The addition of enough non-adsorbing polymer to a hard-sphere suspension causes the particles to aggregate to form a space-Ðlling gel. The integrity of the gel persists for a Ðnite period of time, and then the space-Ðlling structure collapses suddenly to form a denser sediment. This phenomenon of " delayed sedimentation Ï is ubiquitous in many weakly-Ñocculated suspensions. In this work, we observe the processes occurring in the bulk of a colloidÈpolymer gel using dark-Ðeld imaging, and probe the arrangement and dynamics of the particles in the system using two-colour dynamic light scattering. The e †ect of shear is also studied. A number of physical mechanisms relevant to a comprehensive explanation of delayed sedimentation are proposed and discussed.

The addition of non-adsorbing polymer to a suspension of colloidal hard-spheres causes phase separation via the depletion mechanism. At high enough concentration of polymer a variety of non-equilibrium aggregation behaviour is observed. Transient gelation is one such behaviour observed at the highest polymer concentrations. Transient colloid-polymer gels are metastable space-filling particle networks. They persist for some finite time before suddenly collapsing to form a dense sediment. Thus transient gels exhibit "delayed sedimentation" which is a phenomenon observed in many weakly aggregated suspensions. We have studied the collapse process occurring in the bulk of a gelled suspension using dark-field imaging and ultrasonic concentration profiling. At low polymer concentrations we observe delayed sedimentation behaviour. At the highest polymer concentration we observe a change in the settling behaviour. The suspension continuously sediments at a rate comparable to the initial slow settling rate of gels exhibiting delayed sedimentation. This is known as "creeping sedimentation ". We have also investigated the effect of varying suspension height and width on the delay time of a gel. We have found a critical height below which the gel exhibits a size-dependent delay time. Above this critical height the delay time is independent of height. The same behaviour is found when width is varied. We find good agreement between the results of this experimental study and a recent theory. Special thanks go to my supervisors Wilson Poon, Peter Pusey and Margaret Robins who between them have guided me through the jungle that is soft matter physics On the technical front I would like to thank Andy Schofield for making large quantities of PMMA colloid without which none of this work would have been possible. I would also like to thank "Uncle" Steve Illet for showing me the ropes all those years ago and teaching me how to live life in comfort. I'm also indebted to Falk Renth and Jerome Arrault for sharing with me their vast knowledge of all things optical (and doughnuts). Thanks also to David Hibberd at the Institute of Food Research for help and advice in performing the ultrasonics experiments and directing me to the best Thai restaurant in Norwich. For useful discussion and reading various parts of the thesis manuscript I'd like to thank Mark Haw and Mike Evans. I'd also like to acknowledge the rest of the "geisters" with whom I have had interesting scientific discourse: Mike Cates, Steve Meeker, and Abdellatif Mousald. I think that about wraps it up except to say a big thankyou to all the softies and the members of E=M.C.C. for keeping me sane(ish). Finaly, to my family for all their support during the last 25 years and to Mark for just being there.

We present an extension of the two-point optical microrheology technique introduced by Crocker et al. [Phys. Rev. Lett. 85, 888 (2000)] to high frequencies. The correlated fluctuations of two probe spheres held by a pair of optical tweezers within a viscoelastic medium are determined using optical interferometry. A theoretical model is developed to yield the frequency-dependent oneand two-particle response functions from the correlated motion. We demonstrate the validity of this method by determining the one-and two-particle correlations in a semi-dilute solution of polystyrene in decalin. We find that the ratio of the one-and two-particle response functions is anomalous which we interpret as evidence for a slip boundary condition caused by depletion of polymer from the surface of the particle.

Following a quench, colloidal systems with strong, short-ranged, attractive interactions can exhibit transient gelation, instead of the classical phase-ordering mechanisms of spinodal decomposition or nucleation. The particles aggregate into a tenuous, system-spanning network, which, for a time, remains robust to mechanical disturbance. Eventually, the network's ability to recover from destructive deformations becomes compromised, and the gel collapses. A detailed experimental study of gel collapse was reported in the preceding, companion article, leaving several open questions regarding the processes involved. We present a theoretical investigation into the factors affecting a gel's lifetime, concentrating in particular on the surprising influence of the size and shape of the container. We construct a model in which solvent dynamics are replaced by a dissipative coupling of the particulate network to a fixed frame and show that, in the absence of zero-frequency elasticity, such a coupling results in a novel class of matter in which stresses cannot propagate beyond a finite distance. We find our prediction of the characteristic length to be in quantitative agreement with the experimental data, and show how its ratio to the dimensions of the container controls the sedimentation. We discuss some aspects of the ageing mechanism, and suggest that a sudden collapse is ultimately due to erosion with positive feedback.

In recent years optical tracer techniques have been developed to determine the micro-rheology of soft viscoelastic materials. Recent theoretical arguments (Levine A J and Lubensky T C 2001 Phys. Rev. E 65 011501) suggest that the correlated fluctuations of a pair of widely separated probe particles should reflect the bulk rheology of the medium that they are embedded in more accurately than the motion of a single particle. We present a experimental test of these arguments. Using optical tweezers techniques (Henderson S, Mitchell S and Bartlett P 2002 Phys. Rev. Lett. 88 088302), we measure at high spatial and temporal resolution the thermal motion of a pair of colloidal particles suspended in a semi-dilute viscoelastic solution of non-adsorbing polystyrene in decalin. From the measured particle trajectories we determine both the one-and two-particle correlations and extract the local and bulk rheology. A comparison of the two measurements shows significant differences which are interpreted in terms of the depletion of polymer molecules from the particle surface.