Heterodyne Dynamic Light Scattering under Shear (FZJ - ICS3)
DESCRIPTION OF THE FACILITY
The heterodyne dynamic light scattering (HDLS) measures spatially resolved flow velocities in the gap of a Couette shear cell. The analysis of the velocity profile of complex fluids addresses to effects like shear-banding and wall-slip, not known for Newtonian fluids.
The optical set-up is based on a well known differential Laser-Doppler velocimeter as used mainly with single particle detection in turbulent two phase flows. The light of a HeNe laser is split into two beams of equal intensities, which are then focussed into a small volume in the gap of a transparent Couette cell using a photo lens. Light of both beams is scattered by the sheared fluid. Due to the different scattering vectors q related to the two incident beams the frequency of the scattered light is differently shifted (Doppler effect). The interference of this scattered light results in an intensity oscillating with the beat frequency, which is then measured by analysing the intensity autocorrelation function. To adjust the position within the gap the photo lens can be moved along the optical axis by a motorized linear stage. With this special kind of heterodyne light scattering set-up the resulting Doppler frequency is independent of the direction of the observer. Therefore we are able to choose this relatively simple light path in the difficult geometry of the Couette shear cell with many curved surfaces.
The shear cell used in this set-up can be assembled from an outer and inner cylinders with different diameters (dout=48 mm and din=43, 44, 45, 46 mm, respectively) with resulting gaps from 1 to 2.5 mm. The cell has a height of 50 mm. The inner and/or the outer cylinder can be driven by a DC servo motor which together with a gear transmission and the choice of a gap allows shear rates from 0.01s-1 to about 1000s-1 to be applied. The intensity of the HeNe laser with a wavelength of 633nm and a nominal power of 21mW is usually sufficient. The scattering at an angle of 13° corresponds to an absolute value of q of 3 μm-1 . The cross section of the laser beams defines the size of sample volume to about 20*100 μm, when the beams are crossed at an angle of 26°. The intensity autocorrelation function is recorded using an ALV-FastCorr Linear Correlator with 256 channels. The sample time can be adjusted in the range of 75 ns to 408 μs. For longer sample times the correlation function can be calculated by software. An accumulation time of a few seconds is usually sufficient to determine the frequency in the measured correlation function.
Facility available at Forschungszentrum Jülich, Germany
SCIENTIST IN CHARGE
Dr. H. Kriegs
Send an email to Dr. Dr. H. Kriegs