Shear thickening and onion formation of non-ionic surfactant solution and the effect of charge

A shear-induced lamellar to onion transition of a surfactant solution has attracted considerable attention in these decades. Diat and Roux have shown that shear flow induces a transformation from planer lamellar structure to multilamellar vesicles (onions) with a polyhedral shape, which fill all the space without excess water and lead shear thickening. In a nonionic surfactant aqueous solution, pentaethylene glycol monododecyl ether (C₁₂E₅) and D₂O, onion formation associated with shear thickening is observed in 40 wt% C₁₂E₅ solution at T=55° C Effects of charge on the nonionic surfactant mixture were investigated by adding ionic surfactant and onion structure is induced by the suppression of Helfrich undulation of surfactant membranes. [3] Here we investigated the rheological behavior of a dilute solution of C12E5 (10 wt%) and D2O and the effect of charge. We have already shown that a disordered structure of C₁₂E₅ and D₂O at T=59 °C transforms to an ordered lamellar structure by adding an antagonistic salt such as sodium tetraphenylborate (NaBPh₄). An electrostatic interaction between surfactant membranes caused by a heterogeneous distribution of anions and cations originates the transformation as the effect of adding ionic surfactant sodium dodecyl sulfate (SDS). Figure 1 shows molar ratio between SDS and C12E5, S, dependence of viscosity as a function of shear rate. It is clear that shear thickening is observed for all the samples measured including SDS and a shoulder at 2 s-1 is observed for the sample without charged molecules. The present SANS experiment confirmed that the lamellar layers are oriented parallel to the flow direction at low shear rate, while onion structure is formed as evident by isotropic scattering pattern, which is eventually broken by further increase of shear rate. This is the first evidence of the shear thickening and the onion formation in the dilute solution of nonionic surfactant.