116,299 research outputs found
Polymer adsorption on heterogeneous surfaces
The adsorption of a single ideal polymer chain on energetically heterogeneous
and rough surfaces is investigated using a variational procedure introduced by
Garel and Orland (Phys. Rev. B 55 (1997), 226). The mean polymer size is
calculated perpendicular and parallel to the surface and is compared to the
Gaussian conformation and to the results for polymers at flat and energetically
homogeneous surfaces. The disorder-induced enhancement of adsorption is
confirmed and is shown to be much more significant for a heterogeneous
interaction strength than for spatial roughness. This difference also applies
to the localization transition, where the polymer size becomes independent of
the chain length. The localization criterion can be quantified, depending on an
effective interaction strength and the length of the polymer chain.Comment: accepted in EPJB (the Journal formerly known as Journal de Physique
Avalanches, loading and finite size effects in 2D amorphous plasticity: results from a finite element model
Crystalline plasticity is strongly interlinked with dislocation mechanics and
nowadays is relatively well understood. Concepts and physical models of plastic
deformation in amorphous materials on the other hand - where the concept of
linear lattice defects is not applicable - still are lagging behind. We
introduce an eigenstrain-based finite element lattice model for simulations of
shear band formation and strain avalanches. Our model allows us to study the
influence of surfaces and finite size effects on the statistics of avalanches.
We find that even with relatively complex loading conditions and open boundary
conditions, critical exponents describing avalanche statistics are unchanged,
which validates the use of simpler scalar lattice-based models to study these
phenomena.Comment: Journal of Statistical Mechanics: Theory and Experiment, 2015, P0201
Laser Deposition Cladding On-Line Inspection Using 3-D Scanner
Laser deposition directly deposits metal cladding to fabricate and repair components. In
order to finish the fabrication or repair, 3-D shape of the deposition needs to be inspected, and
thus it can be determined if it has sufficient cladding to fabricate a part after deposition process.
In the present hybrid system in the Laser Aided Manufacturing Lab (LAMP) at the University of
Missouri - Rolla, a CMM system is used to do the inspection. A CMM requires point-by-point
contact, which is time consuming and difficult to plan for an irregular deposition geometry. Also,
the CMM is a separate device, which requires removal of the part from the hybrid system, which
can induce fixture errors. The 3-D scanner is a non-contact tool to measure the 3-D shape of laser
deposition cladding which is fast and accurate. In this paper, A prototype non-contact 3-D
scanner approach has been implemented to inspect the free-form and complex parts built by laser
deposition. Registration of the measured model and 3-D CAD model allows
the comparison between the two models. It enables us to determine if the deposition is sufficient
before machining.Mechanical Engineerin
Analysis Of Localization Sites for An Excess Electron In Neutral Methanol Clusters Using Approximate Pseudopotential Quantum-Mechanical Calculations
We have used a recently developed electron–methanol molecule pseudopotential in approximate quantum mechanical calculations to evaluate and statistically analyze the physical properties of an excess electron in the field of equilibrated neutral methanol clusters ((CH3OH)n , n = 50 – 500). The methanol clusters were generated in classical molecular dynamics simulations at nominal 100 K and 200 K temperatures. Topological analysis of the neutral clusters indicates that methyl groups cover the surface of the clusters almost exclusively, while the associated hydroxyl groups point inside. Since the initial neutral clusters are lacking polarity on the surface and compact inside, the excess electron can barely attach to these structures. Nevertheless, most of the investigated cluster configurations do support weakly stabilized cluster anion states. We find that similarly to water clusters, the pre-existing instantaneous dipole moment of the neutral clusters binds the electron. The localizing electrons occupy diffuse, weakly bound surface states that largely engulf the cluster although their centers are located outside the cluster molecular frame. The initial localization of the excess electron is reflected in its larger radius compared to water due to the lack of free OH hydrogens on the cluster surface. The stabilization of the excess electron increases, while the radius decreases monotonically as the clusters grow in size. Stable, interior bound states of the excess electron are not observed to form neither in finite size methanol clusters nor in the equilibrium bulk
A symmetric polymer blend confined into a film with antisymmetric surfaces: interplay between wetting behavior and phase diagram
We study the phase behavior of a symmetric binary polymer blend which is
confined into a thin film. The film surfaces interact with the monomers via
short range potentials. We calculate the phase behavior within the
self-consistent field theory of Gaussian chains. Over a wide range of
parameters we find strong first order wetting transitions for the semi-infinite
system, and the interplay between the wetting/prewetting behavior and the phase
diagram in confined geometry is investigated. Antisymmetric boundaries, where
one surface attracts the A component with the same strength than the opposite
surface attracts the B component, are applied. The phase transition does not
occur close to the bulk critical temperature but in the vicinity of the wetting
transition. For very thin films or weak surface fields one finds a single
critical point at . For thicker films or stronger surface fields
the phase diagram exhibits two critical points and two concomitant coexistence
regions. Only below a triple point there is a single two phase coexistence
region. When we increase the film thickness the two coexistence regions become
the prewetting lines of the semi-infinite system, while the triple temperature
converges towards the wetting transition temperature from above. The behavior
close to the tricritical point, which separates phase diagrams with one and two
critical points, is studied in the framework of a Ginzburg-Landau ansatz.
Two-dimensional profiles of the interface between the laterally coexisting
phases are calculated, and the interfacial and line tensions analyzed. The
effect of fluctuations and corrections to the self-consistent field theory are
discussed.Comment: Phys.Rev.E in prin
Free and forced wave propagation in a Rayleigh-beam grid: flat bands, Dirac cones, and vibration localization vs isotropization
In-plane wave propagation in a periodic rectangular grid beam structure,
which includes rotational inertia (so-called 'Rayleigh beams'), is analyzed
both with a Floquet-Bloch exact formulation for free oscillations and with a
numerical treatment (developed with PML absorbing boundary conditions) for
forced vibrations (including Fourier representation and energy flux
evaluations), induced by a concentrated force or moment. A complex interplay is
observed between axial and flexural vibrations (not found in the common
idealization of out-of-plane motion), giving rise to several forms of vibration
localization: 'X-', 'cross-' and 'star-' shaped, and channel propagation. These
localizations are triggered by several factors, including rotational inertia
and slenderness of the beams and the type of forcing source (concentrated force
or moment). Although the considered grid of beams introduces an orthotropy in
the mechanical response, a surprising 'isotropization' of the vibration is
observed at special frequencies. Moreover, rotational inertia is shown to
'sharpen' degeneracies related to Dirac cones (which become more pronounced
when the aspect ratio of the grid is increased), while the slenderness can be
tuned to achieve a perfectly flat band in the dispersion diagram. The obtained
results can be exploited in the realization of metamaterials designed to
control wave propagation.Comment: 25 pages, 20 figure
Absence of continuous spectral types for certain nonstationary random models
We consider continuum random Schr\"odinger operators of the type with a deterministic background potential .
We establish criteria for the absence of continuous and absolutely continuous
spectrum, respectively, outside the spectrum of . The models we
treat include random surface potentials as well as sparse or slowly decaying
random potentials. In particular, we establish absence of absolutely continuous
surface spectrum for random potentials supported near a one-dimensional surface
(``random tube'') in arbitrary dimension.Comment: 14 pages, 2 figure
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