Colloidal gels which are formed as the result of weak depletion attractions exhibit a two stage settling behaviour under gravity. Initially the space spanning network displays solid like behaviour and is resistant to gravitational effects. This structure persists for a finite period of time defined as the delay time TD . When the age of the gel exceeds its characteristic TD the structural integrity of the network is lost and it begins to rapidly collapse. The microscopic processes which are responsible for this sudden collapse have yet to be established and a better understanding is imperative because a quantitative prediction of gel stability has great implications in the formulation of many commercial products. In this thesis a detailed experimental study of the collapse of emulsion gels is presented. An oil-in-water emulsion system is studied where gelation is induced by the addition of a non-adsorbing polymer, which has a polymer-colloid size ratio of 0.62. Comparison of the measured phase diagram with generalised free volume theory shows that the system behaves as an ideal mixture of hard spheres and non-adsorbing polymer. Research in this area is usually focused on short range attractive systems where the dynamics of phase separation become arrested resulting in an arrested gel network. In contrast, measurements show that there is no period of kinetic arrest in long range attractive systems. Instead, phase separation occurs via a spinodal-like decomposition process until such time that the gel network fails. The gels have a characteristic domain radius Rc which grows as the sample ages and obeys the approximate power law R '" to:, where ? is a strong function of quench depth. The mechanism of phase separation therefore differs from the classical spinodal decomposition of binary liquids, where a is independent of the quench depth. A combination of confocal microscopy, rheology and time lapse video imaging is used to fully explore the collapse of transient gels. Confocal microscopy reveals that network restructuring is continuously occurring during the initial stable period, however there is no large scale restructuring of the network or formation of channels which have been attributed by previous workers to the cause of sudden collapse. The ageing process is microscopically different depending upon whether the initial height (11.0) of the sample is large or small (where 'large' is defined as >;:::; 12 mm). When 11.0 > 12 mm the ageing of the network does not occur uniformly throughout the full height of the network, rather, some areas of the network coarsen at a slightly increased rate resulting in inhomogeneities in the size of Rc . Contrary to this when 11.0 is small the network ages homogeneously throughout the entire height. Time lapse video imaging has revealed that there are two height dependant collapse processes occurring. The network reduces in height at a rate which is well described by the expression h(t - TD)/h(O) ? exp ((t - TD)/Tc)f3. Provided 11.0 remains above 12 mm f3 1.5 and the rate of collapse is a constant independent the height. When 11.0 is small f3 ;:::; 1 which is reminiscent of the collapse of strong gels. The delay time is completely independent of 11.0 in both instances. It is surmised that these results originate from internal stresses being generated by particles moving to more favourable positions in an attempt to reduce the interaction energy. These movements occur at the expense of particles with fewer nearest neighbours. As the network connectivity is reduced beyond some critical level, the gel starts to collapse. The stress inhomogeneities are not found in small 11.0 samples. The phenomenon of network collapse has been widely observed in systems with short range attractions. The measurements reported here provide a framework to quantitatively describe the physical processes which result in the sudden collapse of transient gel networks with long range attractions. When this process is fully understood then the formulation of commercial products can be adjusted accordingly in order to increase the shelf life of products, and maximise financial benefit.
|Date of Award||2011|
|Supervisor||Paul Bartlett (Supervisor)|