Radiation-induced premelting of ice at silica interfaces.

The existence of surface and interfacial melting of ice below 0 degrees C has been confirmed by many different experimental techniques. Here we present a high-energy x-ray reflectivity study of the interfacial melting of ice as a function of both temperature and x-ray irradiation dose. We found a clear increase of the thickness of the quasiliquid layer with the irradiation dose. By a systematic x-ray study, we have been able to unambiguously disentangle thermal and radiation-induced premelting phenomena. We also confirm the previously announced very high water density (1.25 g/cm(3)) within the emerging quasiliquid layer.

Experimental observation and computer simulations of 3D triplet structures in diffusion limited growth of xenon crystals.

Changes of growth morphologies are induced by a perturbation of the thermal diffusion field in the surrounding melt of a growing xenon crystal. Apart from the dendritic morphology, seaweed and doublon morphologies and for the first time transitions from dendritic to triplet structures (first predicted by T. Abel, E. Brener, and H. Müller-Krumbhaar [Phys. Rev. E 55, 7789 (1997)10.1103/PhysRevE.55.7789] were observed experimentally. With 3D phase-field simulations it was possible to reproduce the experimental procedure and to verify that triplet structures can grow in a stable way even in the presence of anisotropic surface free energy as found for experimental substances.

Three-dimensional xenon dendrites: characterization of sidebranch growth.

In our experiments, we investigate in situ the growth of three-dimensional xenon crystals into an undercooled pure melt. Experimental studies have been extended from undisturbed growth conditions to more realistic growth conditions. Methods to characterize the transient growth of sidebranches of dendrites have been developed. Two types of sidebranches have been identified: Sidebranches initiated by selective amplification of thermal noise (type N) and sidebranches induced by macroscopic perturbations (type P). Type N sidebranches start to grow 3-7 tip radii behind the tip and are not correlated at the four fins. It has been verified that the sidebranch amplitude grows exponentially to z(2/5) as predicted by Brener and Temkin [E. Brener and D. Temkin, Phys. Rev. E 51, 351 (1995)]. Type P sidebranches are initiated by macroscopic perturbations. They start to grow at the tip and their amplitudes are significantly higher than the ones of type N sidebranches. The growth of type P sidebranches is symmetric at the four fins. The tip positions of type P sidebranches separate from the tip shape of a dendrite at a distance of about 5R from the tip, while for type N sidebranches this distance is about 10R.

Quantitative description of morphological transitions in diffusion-limited growth of xenon crystals.

Changes of growth morphologies are induced by a perturbation of the temperature distribution in the surrounding of a growing xenon crystal. Apart from the dendritic morphology seaweed and doublon morphologies are found. We present a method which quantitatively describes growth morphologies by means of rotational, scale, and translational invariant transformations. Evolutions of growth morphologies are represented as paths in the morphology space. The presented method could be of some use for other fields of research where qualitative and quantitative information of different classes of images has to be identified.

Interfacial melting of ice in contact with SiO(2).

The physical behavior of condensed matter can be drastically altered in the presence of interfaces. Using a high-energy x-ray transmission-reflection scheme, we have studied ice-SiO2 model interfaces. We observed the formation of a quasiliquid layer below the bulk melting temperature and determined its thickness and density as a function of temperature. The quasiliquid layer has stronger correlations than water and a large density close to rho(HDA)=1.17 g/cm(3) of high-density amorphous ice suggesting a structural relationship with the postulated high-density liquid phase of water.

Generalized length scales for three-dimensional dendritic growth.

The crucial length scale of dendritic growth is the tip radius. Usually it is determined by fitting the data to a theoretical function. We present a method of a generalized tip radius which is entirely based on geometric considerations and is not dependent on an underlying assumption of the shape of the tip. Furthermore the results are stable and the average change of the tip radius between successive images is less than 6%.