734 research outputs found
THz and microwave properties of 3D-printed nanocarbon based multilayers
We propose a new type of light-weight conductive thin film material having good mechanical properties and electromagnetic shielding efficiency. For that 3D printing through layer-to-layer deposition of nanocarbon containing plastic layers and neat polymer layers was combined with hot pressing to obtain 10 μm thick films. We show that such composite has Re ε ≈ Im ε in very broad frequency range, 200GHz-0.6 THz
QED radiative correction to spin-density matrix elements in exclusive vector meson production
QED radiative effects are considered in the case of measurement of
spin-density matrix elements of diffractive -meson electroproduction.
Large radiative correction for is found in the kinematics of
collider experiments at HERA.Comment: 7 pages, 5 figure
Carbon nanotube array as a van der Waals two-dimensional hyperbolic material
We use an ab-initio approach to design and study a novel two-dimensional
material - a planar array of carbon nanotubes separated by an optimal distance
defined by the van der Waals interaction. We show that the energy spectrum for
an array of quasi-metallic nanotubes is described by a strongly anisotropic
hyperbolic dispersion and formulate a model low-energy Hamiltonian for its
semi-analytical treatment. Periodic-potential-induced lifting of the valley
degeneracy for an array of zigzag narrow-gap nanotubes leads to the band gap
collapse. In contrast, the band gap is opened in an array of gapless armchair
tubes. These unusual spectra, marked by pronounced van Hove singularities in
the low-energy density of states, open the opportunity for interesting physical
effects and prospective optoelectronic applications
Behavior of nanoparticle clouds around a magnetized microsphere under magnetic and flow fields
When a micron-sized magnetizable particle is introduced into a suspension of
nanosized magnetic particles, the nanoparticles accumulate around the
microparticle and form thick anisotropic clouds extended in the direction of
the applied magnetic field. This phenomenon promotes colloidal stabilization of
bimodal magnetic suspensions and allows efficient magnetic separation of
nanoparticles used in bioanalysis and water purification. In the present work,
size and shape of nanoparticle clouds under the simultaneous action of an
external uniform magnetic field and the flow have been studied in details. In
experiments, dilute suspension of iron oxide nanoclusters (of a mean diameter
of 60 nm) was pushed through a thin slit channel with the nickel microspheres
(of a mean diameter of 50m) attached to the channel wall. The behavior of
nanocluster clouds was observed in the steady state using an optical
microscope. In the presence of strong enough flow, the size of the clouds
monotonically decreases with increasing flow speed in both longitudinal and
transverse magnetic fields. This is qualitatively explained by enhancement of
hydrodynamic forces washing the nanoclusters away from the clouds. In the
longitudinal field, the flow induces asymmetry of the front and the back
clouds. To explain the flow and the field effects on the clouds, we have
developed a simple model based on the balance of the stresses and particle
fluxes on the cloud surface. This model, applied to the case of the magnetic
field parallel to the flow, captures reasonably well the flow effect on the
size and shape of the cloud and reveals that the only dimensionless parameter
governing the cloud size is the ratio of hydrodynamic-to-magnetic forces - the
Mason number. At strong magnetic interactions considered in the present work
(dipolar coupling parameter ), the Brownian motion seems not to
affect the cloud behavior
Normal stress differences in non-Brownian fiber suspensions
International audienceIn this paper, we present an experimental study of the normal stress differences that arise in non-Brownian rigid fiber suspensions subject to a shear flow. While early measurements of the normal stress in fiber suspensions in Newtonian fluids measured only N 1 − N 2 , the recent work of Snook et al. J. Fluid Mech. 758 486 (2014) and the present paper provide the first measurements of N 1 and N 2 separately. Snook et al perform such measurements with a gap that is very wide compared with the fiber length, whereas the present paper explores the effects of confinement when the gap is 4-10 times the fiber length. The first and the second normal stress differences are measured using a single experiment which consists of determining the radial profile of the second normal stress, along the velocity gradient direction, Σ 22 , in a torsional flow between two parallel discs. Suspensions are made of monodisperse fibers immersed in a neutrally buoyant Newtonian fluid. Two fiber lengths and three aspect ratios a r = L/d, and a wide range of concentrations have been tested. N 1 is found to be positive while N 2 is negative and the magnitude of both normal stress differences increases when nL 2 d increases, n being the number fraction of fibers. The magnitude of N 2 is found to be much smaller than N 1 only for high aspect ratios and low fiber concentrations
Effect of drop-like aggregates on the viscous stress in magnetic suspensions
We present results of theoretical and experimental study of effect of dense drop-like aggregates on the magnetoviscous effects in suspensions of non-Brownian magnetizable particles. Unlike the previous works on this subject, we do not restrict ourselves by the limiting case of highly elongated drops. This allows us to reproduce the experimental rheological curve in wide region of the shear rate of the suspension flow.This work has been supported by the Russian Fund of Fundamental Investigations, Grants 12-01-00132, 13-02-91052, 13-01-96047 and 14-08-00283; by the Act 211 Government of the Russian Federation No. 02.A03.21.0006; by the Junta de Andalucía (Spain), Project P09-FQM-4787; and by the University of Granada (Acción Integrada con Russia; Plan Propio 2011); and CNRS PICS No. 6102 is also acknowledged
Abrupt contraction flow of magnetorheological fluids
International audienceContraction and expansion flows of magnetorheological fluids occur in a variety of smart devices. It is important therefore to learn how these flows can be controlled by means of applied magnetic fields. This paper presents a first investigation into the axisymmetric flow of a magnetorheological fluid through an orifice so-called abrupt contraction flow. The effect of an external magnetic field, longitudinal or transverse to the flow, is examined. In experiments, the pressure-flow rate curves were measured, and the excess pressure drop associated with entrance and exit losses was derived from experimental data through the Bagley correction procedure. The effect of the longitudinal magnetic field is manifested through a significant increase in the slope of the pressure-flow rate curves, while no discernible yield stress occurs. This behavior, observed at shear Mason numbers 10Mnshear100, is interpreted in terms of an enhanced extensional response of magnetorheological fluids accompanied by shrinkage of the entrance flow into a conical funnel. At the same range of Mason numbers, the transverse magnetic field appears not to influence the pressure drop. This can be explained by a total destruction of magnetic particle aggregates by large hydrodynamic forces acting on them when they are perpendicular to the flow. To support these findings, we have developed a theoretical model connecting the microstructure of the magnetorheological fluid to its extensional rheological properties and predicting the pressure-flow rate relations through the solution of the flow equations. In the case of the longitudinal magnetic field, our model describes the experimental results reasonably well
Influence of nanotube length and density on the plasmonic terahertz response of single-walled carbon nanotubes
We measure the conductivity spectra of thin films comprising bundled
single-walled carbon nanotubes (CNTs) of different average lengths in the
frequency range 0.3-1000 THz and temperature interval 10-530 K. The observed
temperature-induced changes in the terahertz conductivity spectra are shown to
depend strongly on the average CNT length, with a conductivity around 1 THz
that increases/decreases as the temperature increases for short/long tubes.
This behaviour originates from the temperature dependence of the electron
scattering rate, which we obtain from Drude fits of the measured conductivity
in the range 0.3-2 THz for 10 m length CNTs. This increasing scattering
rate with temperature results in a subsequent broadening of the observed THz
conductivity peak at higher temperatures and a shift to lower frequencies for
increasing CNT length. Finally, we show that the change in conductivity with
temperature depends not only on tube length, but also varies with tube density.
We record the effective conductivities of composite films comprising mixtures
of WS nanotubes and CNTs vs CNT density for frequencies in the range 0.3-1
THz, finding that the conductivity increases/decreases for low/high density
films as the temperature increases. This effect arises due to the density
dependence of the effective length of conducting pathways in the composite
films, which again leads to a shift and temperature dependent broadening of the
THz conductivity peak.Comment: Submitted to Journal of Physics D. Main manuscript: 9 pages, 8
figures. Supplementary material: 5 pages, 6 figure
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