Immersion Freezing at Topographic Active Sites: Dual-Barrier Prediction of Ice Nucleation Temperatures

Atmospheric ice formation controls cloud glaciation, precipitation, and radiative balance, yet the temperatures <i>T</i>_het at which aerosols trigger freezing remain poorly predicted from first principles. In particular, the role of surface topography─pits, steps, wedges, and pores─in setting <i>T</i>_het has been recognized qualitatively but never quantified. Here, we develop a predictive framework using Classical Nucleation Theory to compute how wedge-like defects control <i>T</i>_het across ice-binding strengths and geometries. Confinement splits ice nucleation into two steps, with distinct barriers: nucleation inside the wedge (Δ<i>G</i>_in) and out of the wedge into the surrounding liquid (Δ<i>G</i>_out). We demonstrate that Δ<i>G</i>_in depends on the angle of the wedge and the strength of binding of its walls to ice, while Δ<i>G</i>_out depends on temperature and opening width, not on the chemistry of the wedge's inner wall. Mapping Δ<i>G</i>_in reveals a volcano-like trend: intermediate-strength nucleants (flat-surface nucleation temperature <i>T</i>_flat ≈ 244-260 K) gain the most from wedge defects (up to ∼25 K). We show that weak nucleants are limited by Δ<i>G</i>_in and strong ones by Δ<i>G</i>_out. The steep temperature dependence of Δ<i>G</i>_out renders potent nucleants insensitive to active-site density. Ice nucleation activity of wedges is strongly angle-selective, with only facet-matching wedge angles achieving maximal enhancement. We release ice-WEDGE, an open-source tool that predicts <i>T</i>_het, the limiting barrier, and nucleus geometry for arbitrary wedge geometry and ice-binding strength. This study supplies design rules for topographies that promote or suppress freezing, enabling physically grounded interpretation and parametrization of atmospheric ice-nucleating particle spectra.