Penetrating flows with gelation due to addition of counter ions to surfactant solution in circular tube

Abstract

Penetrating flows of an aqueous solution of sodium salicylate (NaSal) through an aqueous solution of cetyltrimethylammonium bromide (CTAB) in a circular tube were investigated experimentally. The NaSal solution flowed through the CTAB solution while displacing it. Network structures of wormlike micelles are formed on the interface between the solutions, and gelation occurs there. Although each test fluid has a similar Newtonian viscosity, CTAB/NaSal mixtures had strong viscoelasticity, and their viscosity greatly increased because of the gelation. Consequently, several flow patterns that do not appear in the case of Newtonian/Newtonian penetrating flows were observed. The flow patterns of the core of the penetrating flow were classified into five patterns and were mapped onto a plane of the molar concentration ratio S of NaSal to CTAB and the penetration velocity U. The core exhibited a saw-tooth pattern at small U, which changed to a corkscrew pattern or an elastic recoil pattern as U increased. Elastic recoil patterns appeared at S = 3.0 and 5.0, where the relaxation time of a CTAB/NaSal solution is relatively long. Furthermore, a novel flow pattern in which the core flow exhibits repeated elastic recoiling was observed. In this pattern, the contracting part of the core flow exhibited alternate stretching and recoiling. Moreover, a unique phenomenon of a penetrating flow with gelation was observed. In this phenomenon, a stable tip was newly formed by the gelation after the breakup of a core. The displacement rate decreases with increasing penetration velocity and tends to be small when unstable flow patterns appear.