Effect of a hydrotrope on the viscoelastic properties of polymer-like micellar solutions

Low-viscosity micellar aqueous solutions of cetyltrimethylammonium bromide (CTAB) undergo a major change in the presence of the hydrotrope, potassium 1-phenylmethylsulfate (KPhMS), producing a highly viscoelastic entanglement network of polymer-like micelles. The system studied here shows typical shear banding flow behavior, which tends to disappear with increasing the hydrotrope-to-surfactant concentration ratio (C _H / C _S). The linear rheological response was analyzed with the model of Granek–Cates, whereas the nonlinear behavior was reproduced with the Bautista–Manero–Puig (BMP) model. Both models introduce a kinetic equation to account for the breaking and reformation of the micelles, and they predict the linear and nonlinear rheological data very well. This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006.     

Microstructures and rheology of micellar surfactant solution by Brownian dynamics simulation

In this paper, three-dimensional Brownian dynamics simulation has been conducted for dilute micellar surfactant solution under a steady shear flow. The rodlike micelle in surfactant solution is assumed as a rigid rod made up of lined-up beads. The Lennard–Jones potential and soft-sphere potential are employed and taken as the inter-bead potentials for end–end beads and interior–interior beads, respectively. The motion of the rodlike micelles is determined by solving the translational and rotational equations for each rod under hydrodynamic drag force, Brownian force and inter-rod potential force. Velocity Verlet algorithm has also been exerted in the simulation. The micellar network structure is formed at low shear rates and destroyed by high shear rates. The computed shear viscosities and the first normal stress coefficient represent shear thinning characteristics. The paper reveals the relation between rheology and microstructure of surfactant solution at different shear rates. The effect of surfactant solution concentration rested on the micellar structures and rheological properties has also been investigated.     

An experimental study on the flow behavior of micellar solutions

At higher concentration levels, the inner structure of micellar solutions cannot be detected directly by optical means. Nevertheless, the flow behavior of the micellar solutions reflects their micellar structures. Hence, in this study the material behavior of micellar surfactant solutions was investigated by rheometrical means in steady and oscillatory shear flows. The flow behavior of the solutions was found to be strongly dependent on the concentration of the surfactants. At very low concentrations, the surfactant solution shows Newtonian behavior. With increasing concentration, a transition to shear thinning behavior and increasing viscoelasticity was found. The complex material structure is modeled according to the flow behavior by discrete and continuous relaxation time spectra, depending on the concentration. Received: 3 May 2000/Accepted: 18 September 2000     

Distinguishing shear banding from shear thinning in flows with a shear stress gradient

Shear banding occurs in complex fluids that exhibit a non-monotonic constitutive instability, such as wormlike micelles, and potentially also in polymeric fluids with presumably monotonic constitutive behavior. However, velocity profiles for shear thinning fluids in geometries possessing a stress gradient, such as Taylor-Couette flow, could be misidentified as shear banding. To address this, we present a model-free experimental procedure to distinguish shear banding from strong shear thinning using high-resolution velocimetry. The approach is developed and validated using simulations using the d-Giesekus model and is based upon the behavior of the width of the apparent interface between the high and low shear rate regions. It is then tested using experimental data for model wormlike micellar solutions. The method allows shear banding to be distinguished from shear thinning in cases where this difference is otherwise indistinguishable. As a by-product, it also provides an estimate of the stress diffusivities for shear banding fluids.     

Rheometric and rheo-optical investigation on the effect of the aliphatic chain length of the surfactant on the shear thickening of dilute worm-like micellar solutions

In this study, a new way of understanding the shear-thickening phenomenon in self-assembled solutions is introduced. The near- and out-of-equilibrium behavior is investigated in four aqueous micellar solutions containing surfactants of the same family: the alkyltrimethylammonium bromide mixed with an ionic salt at equimolar concentration. Thus, the four molecules of surfactants have the same polar head but different aliphatic chain length containing 12, 14, 16, and 18 carbon atoms. In aqueous solutions, the attractive forces between the surfactant molecules depend on the length of the aliphatic chain, thus, varying this parameter will have a definite influence on the aggregation number, the shape and dimension of the micelles. According to our results, the evolution of the low shear viscosity is affected by the chain length. The rheometric measurements performed in steady and time-dependent flows show that the emergence, the range, and the amplitude of the shear thickening also depend on this parameter. However, in the domain of high shear rates, after reaching the maximum of viscosity, all flow curves superimpose irrespective of the chain length. The rheo-optic measurements confirm the apparition of the shear-induced structure (SIS); this new phase is locally oriented and the chain length affects strongly the micelles orientation and the birefringence intensity. These results undoubtedly demonstrate that the chain length plays an important role in the behavior near equilibrium and under shear flow of the micellar systems. Paper presented at the third Annual European Rheology Conference (AERC 2006) on April 27–29, 2006, Crete, Greece.     

Rheology and birefringence of Fomblin YR at very high shear rates

The rheological and structural properties of perfluoropolyether (PFPE) lubricant films including viscosity, shear stress, and birefringence were measured at relatively low to extremely high shear rates using a rotational optical rheometer. The viscosity of various films with different thicknesses exhibit Newtonian behavior up to a shear rate 1 × 10⁴ s⁻¹, with a transition to shear-thinning behavior obvious at higher shear rates. Birefringence of these films was also measured for the first time, and these results indicate chain alignment with shear in the shear-thinning regime. The shear rate at which alignment occurs is similar to that of the onset of shear thinning. This correlation between chain alignment and shear thinning provides direct evidence that the ability of PFPEs to lubricate hard drives at high shear rates is a direct consequence of the ability of the applied shear field to align the molecules on a molecular level.     

Viscoelastic properties of polymerlike reverse micelles

A dramatic increase in the viscosity of reverse micellar solutions of lecithin in a variety of organic solvents of up to a factor of 10⁶ upon the addition of a small amount of water can be observed. The formation of viscoelastic solutions can be explained by a water-induced aggregation of lecithin molecules into flexible cylindrical reverse micelles and the subsequent formation of a transient network of entangled micelles. The viscoelastic properties of these solutions are characterized as a function of water content and temperature for different organic solvents by means of dynamic shear viscosity measurements. The results are interpreted by making analogies to the behavior of semidilute polymer solutions and living polymers.Dedicated to Prof. Dr. J. Meissner on the occasion of his 60th birthday.     

Molecular orientation in sheared molten thermotropic random copolyester

The macromolecular alignment and texture orientation in sheared thermotropic copolyester were investigated using in situ wide-angle X-ray scattering (WAXS) and polarizing optical microscopy (POM). The molecular behavior was correlated with viscoelastic properties. The polymer is a random copolyester based on 60 mol% 1,4-hydroxybenzoic acid (B) and 40 mol% ethylene terephthalate (ET) units. X-ray scattering showed that the molecular chains were aligned along the flow direction. The degree of molecular orientation, , is an increasing function of the applied shear rate. However, rheo-optics showed that shear flow could not orient the polydomain texture, i.e., neither defect stretching nor elimination of defects was observed. Instead, shear compressed the microdomains and gave rise to long-range orientation correlations. Rheology showed that the nematic melt is viscoelastic, the loss modulus G″ dominates the elastic modulus G′, and the dynamic viscosity η* is shear thinning. Moreover, the steady shear viscosity, η, also behaved shear thinning, while the first normal stress difference N ₁ remained positive. The empirical Cox–Merz rule did not hold, , within the shear rate range studied. The microscopic and rheological properties suggest that B–ET is a flow-aligning nematic polymer.     

Flow of wormlike micelle solutions through a periodic array of cylinders

Solutions of self-assembled wormlike micelles are used with ever increasing frequency in a multitude of consumer products ranging from cosmetic to industrial applications. Owing to the wide range of applications, flows of interest are often complex in nature; exhibiting both extensional and shear regions that can make modeling and prediction both challenging and valuable. Adding to the complexity, the micellar dynamics are continually changing, resulting in a number of interesting phenomena, such as shear banding and extensional flow instabilities. In this paper, we present the results of our investigation into the flow fields generated by a controllable and idealized porous media: a periodic array of cylinders. Our test channel geometry consists of six equally spaced cylinders, arranged perpendicular to the flow. By systematically varying the Deborah number, the flow kinematics, stability and pressure drop were measured. A combination of particle image velocimetry in conjunction with flush mount pressure transducers were used to characterize the flow, while flow induced birefringence measurements were used to determine micelle deformation and alignment. The pressure drop was found to decrease initially due to the shear thinning of the test fluid, and then exhibit a dramatic upturn as other elastic effects begin to dominate. We present evidence of the onset of an elastic instability in one of the test fluids above a critical Deborah number manifest in fluctuating transient pressure drop measurements and asymmetric streamlines. We argue that this disparity in the two test fluids can be attributed to the measurable differences in their extensional rheology.     

Rheo-optical investigations of lyotropic mesophases of polymeric surfactants

The shear orientation of hexagonal and lamellar liquid crystalline phases of polymeric surfactants was investigated by rheo-optical techniques (flow birefringence (Δn), small-angle light scattering) as well as by nuclear magnetic resonance and optical microscopy. The evolution of birefringence in the hexagonal phase is discussed for simple and oscillatory shear, and an alignment of rodlike micelles along the flow direction was found. A shear induced formation of vesicles (“onions”) is observed with the lamellar phase. They displayed a characteristic four-lobe pattern in depolarized light scattering. Above a critical shear stress vesicles were degraded and perpendicularly aligned lamellae (i.e. with their normal along the vorticity direction) were obtained. A comparison of experiments performed at constant stress and constant rate revealed that the vesicle to planar lamellae transition occurred above a critical shear stress. The behavior of the polysoap lyotropic mesophases under shear, i.e. the strain dependent alignment in the hexagonal phase, the shear induced formation of vesicles, and a transition to planar lamellae in the lamellar phase, is very similar to the behavior of lyotropic mesophases formed by low molar mass surfactants or amphiphilic block copolymers. The geometrical constraints that are introduced when amphiphilic side groups are fixed to a polymer backbone do not significantly alter the response of the mesophase to a shear deformation. Received: 4 May 1999 /Accepted: 19 July 1999     

Droplet deformation of a strongly shear thinning dense suspension of polymeric micelles

We investigated the deformation of a strong shear thinning droplet undergoing simple shear flow in a Newtonian liquid. The droplet was an aqueous solution of poly(ethylene oxide) end capped with an alkyl group that forms spherical micelles in aqueous solution. At high concentrations and below a critical temperature, the jammed micelles showed strong shear thinning behaviour, and neither a yield stress nor a Newtonian viscosity was observed. At small shear rates, the droplet rotated and aligned in the flow, but did not deform or only very weakly. At high shear rates, the droplet deformation increased with increasing shear rate. The deformed droplet did not relax after the shear was stopped except for a modest rounding of the edges. For each shear rate, an apparent viscosity, η _ad, of the equivalent Newtonian droplet was calculated assuming affine deformation. η _ad showed a power law dependence on the capillary number Ca with an exponent of − 1.8 and was larger than the shear viscosity of the micelle suspension at the same shear rates. The results were explained by the existence of a strong gradient of the viscosity inside the droplet leading to a very low viscosity fluid layer near the droplet/matrix interface.     

Instabilities of micellar systems under homogeneous and non-homogeneous flow conditions

The rheological behavior of a cetylpyridinium chloride 100 mmol l^–1/sodium salicylate 60 mmol l^–1 aqueous solution was studied in this work under homogeneous (cone and plate) and non-homogeneous flow conditions (vane-bob and capillary rheometers), respectively. Instabilities consistent with non-monotonic flow curves were observed in all cases and the solution exhibited similar behavior under the different flow conditions. Hysteresis and the sigmoidal flow curve suggested as characteristic of systems that show constitutive instabilities were observed when running cycles of increasing and decreasing stress or shear rate, respectively. This information, together with a detailed determination of steady states at shear stresses close to the onset of the instabilities, allowed one to show unequivocally that "top and bottom jumping" are the mechanisms to trigger the instabilities in this micellar system. It is shown in addition that there is not a true plateau region in between the "top and bottom jumping". Finally, the flow behavior beyond the upturn seemed to be unstable and was found accompanied by an apparent violation of the no-slip boundary condition.     

Wall slip of mayonnaises in viscometers

The shear flow of mayonnaise is generally characterized by an apparent yield stress, shear thinning in steady flow, stress overshoots upon inception of flow and other time-dependent effects. These observations are usually understood to be the result of structural rearrangement within the material. Additionally and separately, the possibility that emulsions may exhibit apparent wall slip on a microscopic scale at a solid-liquid boundary has been reported by some researchers. Thus, observed rheological behavior is likely to be the result of the interplay between these two phenomena. In the present work, it is demonstrated that when measurements are sought to be made on mayonnaise using rotational viscometers visible wall slip occurs, rendering such instruments ineffective for the purpose of making viscosity measurements even at shear rates as low as 10^–3s^–1. The factors that influence the onset and extent of slip are investigated with the help of parallel plate viscometers, and it is concluded that the observed “yielding” of mayonnaise is actually an artifact of the onset of macroscopic slip. Slip effects are also found in capillary flow but are ameliorated with increasing shear rate. To circumvent these problems, it is proposed that extensional viscometry be employed for determining the flow behavior of mayonnaises. Received: 18 August 1997 Accepted: 1 April 1998     

Rheo-optics of cationic surfactant micellar solutions with mixed aromatic counterions

The effects of different ratios of mixed aromatic counterions of sodium salicylate (NaSal) and sodium 4-ethylbenzenesulfonate (NaEBS) at a total counterion concentration of 0.125 M on the rheo-optical behaviors of 0.05 M cetyltrimethylammonium chloride (CTAC) cationic surfactant solutions were studied. The introduction of NaEBS into CTAC–NaSal solution led to a significant deviation from the Cox–Merz rule. Mixing of the two aromatic counterions lowered the CTAC solutions’ zero shear viscosities, increased their plateau shear modulus, and decreased their relaxation time. In addition, the persistence lengths of the threadlike micelles and the mesh size of the micellar network were also significantly reduced. These effects are believed to be due to increased degree of branching and denser meshes of the micellar networks in the mixed counterion systems.     

Surfactant systems for drag reduction: Physico-chemical properties and rheological behaviour

The first part of the work presents an overview of the physical chemistry of surfactants which in aqueous solutions reduce the frictional loss in turbulent pipe flow. It is shown that these surfactants form rodlike micelles above a characteristic concentraionc _t. The experimental evidence for rodlike micelles are reviewed and the prerequisites that the surfactant system must fulfill in order to form rodlike micelles are given. It is demonstrated by electrical conductivity measurements that the critical concentration for the formation of spherical micelles shows little temperature dependence, whereasc _t increases very rapidly with temperature. The length of the rodlike micelles, as determined by electric birefringence, decreases with rising temperature and increases with rising surfactant concentration. The dynamic processes in these micellar systems at rest and the influence of additives such as electrolytes and short chain alcohols are discussed.In the second part, the rheological behaviour of these surfactant solutions under laminar and turbulent flow conditions are investigated. Viscosity measurements in laminar pipe and Couette flow show the build-up of a shear induced viscoelastic state, SIS, from normal Newtonian fluid flow. A complete alignment of the rodlike micelles in the flow direction in the SIS was verified by flow birefringence. In turbulent pipe flow, drag reduction occurs in these surfactant systems as soon as rodlike micelles are present in the solution. The extent and type of drag reduction, i.e. the shape of the friction factor versus Reynolds number curve, depends directly on the size, number and surface charge of the rodlike micelles. The friction factor curve of each surfactant investigated changes in the same characteristic way as a function of temperature. For each surfactant, independent of concentration, an upper absolute temperature limit,T _L, for drag reduction exists which is caused by the micellar dynamics.T _L is influenced by the hydrophobic chain length and the counter-ion of the surfactant system. A first attempt is made to explain the drag reduction of surfactants by combining the results of these rheological measurements with the physico-chemical properties of the micellar systems.     

Conoscopic observations of shear-induced rotations in nematic liquid crystals

Orientational changes in monodomains of flow-aligning liquid crystals, 4-n-pentyl-4′-cyanobiphenyl and N-(4-methoxybenzylidene)-4-butylaniline, were studied during shear and recovery in a linear shearing device fitted to an optical microscope. Planar alignment (director in the shear plane) allows the study of twist effects and was generated by strong planar anchoring at the walls with orientations in a range of 0–90° with the shear direction. While being held back by the anchoring walls, shear caused the bulk director to rotate towards a steady-state alignment angle in the shear direction (Leslie angle θ_L). The transient director rotation was observed with conoscopy. It was found that increasing the initial alignment towards the vorticity direction increased the measured θ_L. Upon stopping the flow, the bulk director returned to its initial state. With initial alignment orientation changing from parallel to perpendicular to the flow direction, the rate of the twist-driven recovery process increases. This rate increase is not seen in the splay-driven recovery which is constant and consistently faster than twist-driven recovery at all orientations studied. Received: 10 December 1998/Accepted: 7 June 1999     

Shear-banding structure orientated in the vorticity direction observed for equimolar micellar solution

 The non-monotonic shear flow of a viscoelastic equimolar aqueous surfactant solution (cetylpyridinium chloride-sodium salicylate) is investigated rheologically and optically in a transparent strain-controlled Taylor Couette flow cell. As reported before, this particular wormlike micellar solution exhibits first a shear thinning and then a pronounced shear-thickening behavior. Once this shear-thickening regime is reached, a transient phase separation/shear banding of the solution into turbid and clear ring-like patterns orientated perpendicular to the vorticity axis, i.e., stacked like pancakes, is observed (Wheeler et al. 1998; Fischer 2000). The solution exhibit several unique features as no induction period of the shear induced phase, no structural build-up at the inner rotating cylinder, jumping pancake structure of clear and turbid ringlike phases, and oscillating shear stresses appear once the pancake structure is present. According to our analysis this flow phenomenon is not purely a mechanical or rheological driven hydrodynamic instability but one has to take into account structural changes of the oriented micellar aggregates (flow induced non-equilibrium phase transition) as proposed by several authors. Although this particular flow behavior and the underlying mixture of shear induced phases and mechanical instabilities is not fully understood yet, some classification characteristics based on a recent theoretical approach by Schmitt et al. (1995) and Porte et al. (1997) where a strong coupling between the flow instability (non-homogeneous flow profile due to the bands) and the structural changes causes the observed transient phenomena can be derived. In reference to the presented model the observed orientation of the rings is typical for complex fluids that undergo a spinodal phase separation coupled with a thermodynamic flow instability. In contrast to other shear banding phenomena, this one is observed in parallel plate, cone-plate, and Couette flow cell as well as under controlled stress and controlled rate conditions. Therefore, it adds an additional aspect to the present discussion on shear banding phenomena, i.e., the coupling of hydrodynamics and phase transition of rheological complex fluids. Received: 8 January 2001 Accepted: 15 May 2001     

Rheology and flow-birefringence from viscoelastic polymer-clay solutions

 The shear orientation of viscoelastic clay-polymer solutions was investigated by means of rheology and flow birefringence (Δn). The polymer chains are in dynamic adsorption/desorption equilibrium with the clay particles to form a “network”. The elastic behavior of the network was characterized by constant stress, oscillatory shear, and stress relaxation experiments. Constant stress experiments indicated a yield stress upon which shear flow started and no strain recovery could be observed. Oscillatory shear experiments showed a broad elastic region followed by flow when a critical strain was reached. Stress relaxation experiments showed several relaxation times when the same critical strain was reached. Experiments under steady flow characterized the transient behavior of the network. With increasing steady shear rate a pronounced minimum in birefringence was observed at a critical shear rate. The shear rate dependent viscosity showed near power law behavior and no corresponding critical feature. While birefringence detects orientational effects on a microscopic length scale, rheology averages over macroscopic changes in the sample. The same degree of orientation could be achieved under constant shear rate or constant stress conditions. Received: 25 January 2001 Accepted: 22 May 2001     

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

We explore the behavior of a wormlike micellar solution under both steady and large amplitude oscillatory shear (LAOS) in a cone–plate geometry through simultaneous bulk rheometry and localized velocimetric measurements. First, particle image velocimetry is used to show that the shear-banded profiles observed in steady shear are in qualitative agreement with previous results for flow in the cone–plate geometry. Then under LAOS, we observe the onset of shear-banded flow in the fluid as it is progressively deformed into the non-linear regime—this onset closely coincides with the appearance of higher harmonics in the periodic stress signal measured by the rheometer. These harmonics are quantified using the higher-order elastic and viscous Chebyshev coefficients e _n and v _n , which are shown to grow as the banding behavior becomes more pronounced. The high resolution of the velocimetric imaging system enables spatiotemporal variations in the structure of the banded flow to be observed in great detail. Specifically, we observe that at large strain amplitudes (γ ₀ ≥ 1), the fluid exhibits a three-banded velocity profile with a high shear rate band located in-between two lower shear rate bands adjacent to each wall. This band persists over the full cycle of the oscillation, resulting in no phase lag being observed between the appearance of the band and the driving strain amplitude. In addition to the kinematic measurements of shear banding, the methods used to prevent wall slip and edge irregularities are discussed in detail, and these methods are shown to have a measurable effect on the stability boundaries of the shear-banded flow.     

Linear and nonlinear rheology of micellar solutions in the isotropic,cubic and hexagonal phase probed by rheo-small-angle light scattering

Aqueous solutions of a branched nonionic surfactant were studied in the isotropic, cubic and hexagonal phase by means of rheological and small-angle light scattering (SALS) experiments. The isotropic phase behaved like a Newtonian liquid. An increase of activation energy of viscous flow was found near the overlap concentration of spherical micelles, but no shear thinning was observed. The viscosity of low concentrated samples increased slightly when the lower critical solution temperature was approached. This increase of viscosity was much smaller compared to common nonionic surfactants. The cubic phases behaved as elastic solids with a high plateau modulus, and shear melting occurred at high shear stresses. The hexagonal phase showed complex behavior. Shear orientation could be achieved by large amplitude oscillatory shear and was proved by rheo-small-angle light scattering. Two orientations were observed, at first perpendicular to the flow direction, i.e., log-rolling state and, secondly, an in-shear-plane orientation parallel to the flow direction. The linear viscoelastic region of the hexagonal phase was extremely small and was detected by simultaneous rheo-small angle light scattering. Shear alignment lead to a decrease of the moduli.