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Database of Optical Properties -- Level: subpage


Optical properties of porous particles


Here we compare the optical properties of porous particles of two kinds:
  • aggregates -- clouds of (cibic) subparticles represented by 1 (or 8, 27, etc) dipole
    (the subparticles are randomly distributed within a sphere);

  • homogeneous spheres with an effective refractive index.

The parameters are
    the refractive index of the subparticles m,
    the number of dipoles forming a subparticle N (at the moment N = 1),
    the porosity (the volume fraction of the subparticles) P,
    the difraction parameter x = 2 pi r / lambda ,
      where lambda is the wavelength of incident radiation
      and r the radius of the quasispherical aggregate
      as well as of the corresponding homogeneous sphere
      (its effective index is calculated from P and m
      by using the effective medium theory, EMT).


To compute the properties of the aggregates and of the homogeneous spheres the DDA method and the Mie theory with the Bruggeman rule of the EMT were utilized, respectively (see for more details the paper of Voshchinnikov et al., 2007).

Below we show the behaviour of the efficiency factors Qext (x), Qsca (x), Qabs (x), the asymmetry parameter g (x) and albedo (x) as well as for x = 3 and 100 the phase function i (theta) and polarization of scattered radiation p (theta) for different values of the parameters m and P.

The refractive index m is chosen to be
    1.200+0.000i (typical of biological particles in the visual part of the spectrum),
    1.330+0.010i (dirty ice),
    1.578+1.038i (center of the 10 micron silicate feature),
    1.680+0.030i (silicate),
    1.750+0.058i (soot),
    1.980+0.230i (amorphous carbon).



The effective refractive indices of the homogeneous spheres are indicated in Table. The porosity is taken to be P = 0.33 and 0.9.

Effective refractive indices calculated using EMT (the Bruggeman rule)
P = 0 P = 0.33 P = 0.9
1.2000+0.0000i 1.1328+0.0000i 1.0193+0.0000i
1.3300+0.0100i 1.2183+0.0066i 1.0310+0.0009i
1.5780+1.0380i 1.3764+0.6710i 1.0762+0.0754i
1.6800+0.0300i 1.4471+0.0196i 1.0588+0.0022i
1.7500+0.5800i 1.4916+0.3781i 1.0707+0.03932
1.9800+0.2300i 1.6431+0.1507i 1.0795+0.0137i

For given porosity, the particles of the same size parameter xporous = xcompact/(1-P)1/3 (see for more details Voshchinnikov et al., 2007) and hence of the same mass are compared in the following figures organized in the tables.



refractive index = 1.20+0.00i
P=0.33 P=0.9
factors i&p
x=10 x=3
Qext(x)
Qsca(x)
g(x)
 i (theta)
p (theta)		 i (theta)
p (theta)
factors i&p
x=10 x=3
Qext(x)
Qsca(x)
g(x)
 i (theta)
p (theta)		 i (theta)
p (theta)




refractive index = 1.33+0.01i
P = 0.33 P = 0.9
factors i & p
x = 3 x = 10
Qext (x)
Qsca (x)
Qabs (x)
g(x)
albedo (x)
 i (theta)
p (theta)		 i (theta)
p (theta)
factors i & p
x = 3 x = 10
Qext (x)
Qsca (x)
Qabs (x)
g (x)
albedo (x)
 i (theta)
p (theta)		 i (theta)
p (theta)


refractive index = 1.578+1.0381i
P=0.33 P=0.9
factors i&p
x=10 x=3
Qext(x)
Qsca(x)
Qabs(x)
g(x)
albedo(x)
 i (theta)
p (theta)		 i (theta)
p (theta)
factors i&p
x=10 x=3
Qext(x)
Qsca(x)
Qabs(x)
g(x)
albedo(x)
 i (theta)
p (theta)		 i (theta)
p (theta)

refractive index = 1.68+0.03i
P=0.33 P=0.9
factors i&p
x=10 x=3
Qext(x)
Qsca(x)(x)
Qabs(x)
g(x)
albedo(x)
 i (theta)
p (theta)		 i (theta)
p (theta)
factors i&p
x=10 x=3
Qext(x)
Qsca(x)
Qabs(x)
g(x)
albedo(x)
 i (theta)
p (theta)		 i (theta)
p (theta)


refractive index = 1.75+0.58i
P=0.33 P=0.9
factors i&p
x=10 x=3
Qext(x)
Qsca(x)
Qabs(x)
g(x)
albedo(x)
 i (theta)
p (theta)		 i (theta)
p (theta)
factors i&p
x=10 x=3
Qext(x)
Qsca(x)
Qabs(x)
g(x)
albedo(x)
 i (theta)
p (theta)		 i (theta)
p (theta)


refractive index = 1.98+0.23i
P=0.33 P=0.9
factors i&p
x=10 x=3
Qext(x)
Qsca(x)
Qabs(x)
g(x)
albedo(x)
 i (theta)
p (theta)		 i (theta)
p (theta)
factors i&p
x=10 x=3
Qext(x)
Qsca(x)
Qabs(x)
g(x)(x)
albedo(x)
 i (theta)
p (theta)		 i (theta)
p (theta)


For more comments see the paper by Nikolai V. Voshchinnikov, Gorden Videen, and Thomas Henning, Effective medium theories for aggregate structures: agglomeration of small particles, Applied Optics, 46, 4065--4072, 2007 .

The work was supported by the RFBR grant 07-02-00831.