|KIT, Campus Nord, IMK-AAF, Gebäude 326, Raum 150|
The freezing of ice from highly supercooled water is a non-equilibrium process. Thus, it is not surprising that a polymorph less stable than hexagonal ice forms first. Indeed, experimental and theoretical work both suggest that the ice produced by freezing super-cooled water is faulty cubic ice, also called stacking disordered ice Isd. In experiments, the ratio of cubic to hexagonal sequences present in Isd increases as the freezing temperature decreases, and approaches 1:1 near water’s homogeneous nucleation limit. In contrast, simulations suggest that at short times and lower temperatures this ratio should be ~2:1, especially in the confined conditions characteristic of nanopores or nanodroplets. Using the X-ray free electron laser at the LCLS, SLAC National Laboratory, we recently conducted in situ Wide Angle X-ray Scattering experiments on nanodroplets with radii of ~10 nm. The droplets were formed in a supersonic nozzle by condensation from the vapor phase and froze at temperatures close to 224 K, i.e. well below the homogeneous nucleation limit for micron size droplets. Fitting the strongest diffraction peak, while accounting for the particles size distribution, suggests the ratio of cubic to hexagonal ice was close to 4:1. On the short timescales (10 – 100 µs) of the experiment we do not observe any transformation from Isd to the more stable hexagonal ice, Ih. This talk will summarize the experimental work and our efforts to understand why ice formed by this approach can be so highly cubic.
|Diese Veranstaltung ist Teil der Reihe Aerosolphysikalisches Seminar|
Ohio State University, Columbus, USA
Department of Chemical and Biomolecular Engineering
Institut für Meteorologie und Klimaforschung (IMK-AAF)
Hermann von Helmholtzplatz 1
E-Mail:susanne bolz∂kit edu