1,804 research outputs found
Depletion forces near a soft surface
We investigate excluded-volume effects in a bidisperse colloidal suspension
near a flexible interface. Inspired by a recent experiment by Dinsmore et al.
(Phys. Rev, Lett. 80, 409 (1998)), we study the adsorption of a mesoscopic bead
on the surface and show that depletion forces could in principle lead to
particle encapsulation. We then consider the effect of surface fluctuations on
the depletion potential itself and construct the density profile of a polymer
solution near a soft interface. Surprisingly we find that the chains accumulate
at the wall, whereas the density displays a deficit of particles at distances
larger than the surface roughness. This non-monotonic behavior demonstrates
that surface fluctuations can have major repercusions on the properties of a
colloidal solution. On average, the additional contribution to the Gibbs
adsorbance is negative. The amplitude of the depletion potential between a
mesoscopic bead and the surface increases accordingly.Comment: 10 pages, 5 figure
Surface-mediated attraction between colloids
We investigate the equilibrium properties of a colloidal solution in contact
with a soft interface. As a result of symmetry breaking, surface effects are
generally prevailing in confined colloidal systems. In this Letter, particular
emphasis is given to surface fluctuations and their consequences on the local
(re)organization of the suspension. It is shown that particles experience a
significant effective interaction in the vicinity of the interface. This
potential of mean force is always attractive, with range controlled by the
surface correlation length. We suggest that, under some circumstances,
surface-induced attraction may have a strong influence on the local particle
distribution
Minimax Estimation of Nonregular Parameters and Discontinuity in Minimax Risk
When a parameter of interest is nondifferentiable in the probability, the
existing theory of semiparametric efficient estimation is not applicable, as it
does not have an influence function. Song (2014) recently developed a local
asymptotic minimax estimation theory for a parameter that is a
nondifferentiable transform of a regular parameter, where the nondifferentiable
transform is a composite map of a continuous piecewise linear map with a single
kink point and a translation-scale equivariant map. The contribution of this
paper is two fold. First, this paper extends the local asymptotic minimax
theory to nondifferentiable transforms that are a composite map of a Lipschitz
continuous map having a finite set of nondifferentiability points and a
translation-scale equivariant map. Second, this paper investigates the
discontinuity of the local asymptotic minimax risk in the true probability and
shows that the proposed estimator remains to be optimal even when the risk is
locally robustified not only over the scores at the true probability, but also
over the true probability itself. However, the local robustification does not
resolve the issue of discontinuity in the local asymptotic minimax risk
Ablation debris control by means of closed thick film filtered water immersion
The performance of laser ablation generated debris control by means of open immersion techniques have been shown to be limited by flow surface ripple effects on the beam and the action of ablation plume pressure loss by splashing of the immersion fluid. To eradicate these issues a closed technique has been developed which ensured a controlled geometry for both the optical interfaces of the flowing liquid film. This had the action of preventing splashing, ensuring repeatable machining conditions and allowed for control of liquid flow velocity. To investigate the performance benefits of this closed immersion technique bisphenol A polycarbonate samples have been machined using filtered water at a number of flow velocities. The results demonstrate the efficacy of the closed immersion technique: a 93% decrease in debris is produced when machining under closed filtered water immersion; the average debris particle size becomes larger, with an equal proportion of small and medium sized debris being produced when laser machining under closed flowing filtered water immersion; large debris is shown to be displaced further by a given flow velocity than smaller debris, showing that the action of flow turbulence in the duct has more impact on smaller debris. Low flow velocities were found to be less effective at controlling the positional trend of deposition of laser ablation generated debris than high flow velocities; but, use of excessive flow velocities resulted in turbulence motivated deposition. This work is of interest to the laser micromachining community and may aide in the manufacture of 2.5D laser etched patterns covering large area wafers and could be applied to a range of wavelengths and laser types
A Smirnov-Bickel-Rosenblatt theorem for compactly-supported wavelets
In nonparametric statistical problems, we wish to find an estimator of an
unknown function f. We can split its error into bias and variance terms;
Smirnov, Bickel and Rosenblatt have shown that, for a histogram or kernel
estimate, the supremum norm of the variance term is asymptotically distributed
as a Gumbel random variable. In the following, we prove a version of this
result for estimators using compactly-supported wavelets, a popular tool in
nonparametric statistics. Our result relies on an assumption on the nature of
the wavelet, which must be verified by provably-good numerical approximations.
We verify our assumption for Daubechies wavelets and symlets, with N = 6, ...,
20 vanishing moments; larger values of N, and other wavelet bases, are easily
checked, and we conjecture that our assumption holds also in those cases
Finite-difference distributions for the Ginibre ensemble
The Ginibre ensemble of complex random matrices is studied. The complex
valued random variable of second difference of complex energy levels is
defined. For the N=3 dimensional ensemble are calculated distributions of
second difference, of real and imaginary parts of second difference, as well as
of its radius and of its argument (angle). For the generic N-dimensional
Ginibre ensemble an exact analytical formula for second difference's
distribution is derived. The comparison with real valued random variable of
second difference of adjacent real valued energy levels for Gaussian
orthogonal, unitary, and symplectic, ensemble of random matrices as well as for
Poisson ensemble is provided.Comment: 8 pages, a number of small changes in the tex
Robust Estimators in Generalized Pareto Models
This paper deals with optimally-robust parameter estimation in generalized
Pareto distributions (GPDs). These arise naturally in many situations where one
is interested in the behavior of extreme events as motivated by the
Pickands-Balkema-de Haan extreme value theorem (PBHT). The application we have
in mind is calculation of the regulatory capital required by Basel II for a
bank to cover operational risk. In this context the tail behavior of the
underlying distribution is crucial. This is where extreme value theory enters,
suggesting to estimate these high quantiles parameterically using, e.g. GPDs.
Robust statistics in this context offers procedures bounding the influence of
single observations, so provides reliable inference in the presence of moderate
deviations from the distributional model assumptions, respectively from the
mechanisms underlying the PBHT.Comment: 26pages, 6 figure
Ab-initio study of BaTiO3 surfaces
We have carried out first-principles total-energy calculations of (001)
surfaces of the tetragonal and cubic phases of BaTiO3. Both BaO-terminated
(type I) and TiO2-terminated (type II) surfaces are considered, and the atomic
configurations have been fully relaxed. We found no deep-gap surface states for
any of the surfaces, in agreement with previous theoretical studies. However,
the gap is reduced for the type-II surface, especially in the cubic phase. The
surface relaxation energies are found to be substantial, i.e., many times
larger than the bulk ferroelectric well depth. Nevertheless, the influence of
the surface upon the ferroelectric order parameter is modest; we find only a
small enhancement of the ferroelectricity near the surface.Comment: 8 pages, two-column style with 4 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#pad_sur
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