Dynamics and unsteady morphologies at ice interfaces driven by D2O-H2O exchange [Chemistry]

The growth dynamics of D₂O ice in liquid H₂O in a microfluidic device were investigated between the melting points of D₂O ice (3.8 °C) and H₂O ice (0 °C). As the temperature was decreased at rates between 0.002 °C/s and 0.1 °C/s, the ice front advanced but retreated immediately upon cessation of cooling, regardless of the temperature. This is a consequence of the competition between diffusion of H₂O into the D₂O ice, which favors melting of the interface, and the driving force for growth supplied by cooling. Raman microscopy tracked H/D exchange across the solid H₂O–solid D₂O interface, with diffusion coefficients consistent with transport of intact H₂O molecules at the D₂O ice interface. At fixed temperatures below 3 °C, the D₂O ice front melted continuously, but at temperatures near 0 °C a scalloped interface morphology appeared with convex and concave sections that cycled between growth and retreat. This behavior, not observed for D₂O ice in contact with D₂O liquid or H₂O ice in contact with H₂O liquid, reflects a complex set of cooperative phenomena, including H/D exchange across the solid–liquid interface, latent heat exchange, local thermal gradients, and the Gibbs–Thomson effect on the melting points of the convex and concave features.