800 research outputs found

    Viviparus acerosus (Bourguignat, 1862) (Gastropoda: Viviparidae), a new exotic snail species for the Dutch fauna

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    Abstract Viviparus acerosus, a Central European species originally inhabiting the Danube drainage system, is reported for the first time from The Netherlands. The species is commercially sold for gardens ponds or aquaria, from which they may have escaped or have been released. On account of its resemblance with the two native Viviparus species the species is easily overlooked

    Stripe-hexagon competition in forced pattern forming systems with broken up-down symmetry

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    We investigate the response of two-dimensional pattern forming systems with a broken up-down symmetry, such as chemical reactions, to spatially resonant forcing and propose related experiments. The nonlinear behavior immediately above threshold is analyzed in terms of amplitude equations suggested for a 1:21:2 and 1:11:1 ratio between the wavelength of the spatial periodic forcing and the wavelength of the pattern of the respective system. Both sets of coupled amplitude equations are derived by a perturbative method from the Lengyel-Epstein model describing a chemical reaction showing Turing patterns, which gives us the opportunity to relate the generic response scenarios to a specific pattern forming system. The nonlinear competition between stripe patterns and distorted hexagons is explored and their range of existence, stability and coexistence is determined. Whereas without modulations hexagonal patterns are always preferred near onset of pattern formation, single mode solutions (stripes) are favored close to threshold for modulation amplitudes beyond some critical value. Hence distorted hexagons only occur in a finite range of the control parameter and their interval of existence shrinks to zero with increasing values of the modulation amplitude. Furthermore depending on the modulation amplitude the transition between stripes and distorted hexagons is either sub- or supercritical.Comment: 10 pages, 12 figures, submitted to Physical Review

    Critical behaviour of the Rouse model for gelling polymers

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    It is shown that the traditionally accepted "Rouse values" for the critical exponents at the gelation transition do not arise from the Rouse model for gelling polymers. The true critical behaviour of the Rouse model for gelling polymers is obtained from spectral properties of the connectivity matrix of the fractal clusters that are formed by the molecules. The required spectral properties are related to the return probability of a "blind ant"-random walk on the critical percolating cluster. The resulting scaling relations express the critical exponents of the shear-stress-relaxation function, and hence those of the shear viscosity and of the first normal stress coefficient, in terms of the spectral dimension dsd_{s} of the critical percolating cluster and the exponents σ\sigma and τ\tau of the cluster-size distribution.Comment: 9 pages, slightly extended version, to appear in J. Phys.

    Dynamics of gelling liquids: a short survey

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    The dynamics of randomly crosslinked liquids is addressed via a Rouse- and a Zimm-type model with crosslink statistics taken either from bond percolation or Erdoes-Renyi random graphs. While the Rouse-type model isolates the effects of the random connectivity on the dynamics of molecular clusters, the Zimm-type model also accounts for hydrodynamic interactions on a preaveraged level. The incoherent intermediate scattering function is computed in thermal equilibrium, its critical behaviour near the sol-gel transition is analysed and related to the scaling of cluster diffusion constants at the critical point. Second, non-equilibrium dynamics is studied by looking at stress relaxation in a simple shear flow. Anomalous stress relaxation and critical rheological properties are derived. Some of the results contradict long-standing scaling arguments, which are shown to be flawed by inconsistencies.Comment: 21 pages, 3 figures; Dedicated to Lothar Schaefer on the occasion of his 60th birthday; Changes: added comments on the gel phase and some reference

    From Social Democracy back to No Ideology? - The Scottish National Party and Ideological Change in a Multi-level Electoral Setting

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    This article examines the development of ideology in the Scottish National Party since its formation in 1934, focusing on the recent period of the party’s history. It examines ideological development within the context of the party’s adaptation to multi-level elections and party system, especially in the decade since devolution began in 1999. It also provides a brief consideration of the party’s office success and governmental performance since 2007 to examine the effect of office on the SNP and its autonomy goal of Scottish independence
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