421 research outputs found
Moving walls accelerate mixing
Mixing in viscous fluids is challenging, but chaotic advection in principle
allows efficient mixing. In the best possible scenario,the decay rate of the
concentration profile of a passive scalar should be exponential in time. In
practice, several authors have found that the no-slip boundary condition at the
walls of a vessel can slow down mixing considerably, turning an exponential
decay into a power law. This slowdown affects the whole mixing region, and not
just the vicinity of the wall. The reason is that when the chaotic mixing
region extends to the wall, a separatrix connects to it. The approach to the
wall along that separatrix is polynomial in time and dominates the long-time
decay. However, if the walls are moved or rotated, closed orbits appear,
separated from the central mixing region by a hyperbolic fixed point with a
homoclinic orbit. The long-time approach to the fixed point is exponential, so
an overall exponential decay is recovered, albeit with a thin unmixed region
near the wall.Comment: 17 pages, 13 figures. PDFLaTeX with RevTeX 4-1 styl
Cascaded four-wave mixing in tapered plasmonic nanoantenna
We study theoretically the cascaded four-wave mixing (FWM) in broadband
tapered plasmonic nanoantennas and demonstrate a 300-fold increase in nonlinear
frequency conversion detected in the main lobe of the nanoantenna far-field
pattern. This is achieved by tuning the elements of the nanoantenna to resonate
frequencies involved into the FWM interaction. Our findings have a potentially
broad application in ultrafast nonlinear spectroscopy, sensing, on-chip optical
frequency conversion, nonlinear optical metamaterials and photon sources
Use of groundwater lifetime expectancy for the performance assessment of a deep geologic waste repository: 1. Theory, illustrations, and implications
Long-term solutions for the disposal of toxic wastes usually involve
isolation of the wastes in a deep subsurface geologic environment. In the case
of spent nuclear fuel, if radionuclide leakage occurs from the engineered
barrier, the geological medium represents the ultimate barrier that is relied
upon to ensure safety. Consequently, an evaluation of radionuclide travel times
from a repository to the biosphere is critically important in a performance
assessment analysis. In this study, we develop a travel time framework based on
the concept of groundwater lifetime expectancy as a safety indicator. Lifetime
expectancy characterizes the time that radionuclides will spend in the
subsurface after their release from the repository and prior to discharging
into the biosphere. The probability density function of lifetime expectancy is
computed throughout the host rock by solving the backward-in-time solute
transport adjoint equation subject to a properly posed set of boundary
conditions. It can then be used to define optimal repository locations. The
risk associated with selected sites can be evaluated by simulating an
appropriate contaminant release history. The utility of the method is
illustrated by means of analytical and numerical examples, which focus on the
effect of fracture networks on the uncertainty of evaluated lifetime
expectancy.Comment: 11 pages, 8 figures; Water Resources Research, Vol. 44, 200
Robust plasmon waveguides in strongly-interacting nanowire arrays
Arrays of parallel metallic nanowires are shown to provide a tunable, robust,
and versatile platform for plasmon interconnects, including high-curvature
turns with minimum signal loss. The proposed guiding mechanism relies on gap
plasmons existing in the region between adjacent nanowires of dimers and
multi-wire arrays. We focus on square and circular silver nanowires in silica,
for which excellent agreement between both boundary element method and multiple
multipolar expansion calculations is obtained. Our work provides the tools for
designing plasmon-based interconnects and achieving high degree of integration
with minimum cross talk between adjacent plasmon guides.Comment: 4 pages, 5 figure
Quantification of the performance of chaotic micromixers on the basis of finite time Lyapunov exponents
Chaotic micromixers such as the staggered herringbone mixer developed by
Stroock et al. allow efficient mixing of fluids even at low Reynolds number by
repeated stretching and folding of the fluid interfaces. The ability of the
fluid to mix well depends on the rate at which "chaotic advection" occurs in
the mixer. An optimization of mixer geometries is a non trivial task which is
often performed by time consuming and expensive trial and error experiments. In
this paper an algorithm is presented that applies the concept of finite-time
Lyapunov exponents to obtain a quantitative measure of the chaotic advection of
the flow and hence the performance of micromixers. By performing lattice
Boltzmann simulations of the flow inside a mixer geometry, introducing massless
and non-interacting tracer particles and following their trajectories the
finite time Lyapunov exponents can be calculated. The applicability of the
method is demonstrated by a comparison of the improved geometrical structure of
the staggered herringbone mixer with available literature data.Comment: 9 pages, 8 figure
Atomic-scale confinement of optical fields
In the presence of matter there is no fundamental limit preventing
confinement of visible light even down to atomic scales. Achieving such
confinement and the corresponding intensity enhancement inevitably requires
simultaneous control over atomic-scale details of material structures and over
the optical modes that such structures support. By means of self-assembly we
have obtained side-by-side aligned gold nanorod dimers with robust
atomically-defined gaps reaching below 0.5 nm. The existence of
atomically-confined light fields in these gaps is demonstrated by observing
extreme Coulomb splitting of corresponding symmetric and anti-symmetric dimer
eigenmodes of more than 800 meV in white-light scattering experiments. Our
results open new perspectives for atomically-resolved spectroscopic imaging,
deeply nonlinear optics, ultra-sensing, cavity optomechanics as well as for the
realization of novel quantum-optical devices
Anyons in a weakly interacting system
We describe a theoretical proposal for a system whose excitations are anyons
with the exchange phase pi/4 and charge -e/2, but, remarkably, can be built by
filling a set of single-particle states of essentially noninteracting
electrons. The system consists of an artificially structured type-II
superconducting film adjacent to a 2D electron gas in the integer quantum Hall
regime with unit filling fraction. The proposal rests on the observation that a
vacancy in an otherwise periodic vortex lattice in the superconductor creates a
bound state in the 2DEG with total charge -e/2. A composite of this
fractionally charged hole and the missing flux due to the vacancy behaves as an
anyon. The proposed setup allows for manipulation of these anyons and could
prove useful in various schemes for fault-tolerant topological quantum
computation.Comment: 7 pages with 3 figures. For related work and info visit
http://www.physics.ubc.ca/~fran
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