(98z) Anomalous Structure and Transport Properties of Soft Colloids

Structure, dynamics and rheology of model nanoparticle suspensions comprising of silica nanoparticles, densely grafted with oligomeric polyethylene glycol (PEG) chains and suspended in similar PEG oligomers are presented. The particle volume fraction corresponding to particle overlap is found to have unique signatures in particle arrangement and dynamics as well as the mechanical rheology of the suspensions. Small angle X-ray scattering measurements reveal that the particle-particle correlations, corresponding to the first maxima in particle structure factors, exhibit a maximum at the volume fraction corresponding to particle contact in conjunction with a transition from a liquid-like to a solid-like response in the linear viscoelastic regime and substantial increments in the viscosities. These anomalous structural trends are in stark contrast to hard sphere suspensions where the particle correlations increase monotonically. In addition, a maximum in particle relaxation time appears at a marginally higher volume fraction; the particles speed up upon further increase in volume fraction. Systems including complex molecular fluids like water and molten silica as well as particles interacting via soft repulsive potentials are known to exhibit similar “cascade of anomalies”, and an analogous competition among configurational entropy and resistance to deformation is argued to lead to the origin of these anomalous trends in all the cases.