Thomas Walmann

Combining optical and electrical impedance techniques for quantitative measurement of confluence in MDCK-I cell cultures

A new method combining optical and electrical impedance measurements is described that enables submicroscopic cell movements to be monitored. The cells are grown on small gold electrodes that are transparent to light. This modified electrical cell-substrate impedance sensor (ECIS) allows simultaneous microscopic recording of both growth and motility, thus enabling cell confluence on the electrodes to be systematically correlated to the impedance in regular time intervals of seconds and for extended periods of time. Furthermore, the technique provides an independent measure of monolayer cell densities that we compare to calculated values from a theoretical model. We have followed the attachment and spreading behavior of epithelial Madin-Darby canine kidney strain I (MDCK-I) cell cultures on microelectrodes for up to 40 h. The studies reveal a high degree of correlation between the measured resistance at 4 kHz and the corresponding cell confluence in 4- to 6-h intervals with typical linear cross-correlation factors of r equaling approximately 0.9. In summary, the impedance measured with the ECIS technique provides a good quantitative measure of cell confluence.

Simulation and characterization of fracture patterns in glaciers

During drainage of subglacial lakes the surface of the glacier subsides and fractures, generating a homogeneous, circular fracture pattern on the rim of the resulting depression. We have analyzed several fracture patterns around the Skafta cauldron on Iceland. A regular fracture spacing was observed on the cauldron rim, indicating that the region was subjected to a uniform strain. In this region, standard image analysis techniques were applied to find the lengths L and open areas A of the fractures. We demonstrate that an understanding of sampling biases allows a scaling relation L ∝ Aβ to be established and the exponent β to be determined. This relation provides a quantitative characterization of interactions between fractures in the population. The value of β is higher for that observed for laboratory experiments on clay, but the uncertainties do not rule out a universal behavior, which applies to ice, rock, and all other solids. The size distribution of fractures did not display a similarly simple crossover behavior, and a power law scaling relation could not be established without a better understanding of the crossovers for large fractures. We introduce a simple simulation model that reproduces the most important visual and statistical properties of the glacier fracture pattern. The direct comparison validates the use of the model to simulate geological fracturing processes.

Modeling and characterization of fracture patterns in the Vatnajökull glacier

The circular fracture pattern in the Vatnajokull glacier generated by a recent subglacial volcanic eruption was analyzed using methods based on fractal analysis. A crossover analysis based on estimates of the sampling bias showed that the fracture length was proportional to a power of the fracture area, providing a quantitative characterization of patterns of interacting fractures. A simple simulation model reproduces the most important visual and statistical characteristics of the observed fracture pattern. The model can be used to understand and separate the roles of material properties, deformation history, and geometry in geological fracture processes.

Construction of a DLA cluster model

On the occasion of the 50th birthday of a distinguished colleague, a three-dimensional wood model of a computer-generated DLA cluster was built. In this paper the design of the model and the construction process are described. The experiment may be carried out in the framework of a classroom experiment to demonstrate some of the fundamental concepts used in current research on growth phenomena. It is suitable as a first introduction to fractal geometry. Zusammenfassung. Anlaslich des 50sten Geburtstag eines bedeutenden Kollegen wurde ein dreidimensionales Holzmodell eines computererzeugten DLA-Clusters gebaut. Hier beschreiben wir die Entwickelung des Modells und den Konstruktionsprozess. Das Experiment kann im Rahmen eines Klassenzimmerexperiments durchgefuhrt werden um einige der fundamentalen Konzepte innerhalb der aktuellen Forschung auf dem Gebiet der Wachstumsphanomene zu demonstrieren. Es eignet sich als eine erste Einfuhrung in die fraktale Geometrie.

Structure of plastically compacting granular packings

In this paper we present results of structural studies of compacting experimental systems of ductile grains in two and three dimensions. The high precision of our two-dimensional experiments enables a detailed study of the evolution of coordination numbers and local crystalline arrangements as a function of the packing fraction. The structure in both dimensions deviates considerably from that of hard disks and spheres, although geometrically, crystalline arrangements dominate on a local scale (in two dimensions). In three dimensions, the evolution of the coordination number is compared to experimental packings of hard and ductile grains from the literature. This comparison shows that the evolution of coordination number with packing fraction is not unique for ductile systems in general, but must depend on rheology and grain size.