Intermittency Route to Rheochaos in Wormlike Micelles with Flow-Concentration Coupling

Abstract

We show experimentally that the route to chaos is via intermittency in a shear-thinning wormlike micellar system of cetyltrimethylammonium tosylate, where the strength of flow-concentration coupling is tuned by the addition of salt sodium chloride. A Poincaré first return map of the time series and the probability distribution of laminar lengths between burst events shows that our data is consistent with type-II intermittency. The coupling of flow to concentration fluctuations is evidenced by the "butterfly" intensity pattern in small angle light scattering (SALS) measurements performed simultaneously with the rheological measurements. The scattered depolarized intensity in SALS, sensitive to orientational order fluctuations, shows the same time dependence (like intermittency) as that of shear stress.

Bubble kinematics in a sheared foam

“…The dynamics are influenced by the structure and interaction at these scales. Examples of such materials include cellular structures such as foams [9], fluids containing worm-like-micelles [10,11] and large molecular mass solutions [12]. The constraints imposed by the structure influence the kinematic response (individual particle motions) at the microscopic scale [13].…”

Novel experimentally observed phenomena in soft matter

“…It was proposed that a modified version of the Johnson-Segalman model [102], incorporating terms accounting for the coupling between the mean micellar length and the shear rate, the dynamics of the mechanical interfaces and the flow-concentration coupling, could be used to model this phenomenon. Subsequent experiments have also demostrated that chaotic flows can be supported by semi-dilute solutions of giant wormlike micelles [88,101,103], dilute, shear-thickening solutions of cylindrical micelles [104], lamellar phases of surfactant solutions [105,106], concentrated colloidal suspensions [107], granular matter [108,109] and foam [110].…”

“…This phenomenon is known as vorticity banding [98]. When salt is added to the sample, the metastable region of the flow curve develops a non-zero slope α (σ ∼γ α ) [88,99] due to an enhanced concentration difference between the shear bands [100].…”

“…In the experiment with salt, the stress varies strongly with shear rate in the metastable region (the flow curve is plotted in the inset of Fig. 5(B) shows a slope α ∼ 0.32) [88,100,111]. Stress relaxation data, acquired by imposing fixed shear rates in the metastable region of the flow curve, are displayed in Fig.…”

“…The effect of a strong coupling between flow and concentration on the development of chaos was investigated in [88]. The flow curve of a salt-free GWM solution (an aqueous solution of the cationic surfactant cetyltrimethylammonium tosylate, CTAT) shows a stress plateau and is represented by solid circles in Fig.…”