Hexagonal Ice Crystals Research Articles

The structure of ice under confinement in periodic mesoporous organosilicas (PMOs).

We investigated the structure of ice under nanoporous confinement in periodic mesoporous organosilicas (PMOs) with different organic functionalities and pore diameters between 3.4 and 4.9nm. X-ray scattering measurements of the system were performed at temperatures between 290 and 150K. We report the emergence of ice I with both hexagonal and cubic characteristics in different porous materials, as well as an alteration of the lattice parameters when compared to bulk ice. This effect is dependent on the pore diameter and the surface chemistry of the respective PMO. Investigations regarding the orientation of hexagonal ice crystals relative to the pore wall using x-ray cross correlation analysis reveal one or more discrete preferred orientation in most of the samples. For a pore diameter of around 3.8nm, stronger correlation peaks are present in more hydrophilically functionalized pores and seem to be connected to stronger shifts in the lattice parameters.

Quantifying the Impact of the Surface Roughness of Hexagonal Ice Crystals on Backscattering Properties for Lidar‐Based Remote Sensing Applications

AbstractImpacts of small‐scale surface irregularities, or surface roughness, of atmospheric ice crystals on lidar backscattering properties are quantified. Geometric ice crystal models with various degrees of surface roughness and state‐of‐the‐science light‐scattering computational capabilities are utilized to simulate the single‐scattering properties across the entire practical size parameter range. The simulated bulk lidar and depolarization ratios of polydisperse ice crystals at wavelength 532 nm are strongly sensitive to the degree of surface roughness. Comparisons of these quantities between the theoretical simulations and counterparts inferred from spaceborne lidar observations for cold cirrus clouds suggest a typical surface‐roughness‐degree range of 0.03–0.15 in the cases of compact hexagonal ice crystals, which is most consistent with direct measurements of scanning electron microscopic images. To properly interpret lidar backscattering observations of ice clouds, it is necessary to account for the degree of surface roughness in light‐scattering computations involving ice crystals.

Brassica juncea leaf cuticle contains xylose and mannose (xylomannan) which inhibit ice recrystallization on the leaf surface.

Conjugated sugars showed antifreeze activity in the cuticle by ice recrystallization inhibition rather than thermal hysteresis, enhancing freezing capacity at the surface of B. juncea leaves. Antifreeze biomolecules play a crucial role in mitigating the physical damage from frost by controlling extracellular ice crystal growth in plants. Antifreeze proteins (AFPs) are reported from the apoplast of different plants. Interestingly, there is no report about antifreeze properties of the cuticle. Here, we report the potential antifreeze activity in the Brassica juncea (BJ) leaf cuticle. Nano LC-MS/MS analysis of a cuticle protein enriched fraction (CPE) predicted over 30 putative AFPs using CryoProtect server and literature survey. Ice crystal morphology (ICM) and ice recrystallization inhibition (IRI) analysis of ABC supernatant showed heat and pronase-resistant, non-protein antifreeze activities as well as hexagonal ice crystals with TH of 0.17°C and IRI 46%. The ZipTip processed ABC supernatant (without peptides) had no effect on TH activity, confirming a non-protein antifreeze molecule contributing to activity. To understand the origin and to confirm the source of antifreeze activity, cuticular membranes were isolated by pectinase and cellulase hydrolysis. FTIR analysis of the intact cuticle showed xylose, mannose, cellulose, and glucose. Xylanase and cellulase treatments of the ZipTip processed ABC supernatant led to an increase in sugar content and 50% loss in antifreeze activity. UV spectroscopy and NMR analysis supported the finding of FTIR and enzyme hydrolysis suggesting the contribution of xylose and mannose to antifreeze activity. By TLC analysis, conjugated sugars were found in the cuticle. This work has opened up a new research area where the antifreeze capacity needs to be established with regard to complete characterization and mechanism of action of the antifreeze carbohydrates (conjugated sugars) on the leaf surface.

Ice Nucleation Promotion Impact on the Ice Recrystallization Inhibition Activity of Polyols.

Heterogeneous ice nucleation occurs vis-à-vis nucleating agents already present in solution yet can occur within a rather broad range of temperatures (0 to ca. -38 °C). Controlling this temperature and the subsequent growth of resulting ice crystals is crucial for the survival of biological organisms (certain insects, fish, and plants that endure subzero temperatures), as well as in the context of medical cryopreservation and food science. In these environments, uncontrolled crystal shape and size can rupture the cell membrane causing irreversible and catastrophic damage. Antifreeze (AF) proteins and synthetic AF analogs address this issue to restrict crystal growth and to shape ice crystals. Yet, if the nucleation temperature is not controlled and occurs in a lower temperature range, nascent ice crystals will have grown to a significantly larger size before the AF agents can be active on their surface to halt or slow the Ostwald ripening process during recrystallization. At a higher nucleation temperature, diffusion of AF macromolecules is enhanced, and dynamic crystal shaping can start earlier, producing smaller crystals overall. While antifreeze proteins, the inspiration for these synthetic analogs, are always applied in a salt buffer aqueous environment (most typically phosphate-buffered saline (PBS) buffer), the heterogeneous nucleation events are stochastic and occur within a wide temperature range. Silver iodide (AgI), however, is a highly effective ice nucleation promoter as its crystal lattice structure is a 98% lattice match to the basal plane of hexagonal ice (Ih) crystals acting as a template for water molecule orientation and decreasing the interfacial free energy. Here, we expose the advantage of purposely seeding such nascent ice crystals with AgI at a defined and higher temperature (-7 °C) in ultrapure water (UPW) such that nucleation can only come from AgI (and also in AgI/PBS), resulting in the most potent synthetic IRI observed to date (at concentrations as low as 0.001 mg·mL-1).

Consistency test of precipitating ice cloud retrieval properties obtained from the observations of different instruments operating at Dome C (Antarctica)

Abstract. Selected case studies of precipitating ice clouds at Dome C (Antarctic Plateau) were used to test a new approach for the estimation of ice cloud reflectivity at 24 GHz (12.37 mm wavelength) using ground-based far infrared spectral measurements from the REFIR-PAD Fourier transform spectroradiometer and backscattering/depolarization lidar profiles. The resulting reflectivity was evaluated with the direct reflectivity measurements provided by a co-located micro rain radar (MRR) operating at 24 GHz, that was able to detect falling crystals with large particle size, typically above 600 µm. To obtain the 24 GHz reflectivity, we used the particle effective diameter and the cloud optical depth retrieved from the far infrared spectral radiances provided by REFIR-PAD and the tropospheric co-located backscattering lidar to calculate the modal radius and the intercept of the particle size distribution. These parameters spanned in the wide ranges between 570–2400 µm and 10−2–104 cm−5, respectively. The retrieved effective sizes and optical depths mostly varied in the ranges 70–250 µm and 0.1–5, respectively. From these parameters, the theoretical reflectivity at 24 GHz was obtained by integrating the size distribution over different cross sections for various habit crystals provided by Eriksson et al. (2018) databases. From the comparison with the radar reflectivity measurements, we found that the hexagonal column-like habits, the columnar crystal aggregates, and the 5/6 branches bullet rosettes showed the best agreement with the MRR observations. The dispersion coefficient of the crystal particle size distribution was assumed in the range 0–2 according to the temperature dependence found in previous studies. The retrieved values of the intercept and slope were found in good agreement with these studies. The presence of the inferred habits was confirmed by the crystal images taken by the ICE-CAMERA, operating in proximity of REFIR-PAD and the MRR. In particular, the occurrence of hexagonal column-like ice crystals was confirmed by the presence of 22∘ solar halos, detected by the HALO-CAMERA. The average crystal lengths obtained from the retrieved size distribution were also compared to those estimated from the ICE-CAMERA images. The agreement between the two results confirmed that the retrieved parameters of the particle size distributions correctly reproduced the observations.

High small ice concentration in stratiform clouds over Eastern China based on aircraft observations: Habit properties and potential roles of secondary ice production

The microphysical properties of supercooled liquid droplets (SLDs) and ice particles of stratiform mixed-phase clouds over Eastern China are characterized using carefully post-processed airborne data. The majority of sampled clouds were precipitating with ice particles and were frequently mixed with SLDs at cold temperature. While the concentration of large ice crystal (> 600 μm in diameter, the same below) was low (up to 3 L−1), the concentration of smaller ice particle (> 50 μm) was high (up to 300 L−1). Such particles with high concentration cannot be a result of the recirculation of pre-existing aged ice and thus secondary ice production (SIP) was likely occurring over the stratiform clouds at temperatures between −16.9 °C and −6.4 °C. The statistical analyses show that concentrations of tiny, hexagonal and irregular ice crystals were significantly greater in updraft than those in downdraft regions, suggesting that updrafts not only provide a favorable environment for the growth of cloud particles, but also promote the multiplication of the above young-age small ice (50–100 μm) where SIP is commonly occurring. Since the criteria for the other SIP mechanisms are difficult to meet for this light-riming stratiform without deep convections, this analysis indicates that shattering during droplet freezing might thus be an important SIP source at temperatures between −15 °C and −9 °C. This study should provide a precise opportunity for parameterizations of mixed-phase stratiform clouds associated with the SIP processes and effects of updraft and temperature.

Light scattering by airborne ice crystals – An inventory of atmospheric halos

Atmospheric halos are a light scattering phenomenon caused by airborne ice crystals in the atmosphere. Halos can be seen by the naked eye. They provide the observer the information on the kinds of ice crystals present in the sky during a halo display. A combination of ice crystals’ shape, their orientation, and light ray paths through the crystals dictates what halos will be observable. All well-known halos are produced by the interaction of light with crystals of hexagonal water ice. However, some of the documented halos cannot be explained using common hexagonal ice crystals. It might be required to consider such factors as abnormal crystal shape, crystals of cubic water ice, or airborne crystals of other minerals to explain the nature of some exotic halos.Halos can be also observed in the atmospheres of other planets or their moons, which provides information on airborne ice crystals or crystals of other minerals. Ice crystal halos have been already photographed in the Martian atmosphere.In this article, we summarize the current knowledge of atmospheric halos and show which halo forms cannot be explained by ordinary hexagonal ice crystals. We list here 119 different identifiable halo forms known by today.

Theoretical Calculations of Directional Scattering Intensities of Small Nonspherical Ice Crystals: Implications for Forward Scattering Probes

In situ aircraft measurements of the sizes and concentrations of liquid cloud droplets and ice crystals with maximum dimensions (Dmax) less than ~50 μm have been measured mainly using forward scattering probes over the past half century. The operating principle of forward scattering probes is that the measured intensity of light scattered by a cloud particle at specific forward scattering angles can be related to the size of that particle assuming the shape and thermodynamic phase of the target are known. Current forward-scattering probes assume spherical liquid cloud droplets and use the Lorenz–Mie theory to convert the scattered light to particle size. Uncertainties in sizing ice crystals using forward scattering probes are unavoidable since the single-scattering properties of ice crystals differ from those of spherical liquid cloud droplets and because their shapes can vary. In this study, directional scattering intensities of four different aspect ratios (ARs = 0.25, 0.50, 1.00, and 2.00) of hexagonal ice crystals with random orientations and of spherical liquid cloud droplets were calculated using the discrete dipole approximation (i.e., ADDA) and Lorenz–Mie code, respectively, to quantify the errors in sizing small ice crystals and cloud droplets using current forward scattering probes and to determine the ranges of optimal scattering angles that would be used in future forward scattering probes. The calculations showed that current forward scattering probes have average 5.0% and 17.4% errors in sizing liquid cloud droplets in the forward (4–12°) and backward (168–176°) direction, respectively. For measurements of hexagonal ice crystals, average sizing errors were 42.1% (23.9%) in the forward (backward) direction and depended on the ARs of hexagonal ice crystals, which are larger than those for liquid cloud droplets. A newly developed size conversion table based on the calculated single-scattering properties of hexagonal ice crystals using the ADDA reduced the sizing errors for the hexagonal ice crystals down to 14.2% (21.9%) in the forward (backward) direction. This study is a purely theoretical examination of the operating principle of forward scattering probes and there are several limitations, such as assumed hexagonal ice crystals with smooth surfaces and random orientations.

Design of ultrastretchable and super-elastic tailorable hydrophilic thermoplastic elastomeric materials

In this work, new ultrastretchable and super-elastic hydrophilic thermoplastic elastomeric (TPEs) materials were developed for the first time from the thermoplastic polyurethane (TPU) and epichlorohydrin-ethylene oxide-allyl glycidyl ether (GECO) rubber using straightforward melt-blending and dynamic vulcanization approaches, which allows the creation of tailorable hydrophilic TPE materials. The resulting materials can absorb up to ∼80% water within 4 h. The absorbed water convert itself into hexagonal ice crystals in the TPE matrix which was directly evident from differential scanning calorimetry and cryo-transmittance electron microscopy. The phase components of developed TPE were found to be interactive which was analysed from the torque and loss tangent values. Interaction was further enhanced after dynamic vulcanization which was predicted from the rheological Palierne model. Interestingly, interfacial tension between the TPU and GECO was significantly reduced after dynamic vulcanization which suggested stronger inter-domain interaction between the phase components. It was revealed that the resulting TPE possess droplet-matrix phase morphological structure which was supported from the theoretical Kerner model prediction. Surprisingly, resulting materials showed extraordinary elastomeric properties than that of TPEs reported earlier, such as very high strain at break (∼800–1000%) and lowest tension set (∼2–6%) which enables the creation of ultrastretchable and super-elastic materials. • A new kind of ultra-stretchable, superelastic and tailorable hydrophilic TPE was developed. • The developed TPE showed extraordinary elastic properties having high strain at failure (1000%) and low-tension set (∼2%). • The hexagonal ice crystals were formed in the TPE matrix by absorbed water.

Mean path length in refractive regular polygons and prisms

We analytically derive the mean path length of light rays diffusely incident on refractive regular polygons of $n$ sides ($n$-gons), analyzed from the dynamical billiards perspective and from ray optics. In polygons with sufficiently low refractive index, the mean path length is found to be equal to that in the invariant scattering case, i.e., the product of the mean chord length with the refractive index. If the refractive index is higher than some critical value, the mean path length is lower than the scattering value due to inaccessibility of trapped modes. Regular odd $n$-gons are found to have the same mean path length as $2n$-gons, when normalized against their mean chord length. There is a discontinuity between the mean path length in high $n$-gons and that in a circle, attributed to quasitrapped modes in high $n$-gons. This difference is removed with a small amount of absorption. We extend the results to refractive prisms with a regular polygon base, for example, hexagonal ice crystals. For prisms with any ergodic shaped base, we derive a simple formula for the mean path length.

Studying Ice with Environmental Scanning Electron Microscopy.

Scanning electron microscopy (SEM) is a powerful imaging technique able to obtain astonishing images of the micro- and the nano-world. Unfortunately, the technique has been limited to vacuum conditions for many years. In the last decades, the ability to introduce water vapor into the SEM chamber and still collect the electrons by the detector, combined with the temperature control of the sample, has enabled the study of ice at nanoscale. Astounding images of hexagonal ice crystals suddenly became real. Since these first images were produced, several studies have been focusing their interest on using SEM to study ice nucleation, morphology, thaw, etc. In this paper, we want to review the different investigations devoted to this goal that have been conducted in recent years in the literature and the kind of information, beyond images, that was obtained. We focus our attention on studies trying to clarify the mechanisms of ice nucleation and those devoted to the study of ice dynamics. We also discuss these findings to elucidate the present and future of SEM applied to this field.

The microscopic mechanism of bulk melting of ice.

We study the initial stages of homogeneous melting of a hexagonal ice crystal at coexistence and at moderate superheating. Our trajectory-based computer simulation approach provides a comprehensive picture of the events that lead to melting, from the initial accumulation of 5+7 defects, via the formation of L-D and interstitial-vacancy pairs, to the formation of a liquid nucleus. Of the different types of defects that we observe to be involved in melting, a particular kind of 5+7 type defect (type 5) plays a prominent role as it often forms prior to the formation of the initial liquid nucleus and close to the site where the nucleus forms. Hence, like other solids, ice homogeneously melts via the prior accumulation of defects.

Liquid Water and Interfacial, Cubic, and Hexagonal Ice Classification through Eclipsed and Staggered Conformation Template Matching.

We propose a novel method based on template matching for the recognition of liquid water, cubic ice (ice Ic), hexagonal ice (ice Ih), clathrate hydrates, and different interfacial structures in atomistic and coarse-grained simulations of water and ice. The two template matrices represent staggered and eclipsed conformations, which are the building blocks of hexagonal and cubic ice and clathrate crystals. The algorithm is rotationally invariant and highly robust against imperfections in the ice structure, and its sensitivity for recognizing ice-like structures can be tuned for different applications. Unlike most other algorithms, it can discriminate between cubic, hexagonal, clathrate, mixed, and other interfacial ice types and is therefore well suited to study complex systems and heterogeneous ice nucleation.

Growth and Equilibrium Morphology of Hydrohalite (NaCl·2H2O) and Its Epitaxy with Hexagonal Ice Crystals

The experimental growth morphology of NaCl·2H2O, mineral hydrohalite (HH), is compared with that calculated at equilibrium, at 0 K. The HH surface structure is theoretically investigated through the Hartman–Perdok method: the growth characters of the different {hkl} forms are determined along with their possible surface profiles in order to calculate, at the DFT level, their specific surface energies in the presence of vacuum. The striking difference between the calculated equilibrium shape (ES) and the observed growth shape (GS) is interpreted on the ground of a privileged epi-adsorption of {00.1}, the most important form of hexagonal ice (Ih) on the observed dominant {010} form of HH. The sharp pseudo-hexagonality of the {010} HH-form (substrate) seems to be the keystone to understand the very good epitaxy with the {00.1} pinacoid (deposit) of Ice (Ih).

Neural Network Classification of Ice-Crystal Images Observed by an Airborne Cloud Imaging Probe

ABSTRACT In the atmosphere, cloud particles have different shapes. The study of cloud particle shapes plays an important role in understanding cloud precipitation processes, radiative transfer, and weather modification. The image resolution and data quality of cloud probes affect the accuracy of the classification of particle shapes. To solve the occlusion of the photosensitive edge of the particle image and achieve automatic, high-precision ice-crystal classification of airborne Cloud Imaging Probe (CIP) ice-crystal images, this study uses a traditional image processing algorithm for data quality control and applies artificial intelligence algorithms to classify ice-crystal images. At present, there are mainly two types of ice-crystal classification methods, one classifies the shape of ice crystals using a pattern parameterization scheme, and the other uses an artificial intelligence network model to classify the shape. Combined with data quality control, the dataset was tested on eight models, and the TL-EfficientNet-b6 model was found to be the most accurate. Therefore, the TL-EfficientNet-b6 classifier model was used in this study, which is a newly developed convolutional neural network (CNN) based on a transfer learning method. Experimental results show that the TL-EfficientNet-b6 model can reach 100% in the single-class precision of tiny and hexagonal ice crystals, and the average precision can reach 98%. These results are more accurate than those using traditional classification methods. This method could be valuable in cloud microphysics research and weather modification.

Atomic Steps Induce the Aligned Growth of Ice Crystals on Graphite Surfaces.

Heterogeneous ice nucleation on atmospheric aerosols strongly affects the earth's climate, and at the microscopic level, surface-irregularity-induced ice crystallization behaviors are common but crucial. Because of the lack of visual evidence and effective experimental methods, the mechanism of atomic-structure-dependent ice formation on aerosol surfaces is poorly understood. Here we chose highly oriented pyrolytic graphite (HOPG) to represent soot (a primary aerosol), and environmental scanning electron microscopy (ESEM) was performed for in situ observations of ice formation. We found that hexagonal ice crystals show an aligned growth pattern via a two-stage pathway with one a axis coinciding with the direction of atomic step edges on the HOPG surface. Additionally, the ice crystals grow at a noticeably higher speed along this direction. This study reveals the role of atomic surface defects in heterogeneous ice nucleation and may pave the way to control icing-related processes in practical applications.

Improvements in the computational efficiency and convergence of the Invariant Imbedding T-matrix method for spheroids and hexagonal prisms.

The invariant-imbedding T-matrix (II-TM) method is a numerical method for accurately computing the single-scattering properties of dielectric particles. Because of the linearity of Maxwell's equations, the incident electric field and the scattered electric field can be related through a transition matrix (T-Matrix). The II-TM method computes the T-matrix through a matrix recurrence formula which stems from an electromagnetic volume integral equation. The recurrence starts with an inscribed sphere within the particle and ends with the circumscribed sphere of the particle. For each iteration, a matrix known as the U-matrix is computed with the Gauss Legendre (GL) quadrature, and matrix inversion is subsequently performed to obtain the T-matrix corresponding to the portion of the particle enclosed by the spherical shell. We modify a commonly used scheme to avoid applying the quadrature scheme to discontinuities. Moreover, we apply a new scheme to generate nodes and weights in conjunction with the GL quadrature. This leads to a considerable improvement on convergence and computational efficiency in the cases of hexagonal prisms and spheroids. The basic shapes of ice crystals in the atmosphere are hexagonal columns and plates. The single-scattering properties of hexagonal ice prisms are important to atmospheric optics, radiative transfer, and remote sensing. We demonstrate that the present approach can significantly accelerate the convergence in simulating light scattering by hexagonal ice crystals.

Derivation of equations of the model of the dynamic behaviour of the three-dimensional atmospheric cloud of electrically charged ice crystals under the influence of electrostatic forces

The purpose of this research is to create a theoretical model describing the dynamics of a cloud of electrically charged ice crystals in a three-dimensional space. The linear motion of the crystals was declined and considerations were limited to rotation of these crystals under the influence of electrostatic forces. The crystals were modeled as thin circular discs to simulate hexagonal plate ice crystals. This has reduced problem to the problem of two degrees of freedom (that allow for rotation around its center of gravity) per crystal. The basic assumption of the model is the crystals in the cloud are synchronized, which means that the differences in rotation of adjacent crystals are small. With this assumption, it was possible to derive continuous differential equations describing the dynamic behavior of such a medium on a macro scale from equilibrium of moments (from electrostatic forces) acting on the single crystal and discrete equations of motion of the cloud. Finally, the system of differential equations is second order with respect to time and space with trigonometric variable coefficients is derived. Examples of solutions of the system of equations are shown and compared with both solutions of similar theoretical problems available in the literature and optical effects in the clouds described by observers. It can be used in the future to better understand the dynamic optical phenomena occurring in the atmosphere.

Analysis algorithm for sky type and ice halo recognition in all-sky images

Abstract. Halo displays, in particular the 22∘ halo, have been captured in long time series of images obtained from total sky imagers (TSIs) at various Atmospheric Radiation Measurement (ARM) sites. Halo displays form if smooth-faced hexagonal ice crystals are present in the optical path. We describe an image analysis algorithm for long time series of TSI images which scores images with respect to the presence of 22∘ halos. Each image is assigned an ice halo score (IHS) for 22∘ halos, as well as a photographic sky type (PST), which differentiates cirrostratus (PST-CS), partially cloudy (PST-PCL), cloudy (PST-CLD), or clear (PST-CLR) within a near-solar image analysis area. The color-resolved radial brightness behavior of the near-solar region is used to define the discriminant properties used to classify photographic sky type and assign an ice halo score. The scoring is based on the tools of multivariate Gaussian analysis applied to a standardized sun-centered image produced from the raw TSI image, following a series of calibrations, rotation, and coordinate transformation. The algorithm is trained based on a training set for each class of images. We present test results on halo observations and photographic sky type for the first 4 months of the year 2018, for TSI images obtained at the Southern Great Plains (SGP) ARM site. A detailed comparison of visual and algorithm scores for the month of March 2018 shows that the algorithm is about 90 % reliable in discriminating the four photographic sky types and identifies 86 % of all visual halos correctly. Numerous instances of halo appearances were identified for the period January through April 2018, with persistence times between 5 and 220 min. Varying by month, we found that between 9 % and 22 % of cirrostratus skies exhibited a full or partial 22∘ halo.

In Situ Observation of Ice Formation from Water Vapor by Environmental SEM

A microscopic understanding of the mechanism of direct ice formation from water vapor has a significant benefit for controlling the processes involving ice condensation and evaporation. However, previous studies on this topic have been limited to theoretical simulations or optical observations. Here, by in situ observation via environmental scanning electron microscopy (ESEM), we revealed that hexagonal ice crystals are developed by a step-by-step pathway in a supersaturated water vapor environment. Furthermore, we also discerned that such steps came from two different origins, which are screw dislocations and initial steps. In addition, the relationship between the edge-length (of hexagonal ice crystals) and the growth time was quantitatively studied at controlled temperatures and pressures by experimental data fitting. This study shows that qualitative and quantitative observations of ice formation can be made with simple setups, and it should inspire future investigations toward important physicochemic...