  Contact freezing is a mode of atmospheric ice nucleation in which a collision
between a dry ice nucleating particle (INP) and a water droplet results in
considerably faster heterogeneous nucleation. The molecular mechanism of such
enhancement is, however, still a mystery. While earlier studies had attributed
it to collision-induced transient perturbations, recent experiments point to
the pivotal role of nanoscale proximity of the INP and the free interface. By
simulating heterogeneous nucleation of ice within INP-supported nanofilms of
two model water-like tetrahedral liquids, we demonstrate that such nanoscale
proximity is sufficient for inducing rate increases commensurate with those
observed in contact freezing experiments, but only if the free interface has a
tendency to enhance homogeneous nucleation. Water is suspected of possessing
this latter property, known as surface freezing propensity. Our findings
therefore establish a connection between surface freezing propensity and
kinetic enhancement during contact nucleation. We also observe that faster
nucleation proceeds through a mechanism markedly distinct from classical
heterogeneous nucleation, involving the formation of hourglass-shaped
crystalline nuclei that conceive at either interface, and that have a lower
free energy of formation due to the nanoscale proximity of the interfaces and
the modulation of the free interfacial structure by the INP. In addition to
providing valuable insights into the physics of contact nucleation, our
findings can assist in improving the accuracy of heterogeneous nucleation rate
measurements in experiments, and in advancing our understanding of ice
nucleation on nonuniform surfaces such as organic, polymeric and biological
materials.
