Late stage coarsening in concentrated ice systems

Abstract

We have developed a dedicated, automated cryomicroscope for the study of ice coarsening in binary aqueous systems. This, together with new image analysis procedures, can provide wide ranging characterisation capabilities for the study of isothermal coarsening of ice crystal ensembles. Single particle tracking has elucidated hitherto unattainable mechanistic detail of coarsening kinetics. Ice crystal faceting has been shown to be an important factor. Ice crystals, with an average size in the mesoscopic range, have a significant tendency to evolve their initial kinetic growth forms towards the equilibrium Wulff shape. By Fourier harmonic analysis, the time evolution of the state of roughness of the prism plane of ice can be monitored, both for growing and dissolving crystals.

Results are presented for fructose/water at three temperatures, -20, -19 and -17°C. On the basis of recent high pressure studies on ice roughening, the highest temperature may be near the onset of a (high order) thermodynamic roughening transition of the prism plane; pronounced faceting of both the prism and the basal plane of ice is expected at the lower temperatures. We describe coarsening of non-interacting, faceted crystals at -19°C, and that of initially percolated networks of aggregated ice crystals at -20°C. The onset of faceting of ice prism planes at -17°C was monitored using Fourier harmonics to characterise the ‘sharpness’ of hexagonal contours of ice crystals viewed normal to their basal plane. A tentative analysis of the results suggests an estimate of the dimensionless step free energy of the prism face, γ, of 7×10^-3.

The coarsening kinetics observed for dilute ice crystal ensembles did not conform with classical continuum theories such as the LSW treatment. This was indicated by the measured size distribution, the scaling dependence of the mean radius and crystal number density with time, and by single particle tracking showing that an asymptotic steady state was not reached. The lack of a sharply defined critical radius, demarcating growing particles from those which dissolve, does not then permit an assignment of excess chemical potentials to individual ice crystals on the basis solely of their observed curvature. While faceting and shape changes are important, the overall kinetics are broadly consistent with diffusion control.

More concentrated ice systems at -20°C form networks of aggregated faceted crystals under the action of van der Waals attractive forces. These initially percolated structures are linear chains of crystals with occasional branches. On ripening, the chains thicken and progressively break. The tendency for linear chains may reflect a preference for basal face ice contacts. This is being investigated further.