Structural Properties of Two-Dimensional Polymers

We present structural properties of two-dimensional polymers as far as they can be described by percolation theory. The percolation threshold, critical exponents and fractal dimensions of clusters are determined by computer simulation and compared to the results of percolation theory. We also describe the dependence of the typical cluster structures on the reaction rate.Comment: 7 pages, LaTeX with RevTeX and epsf styles and PostScript figures included (uuencoded shell archive), TVP-93051

The Crumpling Transition of Dynamically Triangulated Random Surfaces

We present the crumpling transition in three-dimensional Euclidian space of dynamically triangulated random surfaces with edge extrinsic curvature and fixed topology of a sphere as well as simulations of a dynamically triangulated torus. We used longer runs than previous simulations and give new and more accurate estimates of critical exponents. Our data indicate a cusp singularity in the specific heat. The transition temperature, as well as the exponents are topology dependent.Comment: 11pages (PostScript figures are not included. Request author directly.), LaTeX, TPHD-93020

Large-scale Simulation of the Two-dimensional Kinetic Ising Model

We present Monte Carlo simulation results for the dynamical critical exponent $z$ of the two-dimensional kinetic Ising model using a lattice of size $10^6 \times 10^6$ spins. We used Glauber as well as Metropolis dynamics. The $z$-value of $2.16\pm 0.005$ was calculated from the magnetization and energy relaxation from an ordered state towards the equilibrium state at $T_c$.Comment: 6 pages + 2 figures as separate uuencoded compressed tar file, Postscipt also available at http://wwwcp.tphys.uni-heidelberg.de/papers

Deformation of polymer films by bending forces

We study the deformation of nano--scale polymer films which are subject to external bending forces by means of computer simulation. The polymer is represented by a generalized bead--spring--model, intended to reproduce characteristic features of n--alkanes. The film is loaded by the action of a prismatic blade which is pressed into the polymer bulk from above and a pair of columns which support the film from below. The interaction between blade and support columns and the polymer is modelled by the repulsive part of a Lennard-Jones potential. For different system sizes as well as for different chainlengths, this nano--scale experiment is simulated by molecular dynamics methods. Our results allow us to give a first characterization of deformed states for such films. We resolve the kinetic and the dynamic stage of the deformation process in time and access the length scale between discrete particle and continuum mechanics behaviour. For the chainlengths considered here, we find that the deformation process is dominated by shear. We observe strangling effects for the film and deformation fluctuations in the steady state.Comment: 15 pages, 8 figure

Reconstruction of the Free Energy in the Metastable Region using the Path Ensemble

By quenching into the metastable region of the three-dimensional Ising model, we investigate the paths that the magnetization (energy) takes as a function of time. We accumulate the magnetization (energy) paths into time-dependent distributions from which we reconstruct the free energy as a function of the magnetic field, temperature and system size. From the reconstructed free energy, we obtain the free energy barrier that is associated with the transition from a metastable state to the stable equilibrium state. Although mean-field theory predicts a sharp transition between the metastable and the unstable region where the free energy barrier is zero, the results for the nearest-neighbour Ising model show that the free energy barrier does not go zero

Rural NPO Leadership in Time of Volatility: A Study of Nonprofit Organizations in South Dakota

This exploratory study determined the legitimacy of the leadership gap in Nonprofit Organizations (NPOs) in South Dakota by collecting quantitative and qualitative data and then comparing the results with national studies. It aimed to understand if NPOs were more vulnerable to leadership transitions and the costs associated with them. It also provided additional insight for future research opportunities. While there were studies on larger NPOs, such as the United Way, or NPOs in more highly populated regions, there was a gap in research for rural NPOs that potentially would not be financially able to research these trends on their own. This gap in research was addressed through a sequential mixed-methods approach. An electronic survey, developed and designed for this research, was submitted to obtain quantitative data. After which, a smaller subset of small NPO leaders and board members were selected, based on their responses, to participate in a qualitative phone interview. These findings were then compared to national data

Who makes the best of it? Alternative feeding strategies of European perch (Perca fluviatilis) and their consequences

Cannibalism in fish has various effects on the population as well as on individuals. Cannibals derive direct energetic benefit from cannibalism by feeding on their smaller conspecifics, while at the same time reducing competition for the shared resource. Avoiding intra-cohort competition can force fish to use alternative feeding strategies (e.g. becoming early piscivorous feeding on bream larvae) and hence leads to greater specialisation within the population, which may subsequently stimulate cannibalism. Early piscivory and cannibalism leads to the development of a stable bimodality which brings several advantages and adaptations especially for larger fish, such as morphological changes to piscivory, higher energy stores (lipid, protein) and reduced winter mortality. Defined pond and laboratory experiments were provided to gain insights into (1) the mechanism leading to cannibalism via specialisation on alternative food resources and the establishment of a bimodal size distribution, (2) the advantages for the large size-cohort especially considering the first winter as a bottleneck for the recruitment success of young-of-the-year (YOY) fish, (3) whether in the system of early piscivory and intraguild cannibalism, fish larvae can be substituted as prey by alternative food resources and (4) the morphological adaptations on the food resource used, irrespective of the habitat fish prefer. As zooplankton levels decreased, YOY perch (Perca fluviatilis) started to specialise, suggesting that increased intraspecific competition forced fish to try alternative feeding strategies. Larger YOY perch then fed on resources richer on energy (bream larvae and tadpoles) and hence outgrew their smaller conspecifics, becoming large enough to enable cannibalism. This differentiation into two size classes and two different feeding strategies was shown to be a stable pattern within the YOY perch population, as indicated by stable isotope analyses. If bream was substituted by roach, no piscivory occurred and perch were unimodally distributed. Relative to perch, roach hatched earlier than bream and also grew faster, preventing gape-limited perch to feed on roach. However, these perch were significantly larger than the small size-cohort in the cannibalistic system, suggesting that due to predation pressure of cannibals, small perch reduced their activity (which was documented by comparing the gill net catches of the two cohorts) and showed reduced growth rates. The two size-cohorts that developed by cannibalism showed different potential for surviving the winter. In the laboratory only the smallest perch suffered from high mortality rates, even when all were fed with low amounts of food. However, feeding the fish reduced the mortality rate. The pond experiments (cold winter 2005/06 and warmer winter 2006/07) showed that although small perch did not reduce their feeding activity, they suffered from high mortality during both winters. However, those who survived the winter rebuilt their energy stores. Large perch demonstrated low rates of mortality during the cold winter. During the warm winter, however, mortality was nine-fold higher than during the first winter. Low mortality rates were observed in the laboratory although the temperature was comparable to the one of 2006/07. Therefore one can conclude that the loss of ice coverage which can function as a shelter against avian predators might be more responsible for the high mortality rates of large perch than the temperature itself. The food consumed not only changes the energy reserves of fish but also their morphology. Distinct morphological changes related to piscivory and planktivory were documented in field and mesocosm studies. The mouth of the piscivorous YOY year perch became larger, the pectoral fins and the centre of mass of the posterior abdomen were shifted backwards. These findings provide evidence that the type of food is important and might lead to further functional adaptations in morphology. Furthermore fish may differ in habitat-specific occurrence and activity related to size, as was the case for the two size-cohorts in those ponds with cannibalism. As a consequence of this, and also as a result of the fact that the size distribution of YOY perch had broadened considerably, there was sometimes no overlap in the length-frequency distributions between gillnetting and electro-fishing, clearly demonstrating that using a single method is not sufficient to draw a complete picture of the population size structure. Intraguild cannibalism and the related development of bimodal size distribution often might not be recognised, since only 1 to 10 % of the perch population choose this alternative feeding strategy. However, the resultant large size-cohort can have considerable repercussions for the perch population