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

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.