Collapse of transient gels in colloid-polymer mixtures

Abstract

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.

Sudden collapse of a colloidal gel

“…Previous studies [13,24] of gel collapse have suggested that the mechanism of collapse depends sensitively on the initial height h 0 of the gel. Gels formed in short sample cells display steady or 'creeping' sedimentation where the height falls continuously with age at a rate which decays exponentially with time while taller samples show sudden collapse.…”

“…Sudden or 'delayed' network collapse is observed in a wide variety of materials [7][8][9][10][11][12][13][14][15][16] and seems to be ubiquitous at small U c /k B T . However, while sudden collapse * Corresponding author:P.Bartlett@bristol.ac.uk has been attributed to channel formation within the gel [12,13], the microscopic processes operating have never been fully established. A better microscopic understanding of the origin of sudden gel collapse is important not only because the distinctive settling behavior is intriguing from a scientific viewpoint but also because a quantitative prediction of gel stability is a critically important issue in the formulation and manufacture of many commercial products.…”

Highly Nonlinear Dynamics in a Slowly Sedimenting Colloidal Gel

“…Although they exhibit solid-like mechanical properties, colloidal gels are easily disrupted by small perturbations, such as gravitational forces. While a large body of macroscopic observations of gels under gravitational stress exists [5][6][7][8][9][10][11][12][13], very little is known on the microscopic processes at play during sedimentation, thus limiting our ability to understand and predict the behavior of sedimenting gels.Here, we use a novel light scattering method to gain access to the dynamics of a slowly settling colloidal system from the macroscopic deformation of the sample down to the relaxational behavior at the particle scale. We find that the very slow macroscopic deformation occurs via irreversible plastic events at the microscopic scale.…”

Gelation in Model Colloid−Polymer Mixtures

“…37 At high enough particle and/or polymer concentrations, the dynamics is arrested for some time in a gel state before slow restructuring leads to loss of mechanical stability and a rapid collapse to form an amorphous sediment. 25,31,38 For this reason, these structures are also known as transient gels.…”