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Structure and dynamics of biopolymer networks : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics at Massey University, Manawatu, New Zealand
The aim of this work was to further understand the structural and dynamical properties
of pectin-based biopolymer networks. This is pertinent to furthering our understanding
of the plant cell wall and has further implications for the food and pharmaceutical industries
where biopolymer networks play a fundamental role in thickening and stabilizing
food products and controlling the rate of drug release.
Firstly, microrheological studies on an acid-induced pectin network revealed previously
unseen slow motions of the network at times longer than one second. This "slow mode"
is reminiscent of so-called alpha processes that are predicted with mode coupling theory
in colloidal glasses. Such slow motions present in the networks are a signature of an outof-
equilibrium system and lead to further work on studying slow relaxation processes in
Secondly, structural and rheological measurements were performed on the acid-formed
pectin networks. It was found using small-angle x-ray scattering that the network was
composed of flexible cylindrical entities with a radius of 7 Å. At larger length scales
these entities were arranged in a clustered confirmation that upon heating increased in
density, indicating the importance of kinetic trapping for the initial network formation.
Finally, multi-speckle dynamic light scattering experiments were performed on three
different ionotropic pectin gels formed with calcium to study the dependence of the
slow dynamics on the junction length (and binding energy) between pectin chains. It
was found that increasing the junction length slows the dynamics until a point where
the internal stress becomes so large that the dynamics increase again. Spatially resolved
photon correlation spectroscopy measurements revealed previously unmeasured
millimetre sized heterogeneity in the networks. Angle-resolved multi-speckle photon
correlation spectroscopy showed conclusively that the dynamics are driven by internal
stresses and further more allowed the temporal heterogeneity to be measured.