6,167 research outputs found
Charge regulation and ionic screening of patchy surfaces
The properties of surfaces with charge-regulated patches are studied using
non-linear Poisson-Boltzmann theory. Using a mode expansion to solve the
non-linear problem efficiently, we reveal the charging behaviour of
Debye-length sized patches. We find that patches charge up to higher charge
densities if their size is relatively small and if the patches are well
separated. The numerical results are used to construct a basic analytical model
which predicts the average surface charge density on surfaces with patchy
chargeable groups.Comment: 9 figure
A New Class of Cellular Automata for Reaction-Diffusion Systems
We introduce a new class of cellular automata to model reaction-diffusion
systems in a quantitatively correct way. The construction of the CA from the
reaction-diffusion equation relies on a moving average procedure to implement
diffusion, and a probabilistic table-lookup for the reactive part. The
applicability of the new CA is demonstrated using the Ginzburg-Landau equation.Comment: 4 pages, RevTeX 3.0 , 3 Figures 214972 bytes tar, compressed,
uuencode
Nonextensive diffusion as nonlinear response
The porous media equation has been proposed as a phenomenological
``non-extensive'' generalization of classical diffusion. Here, we show that a
very similar equation can be derived, in a systematic manner, for a classical
fluid by assuming nonlinear response, i.e. that the diffusive flux depends on
gradients of a power of the concentration. The present equation distinguishes
from the porous media equation in that it describes \emph{% generalized
classical} diffusion, i.e. with scaling, but with a generalized
Einstein relation, and with power-law probability distributions typical of
nonextensive statistical mechanics
Lattice gas with ``interaction potential''
We present an extension of a simple automaton model to incorporate non-local
interactions extending over a spatial range in lattice gases. {}From the
viewpoint of Statistical Mechanics, the lattice gas with interaction range may
serve as a prototype for non-ideal gas behavior. {}From the density
fluctuations correlation function, we obtain a quantity which is identified as
a potential of mean force. Equilibrium and transport properties are computed
theoretically and by numerical simulations to establish the validity of the
model at macroscopic scale.Comment: 12 pages LaTeX, figures available on demand ([email protected]
Suitability of granular carbon as an anode material for sediment microbial fuel cells
Purpose: Sediment-microbial fuel cells (S-MFC) are bio-electrochemical devices that are able to oxidize organic matter directly into harvestable electrical power. The flux of organic matter into the sediment is rather low, therefore other researchers have introduced plants for a continues supply of organic matter to the anode electrode. Until now only interconnected materials have been considered as anode materials in S-MFC. Here granular carbon materials were investigated for their suitability as anode material in sediment microbial fuel cells.
Materials and methods: Laboratory microcosms with 8 different electrode materials (granules, felts and cloths) were examined with controlled organic matter addition under brackish conditions. Current density, organic matter removal and microbial community composition were monitored using 16S-rRNA gene PCR followed by Denaturing Gradient Gel Electrophoresis (DGGE). The main parameters investigated were the influence of the amount of electrode material applied to the sediment, the size of the granular material and the electrode configuration.
Results and discussion: Felt material had an overall superior performance in terms of current density per amount of applied electrode material i.e. felt and granular anode obtained similar current densities (approx. 50–60 mA/m2) but felt materials required 29% less material to be applied. Yet, when growing plants, granular carbon is more suited because it is considered to restore, upon disturbance, the electrical connectivity within the anode compartment. Small granules (0.25–0.5 mm) gave the highest current density compared to larger granules (1-5 mm) of the same material. Granules with a rough surface had a better performance compared to smooth granules of the same size. The different granular materials lead to a selection of distinct microbial communities for each material, as shown by DGGE.
Conclusions: Granular carbon is suited as anode material for sediment microbial fuel cells. This opens the perspective for application of MFC in cultivated areas. In a wider context, the application of granular carbon electrodes can also be an option for in-situ bioremediation of contaminated soils
Questioning the validity of non-extensive thermodynamics for classical Hamiltonian systems
We examine the non-extensive approach to the statistical mechanics of
Hamiltonian systems with where is the classical kinetic energy. Our
analysis starts from the basics of the formalism by applying the standard
variational method for maximizing the entropy subject to the average energy and
normalization constraints. The analytical results show (i) that the
non-extensive thermodynamics formalism should be called into question to
explain experimental results described by extended exponential distributions
exhibiting long tails, i.e. -exponentials with , and (ii) that in the
thermodynamic limit the theory is only consistent in the range
where the distribution has finite support, thus implying that configurations
with e.g. energy above some limit have zero probability, which is at variance
with the physics of systems in contact with a heat reservoir. We also discuss
the (-dependent) thermodynamic temperature and the generalized specific
heat.Comment: To appear in EuroPhysics Letter
Screening of heterogeneous surfaces: charge renormalization of Janus particles
Nonlinear ionic screening theory for heterogeneously charged spheres is
developed in terms of a mode-decomposition of the surface charge. A far-field
analysis of the resulting electrostatic potential leads to a natural
generalization of charge renormalization from purely monopolar to dipolar,
quadropolar, etc., including mode-couplings. Our novel scheme is generally
applicable to large classes of surface heterogeneities, and is explicitly
applied here to Janus spheres with differently charged upper and lower
hemispheres, revealing strong renormalization effects for all multipoles.Comment: 2 figure
A generalized Chudley-Elliott vibration-jump model in activated atom surface diffusion
Here the authors provide a generalized Chudley-Elliott expression for
activated atom surface diffusion which takes into account the coupling between
both low-frequency vibrational motion (namely, the frustrated translational
modes) and diffusion. This expression is derived within the Gaussian
approximation framework for the intermediate scattering function at low
coverage. Moreover, inelastic contributions (arising from creation and
annihilation processes) to the full width at half maximum of the quasi-elastic
peak are also obtained.Comment: (5 pages, 2 figures; revised version
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