Turbid and transparent shear-induced structures in dilute cationic surfactant solutions

Abstract

The advection of shear-induced structures (SISs) was visualized for cationic dilute surfactant solutions in a rectangular channel flow and an impinging wall jet. The SISs visualized here were transparent and distinguished as the macroscopic dark structures in contrast to the background light-scattering micelles. The SIS phase was separated from the solution and behaved like an elastic gel. In the rectangular channel flow, the threadlike SISs of centimeter order length appeared to be aligned in the direction of the flow, and the average thread length was increased with the Reynolds number and turbulent drag reduction. Although the origin of the threadlike SISs was not clear in the fully developed rectangular channel, the impinging flow developed in the streamwise direction and revealed the growing process of the SISs. The viscoelastic solution with equimolar counterions rolled up the wall SIS layer in the impinging flow, and built the cylindrical sheet-like SIS. In contrast, the solution with excess counterions presented the generation of a transparent gel-like SIS from a turbid wall shear layer, and produced a ring-like SIS. The shapes of the phase-separated SISs were dependent on both flow geometry and fluid viscoelasticity.