than bigger ones. Last, we used the empirical relationship between Px and Df to model the freeze-thaw response in conifer species.     

Intracellular gas supersaturation tolerances of erythrocytes and resealed ghosts.

Intact mammalian, avian, and amphibian erythrocytes were saturated with up to 300 atm nitrogen or argon gas and rapidly decompressed. Despite the profuse nucleation of gas bubbles in the suspending fluid, no evidence of intracellular gas bubble nucleation was found; all or most of the cells remained intact and little or no hemoglobin escaped. Internal bubbles were similarly absent from resealed ghosts of human erythrocytes as shown by lack of disintegration and by retention of an entrapped fluorescent compound. The absence of bubbles may indicate that much of the internal water does not have the same nucleation properties as external water.     

Gas bubbles in rats after heliox saturation and different decompression steps and rates.

Effects of pressure reduction, decompression rate, and repeated exposure on venous gas bubble formation were determined in five groups (GI, GII, GIII, GIV, and GV) of conscious and freely moving rats in a heliox atmosphere. Bubbles were recorded with a Doppler ultrasound probe implanted around the inferior caval vein. Rats were held for 16 h at 0.4 MPa (GI), 0.5 MPa (GII and GIII), 1.7 MPa (GIVa), or 1.9 MPa (GIV and GV), followed by decompression to 0.1 MPa in GI to GIII and to 1.1 MPa in GIV and GV. A greater decompression step, but at the same rate (GII vs. GI and GIVb vs. GIVa), resulted in significantly more bubbles (P < 0.01). A twofold decompression step resulted in equal amount of bubbles when decompressing to 1.1 MPa compared with 0.1 MPa. The faster decompression in GII and GVa (10.0 kPa/s) resulted in significantly more bubbles (P < 0.01) compared with GIII and GVb (2.2 kPa/s). No significant difference was observed in cumulative bubble score when comparing first and second exposure. With the present animal model, different decompression regimes may be evaluated.     

Fate of methane bubbles released by pockmarks in Lake Constance

In the eastern part of Lake Constance, the second largest pre-alpine lake in Europe, about five hundred pockmarks (morphological depressions on the lake floor) were recently discovered of which ~40 % release methane bubbles. The carbon isotopic composition of the escaping gas indicated that the methane is of biogenic origin. In our study, we investigated the fate of the released methane bubbles, i.e., the dissolution, oxidation or transport of the bubbles to the surface. At a littoral pockmark site (PM12, 12 m water depth) and a profundal pockmark (PM80, 80 m water depth), we analysed the dissolved methane concentrations and the methane isotopic carbon signature in the water column. At PM80, higher methane concentrations (up to 1,523 nM), compared to the control site and the surface waters (225 ± 72 nM), were recorded only on some occasions and only in the bottom water, despite the fact that the released bubbles were dissolving within the hypolimnion based on bubble modeling. The isotope data suggest that most of the dissolved methane is oxidized below 40 m water depth. The isotopic signature of the methane in the surface water at PM80, however, differed from that of the methane in the hypolimnion; therefore, the surface methane at this profundal site is most likely an export product from the littoral zone. Assuming an initial bubble diameter of 5 mm, we calculated that these small bubbles would reach the surface, but approximately 96 % of the methane would have dissolved from the bubble into the hypolimnion. At PM12, we observed higher concentrations of dissolved methane (312 ± 52 nM) with no significant differences between seasons or between control sites versus pockmark site. In the shallow water, divers estimated the bubble size to be 10–15 mm, which from a release depth of 12 m would barely dissolved into the water column. The isotopic signature also indicated that there had been almost no methane oxidation in the shallow water column. Thus, the water depth of bubble release as well as the initial bubble size determine whether the methane enters the atmosphere largely unhindered (shallow site) or if the released methane is incorporated into the profundal water column.     

Investigation of the behavior of dissolved gases during freezing

This paper deals with the freezing process of aqueous solutions of gases and the nucleation of gas bubbles at the moving ice—water interface. A cryomicroscope was used to investigate the conditions of nucleation and growth of bubbles after reaching a stationary concentration profile ahead of the phase boundary. The enrichment of gases due to the distribution coefficient was detected by means of a test bubble method, i.e., the increase in the radius of a small bubble being approached by the ice front. A distribution coefficient of 0.048 (at 0 °C) was found for oxygen. Nucleation occurs when stationary growth conditions in the solution are reached. The measured oversaturation is close to 20, i.e., about the inverse of the distribution coefficient. In highly saturated gas solutions, dendritic breakdown of the planar ice-water interface due to gas enrichment could be observed. At these positions also a considerable degree of constitutional supercooling was found. Bubbles were nucleated in interdendritic spaces. Nucleation and growth of gas bubbles was seen to be a periodic process under certain circumstances which can be explained by the continuous buildup and reduction of the concentration field in the remaining solution. The growth kinetics of the bubbles and their maximum size are governed by the velocity of the ice-water interface. During growth the gas bubbles are pushed and partially encapsulated, until they reach a radius in the order of magnitude of the diffusion boundary layer of the concentration profile, and become totally engulfed by the solid phase.     

Effect of oxygen breathing on micro oxygen bubbles in nitrogen-depleted rat adipose tissue at sea level and 25 kPa altitude exposures

The standard treatment of altitude decompression sickness (aDCS) caused by nitrogen bubble formation is oxygen breathing and recompression. However, micro air bubbles (containing 79% nitrogen), injected into adipose tissue, grow and stabilize at 25 kPa regardless of continued oxygen breathing and the tissue nitrogen pressure. To quantify the contribution of oxygen to bubble growth at altitude, micro oxygen bubbles (containing 0% nitrogen) were injected into the adipose tissue of rats depleted from nitrogen by means of preoxygenation (fraction of inspired oxygen = 1.0; 100%) and the bubbles studied at 101.3 kPa (sea level) or at 25 kPa altitude exposures during continued oxygen breathing. In keeping with previous observations and bubble kinetic models, we hypothesize that oxygen breathing may contribute to oxygen bubble growth at altitude. Anesthetized rats were exposed to 3 h of oxygen prebreathing at 101.3 kPa (sea level). Micro oxygen bubbles of 500-800 nl were then injected into the exposed abdominal adipose tissue. The oxygen bubbles were studied for up to 3.5 h during continued oxygen breathing at either 101.3 or 25 kPa ambient pressures. At 101.3 kPa, all bubbles shrank consistently until they disappeared from view at a net disappearance rate (0.02 mm(2) × min(-1)) significantly faster than for similar bubbles at 25 kPa altitude (0.01 mm(2) × min(-1)). At 25 kPa, most bubbles initially grew for 2-40 min, after which they shrank and disappeared. Four bubbles did not disappear while at 25 kPa. The results support bubble kinetic models based on Fick's first law of diffusion, Boyles law, and the oxygen window effect, predicting that oxygen contributes more to bubble volume and growth during hypobaric conditions. As the effect of oxygen increases, the lower the ambient pressure. The results indicate that recompression is instrumental in the treatment of aDCS.     

Analysis of crystal growth of methane hydrate using molecular dynamics simulation

Methane hydrate is a crystalline compound with methane molecules enclosed in cages formed by hydrogen-bonded water molecules. Understanding the mechanism of nucleation and crystal growth from methane vapour and liquid water is important for all hydrate applications. However, processes near the water/methane interface are still unclear. In this work, we focused on the crystal growth of methane hydrate seeds located near the water/methane interface. We performed molecular dynamics (MD) simulation and analysed the crystal growth of the hydrate seed at the interface. New cages formed in the liquid water phase were stabilised when they shared faces with the hydrate seed. We also investigated the crystal growth rate as the time development of the number of methane molecules trapped in hydrate cages, based on the trajectory of the MD simulation. The calculated growth rate in the direction that covers the interface was 1.38 times that in the direction towards the inside of the water phase.     

Quantized Water Clusters Around Apolar Molecules

Abstract

In order to understand the mechanism of gas hydrate kinetics and to explore the existance of other new cavities in the hydrate structure, we have used Molecular Dynamics (MD) simulation to study a system comprising two Lennard-Jones particles and 214 water molecules. Equilibrium structure and properties of twelve cases have been investigated. Our findings were as follows: ? Apolar molecules promote spherical liquid water clusters in a hydrate-like labile cavity.

? The size of the cavity and the coordination number is dependent upon the size of the apolar molecule.

? The coordination number of water molecules is quantized in jumps of four.

? Similarities are observed between the labile cavities and cavities in solid hydrates and in other chemical structures such as Buckminsterfullerene.

? Such a simulation procedure suggests the possibility of other clusters which may exist in yet-to-be-found hydrates. A separate question involves whether such suggested cavities can be combined with other cavities into a space-filling crystal.

     

Bubble formation in crabs induced by limb motions after decompression

In vivo bubble formation was studied in the megalopal stage of the crab Pachygrapsus crassipes. The animals were equilibrated with elevated argon, nitrogen, or helium pressures then rapidly decompressed to atmospheric pressure. Voluntary motions induced bubble nucleation in leg joints after exposures to as low as 2 atm nitrogen (gauge pressure). Delays of several minutes sometimes passed between decompression and bubble formation. Mechanically stimulating the animals to move their legs increased this bubble formation, whereas immobilizing the legs before gas equilibration prevented it, even in animals decompressed from 150 atm nitrogen. We conclude that preformed nuclei are not responsible for bubbles developing in the legs of this animal. Instead, tribonucleation of bubbles apparently occurs as a result of limb motions at relatively low gas supersaturations.     

High Molecular Weight Organic Compounds in the Xylem Sap of Mangroves: Implications for Long-Distance Water Transport

The rise of sap in mangroves has puzzled plant physiologists for many decades. The current consensus is that negative pressures in the xylem exist which are sufficiently high to exceed the osmotic pressure of seawater (2.5 MPa). This implies that the radial reflection coefficients of the mangrove roots are equal to unity. However, direct pressure probe measurements in xylem vessels of the roots and stems of mangrove (Rhizophora mangle) grown in the laboratory or in the field yielded below-atmospheric, positive (absolute) pressure values. Slightly negative pressure values were recorded only occasionally. Xylem pressure did not change significantly when the plants were transferred from tap water to solutions containing up to 1700 mOsmol kg^?1 NaCl. This indicates that the radial reflection coefficient of the roots for salt, and therefore the effective osmotic pressure of the external solution, was essentially zero as already reported for other halophytes. The low values of xylem tension measured with the xylem pressure probe were consistent with previously published data obtained using the vacuum/leafy twig technique. Values of xylem tension determined with these two methods were nearly two orders of magnitude smaller than those estimated for mangrove using the pressure chamber technique (?3 to ?6MPa). Xylem pressure probe measurements and staining experiments with alcian blue and other dyes gave strong evidence that the xylem vessels contained viscous, mucilage- and/or protein-related compounds. Production of these compounds resulting from wound or other artifactual reactions was excluded. The very low sap flow rates of about 20–50 cm h^?1 measured in these mangrove plants were consistent with the presence of high molecular weight polymeric substances in the xylem sap. The presence of viscous substances in the xylem sap of mangroves has the following implications for traditional xylem pressure measurement techniques, development of xylem tension, and longdistance water transport: (1) high external balancing pressures in the pressure chamber are needed to force xylem sap to the cut surface of the twig; (2) stable tensions much larger than 0.1 MPa can be developed only occasionally because viscous solutions provide nucleation sites for gas bubble formation; (3) the frequent presence of small gas bubbles in viscous solutions allows water transport by interfacial, gravity-independent streaming at gas/water interfaces and (4) the increased density of viscous solutions creates (gravity-dependent) convectional flows. Density-driven convectional flows and interfacial streaming, but also the very low radial reflection coefficient of the roots to NaCl are apparently the means by which R. mangle maintains water transport to its leaves despite the high salinity of the environment.     

Molecular dynamics study of interaction of dimyristoyl phosphotidyl choline with water

We present here results of molecular dynamics (MD) simulations on hydrated bilayers of 40 molecules of 1-2-dimyristoyl-sn-glycero-3-phosphatidyl choline (DMPC) in liquid crystalline (Lα) phase using two different models (i) with same (A) conformation for all DMPC molecules, (ii) with alternate rows having different (A and B reported in crystallographic studies on DMPC) conformations. The bilayers were hydrated using 776 and 1064 water molecules. Simulations have been carried out at 310K with AMBER 4.0 program, using united atom force field for 200 pico seconds (ps) after equilibration. During heating and equilibration constant pressure temperature (PT) conditions were maintained while in simulation of equillibrated bilayers constant volume temperature (VT) conditions were used. Subaveraged atomic coordinates were used to calculate geometric parameters of lipid molecules and lipid water interaction. Our results show larger flexibility of polar head group and glycerol region in Lα phase compared to gel or non-hydrated bilayers. Chain disorder was more towards end. Sn-2 chains were more disordered. Use of two types of starting conformations increased disorder. Trans fraction of chain torsional angle was higher in non-hydrated bilayer. However it was more disordered due to ‘swing’ movement of chains because of distortion in torsional angles α2 and 03 due to absence of water molecules. Trans fraction of the chains, order parameter and water penetration showed general agreement with the available experimental results. On the whole MD technique was found to be quite useful for depicting microscopic behaviour of liquid crystalline system and correlating the same with macroscopic changes observed experimentally.     

Investigations of Nematic-Isotropic Transition by Means of Constant Pressure Molecular Dynamics Simulations

Abstract

Constant pressure molecular dynamics simulations, which secure the system to be under hydrostatic pressure, are used to simulate the behavior of liquid crystals consisting of anisotropic molecules with both translational and orientational freedom. In order to investigate to what extent can the properties known to real liquid crystalline phases be explained by the anisotropy of the shape of the molecules alone, the molecular dynamic (MD) simulation uses purely repulsive short-range pair potentials representing soft spherocylinders. A clear change in the microscopic as well as the macroscopic physical properties are observed near the phase transition from nematic liquid crystal to isotropic liquid.     

Characterization of the localized hydrodynamic shear forces and dissolved oxygen distribution in sparged bioreactors

Detailed, high-resolution numerical simulations of the bubbly flows, used for oxygen delivery and mixing in mammalian cell suspensions, have been performed. The hydrodynamics, shear and normal forces, mass transfer and mass transport from and around individual bubbles and bubble clusters were resolved for different operating conditions, that is, Weber, Morton, and Schmidt numbers. Suspended animal (e.g., mammalian, insect) cells are known to be susceptible to damage potentially leading to cell death, caused by hydrodynamic stresses and oxygen deprivation. Better knowledge of the magnitude of the shear forces and the extent of mixing of the dissolved oxygen in sparged bioreactors can have a significant impact on their future design and optimization. Therefore, the computed liquid-phase velocity fields were used to calculate and compare the local shear in different types of single bubble wakes and in bubble clusters. Oxygen mass transfer and dissolved oxygen transport were resolved to examine oxygen supply to the cells in the different types of flows.