Sheared aqueous two-phase biopolymer–surfactant mixtures

Aqueous two-phase biopolymer–surfactant mixtures have so far received only little attention. Here, the behaviour of such mixtures was investigated adopting methods well established for aqueous two-phase biopolymer–biopolymer mixtures. The pullulan–sodium dodecyl sulphate (SDS) mixture, in which phase separation can be induced, and then enhanced by the addition of sodium chloride (NaCl), was chosen as a model system and its morphology and rheology under shear was analysed.Rheological measurements on phase-separated mixtures revealed that the viscosity depends on the chemical composition of the single phases, their viscosity ratio and volume fraction. Optical observations showed droplet-like morphology over a wide range of shear rates at low volume fractions of one phase, and co-continuous structures including string phases at high volume fractions and shear rates. For more concentrated systems, co-continuous structures were observed at all shear rates investigated, whereby in some cases string phases developed at high shear rates.