Capillary rheometry of micellar aqueous solutions

Abstract

In this work, the capillary flow of wormlike micellar solutions was studied in detail. Flow curves obtained with capillaries of different sizes for surfactant aqueous solutions at various concentrations, were analyzed with particular attention to the regions of flow instability. It was found that to observe unstable flow a critical residence time of the solution in the capillary is needed, in addition to a critical shear stress value. At specific surfactant concentrations, a plateau-like region was observed for sufficiently long residence times of the liquid under flow, either in the capillary or in a cone and plate rheometer. For shear rates higher than those of the plateau region, the expected upturn on the flow curves was observed and studied. The flow curves in this regime were dependent on the capillary diameter in an opposite way as found in the presence of slip, i.e., adherence appears to occur. The flow instabilities (observed as unstable pressure at the capillary entrance and a variable flow rate at the exit) were detected even for the lowest concentration solution where the plateau region was not apparent. The flow instabilities took place completely in the absence of slip. A remarkable effect was observed in the solution with the higher surfactant concentration using a flow system that allows long residence times (about 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 increased again up to the initial stress and a new cycle began subsequently. This result is consistent with similar observations made in cone and plate or Couette geometries, which are attributed to the presence of `shear bands'.