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 T_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 T_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 T_het across ice-binding strengths and geometries. Confinement splits ice nucleation into two steps, with distinct barriers: nucleation inside the wedge (ΔG_in) and out of the wedge into the surrounding liquid (ΔG_out). We demonstrate that ΔG_in depends on the angle of the wedge and the strength of binding of its walls to ice, while ΔG_out depends on temperature and opening width, not on the chemistry of the wedge's inner wall. Mapping ΔG_in reveals a volcano-like trend: intermediate-strength nucleants (flat-surface nucleation temperature T_flat ≈ 244-260 K) gain the most from wedge defects (up to ∼25 K). We show that weak nucleants are limited by ΔG_in and strong ones by ΔG_out. The steep temperature dependence of ΔG_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 T_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.