Coherent Backscattering from Multiple Scattering Systems - KOPS ...

4.2 The samples
sample particle size [nm] polydispersity [%] TiO 2 content [%] n eff D [m 2 /s] τ [ns]
a-1 1.38 25 1.05
a-2 1.46 13 1.31
NIX-2 1.38 27 0.43
NIX-3 1.31 41 0.67
R700 (DuPont) 245 22 1.58(6) 15(1) 2.0(1)
R700 + gypsum ≈ 17 1.29 250 0.1
R900 (DuPont) 359 29 1.44 17 1.05
R902 (DuPont) 279 38 1.63 15 0.88
S-25 1.35 21 0.65
Ti-pure (Aldrich) 540 37 1.38 19(1) 6.2(3)
Table 4.1: Colloidal samples. Particle size and polydispersity of the particles were
determined from electron microscope images [47]. The effective refractive indices n eff
were calculated using eqn. 4.1, diffusion coefficient D and absorption time τ were measured
in time of flight experiments at laser wavelength λ = 590 nm. Note that the data
are of strongly varied quality. Errors are given only when it was possible to quantify
them.
4.2.1 Titanium dioxide
Titanium dioxide (TiO 2 ), or titania, is widely used as white pigment for paint, plastics or
papers, in cosmetics, medicines and food, and in many other applications. The reason are its
extreme ‘whiteness’ and opacity, which are due to its high refractive index (n = 2.7 in rutile
phase) and its low absorption. In this work we use it to examine the structure of the coherent
backscattering cone. This requires samples with kl ∗ close to unity, as in this regime the cones
become very wide and their features are easier to observe in the experiment.
We use both custom-made TiO 2 particles that were produced in cooperation with the chemistry
department of the University of Konstanz and commercially available titania powders
which were kindly provided for free by Sigma-Aldrich and DuPont. These contain more or
less spherical particles of rutile titania with diameters between 200 and 600 nm (tab. 4.1). As
conglomerates are rare, the powders can quite easily be compressed to volume fractions of the
order of 40%.
The pure titania samples have diffusion coefficients in the low two-digit range. As we also
intended to do measurements on samples with higher diffusion coefficients, we mixed titania
and ground blackboard chalk (gypsum) in a weight ratio of 1 to 5.
4.2.2 Teflon
Polytetrafluoroethylene (PTFE), best known under its DuPont brand name Teflon, is our reference
sample for backscattering experiments. It is a solid (thus avoiding the problem of
re-creating the sample with exactly the same properties for later measurements, which one
would have with a colloidal reference), but without any particular order (crystalline or otherwise),
has low absorption (though not as low as the TiO 2 samples), and a very large transport
mean free path and therefore an extremely narrow backscattering cone.
39