Rheology of non-Newtonian liquid mixtures and the role of molecular chain length

We have examined mixtures of toluene, a Newtonian liquid, with several non-Newtonian liquids (i.e. surfactants) across the full composition range, from pure toluene to pure surfactant, for the purpose of exploring their rheological properties. Surfactants of similar viscosities but significantly different molecular weights were chosen, in order to verify the role of the molecular chain length. We discovered that the classical mixing rule, based on excess activation energy, succeeds for mixtures with shorter molecular chain, but fails for mixtures with longer ones. We suggest here a hypothesis which explains this difference as the result of expanding-collapsing of flexible long-chain molecules in shear flow. In order to support this hypothesis, we applied longitudinal rheology measurements which reproduce such oscillatory effects under the controlled conditions of an ultrasound longitudinal wave. This method confirms that there are qualitative differences in the rheology of these liquids mixtures with short and long molecular chains.