23,940 research outputs found
Reciprocal relations, bounds, and size effects for composites with highly conducting interface
We provide a reciprocal relation linking the effective conductivity of a composite with highly conducting phase interfaces to that of a composite with the same phase geometry but with an electrical contact resistance at phase interfaces. A field relationship linking the electric field inside a composite with highly conducting phase interfaces to the current in a composite with contact resistance between phases is found. New size effects exhibited by isotropic particulate suspensions with highly conducting interface are obtained. The effective properties of periodic composites are shown to be monotonically increasing as the size of the period cell tends to zero. The role of surface energy for energy minimizing polydisperse suspensions of disks is examined; a necessary condition for isotropic polydisperse suspensions with minimal effective conductivity is found. For monodisperse suspensions of spheres, a critical radius is found for which the electric field is uniform throughout the composite
Dielectric Behavior of Nonspherical Cell Suspensions
Recent experiments revealed that the dielectric dispersion spectrum of
fission yeast cells in a suspension was mainly composed of two sub-dispersions.
The low-frequency sub-dispersion depended on the cell length, whereas the
high-frequency one was independent of it. The cell shape effect was
qualitatively simulated by an ellipsoidal cell model. However, the comparison
between theory and experiment was far from being satisfactory. In an attempt to
close up the gap between theory and experiment, we considered the more
realistic cells of spherocylinders, i.e., circular cylinders with two
hemispherical caps at both ends. We have formulated a Green function formalism
for calculating the spectral representation of cells of finite length. The
Green function can be reduced because of the azimuthal symmetry of the cell.
This simplification enables us to calculate the dispersion spectrum and hence
access the effect of cell structure on the dielectric behavior of cell
suspensions.Comment: Preliminary results have been reported in the 2001 March Meeting of
the American Physical Society. Accepted for publications in J. Phys.:
Condens. Matte
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Measurement of thermal and electrical conductivities of graphene nanofluids
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.The current work experimentally investigates the thermal and electrical conductivities of
nanofluids containing graphene sheets that have very high thermal conductivity. Here, the graphene is prepared from natural graphite by oxidation-reduction process through a single step method. The graphene
nanofluid thus obtained exhibited greater stability even after six months of preparation without addition of any surfactants. The detailed characterization process involving TEM, UV absorption and DLS
measurements revealed the well dispersed nature of nanofluid with sheets appropriately interconnected and entangled. The DLS measurement indicates a trimodal size distribution of graphene sheet ranging from 5nm to 1500nm. It was also found that the absorption peak of the sample was 269 nm. This reveals the complete reduction of graphene oxide to graphene and the value is in good agreement with the literature. The thermal conductivity is measured using the traditional Transient Hot Wire (THW) method and enhancements are substantial even at lower concentrations while such behaviour is not predicted by the classical Maxwell theory. The thermal conductivity of graphene nanofluids are measured for different concentrations of 0.01 - 0.2 volume % at different temperatures. It is observed that the thermal conductivity increases with increase in concentration of grapheme, which is as expected. The maximum enhancement obtained is 27% at 0.2% concentration. The enhancement also shows a strong temperature dependence which is unlike that of its carbon predecessors like CNT and graphene oxide nanofluids. Electrical conductivity is measured using a 4 cell conductivity meter with inbuilt automatic temperature compensation. Electrical conductivity enhancement is found to be linear with increase in graphene volume fraction
Modeling tensorial conductivity of particle suspension networks
Significant microstructural anisotropy is known to develop during shearing
flow of attractive particle suspensions. These suspensions, and their capacity
to form conductive networks, play a key role in flow-battery technology, among
other applications. Herein, we present and test an analytical model for the
tensorial conductivity of attractive particle suspensions. The model utilizes
the mean fabric of the network to characterize the structure, and the
relationship to the conductivity is inspired by a lattice argument. We test the
accuracy of our model against a large number of computer-generated suspension
networks, based on multiple in-house generation protocols, giving rise to
particle networks that emulate the physical system. The model is shown to
adequately capture the tensorial conductivity, both in terms of its invariants
and its mean directionality
Electrokinetic and hydrodynamic properties of charged-particles systems: From small electrolyte ions to large colloids
Dynamic processes in dispersions of charged spherical particles are of
importance both in fundamental science, and in technical and bio-medical
applications. There exists a large variety of charged-particles systems,
ranging from nanometer-sized electrolyte ions to micron-sized charge-stabilized
colloids. We review recent advances in theoretical methods for the calculation
of linear transport coefficients in concentrated particulate systems, with the
focus on hydrodynamic interactions and electrokinetic effects. Considered
transport properties are the dispersion viscosity, self- and collective
diffusion coefficients, sedimentation coefficients, and electrophoretic
mobilities and conductivities of ionic particle species in an external electric
field. Advances by our group are also discussed, including a novel
mode-coupling-theory method for conduction-diffusion and viscoelastic
properties of strong electrolyte solutions. Furthermore, results are presented
for dispersions of solvent-permeable particles, and particles with non-zero
hydrodynamic surface slip. The concentration-dependent swelling of ionic
microgels is discussed, as well as a far-reaching dynamic scaling behavior
relating colloidal long- to short-time dynamics
Ion size effects on the electrokinetics of salt-free concentrated suspensions in ac fields
We analyze the influence of finite ion size effects in the response of a
salt-free concentrated suspension of spherical particles to an oscillating
electric field. Salt-free suspensions are just composed of charged colloidal
particles and the added counterions released by the particles to the solution,
that counterbalance their surface charge. In the frequency domain, we study the
dynamic electrophoretic mobility of the particles and the dielectric response
of the suspension. We find that the Maxwell-Wagner-O'Konski process associated
with the counterions condensation layer, is enhanced for moderate to high
particle charges, yielding an increment of the mobility for such frequencies.
We also find that the increment of the mobility grows with ion size and
particle charge. All these facts show the importance of including ion size
effects in any extension attempting to improve standard electrokinetic models.Comment: J. Colloid Interface Sci., in press, 13 pages, 9 figure
Thermal optical non-linearity of nematic mesophase enhanced by gold nanoparticles – an experimental and numerical investigation
In this work the mechanisms leading to the enhancement of optical nonlinearity of nematic liquid crystalline material through localized heating by doping the liquid crystals (LCs) with gold nanoparticles (GNPs) are investigated. We present some experimental and theoretical results on the effect of voltage and nanoparticle concentration on the nonlinear response of GNP-LC suspensions. The optical nonlinearity of these systems is characterized by diffraction measurements and the second order nonlinear refractive index, n 2 , is used to compare systems with different configurations and operating conditions. A theoretical model based on heat diffusion that takes into account the intensity and finite size of the incident beam, the nanoparticle concentration dependent absorbance of GNP doped LC systems and the presence of bounding substrates is developed and validated. We use the model to discuss the possibilities of further enhancing the optical nonlinearity
Two-step percolation in aggregating systems
The two-step percolation behavior in aggregating systems was studied both
experimentally and by means of Monte Carlo (MC) simulations. In experimental
studies, the electrical conductivity, , of colloidal suspension of
multiwalled carbon nanotubes (CNTs) in decane was measured. The suspension was
submitted to mechanical de-liquoring in a planar filtration-compression
conductometric cell. During de-liquoring, the distance between the measuring
electrodes continuously decreased and the CNT volume fraction
continuously increased (from up to % v/v). The two
percolation thresholds at and can reflect the interpenetration of loose CNT aggregates and
percolation across the compact conducting aggregates, respectively. The MC
computational model accounted for the core-shell structure of conducting
particles or their aggregates, the tendency of a particle for aggregation, the
formation of solvation shells, and the elongated geometry of the conductometric
cell. The MC studies revealed two smoothed percolation transitions in
dependencies that correspond to the percolation through the
shells and cores, respectively. The data demonstrated a noticeable impact of
particle aggregation on anisotropy in electrical conductivity
measured along different directions in the conductometric cell.Comment: 10 pages, 6 figure
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