Computer simulation methods to study interfacial tensions : from the Ising model to colloidal crystals

In condensed matter systems, the interfacial tension plays a central role for a multitude of phenomena. It is the driving force for nucleation processes, determines the shape and structure of crystalline structures and is important for industrial applications. Despite its importance, the interfacial tension is hard to determine in experiments and also in computer simulations. While for liquid-vapor interfacial tensions there exist sophisticated simulation methods to compute the interfacial tension, current methods for solid-liquid interfaces produce unsatisfactory results.rnrnAs a first approach to this topic, the influence of the interfacial tension on nuclei is studied within the three-dimensional Ising model. This model is well suited because despite its simplicity, one can learn much about nucleation of crystalline nuclei. Below the so-called roughening temperature, nuclei in the Ising model are not spherical anymore but become cubic because of the anisotropy of the interfacial tension. This is similar to crystalline nuclei, which are in general not spherical but more like a convex polyhedron with flat facets on the surface. In this context, the problem of distinguishing between the two bulk phases in the vicinity of the diffuse droplet surface is addressed. A new definition is found which correctly determines the volume of a droplet in a given configuration if compared to the volume predicted by simple macroscopic assumptions.rnrnTo compute the interfacial tension of solid-liquid interfaces, a new Monte Carlo method called ensemble switch method'' is presented which allows to compute the interfacial tension of liquid-vapor interfaces as well as solid-liquid interfaces with great accuracy. In the past, the dependence of the interfacial tension on the finite size and shape of the simulation box has often been neglected although there is a nontrivial dependence on the box dimensions. As a consequence, one needs to systematically increase the box size and extrapolate to infinite volume in order to accurately predict the interfacial tension. Therefore, a thorough finite-size scaling analysis is established in this thesis. Logarithmic corrections to the finite-size scaling are motivated and identified, which are of leading order and therefore must not be neglected. The astounding feature of these logarithmic corrections is that they do not depend at all on the model under consideration. Using the ensemble switch method, the validity of a finite-size scaling ansatz containing the aforementioned logarithmic corrections is carefully tested and confirmed. Combining the finite-size scaling theory with the ensemble switch method, the interfacial tension of several model systems, ranging from the Ising model to colloidal systems, is computed with great accuracy.

Feasibility of texture analysis for the assessment of biochemical changes in meniscal tissue on T1 maps calculated from delayed gadolinium-enhanced magnetic resonance imaging of cartilage data: comparison with conventional relaxation time measurements

To (1) establish the feasibility of texture analysis for the in vivo assessment of biochemical changes in meniscal tissue on delayed gadolinium-enhanced magnetic resonance imaging of cartilage (dGEMRIC), and (2) compare textural with conventional T1 relaxation time measurements calculated from dGEMRIC data ("T1(Gd) relaxation times").

Mechanisms of species divergence through visual adaptation and sexual selection: Perspectives from a cichlid model system

The theory of ecological speciation suggests that assortative mating evolves most easily when mating preferences are;directly linked to ecological traits that are subject to divergent selection. Sensory adaptation can play a major role in this process,;because selective mating is often mediated by sexual signals: bright colours, complex song, pheromone blends and so on. When;divergent sensory adaptation affects the perception of such signals, mating patterns may change as an immediate consequence.;Alternatively, mating preferences can diverge as a result of indirect effects: assortative mating may be promoted by selection;against intermediate phenotypes that are maladapted to their (sensory) environment. For Lake Victoria cichlids, the visual environment;constitutes an important selective force that is heterogeneous across geographical and water depth gradients. We investigate;the direct and indirect effects of this heterogeneity on the evolution of female preferences for alternative male nuptial colours;(red and blue) in the genus Pundamilia. Here, we review the current evidence for divergent sensory drive in this system, extract;general principles, and discuss future perspectives

Subconductance states of Cx30 gap junction channels: data from transfected HeLa cells versus data from a mathematical model

Human HeLa cells expressing mouse connexin30 were used to study the electrical properties of gap junction channel substates. Experiments were performed on cell pairs using a dual voltage-clamp method. Single-channel currents revealed discrete levels attributable to a main state, a residual state, and five substates interposed, suggesting the operation of six subgates provided by the six connexins of a gap junction hemichannel. Substate conductances, gamma(j,substate), were unevenly distributed between the main-state and the residual-state conductance (gamma(j,main state) = 141 pS, gamma(j,residual state) = 21 pS). Activation of the first subgate reduced the channel conductance by approximately 30%, and activation of subsequent subgates resulted in conductance decrements of 10-15% each. Current transitions between the states were fast (<2 ms). Substate events were usually demarcated by transitions from and back to the main state; transitions among substates were rare. Hence, subgates are recruited simultaneously rather than sequentially. The incidence of substate events was larger at larger gradients of V(j). Frequency and duration of substate events increased with increasing number of synchronously activated subgates. Our mathematical model, which describes the operation of gap junction channels, was expanded to include channel substates. Based on the established V(j)-sensitivity of gamma(j,main state) and gamma(j,residual state), the simulation yielded unique functions gamma(j,substate) = f(V(j)) for each substate. Hence, the spacing of subconductance levels between the channel main state and residual state were uneven and characteristic for each V(j).

[Active and healthy living in old age--results from a representative survey of community-dwelling senior citizens in Hamburg]

BACKGROUND: The majority of community-dwelling people 60 years and older are independent and live actively. However, there is little information about elderly persons' views on aging, health and health promotion. METHODS: Therefore, an anonymous, written questionnaire survey was performed in a representative sample of inhabitants from a section of the city of Hamburg, 60 years and older; 5 year intervals, 14 subsamples according to 7 age groups of females and males. RESULTS: Questionnaires from 950 participants (29% response) could be evaluated: mean age 71.5 years, 58% women, 34% living alone, 5% with professional healthcare needs as indicated by status according to German nursing care insurance. Senior citizens' positive attitudes towards aging and health were predominant: 69% of respondents felt young, 85% worried about loss of autonomy in old age. CONCLUSIONS: The results provide evidence indicating potential for improving health-promoting lifestyles in parts of the older population by evaluating and strengthening older persons' competencies and by considering their concerns seriously. These results provide valuable information for future plans in the public-health sector in the city of Hamburg where particular health-promoting actions for elderly persons will be considered.