Molecular simulations reveal that heterogeneous ice nucleation occurs at higher temperatures in water under capillary tension

Abstract. Homogeneous ice nucleation rates occur at higher temperatures when water is under tension, otherwise referred to as negative pressure. If also true for heterogeneous ice nucleation rates, then this phenomenon can result in higher heterogeneous freezing temperatures in water capillary bridges, pores, and other geometries where water is subjected to negative Laplace pressure. Using a molecular model of water freezing on a hydrophilic substrate, it is found that heterogeneous ice nucleation rates exhibit a similar temperature increase at negative pressures as homogeneous ice nucleation. For pressures ranging from from 1 atm to −1000 atm, the simulations reveal that the temperature corresponding to the heterogeneous nucleation rate coefficient jhet (m−2 s−1) increases linearly as a function of negative pressure, with a slope that can be approximately predicted by the water density anomaly and the latent heat of fusion at atmospheric pressure. Simulations of water in capillary bridges confirm that negative Laplace pressure within the water corresponds to an increase in heterogeneous freezing temperature. The freezing temperature in the water capillary bridges increases linearly with inverse capillary height (1/h). Varying the height and width of the capillary bridge reveals the role of geometric factors in heterogeneous ice nucleation. When substrate surfaces are separated by less than approximately h = 20 Angstroms the nucleation rate is enhanced and when the width of the capillary bridge is less than approximately 30 Angstroms the nucleation rate is suppressed. Ice nucleation does not occur in the region within 10 Angstroms of the air-water interface and shows a preference for nucleation in the region just beyond 10 Angstroms. These results help unify multiple lines of experimental evidence for enhanced nucleation rates due to reduced pressure, either resulting from surface geometry (Laplace pressure) or mechanical agitation of water droplets. This concept is relevant to the phenomenon of contact nucleation and could potentially play a role in a number of different heterogeneous nucleation or secondary ice mechanisms.