# Light Scattering Equipment

Dynamic light scattering with evanescent illumination is used to study the dynamics of colloidal suspensions near a planar solid/liquid interface. The instrument is based on a three axis diffractometer custom made by Huber Diffraktionstechnik. With this set up it is possible to change the scattering vector component normal and parallel to the interface independent of each other. This allows for the model free distinction between the particle mobility normal and parallel to the interface. The instrument is equipped with a 532 nm Nd/YAG or a 633 nm He/Ne Laser as alternative primary sources. The scattered light is collected with a monomode fiber attached to a splitter, which allows detection with a dual avalanche diode system. The TTL output of the diodes is correlated with an ALV 6010 correlator.

( P. Lang )

We use standard light scattering set-up by ALV-Laservertriebsgesellschaft (Langen, Germany) for static and dynamic light scattering experiments. The instrument is equipped with a Helium-Neon laser (633 nm). The intensity time-correlation function is recorded by an ALV-5000 multi tau digital correlator. The thermostatted sample cell is placed on a motor-driven precision goniometer (±0,01°) which enables the photomultiplier detector to be moved accurately from 20° to 150° scattering angle.

( S. Wiegand )

A grating created by the interference of two laser beams is written in a sample. A small amount of dye, present in the sample, converts the intensity grating into a temperature grating, which in turn causes a concentration grating by the effect of thermal diffusion. Both gratings contribute to a combined refractive index grating that is read out by diffraction of a third laser beam. Analyzing the time dependent diffraction efficiency, the three transport coefficients can be obtained. The thermal diffusivity Dth, the translation diffusion coefficient D, and the thermal diffusion coefficient DT. The ratio of the thermal diffusion coefficient and the translation diffusion coefficient allows the determination of the Soret coefficient ST.

( S. Wiegand )

TIRM is used to investigate the static interaction between a single colloidal sphere and a glass wall. Our setup was constructed from Olympus microscope components and equipped with a photon counting head (Hamamatsu H7421-40), which can be connected alternatively to a National Instruments Counting board or an ALV 6010 digital correlator. We use a 15 mW HeNe Laser as the light source which may be attenuated if necessary. Additionally, a Coherent Verdi V II Laser can be connected via a high power fibre to be used as a two dimensional optical trap for the probe particle.

( P. Lang )

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 velocity measurement is based on the Doppler frequency which is determined from the oscillating intensity autocorrelation function of the scattered light. Further Information: H.Kriegs, P. Lettinga

( H. Kriegs, P.Lettinga )

**Available Shear Cells **

We can measure longitudinal and transversal acoustical phonons in liquid samples as a function of temperature (-20°C to 80°C) and pressure (1bar to 2kbar) by means of dynamic laser light scattering. We access S(q,ω) using a tandem Fabry-Perot interferometer in a frequency range between 5Ghz and 100GHz. Three different scattering angles can be realized especially also in back-scattering geometry. Simultaneously S(q,t) is obtained by means of photon correlation spectroscopy in a time range between 20ns and 100s.

( G. Meier )

We run five different set-ups for high pressure applications. Three are devoted for laser light scattering and two for high pressure microscopy. One of these cells is shown in the figure. The microscope objective can be as close as possible to the sample position. The pressure can go up to 1kbar and is provided by hydraulic oil. The other microscopy cell is constructed for fluorescence correlation spectroscopy applications. One of the set-ups for laser light scattering is specially made for small angle scattering and works up to 4kbar provided by hydraulic oil. The other cells are suited for angle and polarisation dependent measurements and work with compressed nitrogen up to 2kbar. A temperature variation is possible with all cells.

( G. Meier )

**Small Angle Vertical Dynamic Light Scattering**

A Dynamic Light Scattering set up is vertically mounted on a motorized goniometer, and can be used in combination with a home-made in-situ cell with which an electric field can be applied.

This set up can be used to investigate the response of charged colloids in bulk to electric fields. Scattering angles are small, in the range of 4 to 45 degrees.

( K. Kang )