Effective medium approximations Information & Effective medium approximations Links at HealthHaven.com

Effective Medium Approximations are physical models that describe the macroscopic properties of a medium based on the properties and the relative fractions of its components. They can be discrete models such as applied to resistor networks or continuum theories as applied to elasticity or viscosity but most of the current theories have difficulty in describing percolating systems. Indeed, among the numerous effective medium approximations (EMA or EMT), only Bruggeman’s symmetrical theory is able to predict a threshold. This characteristics feature of the latter theory puts it in the same category as other mean field theories of critical phenomena.

There are many different effective medium approximations¹, each of them being more or less accurate in distinct conditions. Nevertheless, they all assume that the macroscopic system is homogeneous and typical of all mean field theories, they fail to predict the properties of a multiphase medium close to the percolation threshold due to the absence of long-range correlations or critical fluctuations in the theory.

The properties under consideration are usually the conductivity $σ$ or the dielectric constant $ε$ of the medium. These parameters are interchangeable in the formulas in a whole range of models due to the wide applicability of the Laplace equation. The problems that fall outside of this class are mainly in the field of elasticity and hydrodynamics, due to the higher order tensorial character of the effective medium constants.

[edit] Bruggeman's Model

[edit] Formulas

Without any loss of generality, we shall consider the study of the effective conductivity (which can be either dc or ac) for a system made up of spherical multicomponent inclusions with different arbitrary conductivities. Then the celebrated Bruggeman formula takes the form:

[edit] Circular and spherical inclusions

$\sum_i\,\delta_i\,\frac{\sigma_i - \sigma_e}{\sigma_i + (n-1) \sigma_e}\,=\,0\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(1)$

In a system of Euclidean spatial dimension $n$ that has an arbitrary number of components², the sum is made over all the constituents. $δ i$ and $σ i$ are respectively the fraction and the conductivity of each component, and $σ e$ is the effective conductivity of the medium. (The sum over the $δ i$ 's is unity.)

[edit] Elliptical and ellipsoidal inclusions

$\frac{1}{n}\,\delta\alpha+\frac{(1-\delta)(\sigma_m - \sigma_e)}{\sigma_m + (n-1)\sigma_e}\,=\,0\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(2)$

This is a generalization of Eq. (1) to a biphasic system with ellipsoidal inclusions of conductivity $σ$ into a matrix of conductivity $σ m$ ³. The fraction of inclusions is $δ$ and the system is $n$ dimensional. For randomly oriented inclusions,

$\alpha\,=\,\sum_{j=1}^{n}\,\frac{\sigma - \sigma_e}{\sigma_e + L_j(\sigma - \sigma_e)}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(3)$

where the $L j$ 's denote the appropriate doublet/triplet of depolarization factors which is governed by the ratios between the axis of the ellipse/ellipsoid. For example: in the case of a circle { $L 1 = 1 / 2$ , $L 2 = 1 / 2$ } and in the case of a sphere { $L 1 = 1 / 3$ , $L 2 = 1 / 3$ , $L 3 = 1 / 3$ }. (The sum over the $L j$ 's is unity.)

[edit] Derivation

The figure illustrates a two-component medium². Let us consider the cross-hatched volume of conductivity $σ 1$ , take it as a sphere of volume $V$ and assume it is embedded in a uniform medium with an effective conductivity $σ e$ . If the electric field far from the inclusion is $\overline{E_0}$ then elementary considerations lead to a dipole moment associated with the volume

$\overline{p}\, \propto \,V\,\frac{\sigma_1 - \sigma_e}{\sigma_1 + 2\sigma_e}\,\overline{E_0}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(4)\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,.$

This polarization produces a deviation from $\overline{E_0}$ . If the average deviation is to vanish, the total polarization summed over the two types of inclusion must vanish. Thus

$\delta_1\frac{\sigma_1 - \sigma_e}{\sigma_1 + 2\sigma_e}\,+\,\delta_2\frac{\sigma_2 - \sigma_e}{\sigma_2 + 2\sigma_e}\,=\,0\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(5)$

where $δ 1$ and $δ 2$ are respectively the volume fraction of material 1 and 2. This can be easily extended to a system of dimension $n$ that has an arbitrary number of components. All cases can be combined to yield Eq. (1).

Eq. (1) can also be obtained by requiring the deviation in current to vanish^4,5. It has been derived here from the assumption that the inclusions are spherical and it can be modified for shapes with other depolarization factors; leading to Eq. (2).

[edit] Modeling of percolating systems

The main approximation is that all the domains are located in an equivalent mean field. Unfortunately, it is not the case close to the percolation threshold where the system is governed by the largest cluster of conductors, which is a fractal, and long-range correlations that are totally abscent from Bruggeman's simple formula. The threshold values are in general not correctly predicted. It is 33% in the EMA, in three dimensions, far from the 16% expected from percolation theory and observed in experiments. However, in two dimensions, the EMA gives a threshold of 50% and has been proven to model percolation relatively well^6,7,8.

[edit] Maxwell Garnett's Equation

[edit] Formula

$\varepsilon_e\,=\,\varepsilon_m\,\frac{\varepsilon_i(1 + 2\delta) - \varepsilon_m(2\delta - 2)}{\varepsilon_m(2 + \delta) + \varepsilon_i(1 - \delta)}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(6)$

where $\varepsilon_e$ is the effective dielectric constant of the medium, $\varepsilon_i$ is the one of the inclusions and $\varepsilon_m$ is the one of the matrix; $δ$ is the volume fraction of the embedded material.

[edit] Validity

In general terms, the Maxwell Garnett EMA is expected to be valid at low volume fractions since it is assumed that the domains are spatially separated⁹.

[edit] See also

Percolation threshold
Tuck, Choy (1999). Effective Medium Theory (1st ed.). Oxford: Oxford University Press. ISBN 978-0-19-851892-1.

Contents