COSINES · Colloidal suspensions in non-ergodic states

Final Activity Report Summary - COSINES (Colloidal Suspensions In Non-Ergodic States)

The scope of the project was the transfer of knowledge in the inter-disciplinary area of colloid science through implementation of a specific research program on non-ergodic colloidal states (glasses and gels), at rest and under shear. The research plan involved model systems with varying interparticle interactions from hard spheres to ultrasoft core-shell or microgel particles exploring their role in non-ergodic transitions and flow using a combination of experimental techniques. The vast majority of the goals set were reached. These were: Selection and characterisation of model particles. A series of particles were selected and characterised including both hard spheres and a variety of soft particles with thick polymeric layers. Complete set-up for ageing experiments of quiescent systems. The Dynamic light scattering study of the slow dynamics and ageing was based on a multispeckle CCD camera detection both for DWS (multiple scattering) conditions and single scattering at various scattering angles. Complete Light scattering under shear set-up (LS echo). The LS echo technique was successfully implemented both on a commercial strain control rheometer and a homemade shear cell. Dynamics and ageing of quiescent systems. The dynamics and ageing of systems at rest consisted of the following topical projects: a) Near wall dynamics of colloid polymer mixtures and concentrated hard sphere suspensions and b) Slow dynamics and ageing of hard sphere glasses. Rheology of colloidal suspensions: Linear response. The rheology of colloidal suspensions performed in different systems addressed both linear and nonlinear rheology in order to establish a strict and reproducible protocol. The topics investigated were: a) Rheology of crystallizing hard sphere glasses colloid polymer gels, b) Rheology of hard and soft particle glasses and c) Rheology of repulsive and attractive colloidal glasses. Complete set-up for ageing experiments under shear. We implemented the multispeckle DWS on a rheometer to study effects of ageing under or after shear in a variety of samples in combination with the LS echo measurements. A fast CCD camera was used in conjunction with the LS echo set-up on a strain control rheometer ARES. Simulations of sheared colloidal glasses. We have performed Brownian Dynamics simulations in collaboration with Prof. John Brady (Caltech) in order to investigate the microscopic mechanisms involved in yielding of colloidal glasses and gels and compare with a large amount of experimental data available. Effects of particle interactions. The effect of interparticle interactions was studied in the rheology of highly concentrated suspension, glasses and gels as well as ageing phenomena and their interplay with mechanical properties and dynamics of the system. Attractive interaction were induced via depletion attraction by dissolving linear polymer chains in a hard sphere suspension. Microscopic rearrangements and yielding: Nonlinear behaviour related with the previous we studied particle rearrangents in hard sphere glasses and crystals and attractive glasses and gels with combined rheology and light scattering as well as BD computer simulations. Ageing under shear: Rejuvenation Slow dynamics and ageing were studied at rest in glasses of various soft spheres such as star polymers, star like micelles as well as core-shell particles with long grafted polymer chains. Applied Colloidal Science Work on Applied Colloidal Science had to be diverted from original plan related to applications in diary and personal care products since the planned exchange of scientists and secondments could not be materialized (Unilever, UK). Instead we have established new collaborations with University of Vigo (Spain) for the study of anisotropic particles with potential application in the electronics industry and with MPIP (Germany) in Hydrogels for surface and biomedical applications.

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