Abnormal gas-liquid-solid phase transition behaviour of water observed with in situ environmental SEM

Abstract

Gas-liquid-solid phase transition behaviour of water is studied with environmental scanning electron microscopy for the first time. Abnormal phenomena are observed. At a fixed pressure of 450 Pa, with the temperature set to −7 °C, direct desublimation happens, and ice grows continuously along the substrate surface. At 550 Pa, although ice is the stable phase according to the phase diagram, metastable liquid droplets first nucleate and grow to ~100–200 μm sizes. Ice crystals nucleate within the large sized droplets, grow up and fill up the droplets. Later, the ice crystals grow continuously through desublimation. At 600 Pa, the metastable liquid grows quickly, with some ice nuclei floating in it, and the liquid-solid coexistence state exists for a long time. By lowering the vapour pressure and/or increasing the substrate temperature, ice sublimates into vapour phase, and especially, the remaining ice forms a porous structure due to preferential sublimation in the concave regions, which can be explained with surface tension effect. Interestingly, although it should be forbidden for ice to transform into liquid phase when the temperature is well below 0 °C, liquid like droplets form during the ice sublimation process, which is attributed to the surface tension effect and the quasiliquid layers.

A neutron scattering and electron microscopy study of the structure, wetting, and freezing behavior of water near hydrophilic CuO-nanostructured surfaces

“…We adjusted the electron beam accelerating voltage (5-20 kV), sample-detector gap distance (10-15 mm), image dwell time (0.3-30 μs), water vapor pressure (∼500 Pa), and sample temperature (268-293 K) for the best signal-to-noise ratio. Similar to the technique used by others, 19 by lowering the sample temperature below 271 K at a pressure of 500 Pa, we could condense supercooled water on untreated-copper surfaces and the CuO nanostructures. Images were formed by electrons backscattered into a gaseous electron detector both before and after the observation of liquid water.…”

Neutron scattering and microscopy studies of the structure and dynamics of water near a nanostructured hydrophilic copper oxide surface

“…While this argument can explain the observation of liquid water in our ESEM images, we must emphasize that the phase diagram is applicable to macroscopic systems in thermodynamic equilibrium [113].…”

“…). Controlling the sample temperature in this way allowed us to initiate condensation and evaporation of water similar to the technique used by others [113].…”

“…To reconcile this issue, we first note that condensation is a non-equilibrium thermodynamic process and therefore the PT-phase diagram in Fig. 2.10 does not accurately reflect the state of the water on the CuO surface during condensation [113]. Furthermore, our analysis of water wetting to CuO is based on images of small (<100 µm diameter) droplets, which are known to supercool due to finite size effects [114].…”

“…Furthermore, our analysis of water wetting to CuO is based on images of small (<100 µm diameter) droplets, which are known to supercool due to finite size effects [114]. Based on image contrast alone, we cannot rule out the possibility of the deposition or growth of solid water on the CuO surface; however, comparing our ESEM images with those of bulk ice at similar conditions [113], the droplets in our system have rounded shapes and tend to coalesce, suggesting the liquid state. To pursue this point, we can apply the Young-Laplace equation, which expresses the pressure discontinuity ∆ at a curved interface [113],…”