Until now the structure and properties of
liquid water have not been fully understood in detail. Within the work
presented here the homogeneous germ formation (nucleation) of ice in
deeply supercooled light (H2O) and heavy (D2O) water has
been investigated. Since only molecules of the metastable phase itself are
involved in homogeneous germ formation, this process is solely influenced by
the structure and dynamics of the liquid. Hence, studying of homogeneous
nucleation may yield further clues how water "works".
Homogeneous nucleation of ice in deeply
supercooled droplets of liquid water plays a major role in cirrus cloud
formation. The kinetics of cloud formation strongly depends on the so-called
nucleation rate. In order to model the processes in the atmosphere
successfully, reliable measured data of the nucleation rate as a function of
temperature are needed. The described measurements represent a valuable
contribution for this purpose.
In order to investigate homogeneous nucleation
in the laboratory, electrodynamically levitated supercooled liquid droplets are
very suitable. In comparison with alternative techniques, this method has a
number of advantages which have been further developed by the work presented
here.
The central part of our apparatus consists of a
coolable electrodynamical double ring trap. The setup has been designed
especially to meet the requirements arising by the measurement of nucleation
rates in deeply supercooled levitated liquid droplets. Very long sequences of
measurements become possible through operation of the experiment in the fully
computer controlled modus. This increases the significance of the results
because nucleation is a random process governed by statistical laws.
During the experiment, the volume of each individual
droplet is continuously determined by analysis of the spatial intensity
distribution of the scattered laser light and recorded as a function of time.
Therefore the size distribution within the investigated ensemble of droplets is
known with high accuracy. This is a major advantage of our method over all
other alternatives (supercooling of emulsions, aerosols etc.).
Within this thesis, the homogeneous nucleation
in supercooled H2O has been observed between 239.4 K and 239.7 K. For
the first time the homogeneous nucleation of ice in D2O has been
investigated in levitated supercooled droplets, namely between 243.7 K and
244.7 K.
The temperature intervals under investigation
have been chosen in such a manner that the nucleation times lasted up to 3 min.
This means that the supercooled state has been maintained in average over a
substantially longer period in comparison to earlier measurements. Considering
freezing events with longer nucleation times, pronounced deviations from the
expected statistical behaviour appear. The reasons for these discrepancies
between theory and experiment could not be explained so far only on the basis
of the given data. It cannot be excluded that the described phenomena are
caused by properties of supercooled water which are not yet known. Our
observations point towards the existence of different modifications of
supercooled water being in equilibrium with each other.
