Coherent Backscattering from Multiple Scattering Systems - KOPS ...

5 Experiments
1
0.8
data − ( n water
= 1.334 , n particle
= 1.519 ) .
data − ( n water
= 1.334 , n particle
= 1.521 )
data − ( n water
= 1.334 , n particle
= 1.523 )
intensity [a.u.]
0.6
0.4
0.2
0
0 0.05 0.1 0.15 0.2
scattering angle θ ms
[deg]
Figure 5.18: Coherent backscattering cone of a fluidized bed. To obtain the coherent
backscattering cone, three Mie distributions (stretched along the angular axis to match
the slope of the first maximum of the data) with different zero levels were subtracted
from the measured intensity distribution depicted in fig. 5.17. The angular width of the
cone can be read from the graph to an accuracy of about 15%.
The graph also shows that the actual distribution of particle sizes in the sample differs slightly
from the one measured beforehand – either because the size distribution in the fluidized bed is
not homogenous or because the size distribution changed during the experiments – as the first
maxima of measured data and Mie theory do not fit. For the evaluation the Mie distribution
was therefore stretched to match the measured intensity distribution. This is acceptable as the
intensity characteristics are always the same at small angles; however, the zero level of the Mie
distribution varies with the input parameters, so that the width of the backscattering cone is
known only to an accuracy of about 15% (fig. 5.18).
The transport mean free path of a fluidized bed
The first backscattering experiments were performed on a fluidized bed of water and soda
lime glass spheres with particle diameters around 150 µm. The transport mean free path was
expected to be proportional to the distance between the particles in the fluidized bed. So the
backscattering was measured for different flux rates in the bed, resulting in particle volume
fractions between 39% and 58% (fig. 5.19). However, due to strong fluctuations in the data no
real trend in the width of the backscattering cone can be observed – which does not necessarily
mean that there is none. Here additional speckles from static particles stuck at the container
surface do have a rather disadvantageous impact.
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