Monday, 7 July 2014
Images, videos, and measurements are presented of nucleating, growing, and sublimating ice crystals within the chamber of an Environmental Scanning Electron Microscope (ESEM) at temperatures ranging from -5°C to -44° C. Once nucleated, crystals typically grow steadily, even at modest supersaturations, but under some conditions, crystal growth appears to stall, even at moderate supersaturation. The environmental conditions and molecular-scale mechanisms for this inhibited growth are explored in detail. All experiments took place within the chanmber of an FEI-Quanta 200, equipped with a Peltier-cooled stage, energy dispersive spectrometer, and backscatter x-ray diffraction. Our results thus far have demonstrated the ubiquity of mesoscopic ice surface roughness across temperatures, morphologies, pressures, and growth rates. In a typical experiment, ice crystals are grown on a substrate in the chamber of the ESEM. The differential pumping and thermoelectric cooling of the substrate requires approximately 10 minutes to reach equilibrium values of vapor pressure near 65 Pa and -25°C. Temperature of the Peltier block is automatically controlled through the FEI instrument software to 0.1°C resolution and the water vapor pressure is controlled and measured to 0.1 Pa resolution. Once the ice crystals reach equilibrium with the environment, the pressure is adjusted in increments of 0.5 Pa until nucleation of one or more ice crystals is observed. Growing ice crystals show roughness on both basal and prism faces. The roughness is observed in a variety of forms and scales including ridges, steps, hollows, undualtions, microfacets, and fans, spanning horizontal scales of 100 nm to 10 μm, with feature depths inferred from 50 nm to 5 μm. Animation of ESEM frame sequences also demonstrates the dynamic evolution of these features, as they spread across the surface in layers or diverge away from a contact point. Crystals typically nucleate at just a few percent above equilibrium vapor pressures and continue to grow steadily in proportion to the magnitude of ice supersaturation. At high supersaturations >150% RHi , nucleation and growth proceeded so quickly that it was difficult to isolate and follow the progression of a single crystal, as the entire stage would be overtaken by intersecting crystals within a few seconds. Therefore, most growth experiments occurred at modest supersaturation usually 105-125% RHi. While we did observe clear surface morphology differences between growing and sublimating crystals, we did not detect a systematic dependence on the degree of supersaturation or the rate of growth. In many instances, cycling of vapor pressure was conducted to observe the sensitivity of surface topography to ambient humidity. In the portions of the cycle below equilibrium vapor pressure, a significantly different character to the surface roughness was observed. Instead of regular or spreading ridges, plateaus, and steps, we observed concave, scalloped depressions away from the original surface. The scalloped depressions took on especially dramatic shapes near former grain boundaries or growth ridges, with sharp peaks often evident during advanced sublimation. If the supersaturation was once again increased above equilibrium, the crystal would typically exhibit micro-faceting that initiated along the ridges bounding adjacent sublimation-scallops. As expected, with growth experiments occuring on large, rough substrates, we observed that most crystals nucleated and grew steadily and at rate in proportion to ice supersaturation values. However, at temperatures below -30°C, we repeatedly observed crystals where growth became completely stalled, even at moderate supersaturations >115% RHi, that had previously induced growth in that same crystal and continued to lead to growth in adjacent cystals. This stalled growth was only observed following a specific cycle of humidity adjustment: the vapor supply of a steadily growing crystal was gradually reduced until reaching equilibrium, with observable growth ceasing, and with no sublimation apparent. This equilibrium condition was held for 1-5 minutes, after which the vapor supply was gradually increased (or temperature decreased) until RHi exceeded equilibrium by a few percent. In many instances, the original crystal would not resume growth, even though adjacent crystals continued to nucleate and grow. The image below shows time-separated panels illustrating one such event were a crystal held at equilibrium failed to grow following a 0.3°C decrease in temperature (~4% supersaturation), despite several nearby crystals nucleating and growing. We interpret this as an example of side-by-side ice crystals subject to 2 different surface conditions: a) one where steady growth continues at emergent dislocations or stacking faults, likely partly induced by the underlying substrate and b) one where a previously rough surface has been reconditioned by the momentary maintainence of equilbrium vapor pressure, leaving the surface to grow only by 2-D nucleation requiring vapor in excess of a critical supersaturation. The stalled crystal surfaces to not appear to be completely absent of mesoscopic roughness, nor are they observably different than the growing surfaces, implying that the surface condition differentiating separate growth mechanisms is determined at a smaller scale than observable here.
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