Nucleation- and Emergence-Limited Growth of Ice from Pores

Nucleation of ice from vapor on atmospheric aerosols has been attributed to the condensation and freezing of supercooled water in small pores. Here we use wedge pores on mica to directly observe the growth of ice in confinement prior to the growth of bulk crystals. We report a transition in behavior with a decreasing temperature: At low temperatures, the limiting step is not nucleation but a free energy barrier associated with the growth of ice through a narrow pore mouth to become a bulk phase.

Figure 1

Illustration of ice growth from a mica pocket: (a) an empty pocket; (b) a capillary condensate of supercooled water forms within the pocket; (c) the water freezes to produce a solid condensate; (d) bulk ice crystals emerge from the pocket corners.

Figure 2

Two schematic cross sections through a liquid condensate in perpendicular planes: (a) a condensate with radius of curvature $r_1$ grows within wedge geometry as the saturation increases; (b) after the condensate freezes, the dashed lines show the evolution the ice interface would have to follow to emerge into a bulk phase (here assuming ${\theta }^i\approx 90°$). The red line shows the position of maximum convex curvature.

Figure 3

Two representative time sequences of growth in the same pocket: at frost points $-37.4°\mathrm{C}$ (a)–(c) and $-48.5°\mathrm{C}$ (d)–(f). For the upper sequence, two bulk crystals emerge simultaneously (b) before visible growth along the wedge apex (c). For the lower sequence, a confined solid phase is visible (e) before the emergence of a single bulk crystal (f).

Figure 4

(a) Temperature and saturation at the first emergence of a bulk crystal. Red symbols represent the simultaneous growth of bulk crystals from both pocket corners, black symbols represent a bulk crystal at a single corner, and blue symbols represent condensation and freezing outside the pocket. The six different symbols represent six unique mica pockets. Tails on symbols are lines of constant frost point, representing periods where a condensate was visible within a pocket prior to emergence. The turquoise line is from Eq. (3), above which a liquid condensate may immediately emerge upon freezing. The orange dashed line is an approximate prediction of the nucleation point in a typical pocket. The gray dotted lines represent frost points $-43°$, $-38°$, and $-35°\mathrm{C}$.