INDEX void fraction

Optical systems can be used in multiphase flows at a very low volume fraction of the dispersed phase. Through a refractory index matching of hquid-liquid or liquid-solid systems, it is also possible to measure at high void fractions. However, it is not possible to obtain complete refractory index matching since the molecules at the phase boundary have different optical properties than the molecules in the bulk. Consequently, it is possible to measure at a higher fraction of the dispersed phase with larger drops and particles because of the lower surface area per volume fluid. 

The refractive index of amorphous silicon is. within certain limits, a good measure for the density of the material. If we may consider the material to consist of a tightly bonded structure containing voids, the density of the material follows from the void fraction. This fraction / can be computed from the relative dielectric constant e. Assuming that the voids have a spherical shape, / is given by Bruggeman [61] ... 

Fig. 1. Void fraction vs. refraction index values for HPA and SN films.

Figure 8. Gel filtration of ethylated (/ -0-4)-(/ -/ )-DHP 16. Solid line Ethylated (/ -0-4)-(/ -/ )-DHP 16 after removal of low molecular weight fractions. The column was calibrated with (/ -0-4)-(/ -/ ) lignin substructure model trimer 6 (molecular weight 642) /3-0-4 lignin model dimer 1 (molecular weight 348) and polystyrenes of molecular weight 9000, 4000 (void), 2200 (indicated by A). Column Sephadex LH-20, 1.1 x 48 cm. Eluent DMF, 13.5-14.4 ml/hr. Detector Refractive index detector RI-2 (Japan Analytical Industry Co., Ltd.).

Porosity (or volume fraction of voids, Vf) is defined as the percentage of the foam volume that is occupied by the gaseous phase. It can be calculated from refractive index measurements [83] or from the density of the foam (pf) and the solid precursor (Ps) using Eq. (9.3) ... 

From Equation 15.2, ( ) is an index of summation that corresponds to individual materials in the mixture and (AO is the upper bound of the summation and corresponds to the overall quantity of materials present in the mixture [21 ]. Also p and M represent the material density and its mass fraction in the mixture, respectively. A mixture that has 60% TMD is therefore 60% solids and 40% voids (air space). Loose powders are typically <20% TMD while compressed pellets typically range from 20% to 75% TMD. 

If the size of the voids is below the wavelength of light, then the refractive index of a layer will depend on the volume fraction of voids, as predicted in an effective medium approximation (EMA). Given that Wp is the refractive index of the fully dense polymer at a certain wavelength (measured independently), and that is the refractive index of air voids (tty = 1.00), it is possible to calculate the void concentration from a measurement of the index, n, of a composite film consisting of polymer and voids. According to an EMA model, the volume fraction... 

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