The morphology of ice and liquid brine in an environmental scanning electron microscope: a study of the freezing methods

Abstract

Abstract. The microstructure of polycrystalline ice with a threading solution of brine controls its numerous characteristics, including the ice mechanical properties, ice–atmosphere interactions, sea ice albedo, and (photo)chemical behavior in and on the ice. Ice samples were previously prepared in laboratories in order to study various facets of ice–impurity interactions and (photo)reactions to model natural ice–impurity behavior. We examine the impact of the freezing conditions and solute (CsCl used as a proxy for naturally occurring salts) concentrations on the microscopic structure of ice samples via an environmental scanning electron microscope. The method allows us to observe the ice surfaces in detail, namely, the free ice, brine puddles, brine-containing grain boundary grooves, individual ice crystals, and imprints left by entrapped air bubbles at temperatures higher than −25 ∘C. The amount of brine on the external surface is found proportional to the solute concentration and is strongly dependent on the sample preparation method. Time-lapse images in the condition of slight sublimation reveal subsurface association of air bubbles with brine. With rising temperatures (up to −14 ∘C), the brine surface coverage increases to remain enhanced during the subsequent cooling and until the final crystallization below the eutectic temperature. The ice recrystallization dynamics identify the role of surface spikes in retarding the ice boundaries' propagation (Zener pinning). The findings thus quantify the amounts of brine exposed to incoming radiation, available for the gas exchange, and influencing other mechanical and optical properties of ice. The results have straightforward and indirect implications for artificially prepared and naturally occurring salty ice, respectively.

The ice–vapour interface during growth and sublimation

“…During our measurements on polycrystalline ice films, we observed features that appear as small circular dark spots, which we term "pores", aligned inside the grooves (which are not to be confused with ice veins; Mader, 1992;Mader et al, 2006;Buford Price, 2000). Analogous features to our pores have been observed at ice-brine interfaces (Vetráková et al, 2019). The pores appear in ESEM as black dots (see Fig.…”

The ice-vapor interface during growth and sublimation

“…During our measurements on polycrystalline ice films, we observed features that appear as small circular dark spots, which we term "pores", aligned inside the grain boundaries (which are not to be confused with "ice veins" (Mader, 1992,;Mader et al, 2006;Buford Price, 2000). Analogous features to our pores have been observed at ice/brine interfaces (Vetráková et al, 2019).…”

“…Ambient pressures would require a pressure cell with much loss of resolution. Another possibility would be the addition of traces of ice nucleators such as minerals (Vetráková et al, 2019) or biogenic matter nucleation (Cascajo-Castresana et al, 2020), to research cloud nucleation. An in-situ method such as the one proposed in our study would allow to observe changes in morphology, determine the role of grain boundaries as well as impurities in ice in real time.…”

“…as droplet), or located at the ice surface as a "quasi-liquid layer". In addition, the ESEM environment could offer flexibility to approach realistic scenarios, such as brine-filled veins in ice (Vetráková et al, 2019).…”

Partitioning of Crystalline and Amorphous Phases During Freezing of Simulated Enceladus Ocean Fluids

“…If particles cool to this point, they would be erupted into space and continue to lose heat. How brine vein partitioning scales to smaller droplet sizes is unknown, but partitioning in some form is still expected; for example, vein networks have been observed in flash‐frozen droplets of CsCl brine at sizes closer to that of Enceladus' plume particles ( r ≈ 80 µm) (Vetráková et al., 2019). Thus, the conditions for crystal inhibition due to confinement may be met.…”