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The structure of particle gels as studied with confocal microscopy and computer simulations.

This thesis contains the results of a PhD-study on the structure of particle gels. Part of it is directed at a quantification of this structure from measured data, part of it at modelling the aggregation processes that lead to particle gels. Chapter 1 of this thesis is a general introduction describing the aim of this study. Chapter 2 introduces aggregation and gelation of particles. As to the modelling of aggregation we conclude that there are two types of approach: one based on thermodynamics and one based on fractal aggregation (Diffusion Limited Cluster Aggregation or DLCA). The former is appropriate for reversible aggregation, the latter for irreversible aggregation. For all types of colloidal aggregation which are in between these two extremes, an understanding based on both approaches is needed. We use fractal aggregation models as a starting point, but recognise the importance of cluster reorganisation, which will cause gels with structures different than predicted in DLCA. As to quantification of fractal structure, a lower cutoff length scale r 0 is suggested as a valuable addition to the fractal dimensionality d f . Chapter 3 describes Confocal Scanning Laser Microscopy (CSLM) as a tool to study the structure of various particle gels. An in-depth optical treatment of imaging in CSLM is presented, along with a new way of image enhancement using calculated three dimensional point spread functions. It is concluded that image enhancement is essential in order to get useful results from an analysis of CSLM micrographs. Chapter 4 shows the results on the fractal structure of particle gels derived from CSLM micrographs of colloidal model systems. It explains thoroughly the details of the image analysis procedure and identifies possible problems, most notably background intensities. From the results it appears that all fractal dimensionalities are about equal, with values larger than the DLCA limit. The values of r 0 show that there are significant differences between gel structures despite the similarities in d f . These differences are difficult to connect with properties of the colloidal model systems. A hypothesis about a ' pre- aggregation stage' is formulated which might serve as a starting point for further research. In Chapters 5 and 6 Brownian Dynamics simulations are used as a tool for forming a better connection between aggregation and gel structure. For aggregating Lennard-Jones systems (Chapter 5) we find percolating networks at high volume fractions; these appear to be formed by reorganisation of large aggregates, during which branched strands are formed with voids in between. The gel structure is influenced by both cluster growth and cluster reorganisation; fractal analysis gives non-universal, i.e. time dependent results. For the Lennard-Jones potential, the aggregation is dominated by cluster reorganisation. Aggregating particles with a shorter potential range (Chapter 6) also show time dependent fractal results, but by decreasing the potential range the balance between cluster growth and cluster reorganisation becomes more subtle. Reorganisation of clusters occurs at a lower rate when the potential range is decreased. A large potential well depth eboth promotes reorganisation and inhibits it because of irreversible clustering. A thermodynamical analysis shows that this effect will become more pronounced for shorter ranged potentials. Chapter 7 generalises the conclusions of all previous chapters and gives suggestions for further research. All results in this thesis have used available computing resources up to the maximum. Given the ever growing computing power, significantly more sophisticated techniques for simulation or image analysis will become available within years. The work described in this thesis has led to the following publications: J.H.J. van Opheusden, M.T.A. Bos, 'Induced flocculation of casein micelles - a Brownian Dynamics simulation on the Parsytec Gcel MPP', Future. Gener. Comp. Sy. 11123-133, 1995 BE. Bijsterbosch, M.T.A. Bos, E. Dickinson, J.H.J. van Opheusden, P. Walstra, "Brownian dynamics simulation of particle gel formation: From argon to yoghurt", Faraday Discuss. 10151-64, 1995 M.T.A. Bos, J.H.J van Opheusden, "Brownian dynamics simulation of gelation and aging in interacting colloidal systems", Phys Rev E 53 5044-5050, 1996