Optical Microrheology of Gelling Biopolymer Solutions Based on Diffusing Wave Spectroscopy - Food Colloids: Self-Assembly and Material Science - page 319

Chapter 21
Optical Microrheology of Gelling
Biopolymer Solutions Based on Diffusing
Wave Spectroscopy
Fre´ de´ ric Cardinaux, Hugo Bissig, Peter Schurtenberger and
Frank Scheffold
DEPARTMENT OF PHYSICS, UNIVERSITY OF FRIBOURG, CH-1700
FRIBOURG, SWITZERLAND
21.1 Introduction
Optical microrheology has emerged as one of the most important new tech-
niques to study and characterize rheological properties of complex fluids. The
main principle of passive microrheology is to study the thermal response of
small (colloidal) particles embedded in the system under study. 1-4 The particles
are either artificially introduced, which is then called 'tracer microrheology', or
they can be part of the system itself, e.g., as in the case of yoghurt. By analysing
the thermal motion of the particles, it is possible to obtain quantitative
information about the storage and loss moduli, G 0 ( o ) and G 00 ( o ), over an
extended range of frequency o .
One of the most popular techniques to study the thermal motion of particles
is diffusing wave spectroscopy (DWS), which is an extension of the standard
technique of photon correlation spectroscopy to turbid media. 5,6 DWS-based
microrheology is perfectly suited to a fast and non-invasive determination of
the key rheological properties as given by the frequency dependence of G 0 (o)
and G 00 (o). In this article we demonstrate how optical microrheology based on
DWS can be used to characterize the critical gelation behaviour in physical gels
that rapidly evolve in time.
21.2 Critical Gelation Microrheology
The sol-gel transition of a polymeric system is characterized by the instant at
which the connectivity of the network extends over the entire sample; this
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