1,085 research outputs found
Casimir forces in the time domain II: Applications
Our preceding paper introduced a method to compute Casimir forces in
arbitrary geometries and for arbitrary materials that was based on a
finite-difference time-domain (FDTD) scheme. In this manuscript, we focus on
the efficient implementation of our method for geometries of practical interest
and extend our previous proof-of-concept algorithm in one dimension to problems
in two and three dimensions, introducing a number of new optimizations. We
consider Casimir piston-like problems with nonmonotonic and monotonic force
dependence on sidewall separation, both for previously solved geometries to
validate our method and also for new geometries involving magnetic sidewalls
and/or cylindrical pistons. We include realistic dielectric materials to
calculate the force between suspended silicon waveguides or on a suspended
membrane with periodic grooves, also demonstrating the application of PML
absorbing boundaries and/or periodic boundaries. In addition we apply this
method to a realizable three-dimensional system in which a silica sphere is
stably suspended in a fluid above an indented metallic substrate. More
generally, the method allows off-the-shelf FDTD software, already supporting a
wide variety of materials (including dielectric, magnetic, and even anisotropic
materials) and boundary conditions, to be exploited for the Casimir problem.Comment: 11 pages, 12 figures. Includes additional examples (dispersive
materials and fully three-dimensional systems
Assessment of cockpit interface concepts for data link retrofit
The problem is examined of retrofitting older generation aircraft with data link capability. The approach taken analyzes requirements for the cockpit interface, based on review of prior research and opinions obtained from subject matter experts. With this background, essential functions and constraints for a retrofit installation are defined. After an assessment of the technology available to meet the functions and constraints, candidate design concepts are developed. The most promising design concept is described in detail. Finally, needs for further research and development are identified
Achieving a Strongly Temperature-Dependent Casimir Effect
We propose a method of achieving large temperature sensitivity in the Casimir
force that involves measuring the stable separation between dielectric objects
immersed in fluid. We study the Casimir force between slabs and spheres using
realistic material models, and find large > 2nm/K variations in their stable
separations (hundreds of nanometers) near room temperature. In addition, we
analyze the effects of Brownian motion on suspended objects, and show that the
average separation is also sensitive to changes in temperature . Finally, this
approach also leads to rich qualitative phenomena, such as irreversible
transitions, from suspension to stiction, as the temperature is varied
Structural anisotropy and orientation-induced Casimir repulsion in fluids
In this work we theoretically consider the Casimir force between two periodic
arrays of nanowires (both in vacuum, and on a substrate separated by a fluid)
at separations comparable to the period. Specifically, we compute the
dependence of the exact Casimir force between the arrays under both lateral
translations and rotations. Although typically the force between such
structures is well-characterized by the Proximity Force Approximation (PFA), we
find that in the present case the microstructure modulates the force in a way
qualitatively inconsistent with PFA. We find instead that effective-medium
theory, in which the slabs are treated as homogeneous, anisotropic dielectrics,
gives a surprisingly accurate picture of the force, down to separations of half
the period. This includes a situation for identical, fluid-separated slabs in
which the exact force changes sign with the orientation of the wire arrays,
whereas PFA predicts attraction. We discuss the possibility of detecting these
effects in experiments, concluding that this effect is strong enough to make
detection possible in the near future.Comment: 12 pages, 9, figure. Published version with expanded discussio
Structural anisotropy and orientation-induced Casimir repulsion in fluids
In this work we theoretically consider the Casimir force between two periodic
arrays of nanowires (both in vacuum, and on a substrate separated by a fluid)
at separations comparable to the period. Specifically, we compute the
dependence of the exact Casimir force between the arrays under both lateral
translations and rotations. Although typically the force between such
structures is well-characterized by the Proximity Force Approximation (PFA), we
find that in the present case the microstructure modulates the force in a way
qualitatively inconsistent with PFA. We find instead that effective-medium
theory, in which the slabs are treated as homogeneous, anisotropic dielectrics,
gives a surprisingly accurate picture of the force, down to separations of half
the period. This includes a situation for identical, fluid-separated slabs in
which the exact force changes sign with the orientation of the wire arrays,
whereas PFA predicts attraction. We discuss the possibility of detecting these
effects in experiments, concluding that this effect is strong enough to make
detection possible in the near future.Comment: 12 pages, 9, figure. Published version with expanded discussio
Selection of variant viruses during replication and transmission of H7N1 viruses in chickens and turkeys
AbstractThe influence of different glycosylation patterns of the haemagglutinin glycoprotein of H7N1 avian influenza viruses on virus replication in vivo was examined. Experimental infection of chickens and turkeys was carried out with H7N1 avian influenza viruses with alternative sites of glycosylation in the haemagglutinin and infected birds were sampled daily by swabbing the buccal and cloacal cavities. cDNAs of the HA1 coding region of the HA gene were prepared from the swabs and cloned into plasmids. Sequencing multiple plasmids made from individual swabs taken over the period of virus shedding showed that viruses with specific patterns of glycosylation near the receptor binding site were stable when birds were infected with a single variant, but when presented with a mixed population of viruses encoding differing patterns of glycosylation a specific variant was rapidly selected in the infected host
Microstructure Effects for Casimir Forces in Chiral Metamaterials
We examine a recent prediction for the chirality-dependence of the Casimir
force in chiral metamaterials by numerical computation of the forces between
the exact microstructures, rather than homogeneous approximations. We compute
the exact force for a chiral bent-cross pattern, as well as forces for an
idealized "omega"-particle medium in the dilute approximation and identify the
effects of structural inhomogeneity (i.e. proximity forces and anisotropy). We
find that these microstructure effects dominate the force for separations where
chirality was predicted to have a strong influence. To get observations of
chirality free from microstructure effects, one must go to large separations
where the effect of chirality is at most of the total force.Comment: 5 pages, 4 figure
Identifying the genetic basis of antigenic change in influenza A(H1N1)
Determining phenotype from genetic data is a fundamental challenge. Influenza
A viruses undergo rapid antigenic drift and identification of emerging
antigenic variants is critical to the vaccine selection process. Using former
seasonal influenza A(H1N1) viruses, hemagglutinin sequence and corresponding
antigenic data were analyzed in combination with 3-D structural information. We
attributed variation in hemagglutination inhibition to individual amino acid
substitutions and quantified their antigenic impact, validating a subset
experimentally using reverse genetics. Substitutions identified as low-impact
were shown to be a critical component of influenza antigenic evolution and by
including these, as well as the high-impact substitutions often focused on, the
accuracy of predicting antigenic phenotypes of emerging viruses from genotype
was doubled. The ability to quantify the phenotypic impact of specific amino
acid substitutions should help refine techniques that predict the fitness and
evolutionary success of variant viruses, leading to stronger theoretical
foundations for selection of candidate vaccine viruses
- …