16,386 research outputs found
On the theoretical description of weakly charged surfaces
It is widely accepted that the Poisson-Boltzmann (PB) theory provides a valid
description for charged surfaces in the so-called weak coupling limit. Here, we
show that the image charge repulsion creates a depletion boundary layer that
cannot be captured by a regular perturbation approach. The correct
weak-coupling theory must include the self-energy of the ion due to the image
charge interaction. The image force qualitatively alters the double layer
structure and properties, and gives rise to many non-PB effects, such as
nonmonotonic dependence of the surface energy on concentration and charge
inversion. In the presence of dielectric discontinuity, there is no limiting
condition for which the PB theory is valid
Discrete curvature approximations and segmentation of polyhedral surfaces
The segmentation of digitized data to divide a free form surface into patches is one of the key steps required to perform a reverse engineering process of an object. To this end, discrete curvature approximations are introduced as the basis of a segmentation process that lead to a decomposition of digitized data into areas that will help the construction of parametric surface patches. The approach proposed relies on the use of a polyhedral representation of the object built from the digitized data input. Then, it is shown how noise reduction, edge swapping techniques and adapted remeshing schemes can participate to different preparation phases to provide a geometry that highlights useful characteristics for the segmentation process. The segmentation process is performed with various approximations of discrete curvatures evaluated on the polyhedron produced during the preparation phases. The segmentation process proposed involves two phases: the identification of characteristic polygonal lines and the identification of polyhedral areas useful for a patch construction process. Discrete curvature criteria are adapted to each phase and the concept of invariant evaluation of curvatures is introduced to generate criteria that are constant over equivalent meshes. A description of the segmentation procedure is provided together with examples of results for free form object surfaces
Surface Tension of Electrolyte Solutions: A Self-consistent Theory
We study the surface tension of electrolyte solutions at the air/water and
oil/water interfaces. Employing field-theoretical methods and considering
short-range interactions of anions with the surface, we expand the Helmholtz
free energy to first-order in a loop expansion and calculate the excess surface
tension. Our approach is self-consistent and yields an analytical prediction
that reunites the Onsager-Samaras pioneering result (which does not agree with
experimental data), with the ionic specificity of the Hofmeister series. We
obtain analytically the surface-tension dependence on the ionic strength, ionic
size and ion-surface interaction, and show consequently that the
Onsager-Samaras result is consistent with the one-loop correction beyond the
mean-field result. Our theory fits well a wide range of concentrations for
different salts using one fit parameter, reproducing the reverse Hofmeister
series for anions at the air/water and oil/water interfaces.10.1029Comment: 5 pages, 2 figure
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View-dependent adaptive cloth simulation
This paper describes a method for view-dependent cloth simulation using dynamically adaptive mesh refinement and coarsening. Given a prescribed camera motion, the method adjusts the criteria controlling refinement to account for visibility and apparent size in the camera's view. Objectionable dynamic artifacts are avoided by anticipative refinement and smoothed coarsening. This approach preserves the appearance of detailed cloth throughout the animation while avoiding the wasted effort of simulating details that would not be discernible to the viewer. The computational savings realized by this method increase as scene complexity grows, producing a 2× speed-up for a single character and more than 4× for a small group
A detection method of intersections for determining overlapping using active vision
Sometimes, the presence of objects difficult the observation of other neighboring objects. This is because
part of the surface of an object occludes partially the surface of another, increasing the complexitiy in the
recognition process. Therefore, the information which is acquired from scene to describe the objects is often
incomplete and depends a great deal on the view point of the observation. Thus, when any real scene is
observed, the regions and the boundaries which delimit and dissociate objects from others are not perceived
easily. In this paper, a method to discern objects from others, delimiting where the surface of each object
begins and finishes is presented. Really, here, we look for detecting the overlapping and occlusion zones of
two or more objects which interact among each other in a same scene. This is very useful, on the one hand,
to distinguish some objects from others when the features like texture colour and geometric form are not
sufficient to separate them with a segmentation process. On the other hand, it is also important to identify
occluded zones without a previous knowledge of the type of objects which are wished to recognize. The
proposed approach is based on the detection of occluded zones by means of structured light patterns
projected on the object surfaces in a scene. These light patterns determine certain discontinuities of the
beam projections when they hit against the surfaces becoming deformed themselves. So that, such
discontinuities are taken like zones of boundary of occlusion candidate regions
Variational approach for electrolyte solutions: from dielectric interfaces to charged nanopores
A variational theory is developed to study electrolyte solutions, composed of
interacting point-like ions in a solvent, in the presence of dielectric
discontinuities and charges at the boundaries. Three important and non-linear
electrostatic effects induced by these interfaces are taken into account:
surface charge induced electrostatic field, solvation energies due to the ionic
cloud, and image charge repulsion. Our variational equations thus go beyond the
mean-field theory. The influence of salt concentration, ion valency, dielectric
jumps, and surface charge is studied in two geometries. i) A single neutral
air-water interface with an asymmetric electrolyte. A charge separation and
thus an electrostatic field gets established due to the different image charge
repulsions for coions and counterions. Both charge distributions and surface
tension are computed and compared to previous approximate calculations. For
symmetric electrolyte solutions close to a charged surface, two zones are
characterized. In the first one, with size proportional to the logarithm of the
coupling parameter, strong image forces impose a total ion exclusion, while in
the second zone the mean-field approach applies. ii) A symmetric electrolyte
confined between two dielectric interfaces as a simple model of ion rejection
from nanopores. The competition between image charge repulsion and attraction
of counterions by the membrane charge is studied. For small surface charge, the
counterion partition coefficient decreases with increasing pore size up to a
critical pore size, contrary to neutral membranes. For larger pore sizes, the
whole system behaves like a neutral pore. The prediction of the variational
method is also compared with MC simulations and a good agreement is observed.Comment: This version is accepted for publication in Phys. Rev. E
Macroion solutions in the cell model studied by field theory and Monte Carlo simulations
Aqueous solutions of charged spherical macroions with variable dielectric permittivity and their associated counterions are examined within the cell model using a field theory and Monte Carlo simulations. The field theory is based on separation of fields into short- and long-wavelength terms, which are subjected to different statistical-mechanical treatments. The simulations were performed by using a new, accurate, and fast algorithm for numerical evaluation of the electrostatic polarization interaction. The field theory provides counterion distributions outside a macroion in good agreement with the simulation results over the full range from weak to strong electrostatic coupling. A low-dielectric macroion leads to a displacement of the counterions away from the macroion
Studies on the interference of wings and propeller slipstreams
The small disturbance potential flow theory is applied to determine the lift of an airfoil in a nonuniform parallel stream. The given stream is replaced by an equivalent stream with a certain number of velocity discontinuities, and the influence of these discontinuities is obtained by the method of images. Next, this method is extended to the problem of an airfoil in a nonuniform stream of smooth velocity profile. This model allows perturbation velocity potential in a rotational undisturbed stream. A comparison of these results with numerical solutions of Euler equations indicates that, although approximate, the present method provides useful information about the interaction problem while avoiding the need to solve the Euler equations
Modelling elastic structures with strong nonlinearities with application to stick-slip friction
An exact transformation method is introduced that reduces the governing
equations of a continuum structure coupled to strong nonlinearities to a low
dimensional equation with memory. The method is general and well suited to
problems with point discontinuities such as friction and impact at point
contact. It is assumed that the structure is composed of two parts: a continuum
but linear structure and finitely many discrete but strong nonlinearites acting
at various contact points of the elastic structure. The localised
nonlinearities include discontinuities, e.g., the Coulomb friction law. Despite
the discontinuities in the model, we demonstrate that contact forces are
Lipschitz continuous in time at the onset of sticking for certain classes of
structures. The general formalism is illustrated for a continuum elastic body
coupled to a Coulomb-like friction model
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