Flow birefringence experiments showing a shear-banding structure in a CTAB solution

Richness of Side-Chain Liquid-Crystal Polymers: From Isotropic Phase towards the Identification of Neglected Solid-Like Properties in Liquids

“…The shear stress versus shear rate curve exhibits non-linear behavior that coincides, at the same gap thickness, with the emergence of the shear-induced birefringence. However, by varying the gap thickness or the nature of the substrate (Figure 4(b,c)), the flow curve is not invariant, and therefore cannot be interpreted as a constitutive phase transition related to the coexistence of two liquids [10,[16][17][18][19][20], but as a sliding transition. The slip phenomenon is identified at the lowest shear rates in a regime considered as Newtonian.…”

“…It reveals elastic collective effects so far neglected in the liquid state and has permitted the identification of a solid-like response away from any phase transition. The treatment of the shear-induced transition in lyotropic systems benefited from an abundant body of literature, particularly because of the numerous experimental developments [16][17][18][19][20]. It is usually interpreted by a "nucleation-unidirectional growth" process assisted by the pre-transitional fluctuations [10,11,[16][17][18][19][20][21][22] which leads to the coexistence of two defined shear bands related to the isotropic and the shear-induced phases.…”

“…The treatment of the shear-induced transition in lyotropic systems benefited from an abundant body of literature, particularly because of the numerous experimental developments [16][17][18][19][20]. It is usually interpreted by a "nucleation-unidirectional growth" process assisted by the pre-transitional fluctuations [10,11,[16][17][18][19][20][21][22] which leads to the coexistence of two defined shear bands related to the isotropic and the shear-induced phases. However, this scenario remains under debate, since the high shear branch of the flow curve, associated to the induced nematic phase, deviates from Newtonian fluid behavior [21,22] and the velocity cartography determined by NMR [23][24][25] suggests that the liquid moves rather as a gel.…”

“…Finally, at high shear rates, a new Newtonian branch should emerge corresponding to the induced nematic phase [10]. Concentrated systems of wormlike micelles [16][17][18][19][20][21][22]38,39] near a nematic transition are known to yield a non-monotonic flow curve of this form [10,[16][17][18][19][20]; while more dilute systems exhibit a non-monotonic flow curve interpreted as an effect of flow on the reptation-reaction dynamics [40]. In this description, the stress plateau displayed upon applying an increasing shear rate is supposed to depend on the nature of the interface between the coexisting phases [10,12,13] but not on the nature of the substrate.…”

Novel experimentally observed phenomena in soft matter

“…Experimentally, this metastability shows up as a stress plateau in the flow curve. The existence of shear bands was confirmed experimentally with a velocity profiling technique using nuclear magnetic resonance (NMR) [93], birefringence measurements, rheology and small angle neutron scattering (SANS) [94,95]. When a constant shear rate is imposed in the metastable region of the flow curve, the sample splits up into bands that form layers along the direction of the flow gradient.…”