Transient and Steady-State Shear Banding in a Lamellar Phase as Studied by Rheo-NMR

Abstract

Flow fields and shear-induced structures in the lamellar (L α ) phase of the system triethylene glycol mono n-decyl ether (C 10 E 3 )/water were investigated by NMR velocimetry, diffusometry, and 2 H NMR spectroscopy. The transformation from multilamellar vesicles (MLVs) to aligned planar lamellae is accompanied by a transient gradient shear banding. A high-shear-rate band of aligned lamellae forms next to the moving inner wall of the cylindrical Couette shear cell while a low-shear-rate band of the initial MLV structure remains close to the outer stationary wall. The band of layers grows at the expense of the band of MLVs until the transformation is completed. This process scales with the applied strain. Wall slip is a characteristic of the MLV state, while aligned layers show no deviation from Newtonian flow. The homogeneous nature of the opposite transformation from well aligned layers to MLVs via an intermediate structure resembling undulated multilamellar cylinders is confirmed. The strain dependence of this transformation appears to be independent of temperature. The shear diagram, which represents the shear-induced structures as a function of temperature and shear rate, contains a transition region between stable layers and stable MLVs. The steady-state structures in the transition region show a continuous change from layer-like at high temperature to MLV-like at lower temperature. These structures are homogeneous on a length scale above a few micrometers.

Anomalous shear banding revisited with Rheo-NMR and Rheo-USV

“…Following a common model (Cates and Fielding 2006), the phenomena of shear banding can be explained by microscopic changes in the fluid structure that lead to a non-monotonous region in the flow curve where the flow becomes inhomogeneous. This effect has been observed in a wide range of materials including wormlike micelles (Britton and Callaghan 1997;Bécu et al 2004), lyotropic lamellar systems (Medronho et al 2012), star polymers (Rogers et al 2008), telechelic polymers (Berret and Séréro 2001;Manneville et al 2007), and various other soft glassy systems (Ovarlez et al 2009). …”

Phase Coexistence in a Dynamic Phase Diagram

“…[7,23,24] At 40 8C, the assumption from the diffusion experiments is confirmed, as the 2 HNMR spectra at 2a nd 10 s À1 can be related to aw ell-aligned lamellar phase. This happensa lso in the case of the C 10 E 3 system [13,28] between 38 and 40 8C. However,i nt he case of C 10 E 3 ,t he lamellar phase persists also at high shear rates;a tt he end of the transient experiment at 60 s À1 the lamellar-MLVp hase coexistence was confirmed.…”

“…2 HNMR is as ensitive technique to for analyzing lyotropic liquid crystals, providing relevant information on the microstructure [44][45][46] also under shear flow. [13,15,28,29] Diffusion Flow NMR Self-diffusion coefficients were measured using am icroimaging probe having at hree-axis gradient facility and am aximum gradient strength of 100 Gcm À1 on aB ruker Avance 300 spectrometer operating at 300.0 MHz ( 1 Hn uclei). The diffusion experiments were performed using the longitudinal eddy-current delay-pulse gradient stimulated echo (LED-PGSTE) sequence.…”

“…For all cases, only the diffusionc oefficient with the highest f n value was considered, as the other diffusion coefficient was attributed to the slight structural heterogeneities across the sample. [28] At 40 8Cafter 60 s À1 (40 wt %sample), two diffusion coefficients were considered and at hird excluded (having less weight). The diffusion experiments were performed before and after the applied shear flow in situ.…”

“…Medronho et al [13,28] reported the evolution of the water self-diffusion coefficient D,w ith strain for the three spatial directions duringM LV formation.I nitially,t he lamellar phase shows a D x value of more than one order of magnitude smaller than D z and D y .T his is due to the alignment of layers of the la- www.chemphyschem.org mellar phase along the flow direction.T he initial state of the C 12 E 3 lamellar phase waso btainedb ya pplying as hear rate of 10 s À1 for 1h at 55 8Ci nt he experiments performed here. Figure 2s hows the water self-diffusion coefficients obtained after the applied shear deformation, that is, after the transient experiments at 2, 10, 60 and 100 s À1 at 24 and 34 8C.…”