On the Physics of Ice Nucleation and Growth in Nanoscale

Fundamental understanding of ice formation on a surface, i.e. heterogeneous formation, is critical to suppress ice accretion on the surfaces. Ice formation on a surface includes two steps of ice nucleation and further ice growth. As water droplet is placed on a sub-zero surface, with a time delay, ice nucleolus form on the surface. Ice nucleation is governed by thermodynamics of ice-water-surface system and it is described by Gibbs energy barrier, ∆�∗, which strongly depends on surface factor, �(�, �). Surface factor is a function of surface geometry, i.e. nano or micro, as well as surface free energy and through manipulating these parameters, ice nucleation can be controlled. After ice nucleation, ice further grows in a process which is controlled through heat transfer. Ice growth could be described by two extreme scenarios. In the first one, ice formation occurs with no airflow around where heat transfer through the substrate determines ice growth rate. In the second scenario, ice growth occurs in an environment with external airflow in which ice growth rate is controlled mainly by convective heat transfer. Water-ice transformation of few nm nanodroplets plays a critical role in nature including climate change, microphysics of clouds, survival mechanism of animals in cold environments, and a broad spectrum of technologies. In most of these scenarios, water-ice transformation occurs in a heterogenous mode where nanodroplets are in contact with another medium. Despite computational efforts, experimental probing of this transformation at few nm scales remains unresolved. Here, we report direct probing of water-ice transformation down to 2 nm scale and the length-scale dependence of transformation temperature through two independent metrologies. The transformation temperature shows a sharp length dependence in nanodroplets smaller than 10 nm and for 2 nm droplet, this temperature falls below the homogenous bulk nucleation limit. The formed ice phase even down to 2 nm is Ih (hexagonal). Contrary to nucleation on curved stiff solid surfaces, ice formation on soft interfaces (omnipresent in nature) could deform the interface leading to suppression of ice nucleation. Considering the interfacial deformation, the findings are in good agreement with predictions of classical nucleation theory. This understanding contributes to a greater knowledge of natural phenomena and rational design of anti-icing systems for aviation, wind energy and infrastructures.