Competition between Shear Banding and Wall Slip in Wormlike Micelles

Abstract

The interplay between shear band (SB) formation and boundary conditions (BC) is investigated in wormlike micellar systems (CPyCl-NaSal) using ultrasonic velocimetry coupled to standard rheology in Couette geometry. Time-resolved velocity profiles are recorded during transient strain-controlled experiments in smooth and sand-blasted geometries. For stick BC standard SB is observed, although depending on the degree of micellar entanglement temporal fluctuations are reported in the highly sheared band. For slip BC wall slip occurs only for shear rates larger than the start of the stress plateau. At low entanglement, SB formation is shifted by a constant ∆γ, while for more entangled systems SB constantly "nucleate and melt." Micellar orientation gradients at the walls may account for these original features.PACS numbers: 83.80.Qr, 83.50.Rp, During the past two decades, shear banding (SB), i.e. the shear-induced coexistence of macroscopic bands with widely different viscosities, has been evidenced in a large range of complex fluids [1]. Sheared dispersions of surfactant wormlike micelles have attracted considerable attention due to their practical use in industry, but also because they challenged the physicists to address a non-equilibrium problem with concepts from thermodynamics [1][2][3]. Indeed, rheological measurements show that the flow curve of shear-banding systems, i.e. the measured shear stress σ vs. the applied shear rateγ, presents a plateau at a well-defined shear stress σ over a given range of shear rates [4], very similar to the plateau in pressure as a function of overall concentration of a demixed system. As for equilibrium phase transitions, it has been suggested that the flow can be either metastable or unstable for SB formation, depending on the applied shear rate [2,[5][6][7]. The formation of two coexisting SB, bearing the local shear ratesγ 1 andγ 2 that mark respectively the lower and upper limits of the stress plateau, constitutes a pathway for the relaxation of the excess stress in the initially linear flow. Stress relaxation can, however, also occur through apparent wall slip. Slip phenomena are ubiquitous in polymers [8,9] and soft glassy materials [10]. Wall slip has also been reported in shearthinning wormlike micellar systems [11,12] but its connection with SB has been underexposed. Still, information on the interplay between wall slip and a flow instability like SB are essential for fully understanding the behavior of complex fluids.In this Letter, wall slip is shown to compete with SB formation by offering an alternative route for stress relaxation. We use tunable boundary conditions (BC) at the walls as an experimental tool to probe the effect of wall slip on the flow behavior of cetylpyridinium chloride/sodium salicylate (CPyCl-NaSal) micellar solutions at 6 and 10 wt. % in 0.5 M NaCl brine at 23 • C. We enforce "stick" BC by using a rough sand-blasted Plexiglas Couette cell and partial "slip" BC by using a smooth Plexiglas cell [13]. The competition between SB formation and wall s...

Rheo-PIV of a shear-banding wormlike micellar solution under large amplitude oscillatory shear

“…Finally, we note that some recent studies have suggested that both wall slip (Boukany and Wang, 2008;Lettinga and Manneville, 2009;Feindel and Callaghan, 2010) and unsteady secondary flows (Fardin et al, 2009(Fardin et al, , 2010 play an important role in the shear banding behavior of wormlike micellar solutions. Motivated by these most recent results, the work in this paper is a careful reassessment of the nonlinear rheological response of a model wormlike micellar solution in a cone and plate geometry, taking recent developments concerning wall slip and secondary flow into account.…”

“…Table 1 compares the contact angle of a sessile drop of CPyCl on the quartz surface with and without the adhesive plastic film, as well as the measured surface roughness. As we show below, a result of this modified surface is that slip effects which are often observed for CPyCl solutions at high shear rates (such as those seen by Lettinga and Manneville (2009)) can be suppressed to a substantial degree. The effect of surface roughness and hydrophobicity on inhibition of slip is not surprising, and these effects have been documented in previous studies (Masselon et al, 2010).…”

“…All measurements of the rheology of the CPyCl fluid in this paper (rheometry and combined Rheo-PIV) were carried out on a TA instruments ARG2 stress controlled rheometer equipped with a 50mm diameter, 4 • cone and plate geometry (quartz plate and aluminum cone) and with the temperature controlled to 25 • C. This particular test fluid differs from some of the fluids used in other shear banding studies (e.g. those by Salmon et al (2003) and Lettinga and Manneville (2009)) because it is not prepared in brine and does not contain any added sodium chloride. However, an identical fluid composition (i.e.…”

“…Many workers have observed this behavior directly using a variety of velocimetric techniques and under a number of different flow configurations (Lettinga and Manneville, 2009;Salmon et al, 2003;Lopez-Gonzalez et al, 2006;Britton and Callaghan, 1997;Callaghan, 2008;Boukany and Wang, 2008). Callaghan (1997, 1999) showed, using NMR velocimetry, that a CPyCl:NaSal 100:60 mM system (identical to the one used in this work) exhibited a three-banded velocity profile, in which a high shear rate band is observed in the middle of the gap, connected to two lower shear rate regions near the upper and lower surfaces.…”

“…Numerous classes of non-Newtonian fluids are known to exhibit shear banding behavior (Olmsted, 2008), from various kinds of yield stress fluids (Møller et al, 2008;Divoux et al, 2010) to entangled polymer melts (Tapadia et al, 2006;Hu, 2010) and wormlike micellar solutions (Britton and Callaghan, 1997;Salmon et al, 2003;Manneville et al, 2004b,a;Lopez-Gonzalez et al, 2006;Boukany and Wang, 2008;Lettinga and Manneville, 2009). Wormlike micelles are a particularly interesting class of shear banding systems because they are widely used in consumer products, and they have become a canonical model system for probing shear banding.…”