Crystal Nucleation without Supersaturation.

Classical nucleation theory (CNT) has been extensively employed to interpret crystal nucleation phenomena and postulates the formation of an ordered crystalline nucleus directly from vapor or solution. Here, we provide the first experimental demonstration of a two-step mechanism that facilitates deposition of crystals on solid surfaces from vapor. Crucially, this occurs from saturated vapor without the need for supersaturation, conditions that, according to CNT, cannot lead to direct deposition of crystals from vapor. Instead, the process relies on condensation of supercooled liquid in surface cavities below the melting point. Crystals then nucleate in this liquid, leading to rapid deposition of more solid. Such a mechanism has been postulated for atmospheric nucleation of ice on aerosol particles and may have analogies in the crystallization of biominerals via amorphous precursor phases.