1,078 research outputs found
Force calculation on walls and embedded particles in multiparticle collision dynamics simulations
Colloidal solutions posses a wide range of time and length scales, so that it
is unfeasible to keep track of all of them within a single simulation. As a
consequence some form of coarse-graining must be applied. In this work we use
the Multi-Particle Collision Dynamics scheme. We describe a particular
implementation of no-slip boundary conditions upon a solid surface, capable of
providing correct force s on the solid bypassing the calculation of the
velocity profile or the stre ss tensor in the fluid near the surface. As an
application we measure the friction on a spherical particle, when it is placed
in a bulk fluid and when it is confined in a slit. We show that the
implementation of the no-slip boundary conditions leads to an enhanced Ensko g
friction, which can be understood analytically. Because of the long-range
nature of hydrodynamic interactions, the Stokes friction obtained from the
simulations is sensitive of the simulation box size. We address this topic for
the slit geometry, showing that that the dependence on the system size differs
very much from what is expected in a 3D system, where periodic boundary
conditions are used in all directions.Comment: To appear in Physical Review
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Analytical treatments of micro-channel and micro-capillary flows
This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Extensive work in the field of micro-channel and micro-capillary flows using the extended Navier-Stokes equations are carried out in this paper by taking the diffusive mass transport into account and provided the basis for analytical treatments of these flows. The results are compared with experimental results for micro-channels and showed good agreement. It is found that a characteristic pressure is useful to explain the comparisons. In addition, the work on micro-channel flows is extended to micro-capillary flows, to provide analytical treatments of this class of flows. The analytical results show similar behavior to that of micro-channel flows. Comparisons between the analytical results and experimental findings are also presented and discussed by introducing the characteristic pressure
The complement system in renal homograft recipients
The whole serum complement and its components were studied in 24 recipients of 27 renal homografts. In 12 of 13 instances in which homograft rejection was diagnosed, it was accompanied by significant declines in CH50, IA50, C4, and C3 levels, and to a lesser degree in C1 and C2 levels. Fourteen patients had normal graft function during the postoperative course of study, and in 13 of the 14 the complement levels were within the normal range throughout. In two recipients with systemic lupus erythematosus, very low initial complement levels increased to normal levels following removal of the native kidneys, splenectomy, and the provision of a well-functioning homograft. Anticomplement activity and elevated titers of C1 and C3 inactivators were observed in some patients, but these did not correlate with the changes in CH50. The findings confirm that the complement system participates in renal homograft rejection. © 1972
Radiation-Induced Magnetoresistance Oscillations in a 2D Electron Gas
Recent measurements of a 2D electron gas subjected to microwave radiation
reveal a magnetoresistance with an oscillatory dependence on the ratio of
radiation frequency to cyclotron frequency. We perform a diagrammatic
calculation and find radiation-induced resistivity oscillations with the
correct period and phase. Results are explained via a simple picture of current
induced by photo-excited disorder-scattered electrons. The oscillations
increase with radiation intensity, easily exceeding the dark resistivity and
resulting in negative-resistivity minima. At high intensity, we identify
additional features, likely due to multi-photon processes, which have yet to be
observed experimentally.Comment: 5 pages, 3 figures; final version as published in Phys Rev Let
Transient magnetoconductivity of photoexcited electrons
Transient magnetotransport of two-dimensional electrons with
partially-inverted distribution excited by an ultrashort optical pulse is
studied theoretically. The time-dependent photoconductivity is calculated for
GaAs-based quantum wells by taking into account the relaxation of electron
distribution caused by non-elastic electron-phonon interaction and the
retardation of the response due to momentum relaxation and due to a finite
capacitance of the sample. We predict large-amplitude transient oscillations of
the current density and Hall field (Hall oscillations) with frequencies
corresponding to magnetoplasmon range, which are initiated by the instability
owing to the absolute negative conductivity effect.Comment: 21 pages, 6 fig
Structural and Financial Characteristics of U.S. Farms: 2001 Family Farm Report
Family farms vary widely in size and other characteristics, ranging from very small retirement and residential farms to establishments with sales in the millions of dollars. The farm typology developed by the Economic Research Service (ERS) categorizes farms into groups based primarily on occupation of the operator and sales class of the farm. The typology groups reflect operators' expectations from farming, position in the life cycle, and dependence on agriculture. The groups differ in their importance to the farm sector, product specialization, program participation, and dependence on farm income. These (and other) differences are discussed in this report.Agricultural Resource Management Study (ARMS), family farms, farm businesses, farm financial situation, farm operator household income, farm operators, farm structure, farm typology, female farm operators, government payments, spouses of farm operators, taxes, Agricultural Finance, Farm Management,
Observation of Apparently Zero-Conductance States in Corbino Samples
Using Corbino samples we have observed oscillatory conductance in a
high-mobility two-dimensional electron system subjected to crossed microwave
and magnetic fields. On the strongest of the oscillation minima the conductance
is found to be vanishingly small, possibly indicating an insulating state
associated with these minima.Comment: 4 pages, 3 figures, RevTex
Spectral and Transport Properties of d-Wave Superconductors With Strong Impurities
One of the remarkable features of disordered d-wave superconductors is strong
sensitivity of long range properties to the microscopic realization of the
disorder potential. Particularly rich phenomenology is observed for the --
experimentally relevant -- case of dilute distributions of isolated impurity
centers. Building on earlier diagrammatic analyses, the present paper derives
and analyses a low energy effective field theory of this system. Specifically,
the results of previous diagrammatic T-matrix approaches are extended into the
perturbatively inaccessible low energy regimes, and the long range (thermal)
transport behaviour of the system is discussed. It turns out that in the
extreme case of a half-filled tight binding band and infinitely strong
impurities (impurities at the unitary limit), the system is in a delocalized
phase.Comment: 14 pages, two figures include
Bound Magnetic Polaron Interactions in Insulating Doped Diluted Magnetic Semiconductors
The magnetic behavior of insulating doped diluted magnetic semiconductors
(DMS) is characterized by the interaction of large collective spins known as
bound magnetic polarons. Experimental measurements of the susceptibility of
these materials have suggested that the polaron-polaron interaction is
ferromagnetic, in contrast to the antiferromagnetic carrier-carrier
interactions that are characteristic of nonmagnetic semiconductors. To explain
this behavior, a model has been developed in which polarons interact via both
the standard direct carrier-carrier exchange interaction (due to virtual
carrier hopping) and an indirect carrier-ion-carrier exchange interaction (due
to the interactions of polarons with magnetic ions in an interstitial region).
Using a variational procedure, the optimal values of the model parameters were
determined as a function of temperature. At temperatures of interest, the
parameters describing polaron-polaron interactions were found to be nearly
temperature-independent. For reasonable values of these constant parameters, we
find that indirect ferromagnetic interactions can dominate the direct
antiferromagnetic interactions and cause the polarons to align. This result
supports the experimental evidence for ferromagnetism in insulating doped DMS.Comment: 11 pages, 7 figure
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