10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Effect of Surface Geometry on Heterogeneous Ice Nucleation
Olli Pakarinen, Golnaz Roudsari, EVGENI ZAPADINSKY, Hanna Vehkamäki, University of Helsinki
Abstract Number: 1244 Working Group: Unraveling the Many Facets of Ice Nucleating Particles and Their Interactions with Clouds
Abstract Understanding the way in which ice forms is of great importance to many fields of science. Pure water droplets in the atmosphere can remain unfrozen to nearly -40º C. Crystallization of ice in the atmosphere therefore typically occurs in the presence of aerosol particles, such as mineral dust, soot or organic particles. These ice nucleating particles (INPs) cause heterogeneous ice nucleation at clearly higher temperatures than possible through homogeneous ice nucleation. Therefore, a better understanding of how the various types of aerosol particles present in the atmosphere affect ice nucleation (IN) in clouds would improve our ability to measure, model and parameterize the key processes related to ice- and mixed-phase cloud formation, and would advance the field of atmospheric science.
Experiments have shown in great detail what is the IN activity of different types of compounds, and recently also clarified the importance of small surface features such as surface defects. The molecular-scale processes responsible for ice nucleation are still not well known, however, and difficulties in atomic-scale characterization of complex and imperfect surfaces means that a full understanding of these processes from solely experimental evidence is still a distant goal. In recent years, several computational studies have been published on heterogeneous ice nucleation, advancing our understanding of the details of ice nucleation in many materials. The role of defects has been studied less, but recently the importance of feldspar microstructure and different crystallographic faces of feldspar were shown to be responsible for IN activity of feldspars (Kiselev et al., 2017). Simulations also showed enhanced ice nucleation efficiency in confined geometry such as wedges or pits (Bi, Cao and Li, 2017).
We are studying these topics by utilizing the monatomic water model (Molinero and Moore, 2009) for unbiased molecular dynamics (MD) simulations, where a system including a defected surface, such as pyramidal pits, steps or surface cracks, immersed in water, is cooled continuously below the melting point over tens of nanoseconds of simulation time and crystallization is followed.
Results of simulations on pyramidal pits on Si (100) surfaces, for example, show a clear (ΔT > 10º C) enhancement of ice nucleation compared to flat Si (100) or Si (111) surfaces. Understanding the enhanced activity in such confined geometry may lead to characterization of active sites on some ice nucleating materials, as well as to development of optimal cloud seeding materials.
References: [1] Kiselev, A. et al. (2017). Science 355, 367. [2] Bi, Y., Cao, B. and Li, T. (2017). Nat. Commun. 8, 15372. [3] Molinero, V. and Moore, E. B. (2009). J. Phys. Chem. B 113, 4008.