Capillary rheometry of micellar aqueous solutions

In this work, the capillary flow of wormlike micellar solutions was studied in detail. Flow curves obtained with capillaries of different sizes for su rfactant aqueous solutions at various concentrations, were analyzed with pa rticular attention to the regions of flow instability. It was found that to observe unstable flow a critical residence time of the solution in the cap illary is needed, in addition to a critical shear stress value. At specific surfactant concentrations, a plateau-like; region was observed for suffici ently long residence times of the liquid under flow, either in the capillar y or in a cone and plate rheometer. For shear rates higher than those of th e plateau region, the expected upturn on the flow curves was observed and s tudied. The flow curves in this regime were dependent on the capillary diam eter in an opposite way as found in the presence of slip, i.e., adherence a ppears to occur. The flow instabilities (observed as unstable pressure at t he capillary entrance and a variable flow rate at the exit) were detected e ven for the lowest concentration solution where the plateau region was not apparent. The flow instabilities took place completely in the absence of sl ip. A remarkable effect was observed in the solution with the higher surfac tant concentration using a flow system that allows long residence rimes (ab out 1 h). In this case, within a small window of shear stresses, a dramatic increase in flow rate accompanied by a large drop in the pressure measured at the capillary entrance was observed. After a minimum, the pressure incr eased again up to the initial stress and a new cycle began subsequently. Th is result is consistent with similar observations made in cone and plate or Couette geometries, which are attributed to the presence of 'shear bands'. (C) 1999 Elsevier Science B.V. All rights reserved.