Gabriele, Andrea (2011)
Ph.D. thesis, University of Birmingham.
Gelation of hydrocolloids under shear conditions results in a weak gel which is capable of sustaining elastic mechanical deformation at small strains but which flows if subjected to higher deformations (fluid gels). I have investigated the formation, production and lubrication properties of fluid gels from the gelation of agarose and -carrageenan. The formation of fluid gels in a rheometer, using well-defined, flow patterns and cooling rates have been described. Under these conditions narrow particle size distributions were achieved by inducing spinodal decomposition. The increase in viscosity due to particles formation was found to be inversely proportional to the shear rate. The power law exponents describing this relationship are provided for several hydrocolloid concentrations. The studies on the formation of fluid gels from -carrageenan, showed that it was possible to form homogeneous particle size fluid gels with mean diameter smaller than 5 μm. The fluid gels produced showed strong and highly recoverable particle bridging. A model is presented for the kinetics of aggregation of these particles, providing information on the mechanisms involved.
The use of the pin stirrer heat exchanger was previously reported as the large-scale method for the production of agar fluid gels. However little work has been done in describing fluid dynamics, and specifically the shear rates. Positron Emission Particle Tracking (PEPT) was used to characterise flow phenomena occurring in a pin stirrer heat exchanger for a 1 Pa s glycerol solution and a 2% agar fluid gel. Internal recirculation paths and axial-occupancy show a compartmental behaviour, with the tracer selectively occupying specific areas. Analysis of particle paths revealed that for a rotational speed of 900 rpm the average shear rate experienced by a particle was approximately 200 s-1. Areas of poor mixing in dead-zones were also identified. These findings can contribute to a rational design of equipment and processes for industrial scale fluid gel production.
Bulk rheological properties of fluid gels are similar to emulsions used in a large number of applications in everyday food products and cosmetics (Brown, Cuttler et al., 1990; Bialek, Jones et al., 2000), in order to replace emulsions their lubrication behaviour should also be fully understood. In the proposed lubrication mechanism the agarose fluid gel micro particles under specific conditions of entrainment velocity U, normal load W and elasticity E, interact with the lubricating surfaces, resulting in a localised increase in friction. A wide range of agarose concentration was studied (1% to 4%) resulting in fluid gel particles having different values of elasticity E, ranging from 5 kPa to 140 kPa, which were shown to alter their tribological behaviour, modifying the critical velocity required to induce entrainment of the particles, Uentr. For an identical particle’s elasticity, the critical velocity was found to decrease by 50% when the normal load was increased by 30%, while the opposite trend was observed when the normal load maintained constant. Fluid gel systems of the same elasticity E but of different particles sizes were also investigated. The maximum friction measured upon entrainment of the particles decreased from 0.3 to 0.2 when the average Sauter mean diameter of the particles passed from 102 m to 83 m.
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