Delayed sedimentation of transient gels in colloid–polymer mixtures: dark-field observation, rheology and dynamic light scattering studies

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