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 Thet 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 Thet has been recognized qualitatively but never quantified. Here, we develop a predictive framework using Classical Nucleation Theory to compute how wedge-like defects control Thet across ice-binding strengths and geometries. Confinement splits ice nucleation into two steps, with distinct barriers: nucleation inside the wedge (ΔGin) and out of the wedge into the surrounding liquid (ΔGout). We demonstrate that ΔGin depends on the angle of the wedge and the strength of binding of its walls to ice, while ΔGout depends on temperature and opening width, not on the chemistry of the wedge's inner wall. Mapping ΔGin reveals a volcano-like trend: intermediate-strength nucleants (flat-surface nucleation temperature Tflat ≈ 244-260 K) gain the most from wedge defects (up to ∼25 K). We show that weak nucleants are limited by ΔGin and strong ones by ΔGout. The steep temperature dependence of ΔGout 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 Thet, 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.