(383c) Structure, Rheology, and Network Formation in Wormlike Micelle-Nanoparticle Mixtures

Wormlike micelles (WLMs) have become widely used in a number of industrial and consumer products and processes where they come in contact with colloidal species, yet relatively little is understood regarding the interactions between WLMs and colloids and resulting changes in macroscopic properties. Here, we present the first systematic study of structure and rheology of ionic WLMs in the presence of model electrostatically stabilized nanoparticles. Structural and thermodynamic measurements in dilute surfactant-nanoparticle mixtures reveal that nanoparticle-induced modification of WLMs is critically linked to interactions at the solution-nanoparticle interface, whereby the adsorption of micelles to an adsorbed surfactant layer leads to the formation of micelle-nanoparticle junctions. These micelle-nanoparticle junctions act as physical cross-links between micelles, and give rise to significant rheological modification of WLMs upon nanoparticle addition. In semi-dilute solutions, this results in a decrease of the entanglement concentration and concomitant increase in viscosity by as much as an order of magnitude for particle concentrations as little as 1 vol%. Additionally, nanoparticle addition imparts viscoelasticity in otherwise Newtonian WLMs, and enhances the viscoelasticity of entangled WLMs. Thus, nanoparticle addition gives rise to a so-called ?double network?, where the viscoelasticity can be tuned through two energetic scales, the micellar end cap energy and micelle-nanoparticle junction energy, which are readily manipulated by adjusting solution conditions. In concentrated WLMs, nanoparticle addition leads to suppression of the shear banding instability in WLMs. Combined results of rheometry, velocimetry, and structural measurements under shear reveal that suppression of shear banding results from hindered orientational mobility of micelles due to micelle-nanoparticle junctions. The fidelity of these effects over a range of solution conditions and different nanoparticles demonstrates that nanoparticle addition provides robust and unique control over the rheology of wormlike micellar solutions.