How nanoscale surface steps promote ice growth on feldspar: microscopy observation of morphology-enhanced condensation and freezing

Determining Roles of Potassium-Feldspar Surface Characters in Affecting Ice Nucleation

The roles of surface characteristics of the feldspar surface on ice nucleation have remained elusive. Here, simple strategies are reported to quantitatively analyze the effects of the surface morphology and molecular composition of the potassium-feldspar surface on ice nucleation. The steps are found to be responsible to the high ice nucleation efficacy according to the fact that water drop freezing temperature increases by about 4.5 °C atop the freshly cleavage feldspar surface being rich of steps comparing to the flattened ones. After the molecular component and atomic structure are destroyed by the fluorination, a tremendous decrease of the ice nucleation temperatures by around 9.0 °C is observed on both cleavage and flattened surfaces, and the steps still improve the ice nucleation activity of the hydrophobic cleavage surfaces. The influence of the surface composition also implies the importance of the molecular component and structure specificity on K-feldspar in facilitating ice nucleation.

Atomic structure and water arrangement on K-feldspar microcline (001)

The properties of clouds, such as their reflectivity or their likelihood to precipitate, depend on whether the cloud droplets are liquid or frozen. Thus, understanding the ice nucleation mechanisms is essential for the development of reliable climate models. Most ice nucleation in the atmosphere is heterogeneous, i.e., caused by ice nucleating particles such as mineral dusts or organic aerosols. In this regard, K-feldspar minerals have attracted great interest recently as they have been identified as one of the most important ice nucleating particles under mixed-phase cloud conditions. The mechanism by which feldspar minerals facilitate ice nucleation remains, however, elusive. Here, we present atomic force microscopy (AFM) experiments on microcline (001) performed in an ultrahigh vacuum and at the solid-water interface together with density functional theory (DFT) and molecular dynamics (MD) calculations. Our ultrahigh vacuum data reveal features consistent with a hydroxyl-terminated surface. This finding suggests that water in the residual gas readily reacts with the surface. Indeed, the corresponding DFT calculations confirm a dissociative water adsorption. Three-dimensional AFM measurements performed at the mineral-water interface unravel a layered hydration structure with two features per surface unit cell. A comparison with MD calculations suggests that the structure observed in AFM corresponds to the second hydration layer rather than the first water layer. In agreement with previous computation results, no ice-like structure is seen, questioning an explanation of the ice nucleation ability by lattice match. Our results provide an atomic-scale benchmark for the clean and water-covered microcline (001) plane, which is mandatory for understanding the ice nucleation mechanism on feldspar minerals.

How Water Binds to Microcline Feldspar (001)

Microcline feldspar (KAlSi3O8) is a common mineral with important roles in Earth's ecological balance. It participates in carbon, potassium, and water cycles, contributing to CO2 sequestration, soil formation, and atmospheric ice nucleation. To understand the fundamentals of these processes, it is essential to establish microcline's surface atomic structure and its interaction with the omnipresent water molecules. This work presents atomic-scale results on microcline's lowest-energy surface and its interaction with water, combining ultrahigh vacuum investigations by noncontact atomic force microscopy and X-ray photoelectron spectroscopy with density functional theory calculations. An ordered array of hydroxyls bonded to silicon or aluminum readily forms on the cleaved surface at room temperature. The distinct proton affinities of these hydroxyls influence the arrangement and orientation of the first water molecules binding to the surface, holding potential implications for the subsequent condensation of water.

Heterogeneous Ice Nucleation in Model Crystalline Porous Organic Polymers: Influence of Pore Size on Immersion Freezing

Heterogeneous ice nucleation activity is affected by aerosol particle composition, crystallinity, pore size, and surface area. However, these surface properties are not well understood, regarding how they act to promote ice nucleation and growth to form ice clouds. Therefore, synthesized materials for which surface properties can be tuned were examined in immersion freezing mode in this study. To establish the relationship between particle surface properties and efficiency of ice nucleation, materials, here, covalent organic frameworks (COFs), with different pore diameters and degrees of crystallinity (ordering), were characterized. Results showed that out of all the highly crystalline COFs, the sample with a pore diameter between 2 and 3 nm exhibited the most efficient ice nucleation activity. We posit that the highly crystalline structures with ordered pores have an optimal pore diameter where the ice nucleation activity is maximized and that the not highly crystalline structures with nonordered pores have more sites for ice nucleation. The results were compared and discussed in the context of other synthesized porous particle systems. Such studies give insight into how material features impact ice nucleation activity.

Silica as a Model Ice-Nucleating Particle to Study the Effects of Crystallinity, Porosity, and Low-Density Surface Functional Groups on Immersion Freezing

Aerosol particles can facilitate heterogeneous ice formation in the troposphere and stratosphere by acting as ice-nucleating particles, modulating cloud formation/dissipation, precipitation, and their microphysical properties. Heterogeneous ice nucleation is driven by ice embryo formation on the particle surface, which can be influenced by features of the surface such as crystallinity, surface structure, lattice structure, defects, and functional groups. To characterize the effect of crystallinity, pores, and surface functional groups toward ice nucleation, samples of comparable silica systems, specifically, quartz, ordered and nonordered porous amorphous silica samples with a range of pore sizes (2-11 nm), and nonporous functionalized silica spheres, were used as models for mineral dust aerosol particles. The ice nucleation activity of these samples was investigated by using an immersion freezing chamber. The results suggest that crystallinity has a larger effect than porosity on ice nucleation activity, as all of the porous silica samples investigated had lower onset freezing temperatures and lower ice nucleation activities than quartz. Our findings also suggest that pores alone are not sufficient to serve as effective active sites and need some additional chemical or physical property, like crystallinity, to nucleate ice in immersion mode freezing. The addition of a low density of organic functional groups to nonporous samples showed little enhancement compared to the inherent nucleation activity of silica with native surface hydroxyl groups. The density of functional groups investigated in this work suggests that a different arrangement of surface groups may be needed for enhanced immersion mode ice nucleation activity. In summary, crystallinity dictates the ice nucleation activity of silica samples rather than porosity or low-density surface functional groups. This work has broader implications regarding the climate impacts resulting from ice cloud formation.

Atomic Steps Induce the Aligned Growth of Ice Crystals on Graphite Surfaces

Heterogeneous ice nucleation on atmospheric aerosols strongly affects the earth's climate, and at the microscopic level, surface-irregularity-induced ice crystallization behaviors are common but crucial. Because of the lack of visual evidence and effective experimental methods, the mechanism of atomic-structure-dependent ice formation on aerosol surfaces is poorly understood. Here we chose highly oriented pyrolytic graphite (HOPG) to represent soot (a primary aerosol), and environmental scanning electron microscopy (ESEM) was performed for in situ observations of ice formation. We found that hexagonal ice crystals show an aligned growth pattern via a two-stage pathway with one a axis coinciding with the direction of atomic step edges on the HOPG surface. Additionally, the ice crystals grow at a noticeably higher speed along this direction. This study reveals the role of atomic surface defects in heterogeneous ice nucleation and may pave the way to control icing-related processes in practical applications.