Colloidal metastability

• Main Author: Sedgwick, Helen
• Published: University of Edinburgh 2003
• Subjects: 541.33
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.661723

The aggregation and gelation of colloidal systems has been studied using confocal microscopy. For a model colloid-polymer mixture in a density matched solvent, fluid and gel were found to be the only two phases observed for sufficiently short-ranged attractions. The experimental gel boundary in this system is associated with an ergodic to non-ergodic transition line. Starting from this ‘standard model’ the effects of charge and gravity on the gelation of colloidal systems have been explored. When the colloids in this aggregating system were charged, the electrostatic repulsion was found to stabilise the clusters against further growth. This results in a stable and equilibrium long-lived cluster phase. At sufficiently high inter-particle attraction the electrostatic repulsion is overcome and the clusters form a gel. The experimentally observed gel boundary is shifted to higher inter-particle attractions due to the charge on the particles. When gravity is introduced to the system sedimentation may prevent a gel from forming. The onset of gelation is marked by the formation of small clusters which grow and eventually form a gel. If the rate of sedimentation is faster than the rate of cluster growth a sample-spanning gel is not formed. The experimental gel boundary was observed at higher inter-particle attractions due to the presence of gravity. The phase behaviour of protein solutions has been studied. The addition of salt to a protein solution causes an effective attractive interaction between the proteins, and similarly to colloid-polymer mixtures non-equilibrium behaviour is observed at sufficiently high attractions. Optical microscopy revealed four regimes: gas-liquid phase separation, non-coalescing ‘beads’, large aggregates and transient gelation. The interaction between the gas-liquid binodal and the ergodic to non-ergodic transition line is found to be essential in understanding the non-equilibrium phase behaviour.