Determination of anisotropy in turbid media

Concentrated suspensions of anisotropic or deformable particles show partial orientation when submitted to an external field, like shear, as for instance, transition from isotrope to nematic phase. Apart from these two extreme phases, partial orientation can occur. Direct information of this intermediate regime is difficult for micrometric suspensions, since the medium is generally turbid. Indeed, microscopy provides a partial information at interface, that is not representative of the average volumic orientation at the sample scale. A good and pertinent tool is 2D Small Angle X Scattering (SAXS), leading to the determination of the average orientation of single particles. The aim of this presentation is to show that anisotropic (suggested: scattering) of incoherent light can also give an average information on the anisotropy of the medium. The experimental device is composed by a focused laser (100µm) at the sample surface and incoherent light transport is then measured by a camera in the backscattering geometry (1,2). The acquired image size is 1cm, very large compared to the impinging laser. For spherical particles, or randomly oriented anisotropic particles, the acquired image possesses an angular symmetry. When the scatterers are globally oriented, this incoherent image is deformed. We have performed experiment on a sepiolite suspension (0.5%), known as developing a fiber like system under shear (3). We show here that the measurement of its anisotropy through incoherent light transport is very similar to the order parameter obtained by 2D SAXS measurements performed in shear flow geometry at the European Synchrotron Radiation Facility, on beamline ID02 (fig.1). This shows that the average time and volumic information obtained by both techniques are identical. This very simple technique is then used in another system composed by non Brownian calibrated glass fibers (6µm diameter, 12µm length). Average orientation of particles is followed versus shear at various concentrations