Shear-thickening transition in surfactant solutions: New experimental features from rheology and flow birefringence

Abstract

We report on the shear-thickening transition observed in dilute aqueous solutions of cetyltrimethylammonium tosylate (CTAT) at concentrations φ ∼ 0.2%. We have re-examined the kinetics of the shear-thickening transition using start-up experiments at rates above the critical shear rateγc. Using simple well-defined protocols, we have found that the transient mechanical response depends dramatically on the thermal and on the shear histories. Using the same protocols, flow birefringence experiments were carried out. The gap of a Couette cell containing the sheared solution has been visualized between crossed polarizers in steady shear conditions, as well as in start-up experiments. We show that the birefringent shear-induced phase starts from the inner cylinder and grows along the velocity gradient direction, as in a shear banding situation. However, aroundγ c we have not observed a regime of phase coexistence (isotropic and birefringent).

Ring-driven shear thickening in wormlike micelles?

“…The latency time τ lat gets larger as one approachesγ c from above [18], but in any case is usually much bigger thanγ −1 c , so that a large total strain γ c needs to be applied before the thickening occurs. (For example, τ lat ≃ 100s in [17].) The latency period increases with the gap width of the shear cell [18] and in some cases, before it is over, there is clear spatial inhomogeneity [17,13].…”

“…(For example, τ lat ≃ 100s in [17].) The latency period increases with the gap width of the shear cell [18] and in some cases, before it is over, there is clear spatial inhomogeneity [17,13].…”

Rheology of giant micelles

“…Of equal fascination to the Maxwellian and shear-thinning behaviour described above, is the phenomenon of shear thickening [12,[74][75][76][77][78]. This is seen in a window of volume fraction around the onset of viscoelasticity at φ ≃φ; an initially inviscid or barely viscoelastic system is found, after a period of prolonged shearing above a critical shear rateγ c , to convert into a much more viscous state.…”

“…Among effects observed in [12,78] are the following. In some shear-thickening micellar systems subjected to steady shearing, there is an initial latency time τ lat (seconds or minutes) for the thickening to occur.…”

“…It is possible, though not yet certain, that structural memory plays a major role in the exotic rheological phenomena, such as rheochaos, considered in Section 1.5 below. Though hinted at anecdotally from the earliest days of the subject [48,74], it is only very recently that structural memory effects in micellar systems have been studied in systematic detail [12,78].…”