SHEAR INDUCED MICELLAR STRUCTURES

Mixed solutions of the anionic surfactant Sodiumdodecylsulfate (SDS) and the zwitterionic Tetradecyl-dimethylaminoxid (TDMAO) were studied under shear in a Couette-type apparatus. For shear rates exceeding a critical shear gradient gamma-c the solutions show birefringence which approaches a constant value DELTA-n(MAX) with increasing shear rate. At the same time properties like the shear viscosity and the normal stress difference increase dramatically. A structural modification must thus have taken place for gamma > gamma-c: a shear-induced structure (SIS) has developed. - The structure of the micelles in the solution at rest was determined by SANS-, electric birefringence- and viscosity experiments for different mixing ratios, concentrations and ionic strength. The solutions under shear were studied by SANS, flow-birefringence and rheological measurements. - At rest small, rodlike micelles which consist of mixed surfactants are present. The length of the micelles are smaller than their mean distance and the viscosity is low in this case. The dimensions of the micelles were determined from the positions of the correlation peaks in the SANS-patterns which are due to repulsive forces between the charged aggregates. At a mixing ratio of 9:1 (TDMAO:SDS; C = 100 mM) the solutions are viscoelastic and no SIS is detected. The SIS is thus formed only in solutions with surfactant concentrations which are below the overlap concentration. At gamma-c - at which the SIS is beginning to form - the product of gamma-c and tau-ROT, the rotational relaxation time of the rods, is much smaller than one. - As soon as a critical shear gradient is reached these former isotropic micellar solutions show birefringence, an extinction angle of zero degrees, normal stress difference and an increased viscosity. Former isotropic SANS-patterns deform and peaks show the appearance of a highly ordered structure in equilibrium with unordered micelles. All micelles taking part in the SIS are completely aligned in the direction of flow. Formation of the SIS is highly dependent on concentration, mixing ratio and ionic strength. Finally, two proposals for the nature of SIS are presented: A shear induced phase transition into domains of (pseudo)nematic phases seems most likely to us and is consistent with our experimental results. But also a pearl-string like array in domains of oriented micelles is conceivable.