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H. Grothe:
"Biological Ice Nucleation in the Atmosphere and the Biosphere";
Hauptvortrag: CECAM workshop: Water at interfaces: from proteins to devices, Vienna (eingeladen); 29.11.2016 - 02.12.2016; in: "Water at interfaces: from proteins to devices", (2016).

From the thermodynamic point of view, ice and snow can form already at temperatures slightly below its melting point, i.e. below zero degrees Celsius. Actually, ultrapure, liquid water can be supercooled down to minus forty degrees Celsius without freezing. The reason is a kinetic activation barrier, which hinders the phase transition. However, water impurities, e.g. biological material or organic particles, can lower the activation barrier and can thus catalyze the phase transition. This process is called heterogeneous ice nucleation and it plays an important role in many biological, meteorological and technical processes, e.g. in the formation of atmospheric ice clouds. The most effective ice nucleus is ice itself, since it provides the own hexagonal structure, on which water molecules from the liquid phase can be oriented in order to form further ice phase. The most effective heterogeneous ice nucleus is the bacterium Pseudomonas Syringae. The reason is a protein at its outer cell membrane, which exhibits a hexagonal, icelike structure. Furthermore, fungal spores, pollen, and carbonaceous particles are also very effective ice nuclei [1, 2]. In many cases, the physical and chemical reasons for the ice nucleation activity are understood only rudimentary. Thus, the search for the perfect ice nucleus is still an open issue [3].The talk will explain the fundamentals of heterogeneous ice nucleation and will give examples from the field and the laboratory.

[1] B.G. Pummer, H. Bauer, J. Bernardi, S. Bleicher, and H. Grothe, Atm. Chem. Phys., 12 2541 (2012).
[2] B.G. Pummer, C. Budke, S. Augustin-Bauditz, D. Niedermeier, L. Felgitsch, C. Kampf, R. Huber, K. Liedl, T. Loerting, T. Moschen, M. Schauperl, M. Tollinger, C. Morris, H. Wex, H. Grothe, U. Pöschl, T. Koop, and J. Fröhlich-Nowoisky, Atm. Chem. Phys. 15 4077 (2015).
[3] T. Bartels-Rausch, V. Bergeron, J. Cartwright, R. Escribano, J. Finney, H. Grothe, P. Gutierrez, J. Haapala, W. Kuhs, J. Pettersson, S. Price, C. Sainz-Diaz, D Stokes, G. Strazzulla, E. Thomson, H. Trinks, and N. Uras-Aytemiz, Rev. Mod. Phys. 84 885 (2012).

http://publik.tuwien.ac.at/files/publik_255557.pdf