Ice Nucleation Mechanisms Research Articles

Cell-culture compatible silk fibroin scaffolds concomitantly patterned by freezing conditions and salt concentration

The morphology of freeze-dried silk fibroin 3D-scaffolds was modified by varying both the NaCl concentration and the freezing temperature of the silk fibroin solution prior to lyophilization. Scanning electron micrographs showed that slow freezing at −22 °C generated sponge-like interconnected porous networks, whereas fast freezing at −73 °C formed stacked leaflet structures. The presence of millimolar NaCl (50–250 mM) increased the porosity of the scaffolds and generated small outgrowths at their surface, depending on the freezing regime. Our results suggest that the morphological differences seen between the materials likely depend on ice and NaCl hydrate crystal nucleation and growth mechanisms. Infrared spectroscopy and X-ray diffraction analyses revealed that the salt concentration and freezing conditions induced no structural changes in fibroin. The seeding of P19 embryonic carcinoma cells showed that the presence of salt and freezing conditions influenced the cell distribution into the scaffolds, with salt addition increasing the access of cells to deeper regions.

Evidence of liquid dependent ice nucleation in high-latitude stratiform clouds from surface remote sensors

[1] Ground-based lidar, radar and microwave radiometer observations at Eureka, Canada, Barrow, Alaska and over the western Arctic Ocean measure physical characteristics and morphology of stratiform clouds. Despite transition of a cold atmosphere (−15 C)through ice supersaturated conditions, ice is not observed until soon after a liquid layer. Several cases illustrating this phenomenon are presented in addition to long-term observations from three measurement sites characterizing cloud phase frequency. This analysis demonstrates that clouds composed entirely of ice occur less frequently than liquid-topped mixed-phase clouds at temperatures warmer than −25 to −30 C. These results indicate ice formation generally occurs in conjunction with liquid at these temperatures, and suggest the importance of liquid-dependent ice nucleation mechanisms.

Global simulations of ice nucleation and ice supersaturation with an improved cloud scheme in the Community Atmosphere Model

A process‐based treatment of ice supersaturation and ice nucleation is implemented in the National Center for Atmospheric Research Community Atmosphere Model (CAM). The new scheme is designed to allow (1) supersaturation with respect to ice, (2) ice nucleation by aerosol particles, and (3) ice cloud cover consistent with ice microphysics. The scheme is implemented with a two‐moment microphysics code and is used to evaluate ice cloud nucleation mechanisms and supersaturation in CAM. The new model is able to reproduce field observations of ice mass and mixed phase cloud occurrence better than previous versions. The model is able to reproduce observed patterns and frequency of ice supersaturation. Simulations indicate homogeneous freezing of sulfate and heterogeneous freezing on dust are both important ice nucleation mechanisms, in different regions. Simulated cloud forcing and climate is sensitive to different formulations of the ice microphysics. Arctic surface radiative fluxes are sensitive to the parameterization of ice clouds. These results indicate that ice clouds are potentially an important part of understanding cloud forcing and potential cloud feedbacks, particularly in the Arctic.

Biotransport Phenomena in Freezing Mammalian Oocytes

Water transport across the cell plasma membrane and intracellular ice formation (IIF)-the two biophysical events that may cause cell injury during cryopreservation-were studied by cryomicroscopy and modeling using mammalian (Peromyscus) oocytes. Unusually high activation energy for water transport across the cell plasma membrane was identified indicating that the water transport process is unusually sensitive to temperature (and cooling rate). Although literally all studies on IIF were conducted using protocols with ice-seeding (seeding extracellular ice usually at≥-7°C), it is not used for cell cryopreservation by vitrification that is becoming increasingly popular today. In this article, we show that ice-seeding has a significant impact on IIF. With ice-seeding and cooling at 60°C/min, IIF was observed to occur over a wide range from approximately -8 to -48°C with a clear change of the ice nucleation mechanism (from surface- to volume-catalyzed nucleation) at approximately -43°C. On the contrary, without ice-seeding, IIF occurred over a much narrower range from approximately -19 to -27°C without a noticeable change of the nucleation mechanism. Moreover, the kinetics of IIF without ice-seeding was found to be strongly temperature (and cooling rate) dependent. These findings indicate the importance of quantifying the IIF kinetics in the absence of ice-seeding during cooling for development of optimal vitrification protocols of cell cryopreservation.

Evolution of water-in-oil emulsions stabilized with solid particles: Influence of added emulsifier

Water-in-oil emulsions made of water droplets dispersed in a continuous paraffin oil phase were prepared and stabilized with hydrophobic silica particles alone. Differential scanning calorimetry (DSC) experiments were carried out to characterize the water droplets freezing transitions and their evolution with time. Water droplet size distribution, oil–water interfacial tension, and rheological stress–strain properties were determined alongside to better understand the role of particles in the formation and stabilization of emulsions. The results obtained were compared to the properties of emulsions prepared with a non-ionic emulsifier, sorbitan monooleate. No interfacial tension reduction was observed at the oil/water interface in presence of particles. As a consequence, the fragmentation of water into droplets required more energy in absence of surface-active emulsifiers. The resulting emulsion droplet size distribution is polydisperse and contains large droplets. Rheology measurements showed that the stability of the emulsion prepared with particles originates from the formation of a 3D network of particles in the continuous oil phase. In emulsion samples containing sorbitan monooleate, calorimetry experiments revealed a progressive displacement of the water droplet freezing transition due to a change in ice nucleation mechanism. The interpretation points out a possible reorganization of the emulsifier film at the water/oil interface, which could modify the conditions of crystallization of dispersed water droplets and affect the emulsion long-term stability. Calorimetry, when used together with other techniques, such as laser light scattering, provides complementary information on the evolution with time of the structure of the emulsion.

Understanding ice supersaturation, particle growth, and number concentration in cirrus clouds

Many factors control the ice supersaturation and microphysical properties in cirrus clouds. We explore the effects of dynamic forcing, ice nucleation mechanisms, and ice crystal growth rate on the evolution and distribution of water vapor and cloud properties in nighttime cirrus clouds using a one‐dimensional cloud model with bin microphysics and remote sensing measurements obtained at the Department of Energy's Atmospheric Radiation Measurement (ARM) Climate Research Facility located near Lamont, OK. We forced the model using both large‐scale vertical ascent and, for the first time, mean mesoscale velocity derived from radar Doppler velocity measurements. Both heterogeneous and homogeneous nucleation processes are explored, where a classical theory heterogeneous scheme is compared with empirical representations. We evaluated model simulations by examining both bulk cloud properties and distributions of measured radar reflectivity, lidar extinction, and water vapor profiles, as well as retrieved cloud microphysical properties. Our results suggest that mesoscale variability is the primary mechanism needed to reproduce observed quantities. Model sensitivity to the ice growth rate is also investigated. The most realistic simulations as compared with observations are forced using mesoscale waves, include fast ice crystal growth, and initiate ice by either homogeneous or heterogeneous nucleation. Simulated ice crystal number concentrations (tens to hundreds particles per liter) are typically two orders of magnitude smaller than previously published results based on aircraft measurements in cirrus clouds, although higher concentrations are possible in isolated pockets within the nucleation zone.

Water Modeled As an Intermediate Element between Carbon and Silicon

Water and silicon are chemically dissimilar substances with common physical properties. Their liquids display a temperature of maximum density, increased diffusivity on compression, and they form tetrahedral crystals and tetrahedral amorphous phases. The common feature to water, silicon, and carbon is the formation of tetrahedrally coordinated units. We exploit these similarities to develop a coarse-grained model of water (mW) that is essentially an atom with tetrahedrality intermediate between carbon and silicon. mW mimics the hydrogen-bonded structure of water through the introduction of a nonbond angular dependent term that encourages tetrahedral configurations. The model departs from the prevailing paradigm in water modeling: the use of long-ranged forces (electrostatics) to produce short-ranged (hydrogen-bonded) structure. mW has only short-range interactions yet it reproduces the energetics, density and structure of liquid water, and its anomalies and phase transitions with comparable or better accuracy than the most popular atomistic models of water, at less than 1% of the computational cost. We conclude that it is not the nature of the interactions but the connectivity of the molecules that determines the structural and thermodynamic behavior of water. The speedup in computing time provided by mW makes it particularly useful for the study of slow processes in deeply supercooled water, the mechanism of ice nucleation, wetting-drying transitions, and as a realistic water model for coarse-grained simulations of biomolecules and complex materials.

Experimental investigation of ice nucleation by different types of aerosols in the aerosol chamber AIDA: implications to microphysics of cirrus clouds

The aerosol chamber AIDA was used as a moderate expansion cloud chamber with cooling rates at the onset of ice nucleation between -1.3 and -3.0 K min -1 to investigate the nucleation and growth of ice crystals in sulphuric acid, ammonium sulphate, and mineral dust aerosols at temperatures between 196 and 224 K. Supercooled sulphuric acid droplets with mean diameters of about 0.2 to 0.3 μm nucleated ice by homogeneous freezing at RH ice increasing from 144 to 166 % with temperatures from 220 and 196 K. This is in good agreement both with previous results of AIDA experiments and literature data. In contrast, ammonium sulphate particles of similar size nucleated ice at the significantly lower RH ice of 120 to 127 % in the same temperature range. Fourier-Transform infrared (FTIR) extinction spectra of the aerosol revealed that the ammonium sulphate particles, mainly consisted of the liquid phase. The number concentration of ice crystals formed during the homogeneous freezing experiments agree well with model results from the literature. Higher ice crystal number concentrations formed during the ammonium sulphate, compared to the sulphuric acid experiments, can be explained by the also somewhat higher cooling rates at ice nucleation. Deposition ice nucleation on mineral dust particles turned out to be the most efficient ice nucleation mechanism both with respect to RH ice at the onset of ice nucleation (102 to 105 % in the temperature range 209 to 224 K) and the ice crystal number concentration. Almost all mineral dust particles nucleated ice at the lower temperatures.

Ice nucleation on BaF2(111)

The mechanism of heterogeneous ice nucleation on inorganic substrates is not well understood despite work on AgI and other materials over the past 50 years. We have selected BaF(2) as a model substrate for study since its (111) surface makes a near perfect match with the lattice of the basal face of I(h) ice and would appear to be an ideal nucleating agent. Two series of experiments were undertaken. In one, nucleation of thin film water formed from deposition of vapor on BaF(2)(111) faces was explored with the finding that supercooling to -30 degrees C was required before freezing occurred. In the other series, nucleation of liquid water on submerged BaF(2) crystals was studied. Here supercooling to -15 degrees C was needed before ice formed. The reason why BaF(2) is such a poor nucleating agent contains clues to realistic mechanisms of heterogeneous nucleation. Our explanation of these results follows the model of Fletcher [J. Chem. Phys. 29, 572 (1958)] who showed that heterogeneous ice nucleating ability depends on how well ice wets a substrate. In this view, a smooth BaF(2)(111) face is poor at nucleation because ice only partially wets its surface. In an extension of Fletcher's model, our calculations, consistent with the experimental results demonstrate that the pitting of a submerged BaF(2) crystal dramatically improves its ice nucleating ability.

Sensitivity studies of cirrus clouds formed by heterogeneous freezing in the ECHAM GCM

Cirrus clouds can form by homogeneous and heterogeneous ice nucleation mechanisms at temperatures below 235 K. Here we evaluate the effectiveness of heterogeneous freezing versus homogeneous freezing using a newly developed parameterization of heterogeneous freezing that is restricted to immersion freezing as the most likely pathway for heterogeneous ice formation in cirrus conditions [Kärcher and Lohmann, 2003]. In addition to a reference simulation considering homogeneous nucleation with temperature‐dependent freezing thresholds, we discuss two idealized model experiments. We conduct a scenario that hypothetically assumes that the aerosol particles available for homogeneous freezing could act as freezing nuclei commencing freezing at 130% with respect to ice and contrast that by a scenario that only considers black carbon and mineral dust as immersion nuclei with the same freezing relative humidity of 130%. These idealized simulations serve to delimit possible climate responses. If the number of freezing nuclei is limited by the number of black carbon and dust aerosols, then heterogeneous freezing results in fewer ice crystals than formed by homogeneous freezing. These fewer ice crystals grow more readily to precipitation size and with that increase the global mean precipitation, decrease the ice water path, and trap less outgoing longwave radiation at the top of the atmosphere.

Wetting and hydration of insoluble soot particles in the upper troposphere

Wettability and hydration are determined for aircraft combustor and laboratory-made soots which are used as surrogates for the insoluble part of aircraft-generated black carbon particles in the upper troposphere (UT). The measured water/ice contact angles on the soot surfaces are in the range 60-80 degrees. Factors influencing the soot wetting show a tremendous dependence on the surface chemical composition and microstructure. Wetting characteristics of soots are directly related to its hydrophilicity. The inverse Kelvin effect is considered as a mechanism of ice nucleation which is facilitated by the soot agglomerated structure with interparticle cavities in which condensation takes place on the insoluble surface with a high water contact angle. Estimations of the critical supersaturations needed for the ice condensation growth of particles are provided to determine which of the wetting characteristics are required for cirrus cloud formation in ice saturated regions of the UT.

Ice condensation on sulfuric acid tetrahydrate: Implications for polar stratospheric ice clouds

Abstract. The mechanism of ice nucleation to form Type 2 PSCs is important for controlling the ice particle size and hence the possible dehydration in the polar winter stratosphere. This paper probes heterogeneous ice nucleation on sulfuric acid tetrahydrate (SAT). Laboratory experiments were performed using a thin-film, high-vacuum apparatus in which the condensed phase is monitored via Fourier transform infrared spectroscopy and water pressure is monitored with the combination of an MKS baratron and an ionization gauge. Results show that SAT is an efficient ice nucleus with a critical ice saturation ratio of S*ice = 1.3 to 1.02 over the temperature range 169.8-194.5 K. This corresponds to a necessary supercooling of 0.1-1.3 K below the ice frost point. The laboratory data is used as input for a microphysical/photochemical model to probe the effect that this heterogeneous nucleation mechanism could have on Type 2 PSC formation and stratospheric dehydration. In the model simulations, even a very small number of SAT particles (e.g., 10-3 cm-3) result in ice nucleation on SAT as the dominant mechanism for Type 2 PSC formation. As a result, Type 2 PSC formation is more widespread, leading to larger-scale dehydration. The characteristics of the clouds are controlled by the assumed number of SAT particles present, demonstrating that a proper treatment of SAT is critical for correctly modeling Type 2 PSC formation and stratospheric dehydration.

A New Hypothesis for the Mechanism of Ice Nucleation on Wetted AgI and AgI·AgCl Particulate Aerosols

A potential molecular mechanism of ice nucleation on AgI and AgI · AgCl particulates involves rearrangement of ordered water molecule clusters associated with hydrated Ag+ ion patches. This nucleation mechanism is thought to occur rapidly at −5° to −20°C on substrate particles wetted by water. This hypothesis is based on analysis of the rates of ice crystal formation and ice nucleus activities observed in experiments previously conducted in a 1-m3 isothermal cloud chamber using chemical kinetics. These experiments examined the chemistry of wetted AgI colloid particles involving electric charge separations between the particles and adjacent solution. The rate of nucleation is slowed by the presence of alkali and alkaline earth halides in concentrations greater than approximately 10−3 M in the water wetting the particles. Match of the crystal structure of the substrate with that of the ice may have no effect on this mechanism.

Cirrus Parcel Model Comparison Project. Phase 1: The Critical Components to Simulate Cirrus Initiation Explicitly

Abstract The Cirrus Parcel Model Comparison Project, a project of the GCSS [Global Energy and Water Cycle Experiment (GEWEX) Cloud System Studies] Working Group on Cirrus Cloud Systems, involves the systematic comparison of current models of ice crystal nucleation and growth for specified, typical, cirrus cloud environments. In Phase 1 of the project reported here, simulated cirrus cloud microphysical properties from seven models are compared for “warm” (−40°C) and “cold” (−60°C) cirrus, each subject to updrafts of 0.04, 0.2, and 1 m s−1. The models employ explicit microphysical schemes wherein the size distribution of each class of particles (aerosols and ice crystals) is resolved into bins or the evolution of each individual particle is traced. Simulations are made including both homogeneous and heterogeneous ice nucleation mechanisms (all-mode simulations). A single initial aerosol population of sulfuric acid particles is prescribed for all simulations. Heterogeneous nucleation is disabled for a second...

Simulations of ice clouds during FIRE ACE using the CCCMA single‐column model

The single‐column model (SCM) of the Canadian Centre for Climate Modelling and Analysis (CCCMA) solves prognostic equations for the number concentration and mass mixing ratios of ice crystals. Ice crystal formation is specified via different ice nucleation mechanisms. The CCCMA SCM was used to simulate the evolution of ice clouds for three different flights during the First ISCCP Regional Experiment Arctic Cloud Experiment (FIRE ACE) in April 1998, where measurements of cloud droplets, ice crystals, and aerosols >0.08 μm in radius were conducted. The CCCMA SCM in its original setup predicts larger ice crystal concentrations than measured with the 2DC probe. The agreement with observations of ice crystals detectable with the 2DC probe was improved if a condensation freezing parameterization depending on supersaturation with respect to ice was used. The best correlation between observed and simulated ice crystal number, which still showed a lot of scatter, was obtained if an empirical linear relationship between the number of aerosols and the number of ice crystals deduced from FIRE ACE was used.

An Investigation of Ice Production Mechanisms in Small Cumuliform Clouds Using a 3D Model with Explicit Microphysics. Part I: Model Description

A new cloud model that combines a three-dimensional nonhydrostatic dynamical framework with explicit liquid- and ice-phase microphysics and a detailed treatment of ice nucleation and multiplication processes is presented. The use of 28 size bins to describe each cloud drop and ice particle spectra allows the authors to account for all major microphysical processes. A detailed scavenging model has been implemented to calculate the collection rate of contact ice nuclei by cloud drops. In addition to contact nucleation, the model accounts for deposition, condensation-freezing, and immersion ice nucleation mechanisms, as well as for secondary ice production via rime splintering. The model performance is illustrated by a simulation of a New Mexican cumulus cloud. Combination of high 100-m spatial resolution with a new initialization procedure that promotes development of small eddies results in a cloud with a more realistic distribution of liquid water content compared to simulations initialized by the standard “bubble” approach.

Ice nucleation in cirrus clouds: A model study of the homogeneous and heterogeneous modes

Parcel cloud model simulations have explored the feasibility of a unified approach to describe the homogeneous and heterogeneous ice nucleation mechanisms in cirrus clouds. It was assumed that the ice crystal generation process involved the freezing of ammonium sulfate haze particles, as modulated by an effective temperature proportional to molality in both cases. Treated as variables in the ‐36° to ‐60°C model domain are the updraft velocity, and the terms in a modified Fletcher equation used to approximate heterogeneous freezing. Parameterizations drawn from the homogeneous freezing runs are provided for the threshold relative humidity and number of crystals nucleated, as functions of both temperature and updraft velocity. We find that heterogeneous nucleation can compete or dominate at relatively weak updrafts, but only for unusually high ice nuclei concentration and temperature activation terms.

Rates and Mechanisms of Heterogeneous Ice Nucleation on Silver Iodide and Silver Chloroiodide Particulate Substrates

Information on two molecular mechanisms of ice nucleation on silver iodide and silver chloroiodide particulates has been gained by a chemical kinetics interpretation of the rates and mechanisms of ice crystal formation from experiments conducted in a 1 m3isothermal cloud chamber. The interpretation required the consideration of the Langmuir surface adsorption of water molecules by AgI from the vapor and the chemistry of AgI colloids and suspensions in water with and without the influences of supporting electrolytes. One mechanism is a slow rearrangement of three-dimensional water molecule clusters, adsorbed at water saturation and −20°C and lower on 500 Å and larger substrate particles, to ice embryos. The second occurs rapidly at −5 to −20°C on substrate particles wetted by liquid water. This mechanism is hypothesized to involve the rearrangement of ordered water molecule clusters associated with hydrated Ag+ion patches on the substrate surface. The rate of nucleation is slowed by the presence of alkali and alkaline earth halides. Crystal structure match of the substrate with that of ice may help the first mechanism but, perhaps, has no effect on the second.

The construction and application of numerical models to the study of cloud dynamics and the structure of winter frontal rainbands

The objectives of the present research are to construct a set of equations for simulating the evolution of frontal systems, to develop a finite-difference scheme of their solution, and to carry out numerical modelling. Numerical models with detailed descriptions of cloud particle evolution (nuclei, drops, crystals, aggregates, etc.) including condensation (sublimation) and coagulation processes are constructed to study the inner structure of frontal mixed stratiform clouds and precipitation development. Two- and three-dimensional diagnostic and prognostic simulations of two occluded winter frontal systems that passed over Ukraine and a two-dimensional prognostic model of an occluded frontal system that passed over USA are presented. A two-dimensional model with nested and stretched grids is used for the simulation of convective cells embedded in stratiform. The structure of the simulated atmospheric fronts in occluded cyclones exhibits many common features described in other previous studies, for other mid-latitude regions: hyperbaroclinic zones of different scales within the mesoscale range, small and large mesoscale rainbands, embedded convective cells, wave-like features of cloud and precipitation evolution, etc. The qualitative estimations of instability in frontal zones implies that large rainbands can apparently be generated in baroclinic zones with stable stratification. In unstable zones, small mesoscale rainbands with embedded convective cells are mainly formed. The clouds' precipitation is determined largely by dynamical features. The mechanisms of ice nucleation, sublimation, freezing or coagulation processes have a crucial role in cloud and precipitation formation if there is an inability to realise all of the precipitable moisture.

Endothermal wetting effect and ice nucleation mechanisms of silver iodide

The thermodynamics of the water wetting of energetically heterogeneous surfaces of mixed aerosol particles (of type AgI) and ice nucleation are investigated. A structural substantiation of the endothermal effect in the “heterophobic wetting” has been proposed. Discussions on the ice-forming mechanism of AgI are generalized.