Influence of Planar Elongation Strain on Flow-Induced Structure and Flow Instability of CTAB/NaSal Aqueous Solution

Abstract

An aqueous solution of CTAB/NaSal forms wormlike micelles and shows viscoelastic properties. It also shows flow-induced structure and flow instability in high shear rate flows. In this study, the occurrences of flow-induced structure and flow instability in planar elongational flows in a 4-roll mill flow cell were examined. An empirical formula of planar elongation strain as a function of elapsed time was proposed by results of particle trace velocimetry technique. In the planar elongational flow, the fluid was observed to become turbid, caused by the structure change, beyond a certain elongation rate. Subsequently, the elastic recoil was observed and simultaneously the turbid region disappeared. The change of turbidity is closely related to the planar elongation strain and the occurrence of the elastic recoil is related with a certain critical elastic energy. Furthermore, both critical rate and critical strain of the occurrence of these phenomena are considerably low in the planar elongation flow as compared with the shear flow.