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\ncontrols its numerous characteristics, including the ice mechanical\nproperties, ice–atmosphere interactions, sea ice albedo, and (photo)chemical\nbehavior in and on the ice. Ice samples were previously prepared in laboratories\nin order to study various facets of ice–impurity interactions and (photo)reactions\nto model natural ice–impurity behavior. We examine the impact of the\nfreezing conditions and solute (CsCl used as a proxy for naturally occurring\nsalts) concentrations on the microscopic structure of ice samples via an\nenvironmental scanning electron microscope. The method allows us to observe\nthe ice surfaces in detail, namely, the free ice, brine puddles,\nbrine-containing grain boundary grooves, individual ice crystals, and\nimprints left by entrapped air bubbles at temperatures higher than\n −25 ∘ C. The amount of brine on the external surface is found\nproportional to the solute concentration and is strongly dependent on the\nsample preparation method. Time-lapse images in the condition of slight\nsublimation reveal subsurface association of air bubbles with brine. With\nrising temperatures (up to −14 ∘ C), the brine surface coverage\nincreases to remain enhanced during the subsequent cooling and until the\nfinal crystallization below the eutectic temperature. The ice\nrecrystallization dynamics identify the role of surface spikes in retarding\nthe ice boundaries propagation (Zener pinning). The findings thus quantify\nthe amounts of brine exposed to incoming radiation, available for the gas\nexchange, and influencing other mechanical and optical properties of ice.\nThe results have straightforward and indirect implications for artificially\nprepared and naturally occurring salty ice, respectively.