Browsing by Author "Mansel, Bradley William"

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Setup and calibration of a suite of state-of-the-art microrheology techniques : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Physics at Massey University, Palmerston North, New Zealand
(Massey University, 2011) Mansel, Bradley William; Mansel, Bradley William
Microrheology is the study of the flow and deformation of fluids on the micrometre scale. It has many benefits over the use of traditional rheomoeters to measure the mechanical properties of fluids. Microrheology has small sample sizes, can extract information about the underlying heterogeneities, often has a lower setup cost, can measure to higher frequencies and can measure the viscoelasticity of in-vivo samples. Work has been carried out to setup and calibrate four different microrheology techniques, namely: diffusing wave spectroscopy, dynamic light scattering, multiple particle tracking and probe laser tracking with a quadrant photodiode and optical traps. This resulted in the ability to measure the viscoelastic properties of a material over approximately eight orders of magnitude, with nanometre resolution on the most sensitive technique; diffusing wave spectroscopy. The link between free Brownian motion and a particle diffusing in a harmonic potential was used to calibrate the trap strength of the optical tweezers, enabling a comparison of three different trap calibration techniques. Calibration of the trap strength in optical tweezers resulted in a good agreement between different methods, although, the power spectral density method proved easier to implement and more accurate over the range of laser powers, making it the superior method to use. To illustrate the power of microrheology techniques, the mechanical properties of standard viscous and viscoelastic fluids were first compared. Also organelles in pollen tubes were tracked to simply and accurately measure properties of a complex biological system in-vivo.
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
(Massey University, 2015) Mansel, Bradley William
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 pectin networks. 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.