  What makes a material a good ice nucleating agent? Despite the importance of
heterogeneous ice nucleation to a variety of fields, from cloud science to
microbiology, major gaps in our understanding of this ubiquitous process still
prevent us from answering this question. In this work, we have examined the
ability of generic crystalline substrates to promote ice nucleation as a
function of the hydrophobicity and the morphology of the surface. Nucleation
rates have been obtained by brute-force molecular dynamics simulations of
coarse-grained water on top of different surfaces of a model fcc crystal,
varying the water-surface interaction and the surface lattice parameter. It
turns out that the lattice mismatch of the surface with respect to ice,
customarily regarded as the most important requirement for a good ice
nucleating agent, is at most desirable but not a requirement. On the other
hand, the balance between the morphology of the surface and its hydrophobicity
can significantly alter the ice nucleation rate and can also lead to the
formation of up to three different faces of ice on the same substrate. We have
pinpointed three circumstances where heterogeneous ice nucleation can be
promoted by the crystalline surface: (i) the formation of a water overlayer
that acts as an in-plane template; (ii) the emergence of a contact layer
buckled in an ice-like manner; and (iii) nucleation on compact surfaces with
very high interaction strength. We hope that this extensive systematic study
will foster future experimental work aimed at testing the physiochemical
understanding presented herein.
