Survival of freezing by free-living Antarctic soil nematodes

Free-living microbivorous nematodes become numerically dominant in Antarctic terrestrial faunas as environmental conditions become more severe, while also reaching very high levels of abundance in moist, vegetated habitats. Nematodes have little resistance to freezing via exogenous ice nucleation, such as would occur as their microhabitat freezes. We report the results of experiments testing the ability of seven maritime Antarctic nematode taxa to survive freezing in small water droplets at high sub-zero temperatures. Isolated individuals of these species possessed supercooling characteristics similar to those previously reported (supercooling points -6 to -25 degree C). When frozen in water at -3 to -6 degree C, most showed high (> 70%) survival both (i) after rapid cooling (1 degree C/min) to c. -60 degree C followed by immediate rewarming, and (ii) when held for 7-12 h at either -10 or -30 degree C, although the proportions surviving varied between species. We propose that the ability to survive freezing while fully hydrated at high sub-zero temperatures is one of the most important aspects of these species' survival tactics.     

Testing an ensemble model of cirrus ice crystals using midlatitude in situ estimates of ice water content,volume extinction coefficient and the total solar optical depth

In this paper an ensemble model of cirrus ice crystals is tested against midlatitude in situ estimates of ice water content, volume extinction coefficient and the total solar optical depth. During the Winter of 2005 and Spring 2006 the FAAM (Facility for Airborne Atmospheric Measurements) BAE-146 G-LUXE aircraft flew three flights as part of the CAESAR (Cirrus and Anvils: European Satellite and Airborne Radiation measurements project) campaign of flying in cirrus around the UK. The suite of microphysical instrumentation onboard the aircraft included the PMS 2D-C probe and the Stratton Park Engineering Company (SPEC) cloud particle imager (CPI). The campaign characterized cirrus properties such as ice water content, volume extinction coefficient, ice crystal geometric shape and ice crystal effective dimension. Cirrus cloud temperatures ranged approximately between 215 and 240 K. From the CPI instrument 60–80% of the ice crystal habits were estimated to be either indeterminate or ‘irregular’ (though such irregular crystals could be composed of pristine components) of some form with hexagonal columns and hexagonal plates accounting for generally much less than 3% of the ice crystal population. The CPI estimated integrated ice water content ranged between 5±2 and 45±22 gm^?2, whilst the CPI estimate of the total solar optical depth was found to lie between 0.2±0.1 and 1.0±0.5. The CPI estimate of the mean ice crystal effective dimension was found to range between about 59±9 and 90±75 μm.The particle size distribution (PSD) function was estimated using a PSD scheme that requires as input the in situ estimated IWC and measured in-cloud temperature. The CPI estimates of the bulk and microphysical properties of the midlatitude cirrus are used to test whether an ensemble model of cirrus ice crystals together with a PSD scheme can predict CPI in situ estimates to within the experimental uncertainty. This paper demonstrates that the ensemble model coupled with a PSD scheme can predict the ice water content and the integrated ice water content to generally well within the experimental uncertainty if a varying density with respect to size is assumed. The ensemble model together with a PSD scheme is also shown to predict the CPI estimated volume extinction coefficient and the derived total solar optical depth to generally well within the experimental uncertainty. The paper demonstrates that an ensemble model of cirrus combined with a PSD scheme can predict the radiative properties of cirrus without the need to invoke the concept of an ice crystal effective dimension.     

Sonochemical water splitting in the presence of powdered metal oxides

Kinetics of hydrogen formation was explored as a new chemical dosimeter allowing probing the sonochemical activity of argon-saturated water in the presence of micro- and nano-sized metal oxide particles exhibiting catalytic properties (ThO₂, ZrO₂, and TiO₂). It was shown that the conventional sonochemical dosimeter based on H₂O₂ formation is hardly applicable in such systems due to catalytic degradation of H₂O₂ at oxide surface. The study of H₂ generation revealed that at low-frequency ultrasound (20 kHz) the sonochemical water splitting is greatly improved for all studied metal oxides. The highest efficiency is observed for relatively large micrometric particles of ThO₂ which is assigned to ultrasonically-driven particle fragmentation accompanied by mechanochemical water molecule splitting. The nanosized metal oxides do not exhibit particle size reduction under ultrasonic treatment but nevertheless yield higher quantities of H₂. The enhancement of sonochemical water splitting in this case is most probably resulting from better bubble nucleation in heterogeneous systems. At high-frequency ultrasound (362 kHz), the effect of metal oxide particles results in a combination of nucleation and ultrasound attenuation. In contrast to 20 kHz, micrometric particles slowdown the sonolysis of water at 362 kHz due to stronger attenuation of ultrasonic waves while smaller particles show a relatively weak and various directional effects.     

Thermodynymic analysis of the mechanism of deep supercooling of tissue water in winter-hardy plants

The temperature at which ice grows through narrow, hydrophilic capillary is known to be depressed. Further, the nucleation temperature near a hydrophilic surface varies with the size, geometry and the properties of a particle. In this paper we show how these two effects are additive for the water that freezes on the wall of a capillary without the presence of pre-existing ice. The combined effect is a substantial lowering of nucleation temperature that could, if this analysis is correct, have important cryobiological consequences.     

Substances which inhibit ice nucleation: a review

There are a number of substances described in the published literature which inhibit ice nucleation. Certain bacterial strains, mostly found among the nonfluorescent pseudomonade species, release material into the growth medium which reduces the nucleation temperature of water droplets to below that of distilled water. Extracts from the seeds of food crops including apricot, peach and plum can reduce the nucleation temperature of water droplets and dispersions of silver iodide. Antifreeze glycoproteins can reduce the nucleation temperature of saline solutions. Antifreeze proteins can inhibit the activity of some biological ice nucleators but not others. Certain novel polymers have been shown to inhibit the nucleation activity of dispersions of silver iodide and ice-nucleating bacteria.     

Membrane cleaning with ultrasonically driven bubbles

A laboratory filtration plant for drinking water treatment is constructed to study the conditions for purely mechanical in situ cleaning of fouled polymeric membranes by the application of ultrasound. The filtration is done by suction of water with defined constant contamination through a membrane module, a stack of five pairs of flat-sheet ultrafiltration membranes. The short cleaning cycle to remove the cake layer from the membranes includes backwashing, the application of ultrasound and air flushing. A special geometry for sound irradiation of the membranes parallel to their surfaces is chosen. Two frequencies, 35 kHz and 130 kHz, and different driving powers are tested for their cleaning effectiveness. No cleaning is found for 35 kHz, whereas good cleaning results are obtained for 130 kHz, with an optimum cleaning effectiveness at moderate driving powers. Acoustic and optic measurements in space and time as well as analytical considerations and numerical calculations reveal the reasons and confirm the experimental results. The sound field is measured in high resolution and bubble structures are high-speed imaged on their nucleation sites as well as during their cleaning work at the membrane surface. The microscopic inspection of the membrane surface after cleaning shows distinct cleaning types in the cake layer that are related to specific bubble behaviour on the membrane. The membrane integrity and permeate quality are checked on-line by particle counting and turbidity measurement of the permeate. No signs of membrane damage or irreversible membrane degradation in permeability are detected and an excellent water permeate quality is retained.     

Theoretical model of ice nucleation induced by inertial acoustic cavitation. Part 2: Number of ice nuclei generated by a single bubble

In the preceding paper (part 1), the pressure and temperature fields close to a bubble undergoing inertial acoustic cavitation were presented. It was shown that extremely high liquid water pressures but quite moderate temperatures were attained near the bubble wall just after the collapse providing the necessary conditions for ice nucleation. In this paper (part 2), the nucleation rate and the nuclei number generated by a single collapsing bubble were determined. The calculations were performed for different driving acoustic pressures, liquid ambient temperatures and bubble initial radius. An optimal acoustic pressure range and a nucleation temperature threshold as function of bubble radius were determined. The capability of moderate power ultrasound to trigger ice nucleation at low undercooling level and for a wide distribution of bubble sizes has thus been assessed on the theoretical ground.     

The impact of Martian aerosols on the retrieval of temperature profiles from PFS measurements

Our purpose was a qualitative assessment of the impact of dust and water ice aerosols on the retrieved temperature profiles and the retrieval process itself in the Martian atmosphere. It aims to quantify the related uncertainties in the atmospheric temperature profiles derived from radiance measurements of the Planetary Fourier Spectrometer (PFS), currently operating on the Mars Express orbiter. In this study the effects of aerosol opacities on simulated data and retrieved temperature profiles were also investigated.From the analysis of the model atmosphere including dust and water ice with different size distributions it results that the dust component affects weighting functions and brightness temperatures less than water ice. A similar situation is also observed when different vertical distributions are considered. Unlike dust, water ice with different sizes of crystals evidently influences weighting functions and brightness temperatures. The impact of the considered water ice vertical distributions on brightness temperatures is noticeable only near 840 cm⁻¹.Considering different dust opacities, the largest differences—5 K maximum—between retrieved temperature profiles were observed close to the surface, regardless assumptions on a size distribution or the refractive indices. Contrary to dust, the different sizes of water ice particles assumed during retrieval stronger affected the retrieved temperature profiles than water ice opacities. Moreover, the differences in the retrieved temperature profiles were amplified while wrong optical properties for dust as well as for water ice aerosol were assumed instead of the nominal case. This means that the wrong assumption can induce an additional source of the retrieval error and lead to unreasonable temperature profiles. In the cases of expected heavily loads water ice crystals, their size distribution in the Martian atmosphere should be known from other observations before the retrieval of the temperature profile is attempted.For the analyzed examples of real PFS measurements the impact of different dust vertical distributions on the retrieval of temperature profile is prominent only in layers close to the surface. However, these differences remain comparable with retrieval errors. All influences of dust on weighting functions, brightness temperatures and during retrieval can be neglected if the noise equivalent radiance (NER) of PFS is taken into account.     

The effect of ultrasonic waves on the nucleation of pure water and degassed water

In order to clarify the mechanism of nucleation of ice induced by ultrasound, ultrasonic waves have been applied to supercooled pure water and degassed water, respectively. For each experiment, water sample is cooled at a constant cooling rate of 0.15 °C/min and the ultrasonic waves are applied from the water temperature of 0 °C until the water in a sample vessel nucleates. This nucleation temperature is measured. The use of ultrasound increased the nucleation temperature of both degassed water and pure water. However, the undercooling temperature for pure water to nucleate is less than that of degassed water. It is concluded that cavitation and fluctuations of density, energy and temperature induced by ultrasound are factors that affect the nucleation of water. Cavitation is a major factor for sonocrystallisation of ice.     

Monitoring the presence of water and water–sand droplets in a horizontal pipe with Acoustic Emission technology

Monitoring of multiphase flow is a process that has been established over several decades. This paper demonstrates the use of Acoustic Emission (AE) technology to detect and monitor moving water and water–sand droplets in a horizontal pipe. The experimental investigation considered two types of droplets, water and water–sand with average droplet volumes ranging from 1 ml to 5 ml. The experimental findings show good correlation between AE energy, droplet volume and the superficial gas velocity (V_SG).     

Ultrasonic-induced nucleation of ice in water containing air bubbles

Cavitation induced by ultrasonic vibrations can cause nucleation of ice in supercooled water. In this study, the time required for ultrasonic-induced nucleation of ice was measured for water containing two different size distributions of air bubbles. When the water was supersaturated with air bubbles, there was a time lag of about 0.5 s between the onset of ultrasonic irradiation and the onset of ice nucleation, and the probability of ice nucleation was unusually high within 0.5-1.1 s after the onset of ultrasonic irradiation. These results cannot be explained by conventional models alone, in which the collapse of a cavitation bubble triggers the nucleation of ice. Secondary effects appear to also influence ice nucleation.     

In situ investigation of ice formation on surfaces with representative wettability

In this work, we have prepared a series of samples with five representative surface wettabilities: i.e. superhydrophilic, hydrophilic, critical, hydrophobic and superhydrophobic. These samples were in situ observed the freezing process of water droplets on clean and artificially contaminated surfaces to investigate the relationship between surface wettability and ice formation. Ice accretion was also tested by spraying supercooled water to samples at different horizontal inclination angles (HIA). Surface topography was proved to be essential to the icing through heterogeneous nucleation. However, the correlation between surface wettability and ice formation was not observed. Finally, we found that the superhydrophobic surface clearly exhibited reduced ice accumulation in the initial stage of ice formation associated with the lower sliding angle (SA) of water droplets.     

Nucleation and growth of SWNT: TEM studies of the role of the catalyst

This paper reviews transmission electron microscopy studies, combining high resolution imaging and electron energy loss spectroscopy, of the nucleation and growth of carbon single wall nanotubes with a particular emphasis on the nanotubes obtained from the evaporation-based elaboration techniques. Inspection of samples obtained from different synthesis routes shows that in all cases nanotubes are found to emerge from catalyst particles and that they have grown perpendicular or parallel to the surface according to whether they have been synthesized via evaporation-based methods or CCVD methods. Whereas the latter case corresponds to the well-known situation of carbon filaments growth, the former case strongly suggests another formation and growth process, which is described and its different steps discussed in detail. In this model, formation of the nanotubes proceeds via solvation of carbon into liquid metal droplets, followed by precipitation, at the surface of the particles, of excess carbon in the form of nanotubes through a nucleation and root growth process. It is argued that the nucleation of the nanotubes, which compete with the formation of graphene sheets wrapping the surface of the particle, necessarily results from a surface instability induced by the conditions of segregation. The nature and the origin of this instability was studied in the case of the class of catalyst Ni–R.E. (R.E.=Y, La, Ce, …) in order to identify the influence of the nature of the catalyst. The respective roles played by Ni and R.E. have been identified. It is shown that carbon and rear-earth co-segregate and self-assemble at the surface of the particle in order to form a surface layer destabilizing the formation of graphene sheets and providing nucleation sites for nanotubes growing perpendicular to the surface. To cite this article: A. Loiseau et al., C. R. Physique 4 (2003).     

High undercooling of bulk water during acoustic levitation

The experiments on undercooling of acoustically levitated water drops with the radius of 5-8 mm are carried out, and the maximum undercooling of 24 K is obtained in such a containerless state. Various factors influencing the undercoolability of water under acoustic levitation are synthetically analyzed. The experimental results indicate that impurities tend to decrease the undercooling level, whereas the dominant factor is the effect of ultrasound. The stirring and cavitation effects of ultrasound tend to stimulate the nucleation of water and prevent further bulk undercooling in experiments. The stirring effect provides some extra energy fluctuation to overcome the thermodynamic barrier for nucleation. The local high pressure caused by cavitation effect increases the local undercooling in water and stimulates nucleation before the achievement of a large bulk undercooling. According to the cooling curves, the dendrite growth velocity of ice is estimated, which is in good agreement with the theoretical prediction at the lower undercooling. The theoretical calculation predicts a dendrite growth velocity of 0.23 m/s corresponding to the maximum undercooling of 24 K, at which the rapid solidification of ice occurs.     

Control of nanoparticle size and agglomeration through electric-field-enhanced flame synthesis

The isolated study of electrophoretic transport of nanoparticles (that are innately charged through thermionic emission), with no ionic wind, has been conducted under uniform electric fields. Titania nanoparticles are produced using a burner-supported low-pressure premixed flame in a stagnation-point geometry from corresponding organometallic vapor precursor. The material processing flow field is probed in-situ using laser-induced fluorescence (LIF) to map OH-radical concentrations and gas-phase temperatures. The experimental results of particle growth under different applied electric fields are compared with computations using monodisperse and sectional models. The results show that such electric field application can decrease aggregate particle size (e.g. from 40 to 18 nm), maintain metastable phases and particle crystallinity, and non-monotonically affect primary particle size (e.g. from 6 to 5 nm) and powder surface area. A specific surface area (SSA) for anatase titania nanopowder of 310 m²/g, when synthesized under an applied electric field of 125 V/cm, is reported. Results are also given for the synthesis of alumina nanoparticles.     

Evidence for the transition from the diffusion-limit in aluminum particle combustion

This work presents experimental evidence that the transition from gas-phase diffusion-limited combustion for aluminum particles begins to occur at a particle size of 10 μm at a pressure of 8.5 atm. Measurements of the particle temperature by AlO spectroscopy and three-color pyrometry indicate that the peak temperature surrounding a burning particle approaches the aluminum boiling temperature as particle size is decreased to 10 μm when oxygen is the oxidizer. This reduction indicates that reactions are occurring at or near the particle surface rather than in a detached diffusion flame. When CO₂ is the oxidizer, the combustion temperatures remain near the aluminum boiling temperature for particles as large as 40 μm, indicating that the flame is consistently near the surface throughout this size range. Burn time measurements of 10 and 2.8 μm powders indicate that burn time is roughly proportional to particle diameter to the first power. The burn rates of micron- and nano-particles also show strong pressure dependence. These measurements all indicate that the combustion has deviated from the vapor-phase diffusion limit, and that surface or near-surface processes are beginning to affect the rate of burning. Such processes would have to be included in combustion models in order to accurately predict burning characteristics for aluminum with diameter less than 10 μm.     

Correlated nucleation model for simulating nanocluster pattern formation on Si(111)7 × 7 surface

We study the aggregation mechanisms of metal nanoclusters on the Si(111)7 × 7 reconstructed surface using a correlated nucleation model, in which the nucleation and growth behavior of a cluster (irreversible or partially reversible growth) depend on the local environment of the cluster. The kinetic Monte Carlo simulation of the model shows that with increasing temperature, the correlated nucleation effect causes a transition of growth behavior from asymmetric adatom aggregation between faulted and unfaulted half cells with a strong preference of occupation of faulted half cells, to compact cluster aggregation with a low occupation preference at high temperatures. As a result the preference as a function of the temperature exhibits a nonmonotonous behavior, with a maximum located at the temperature at which the transition of growth behavior has been observed. Both the simulated cluster morphologies and the quantitative analysis of the cluster distribution are in good agreement with the results observed from relevant growth experiments.     

Acoustic cavitation in 1-butyl-3-methylimidazolium bis(triflluoromethyl-sulfonyl)imide based ionic liquid

In this work, a comparison between the temperatures/pressures within acoustic cavitation bubble in an imidazolium-based room-temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium bis(triflluoromethyl-sulfonyl)imide ([BMIM][NTf₂]), and in water has been made for a wide range of cavitation parameters including frequency (140–1000 kHz), acoustic intensity (0.5–1 W cm⁻²), liquid temperature (20–50 °C) and external static pressure (0.7–1.5 atm). The used cavitation model takes into account the liquid compressibility as well as the surface tension and the viscosity of the medium. It was found that the bubble temperatures and pressures were always much higher in the ionic liquid compared to those predicted in water. The valuable effect of [BMIM][NTf₂] on the bubble temperature was more pronounced at higher acoustic intensity and liquid temperature and lower frequency and external static pressure. However, confrontation between the predicted and the experimental estimated temperatures in ionic liquids showed an opposite trend as the temperatures measured in some pure ionic liquids are of the same order as those observed in water. The injection of liquid droplets into cavitation bubbles, the pyrolysis of ionic liquids at the bubble-solution interface as well as the lower number of collapsing bubbles in the ionic liquid may be the responsible for the lower measured bubble temperatures in ionic liquids, as compared with water.     

Relationships between cold hardiness, and ice nucleating activity, glycerol and protein contents in the hemolymph of caterpillars, Aporia crataegi L

Insects in Siberia must tolerate some of the coldest conditions on earth. The relationship between hemolymph ice nucleating activity, glycerol and total protein concentrations, and cold hardiness was explored in Aporia crataegi L. (Lepidoptera: Pieridae). Cold-hardened overwintering caterpillars were collected at a time of year when temperatures are regularly below -50 degree C, and warm-acclimated at +22 degree C, to see how changes in the physical and chemical properties of the hemolymph influence their cold hardiness potential. Warm acclimation led to a decrease in glycerol and proteins content in the hemolymph, which was associated with the decrease in ice nucleating activity and dramatic loss of cold hardiness potential of the caterpillars. It is suggested that one of the effects of cryoprotection in the freeze tolerant insects, caused by glycerol, might be associated with its ability to form larger aggregates of ice nucleating polypeptides that initiate the ice nucleation at high subzero temperatures. Such ice nucleating structures seem to ensure a high probability of ice nucleation at relatively high temperatures, which may contribute to the extraordinary cold hardiness of A. crataegi caterpillars, which may tolerate temperatures below -85 degree С.