Research Interests
 

The physical and chemical properties of supercooled liquids and their freezing mechanisms are of major interest in material science and the atmospheric chemistry of clouds. In order to examine supercooled microdroplets under well defined conditions, an electrodynamic trap was built in our lab where single levitated droplets can be stored for infinite time and can be analysed via elastic light scattering or fluorescence spectroscopy.
In the absence of walls which could serve as a freezing nucleus these levitated droplets remain liquid until the temperature for homogeneous nucleation is reached.  The measurement of nucleation rates makes it possible to determine activation energies of the phase transition liquid to solid. From that surface energies between the solid/liquid and solid/gas interface can be calculated.
 

 

Different light scattering pattern of n-Heptadecane. From left to right: a) liquid b) solid phase 1 c) solid phase 2. 
Upper half circle is parallel, lower half circle is perpendicular to the incident laser polarisation

 

The time behaviour of the HCl Gas uptake on sulfuric acid/water droplets at stratospheric temperatures was studied on levitated droplets. This reaction takes place in the stratosphere during the arctic winter where Polar Stratospheric Clouds are formed and is one important step in the chlorine-cycle that yields into the ozone depleation observed during spring time. At higher temperatures and lower sulfuric acid concentations the gas uptake is limited by the diffusion of the HCl molecules in the gas phase. At lower temperatures and higher sulfuric acid concentrations the droplet viscosity increases rapidly. The gas uptake is then limited by the diffusion of the HCl molecules inside the droplet. This change can be seen in the time behavior of the growing levitated droplet.
 
 
 

Freezing behavior of n-alkane microdroplets

The n-alkanes are one of the most basic organic liquids and are therefore very suitable as a model substance. The shorter alkanes exibit several solid phases in which nucleation can occur. The activation energy needed for this phase transition also depends on molecule chain length and the surface tension of the particle/gas interface. Alkanes which exhibit an liquid/air interface form a solid monolayer above the thermodynamic melting point  (Wu et al. Science 1993). Because of this surface layer supercooling of these liquids is not possible in macroscopic bulk probes. Levitated n-alkane microdroplets can be supercooled about 9 °C below their thermodynamic melting temperature. The phase transition liquid to solid exhibits an activation energy, because the creation of a new phase involves the formation of a nucleus first. Such a nucleus can be formed inside the droplet volume or the droplet surface. The existence of a closed solid shell does not lead inevitably to the crystallization of the whole droplet, since also an activation energy is needed for further growing.
 
 

Fluorescence Spectroscopy

There is also the possibility to measure the fluorescence of a levitated droplet. Excitation is done by a N₂-Laser (l=337 nm). For the detection of the fluorescent light a spectrometer and an Optical Multichannel Analyser (OMA) are available. For some fluorescent molecules like Tetraphenylethylene (TPE) the quantum yield increases with increasing viscosity of the surrounding medium. Therefore these molecules are very suitable to detect the beginning of the phase transition.
Another project is the measurement of the CO₂ gas uptake of sea salt aerosols via the pH sensitive fluorescence of quinine sulfate.
 

All research has been done in coorporation with the Group of Prof. Wöste at the Physics Department, Freie Universität Berlin and Prof. Leisner at the Enviromental Physics Department at the TU Ilmenau.