[M-IS14] Interface- and nano-phenomena on crystal growth and dissolution

[MIS14-P06] Why does the growth rate of an ice crystal oscillate periodically under microgravity conditions? (2): Numerical simulation based on phase-field model

*Hitoshi Miura¹ (1.Graduate School of Science, Department of Information and Basic Science, Nagoya City University)

Keywords:Crystal growth, Water ice, Antifreeze protein, Oscillatory phenomenon, Microgravity, Numerical simulation

Fish and insects living in cold regions protect themselves from freezing body fluids by synthesizing proteins in their bodies that inhibit the growth of ice crystals. Proteins with such effects are called antifreeze proteins (AFP, including antifreeze glycoprotein AFGP), and they are expected to be applied to various fields related to the phenomenon of water freezing. An in-situ molecular-level observation experiment of ice crystal basal faces growing in supercooled water was conducted on the International Space Station in 2013-2014 with the aim of clarifying the effect of AFGP on ice crystal growth from a crystal growth perspective (Ice Crystal 2 project) [1]. Analysis of the growth rate of the basal face growing in supercooled water containing AFGP showed that the growth rate oscillated spontaneously with a period of about 10 seconds (spontaneous oscillatory growth), even though the supercooling was set at a constant level of 0.3 K on the instrument side. The growth rate showed a behavior of switching between fast and slow phases within a short period of time. The growth rate in the slow phase was smaller than in the case of pure water (without AFGP), and in the fast phase significantly larger than in the case of pure water. This experimental fact suggests that AFGP adsorbed on the ice-crystal basal face has not only the growth-inhibiting effect but also the growth-promoting effect, and that these effects may cause the spontaneous oscillatory growth. I modeled the growth-inhibiting and growth-promoting effects of AFGP on the basal face and formulated the interaction between the adsorption/desorption of AFGP and the layer growth on the basal face, and theoretically showed that a hysteresis appears in the growth rate as the degree of supercooling increases or decreases (crystal growth hysteresis) [2]. In addition, the release of latent heat of crystallization associated with the ice growth and the heat conduction in the supercooled water were considered, and it was clarified that the necessary change in the degree of supercooling to induce oscillatory growth could occur. However, this theoretical model was based on a mean-field approximation, and it was not known whether the growth-rate oscillation would occur similarly in a system in which the growth rate and AFGP adsorption varied in time and space.

In this study, we investigated whether the spontaneous oscillatory growth occurs by numerically simulating simultaneously the layer growth on the basal face, adsorption and desorption of AFGP, and the time variation of the ambient temperature field due to the release of latent heat of crystallization. The layer growth was calculated based on the phase-field method, and the adsorption/desorption of AFGP was calculated based on the Monte Carlo method. AFGP in supercooled water was assumed to be first reversibly adsorbed on the basal face, and then to become irreversibly bound there by embedded in the growing crystal. Then, the AFGP is assumed to exhibit growth-inhibiting and growth-promoting effects depending on the adsorption state. The time variation of the temperature field around the growing ice crystal was obtained by calculating the spherically symmetric unsteady heat conduction equation with the release of the latent heat of crystallization as the boundary condition. The calculation results indicate that the spontaneous oscillatory growth occurs when the reversibly adsorbed AFGP exhibits the growth-inhibiting activity and the irreversibly adsorbed AFGP does the growth-promoting activity. The mechanism by which the AFGP significantly promotes the growth of the basal face has not yet been elucidated. Our results will provide a new perspective on the action of AFGP on the growth of ice crystals.

Reference: [1] Y. Furukawa et al., Sci. Rep. 7, 43157 (2017). [2] H. Miura and Y. Furukawa, J. Cryst. Growth 603, 127044 (2023).