Optical Microrheology of Gelling Biopolymer Solutions Based on Diffusing Wave Spectroscopy - Food Colloids: Self-Assembly and Material Science - page 322

The measurements were performed with a DWS Rheolab instrument from
LS Instruments (Switzerland). This DWS instrument allows us to record the
intensity correlation function over a wide range of t (i.e.,10 6 -10 s) within
approximately a minute. This superior time resolution can be achieved by a
recently introduced two-cell echo principle. 15 It allows efficient ensemble aver-
aging of the signal even in the case of slow or arrested dynamics. This is
achieved by putting a fast rotating diffuser in the optical path between laser and
sample as seen in Figure 1. A 7 mWHe-Ne laser (JDS Uniphase) operating at l
¼ 632.8 nm is used to illuminate a circular ground glass, which is mounted on a
stepper motor. Through scattering and dephasing, the ground glass creates a
speckle with a nearly Gaussian optical field. The transmitted light from the
ground glass is collected by a lens and focused onto the sample with a resulting
spot-size diameter of roughly 5 mm. The scattered light is then collected with a
monomode fibre and analysed by a photodetector and a digital correlator
(Correlator.com, NJ, USA). The recorded (multi-speckle) correlation echoes
provide an ensemble-averaged signal that does not require any additional time
averaging. Absolute values of the correlation function were obtained by cali-
brating the instrument with a solid block of teflon (for which g 2 (t) 1 1).
Access to the short-time dynamics was obtained with the motor at rest. The
time-averaged correlation function was corrected for non-ergodicity using the
method of Pusey and van Megen. 26 The whole procedure was controlled by the
instruments' software package, including l* calibration, motor control, data
handling, and microrheological analysis. A full intensity correlation function
could be recorded within 1 min.
Figure 1 The DWS experimental arrangement