21,874 research outputs found
AC electrokinetic phenomena over semiconductive surfaces: effective electric boundary conditions and their applications
Electrokinetic boundary conditions are derived for AC electrokinetic (ACEK)
phenomena over leaky dielectric (i.e., semiconducting) surfaces. Such boundary
conditions correlate the electric potentials across the
semiconductor-electrolyte interface (consisting of the electric double layer
(EDL) inside the electrolyte solutions and the space charge layer (SCL) inside
the semiconductors) under AC electric fields with arbitrary wave forms. The
present electrokinetic boundary conditions allow for evaluation of induced zeta
potential contributed by both bond charges (due to electric polarization) and
free charges (due to electric conduction) from the leaky dielectric materials.
Subsequently, we demonstrate the applications of these boundary conditions in
analyzing the ACEK phenomena around a semiconducting cylinder. It is concluded
that the flow circulations exist around the semiconducting cylinder and are
shown to be stronger under an AC field with lower frequency and around a
cylinder with higher conductivity.Comment: 29 pages, 4 figure
Invisibility and Inverse Problems
This survey of recent developments in cloaking and transformation optics is
an expanded version of the lecture by Gunther Uhlmann at the 2008 Annual
Meeting of the American Mathematical Society.Comment: 68 pages, 12 figures. To appear in the Bulletin of the AM
Effects of nano-void density, size, and spatial population on thermal conductivity: a case study of GaN crystal
The thermal conductivity of a crystal is sensitive to the presence of
surfaces and nanoscale defects. While this opens tremendous opportunities to
tailor thermal conductivity, a true "phonon engineering" of nanocrystals for a
specific electronic or thermoelectric application can only be achieved when the
dependence of thermal conductivity on the defect density, size, and spatial
population is understood and quantified. Unfortunately, experimental studies of
effects of nanoscale defects are quite challenging. While molecular dynamics
simulations are effective in calculating thermal conductivity, the defect
density range that can be explored with feasible computing resources is
unrealistically high. As a result, previous work has not generated a fully
detailed understanding of the dependence of thermal conductivity on nanoscale
defects. Using GaN as an example, we have combined physically-motivated
analytical model and highly-converged large scale molecular dynamics
simulations to study effects of defects on thermal conductivity. An analytical
expression for thermal conductivity as a function of void density, size, and
population has been derived and corroborated with the model, simulations, and
experiments
Fluctuations, dissipation and the dynamical Casimir effect
Vacuum fluctuations provide a fundamental source of dissipation for systems
coupled to quantum fields by radiation pressure. In the dynamical Casimir
effect, accelerating neutral bodies in free space give rise to the emission of
real photons while experiencing a damping force which plays the role of a
radiation reaction force. Analog models where non-stationary conditions for the
electromagnetic field simulate the presence of moving plates are currently
under experimental investigation. A dissipative force might also appear in the
case of uniform relative motion between two bodies, thus leading to a new kind
of friction mechanism without mechanical contact. In this paper, we review
recent advances on the dynamical Casimir and non-contact friction effects,
highlighting their common physical origin.Comment: 39 pages, 4 figures. Review paper to appear in Lecture Notes in
Physics, Volume on Casimir Physics, edited by Diego Dalvit, Peter Milonni,
David Roberts, and Felipe da Rosa. Minor changes, a reference adde
Modelling linered engine blocks
Factors that affect heat transfer in the linered aluminium engine block are examined to determine their importance. Conduction is found to be the dominant mode of heat transfer, and the interface is characterised as imperfect contact if there are no surface manufacturing defects larger than 139 microns. A model is proposed to estimate the effective conductivity for imperfect contact. This thermal conductance depends on the area of contact, macroscopic roughness, the contact pressure and the interstitial medium. The transfer of heat and the distribution of stress in line red engine blocks are coupled, and the problem is strongly non-linear. A finite element solution procedure for solving the heat transfer problem in the linered engine block is outlined
Homogenization of the Poisson-Nernst-Planck Equations for Ion Transport in Charged Porous Media
Effective Poisson-Nernst-Planck (PNP) equations are derived for macroscopic
ion transport in charged porous media under periodic fluid flow by an
asymptotic multi-scale expansion with drift. The microscopic setting is a
two-component periodic composite consisting of a dilute electrolyte continuum
(described by standard PNP equations) and a continuous dielectric matrix, which
is impermeable to the ions and carries a given surface charge. Four new
features arise in the upscaled equations: (i) the effective ionic diffusivities
and mobilities become tensors, related to the microstructure; (ii) the
effective permittivity is also a tensor, depending on the electrolyte/matrix
permittivity ratio and the ratio of the Debye screening length to the
macroscopic length of the porous medium; (iii) the microscopic fluidic
convection is replaced by a diffusion-dispersion correction in the effective
diffusion tensor; and (iv) the surface charge per volume appears as a
continuous "background charge density", as in classical membrane models. The
coefficient tensors in the upscaled PNP equations can be calculated from
periodic reference cell problems. For an insulating solid matrix, all gradients
are corrected by the same tensor, and the Einstein relation holds at the
macroscopic scale, which is not generally the case for a polarizable matrix,
unless the permittivity and electric field are suitably defined. In the limit
of thin double layers, Poisson's equation is replaced by macroscopic
electroneutrality (balancing ionic and surface charges). The general form of
the macroscopic PNP equations may also hold for concentrated solution theories,
based on the local-density and mean-field approximations. These results have
broad applicability to ion transport in porous electrodes, separators,
membranes, ion-exchange resins, soils, porous rocks, and biological tissues
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