# The Small Angle Vertical Dynamic Light Scattering

The dynamic light scattering set up is mounted vertically to the horizontally placed electric cell. he scattering-angle range is 5 to about 35 degrees, with an almost homodyne signal for all angles. The light source is a 10 mWatt He-Ne laser, operating at a wavelength of 633 nm. Fig.3 is a schematic of the set up. The detector and correlator are from ALV, where the detector is fiber-based. Essential for this set up is the 75 mm focal-length lens (shown in red in Fig.3) that defines the scattering volume in the bulk in between the two electrodes. Without this lens, the behaviour of the correlation function is affected through scattering of particles near the two walls. As can be seen from Fig.4, such scattering contributions lead to additional relaxation contributions in the tailff the correlation function. The figure shows correlationfunctions for dilute suspensions of silica spheres with a radius of 116 nm, at a scattering angle of 21 degrees. Note that the true scattering angle is not equal to the goniometer angle due to refraction of scattered light at the electrode-air and -sample interface. The goniometer angle should be corrected to obtain the true scattering angle.

Fig.4: Correlation functions with the lens (a) and without the lens (b). The inserts are blown ups of the long-time behaviour of the correlation functions.

( K. Kang )