Image comparisons of snow and ice crystals photographed by light (video) microscopy and low-temperature scanning electron microscopy

Light (video) microscopy and low-temperature scanning electron microscopy (SEM) were used to examine and record images of identical precipitated and metamorphosed snow crystals as well as glacial ice grains. Collection procedures enabled numerous samples from distant locations to be shipped to a laboratory for storage and/or observation. The frozen samples could be imaged with a video microscope in the laboratory at ambient temperatures or with the low-temperature SEM. Stereo images obtained by video microscopy or low-temperature SEM greatly increased the ease of structural interpretations. The preparation procedures that were used for low-temperature SEM did not result in sublimation or melting. However, this technique did provide far greater resolution and depth of focus over that of the video microscope. The advantage of resolution was especially evident when examining the small particles associated with rime and graupel (snow crystals encumbered with frozen water droplets), whereas the greater depth of focus provided clearer photographs of large crystals such as depth hoar, and ice. Because the SEM images contained only surface information while the video images were frequently confounded by surface and internal information, the SEM images also clarified the structural features of depth hoar crystals and ice grains. Low-temperature SEM appears to have considerable promise for future investigations of snow and ice.     

Low-temperature scanning electron microscopy of frozen hydrated mouse lung

A method is presented by which water is preserved as ice during examination of the lung in the scanning electron microscope (SEM). The lung need only be inflated, frozen, transferred to the microscope and examined with the electron beam. Chemical fixation, solvent dehydration, and drying are not necessary. The low-temperature SEM of Pawley and Norton [11] maintains lung at ?180° C, nearly liquid nitrogen temperature, for extended periods with a Joule-Thomson refrigerator built into the stage. It has an integral high-vacuum preparation chamber attached to the microscope column which allows serial fracture, low-magnification stereo light microscopy, radiant etching, and evaporative coating with gold or carbon. The stage can be tilted from 0° to 45° and rotated a full 360°. It is demonstrated that the air-liquid interface in the lung can be examined and that low-temperature SEM can be used to investigate the shape of alveoli, the patency of the pores of Kohn in the hydrated state, and the shrinkage and distortion of lung with drying.     

Low-temperature scanning electron microscopy of particle-exposed mouse lung

Inflated frozen mouse lungs were examined using low-temperature scanning electron microscopy (LTSEM) following bulk fracture under vacuum. Various aspects of pulmonary architecture were identified and correlated with structures revealed by SEM following conventional fixation and preparation techniques. Surface etching of selected samples was performed by radiant heating, revealing characteristic cytoplasmic, nuclear and extracellular lattice patterns resulting from ice crystal formation during freezing. These patterns aided in distinguishing between intra- and extracellular spaces. Pulmonary fluids such as mucus and surfactant were identified. Iron oxide particles were introduced into the lungs of some animals by intratracheal instillation and were subsequently identified in frozen-hydrated lung tissue using characteristic X-ray identification and mapping techniques. Particles were observed both intra-and extracellularly and were commonly found in large deposits. These observations confirm the utility of LTSEM techniques for examination of particles within pulmonary tissue. Particle exposure by intratracheal instillation was found to result in a non-uniform distributional pattern.     

Observations of snow crystals using low-temperature scanning electron microscopy

Low-temperature scanning electron microscopy (SEM) was used to observe precipitation particles commonly known as “snowflakes.” The snowflakes were collected in Beltsville, Maryland, at temperatures ranging from ?6° to +1°C, mounted on SEM stubs, frozen in liquid nitrogen (LN₂), and then transferred to a cryosystem mounted on a field-emission SEM. Neither sputter coating with platinum nor irradiation by the electron beam affected their delicate fine structure. SEM observations revealed that snowflakes consisted of aggregations of snow crystals that occurred as hexagonal plates, prismatic columns, needles, and dendrites. In some cases, the snow crystals contained minute surface structures that consisted of rime, microdroplets, short prismatic columns, and amorphous films. Snow crystals from wet snow, which were collected at temperatures of 0° and +1°C, exhibited varying degrees of metamorphism or melting. The discrete crystalline faces and their sharp intersecting angles were gradually replaced by sinuous surfaces that tended to exhibit more spherical shapes. This study indicates that low-temperature SEM is a valuable technique for studying the formation and metamorphosis of snow crystals. The results suggest that combining low-temperature SEM and x-ray analysis could also provide qualitative elemental information on the nucleation particles of snow crystals as well as on the composition of acid snow.     

A technique for the examination of polar ice using the scanning electron microscope

The microstructure and location of impurities in polar ice are of great relevance to ice core studies. We describe a reliable method to examine ice in the scanning electron microscope (SEM). Specimens were cut in a cold room and could have their surfaces altered by sublimation either before (pre‐etching) or after (etching) introduction to the cryo‐chamber of the SEM. Pre‐etching was used to smooth surfaces, whilst etching stripped away layers from the specimen surface, aiding the location of particles in situ, and allowing embedded structures to be revealed. X‐ray analysis was used to determine the composition of localized impurities, which in some cases had been concentrated on the surface by etching. Examining uncoated surfaces was found to be advantageous and did not detract from qualitative X‐ray analysis. Imaging uncoated was performed at low accelerating voltages and probe currents to avoid problems of surface charging.     

High-resolution low-temperature scanning electron microscopy for observing intracellular structures of quick frozen biological specimens

Intracellular structures of rapidly frozen biological tissues were observed in 3-D under a low-temperature scanning electron microscope using a newly developed side-entry type cryo-holder. The present low-temperature SEM is simple, easy to operate and effective for observing biological materials at high magnification. Biological tissues (the pancreas, small intestine, brown adipose tissue and Harderian gland) freshly removed from the mouse were immediately frozen in liquid propane cooled with liquid nitrogen, and their surfaces were manually fractured using a precooled razor blade in liquid nitrogen before introducing the cryo-holder into the SEM. When intracellular structures were revealed after appropriate sublimation, the specimens were coated with gold using a metal evaporator fitted to the side of the microscope column at one of the specimen chamber ports. The cryo-holder was connected to a copper braid coming from a liquid nitrogen reservoir to maintain a low temperature. Using this method, intracellular structures such as the mitochondria and endoplasmic reticulum were demonstrated at high magnifications. Ribosomal granules were discerned on the rough endoplasmic reticulum of the pancreatic acinar cells. Granular substances, presumably elementary particles, were also recognized on the mitochondrial cristae of the brown adipose tissue. The method was particularly effective for studying the 3-D configuration of lipid droplets which had been difficult to preserve by chemical fixation.     

Static and dynamic experiments in cryo-electron microscopy: comparative observations using high-vacuum, low-voltage and low-vacuum SEM

We carried out a unique comparative study between three modes of cryo‐scanning electron imaging: high‐vacuum, low‐voltage and low‐vacuum, using ice cream as a model system. Specimens were investigated both with and without a conductive coating (Au/Pd) and at temperatures for which ice either remains fully frozen (< ?110 °C) or undergoes sublimation (?110 to ?90 °C). At high magnification, high‐vacuum imaging of coated specimens gave the best results for ‘static’ specimens (i.e. containing fully frozen ice). Low voltages, such as 1 kV, could be used for imaging uncoated specimens at high vacuum, although slight ‘classical’ charging artefacts remained an issue, and the reduced electron beam penetration tended to decrease the definition between different microstructural features. However, this mode was useful for observing in situ sublimation from uncoated specimens. Low‐vacuum mode, involving small partial pressures of nitrogen gas, was particularly suited to in situ sublimation work: when sublimation was carried out in low vacuum in the absence of an anti‐contaminator plate, sublimation rates were significantly reduced. This is attributed to a small partial pressure of sublimated water vapour remaining near the specimen surface, enhancing thermodynamic stability.     

非闭合电极电容层析成像传感器在冻土测试中的应用

为了实现电容层析成像技术对冻土冻结冰峰面的在线、非侵入测试,研制出了满足冻土测试要求的非闭合电极电容层析成像传感器,并对该种传感器的电容分布特性进行了实测;确定出了适合冻土测试的的高、低介电常数的标定物质;搭建了冻土一维冻结实验系统.对含水量8%的湿土样冻结过程的冰峰面迁移进行了电容层析成像测试,并利用IMNSNOF图像重建算法重建出了冻结过程各时刻的冻结截面物质分布图像,由图像可确定出冻土中已冻土、未冻区以及冰峰面的位置.电容层析成像测试结果与温度测试结果相吻合.     

Low-Temperature Friction in the XPS Analytical Ultrahigh Vacuum Tribotester

In this work, we present the capability of ultrahigh vacuum analytical tribometry for studying mechanisms of friction at low temperatures. We investigated the low-temperature frictional behavior of two different materials: ice and polyethylene (PE). We successfully formed a thin layer of ice on a steel surface, at temperature as low as 123 K in an ultrahigh vacuum. The surface characterization technique used for this study was X-ray photoelectron spectroscopy (XPS). We investigated the frictional behavior of such a thin ice layer. The changing friction as a function of temperature indicates that the ice might undergo pre-melting even at temperatures below 123 K. A polyethylene (PE) film previously deposited on a metal surface also showed changing friction as a function of temperature in the range 123 to 400 K. As there is no change in the nature of the surface chemistry of the polymer, as indicated by XPS, the results are therefore interpreted in terms of change in ductile-to-brittle transition of the polymer film over the temperature range. This work enables the fundamental investigation of friction at low temperatures with the help of surface analysis.     

Recent progress in freeze-fracturing of high-pressure frozen samples

Pancreatic tissue, bacteria and lipid vesicles were high‐pressure frozen and freeze‐fractured. In addition to the normal holder, a new type of high‐pressure freezing holder was used that is particularly suitable for suspensions. This holder can take up an EM grid that has been dipped in the suspension and clamped in between two low‐mass copper platelets, as used for propane‐jet freezing. Both the standard and the new suspension holder allowed us to make cryo‐fractures without visible ice crystal damage. High‐pressure frozen rat pancreas tissue samples were cryo‐fractured and cryo‐sectioned with a new type diamond knife in the microtome of a freeze‐etching device. The bulk fracture faces and blockfaces were investigated in the frozen‐hydrated state by use of a cryo‐stage in an in‐lens SEM. Additional structures can be made visible by controlled sublimation of ice (‘etching’), leading to a better understanding of the three‐dimensional organization of organelles, such as the endoplasmic reticulum. With this approach, relevant biological structures can be investigated with a few nanometre resolution in a near life‐like state, preventing the artefacts associated with conventional fixation techniques.     

Freezing of aqueous specimens: an X-ray diffraction study

The effects on water of two cooling methods, immersion in a liquid cryogen and high-pressure freezing, were studied by X-ray cryodiffraction on different sucrose solutions. The nature of the ice formed by each method depends on both the sucrose concentration and the specimen thickness. In order to compare the two methods, we mainly studied specimens having a thickness of 0.2 mm. Under these conditions, freezing by immersion gives rise to hexagonal (IH), cubic (IC) and amorphous (IV) ices when the sucrose concentration (weight/weight) has a value within the range 0–30%, 30–60%, 60% and higher, respectively. The temperature of the phase transitions IV–IC, IC–IH depends on the sucrose concentration. High-pressure freezing gives rise to two specific forms of ice: an amorphous and a crystalline ice (ice III). Ice III is observed when pure water samples are high-pressure frozen provided that the sample temperature does not rise above −150 °C. Above this temperature, ice III transforms into hexagonal ice. Amorphous ice is formed when the sucrose concentration is higher than 20%. The amorphous ice formed under high pressure has a similar, but not identical, X-ray diffraction pattern to that of amorphous ice formed at atmospheric pressure. While the X-ray diffraction pattern of amorphous ice formed at atmospheric pressure (IV) shows a broad ring at a position corresponding to 0.37 nm, that of high-pressure amorphous ice (IVHP) shows a broader ring, located at 0.35 nm. IVHP presents a phase transition (IVHP–IV) at temperatures that depend on the sucrose concentration. We also observed that some precautions have to be taken in order to minimize the alcohol contamination of high-pressure frozen samples. The ice-phase diagram presented in this paper should be taken into account in all methods dedicated to the structural study of frozen biological specimens.     

Progression of fusion during rapid freezing for electron microscopy.

The method used to determine the rate of fusion was based on the large difference in the dielectric constants of water and ice. A thin (50--60 micrometers) slice of a gelatin gel was used as the dielectric in a plate condenser. The slice was placed on a metal electrode built in a specimen carrier which was dropped on a silver freezing surface kept at below 70 K, forming the other plate of the condenser. Freezing of the gelatin causes a marked decrease in a 20,000 cycle current passing through the condenser. Since the thickness of the layer of frozen material was shown to be a function of the reciprocal of the current, it was possible to determine the course of fusion of the section. Freezing started at a high rate which declined during the first 5 ms but then increased again and usually became quite high at the end of fusion.     

Critical-point drying versus freeze drying for scanning electron microscopy: a quantitative and qualitative study on isolated hepatocytes

Critical-point drying and freeze drying were compared both quantitatively and qualitatively as preparative procedures for scanning electron microscopy. Isolated hepatocytes were used as model cells. Nomarski differential interference contrast microscopy was used for light microscopic measurements of the hepatocytes in the unfixed, the glutaraldehyde fixed, the glutaraldehyde + OsO₄ fixed, the critical-point dried and the freeze dried states. Critical-point dried hepatocytes were found to shrink to 38% of glutaraldehyde + OsO₄ fixed volume, whereas optimal freeze dried hepatocytes (frozen in water saturated with chloroform and freeze dried at 183 K for 84 h) were found to shrink to 51% of glutaraldehyde + OsO₄ fixed volume. Transmission and scanning electron micrographs of the critical-point dried cells showed well-preserved ultrastructure and surface structure. Micrographs of the freeze dried cells showed ultrastructure destroyed by internal ice crystals and surface structure destroyed by external ice crystals. Double-fixed isolated hepatocytes were shown to swell during storage in buffer and to shrink during storage after critical-point drying. For low magnification scanning electron microscopy (up to about 3000 times) both critical-point drying and freeze drying can be used. However, for high magnification scanning electron microscopy, critical-point drying is superior to freeze drying.     

The swelling behavior and wet morphology of water-absorbable polymer materials

In this research work, a low-temperature examination method for scanning electron microscopy (SEM) is introduced. A water-absorbable polymer material, the sulfonated polyethylene (SPE) ion-exchange hollow fiber membrane, was used for the experiments. With this low-temperature technique, the wet morphology of the water-absorbable sulfonated polyethylene hollow fiber membrane was revealed. The results obtained from this investigation offer some important information to explain the behavior of the SPE hollow fiber membranes when they are applied in pervaporation separation of water/organic solvent mixtures, such as water/ethylene glycol, water/ethanol, and so forth.     

The swelling behavior and wet morphology of water-absorbable polymer materials

In this research work, a low-temperature examination method for scanning electron microscopy (SEM) is introduced. A water-absorbable polymer material, the sulfonated polyethylene (SPE) ion-exchange hollow fiber membrane, was used for the experiments. With this low-temperature technique, the wet morphology of the water-absorbable sulfonated polyethylene hollow fiber membrane was revealed. The results obtained from this investigation offer some important information to explain the behavior of the SPE hollow fiber membranes when they are applied in pervaporation separation of water/organic solvent mixtures, such as water/ethylene glycol, water/ethanol, and so forth.     

Ultrastructure of Tracheal Surface Liquid: Low-Temperature Scanning Electron Microscopy

A layer of liquid lines the airways in the lung. Previous microscopic studies have suggested that it is in two phases, with a mucous gel lying above a periciliary sol. However, shrinkage artifacts due to chemical fixation, dehydration, and drying have prevented reliable estimates of the depth of these layers. To avoid such problems, we have studied the surface liquid of bovine trachea by low-temperature scanning electron microscopy (LTSEM). A polished copper probe cooled to liquid nitrogen temperature was applied to the mucosal surface of sheets of excised tracheal epithelium to effect rapid freezing of surface liquid. Tissue sheets were then mounted in an LTSEM (AMRay 1000A with Biochamber) which maintains samples at -180°C with a Joule-Thompson refrigerator built into the stage. Tissues were fractured at right angles to the epithelial surface, coated with gold, and viewed, all at 10^?5 to 10^?6 torr without transfer through air. The sample was stable under the electron beam at accelerating voltages up to 20 kV. Epithelial features (nuclei, cilia, microvilli, mucous granules) were well preserved. The mucosal surface of the cells was covered with material on the order of 8 μm in depth. The mucous gel and periciliary sol could be seen as distinct layers and could be distinguished by the size and pattern of ice crystal voids generated by radiant-etching of the fractured surface of the sample.     

Quantitative analysis of a two-dimensional ice accretion on airfoils

This paper presents the development of a code that can determine the shape of accreted ice on a 2D airfoil, verification of the code via quantitative parameters, and the variation in ice accretion according to ambient conditions. First, the 2D panel method is used as the aerodynamic solver, and Messinger’s model is used as the thermodynamic model. Second, the code is quantitatively verified through comparison with existing ice accretion analysis codes under rime, mixture, and glaze ice conditions. The parameters for comparison are the cross-sectional area of the ice, maximum ice thickness, ice heading, and distribution of the ice thickness measured on the airfoil surface. The verification shows that the developed code yields results of similar accuracy to existing analysis codes. Finally, ice shapes, depending on variations in the ambient conditions, are determined and investigated based on these parameters for comparison. The selected ambient condition parameters are freestream velocity, LWC, and temperature. The investigation is carried out for rime and glaze conditions. Increasing the freestream velocity produces an ice horn that increases the area over which the ice encounters liquid water in the air. The ambient temperature is the factor that alters ice accretion behavior; increasing the ambient temperature turns rime ice into glaze ice. Ice accretion area is increased at higher LWC. The LWC and the ice cross-sectional area show a linear relationship.     

Amorphous solid water produced by cryosectioning of crystalline ice at 113 K

Amorphous solid (vitreous) water can be obtained by a number of methods, including quick freezing of a very small volume of pure water, low pressure condensation of water vapour on a cold substrate or transformation of hexagonal ice (the ice which is naturally formed) under very high pressure at liquid nitrogen temperature. Larger volumes can be vitrified if cryoprotectant is added or when samples are frozen under high pressure. We show that a sample of 17.5% dextran solution or mouse brain tissue, respectively, frozen under high pressure (200 MPa) into cubic or hexagonal ice can be transformed into vitreous water by the very process of cryosectioning. The vitreous sections obtained by this procedure differ from cryosections obtained from vitreous samples by the irregular aspect of the sections and by small but significant differences in the electron diffraction patterns. For the growing community of cryo‐ultramicrotomists it is important to know that vitrification can occur at the knife edge. A vitreous sample is considered to show the best possible structural preservation. The sort of vitrification described here, however, can lead to bad structural preservation and is therefore considered to be a pitfall. Furthermore, we compare these sections with other forms of amorphous solid water and find it similar to high density amorphous water produced at very high pressures (about 1 GPa) from hexagonal ice and annealed close to its transformation temperature at 117 K.     

Application of lyophilization to prepare the nitrifying bacterial biofilm for imaging with scanning electron microscopy

Structure of bacterial biofilms may be investigated using several variants of scanning electron microscopy (SEM). We apply lyophilization to prepare nitrifying bacterial biofilm for conventional SEM imaging in high-vacuum mode (CSEM). We therefore replace standard biofilm fixation in glutaraldehyde cross-linking, ethanol dehydration, and critical-point drying (CPD) with less-invasive low-temperature drying by sublimation in vacuum. We compare this approach with: (1) standard preparation with glutaraldehyde fixation, ethanol dehydration, and CPD before CSEM, (2) cryo-sputter preparation of rapidly frozen biofilm in hydrated state (cryo-SEM), and (3) in situ observation without any sample pretreatment in environmental SEM. Combined imaging with these modalities revealed two distinct immobilization patterns on the polyurethane foam: (1) large irregular aggregates (flocs) of bacterial biofilm that exist as irregular biofilm fragments, rope-like structures, or biofilm layers on the foam surface; (2) biofilm threads adherent to the surface of polyurethane foam. Our results indicate that lyophilization was suitable for preservation of bacterial cells and many forms of structure of extracellular matrix. The lyophilized material could be imaged with high resolution (using CSEM) to generate structural information complementary to that obtained with other SEM techniques.