Coherent Backscattering from Multiple Scattering Systems - KOPS ...

4 Samples
Figure 4.4: Titania and teflon samples. (Clockwise from left:) Teflon sample with 4 cm
diameter and 5 cm height, titania sample with 15 mm diameter and < 1 mm height in
sample container, titania powder. Even on the photo one can observe the significant
difference in the albedos or ”whiteness” of Teflon and titania.
With a diameter of 4 cm and a thickness of 5 cm our teflon sample is not exactly a slab,
which however would be the condition for eqn. 2.12 to be valid. The value of D = 27500 m2 /s
measured in time of flight experiments is therefore not correct. In time of flight experiments
on shorter teflon blocks the diffusion coefficient drops to D = 20000 m2 /s for samples with
thickness L ≈ 2 − 3 cm, and even to D = 16500 m2 /s for L ≈ 1 − 1.5 cm. With this last value
eqns. 2.7 and 2.16 propose a transport mean free path of about 220 µm and a cone width
of approximately 0.01 ◦ . This is very close to the results of the backscattering experiments
reported in sec. 5.2.2, which yield l ∗ = 180 µm, and correspondingly a diffusion coefficient
D = 13300 m2 /s, which however could not be measured in the time of flight experiments due
to the still insufficient size of the teflon samples. a For the absorption time τ the time of flight
data yield 3.3 ns, the refractive index of teflon is given in literature as n = 1.35 [4].
4.2.3 Fluidized beds
A fluidized bed is a mixture of a fluid and a granular medium that has fluid-like properties. As
a dispersion of particles in an upward flux of low-viscosity fluid (gas or liquid) it represents an
intermediate system between suspensions in viscous fluids and dry granular materials. It has
excellent heat and mass transfer characteristics, and the fluid-solid mixture is easy to remove
[a] For technical reasons – the samples have to fit into the time of flight setup – it is impossible to use teflon samples
with larger radii that can reasonably be described as an infinite slab. The adaption of the setup to accommodate
larger samples seems to be the only solution.
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