Volume 58, Issue 6, November 2014
Index of content:
Rheological studies of thermotropic phase transitions in cationic vesicle suspensions: Instantaneous “jamming” and aging behavior58(2014); http://dx.doi.org/10.1122/1.4886175View Description Hide Description
Cationic double-tailed surfactants exhibit a rich thermotropic phase behavior. Here, we examine the effect of thermal gradients and processing history on the rheology and microstructure of concentrated multilamellar vesicle (MLV) suspensions made from a double-tailed cationic surfactant (diC18:0 DEEDMAC), whose bilayers are in the crystalline (solid) state at room temperature. The rheological properties of the MLV suspensions are found to be strongly dependent on the thermal behavior of the constituent bilayer with the visco-elastic moduli exhibiting a nonmonotonic variation with temperature, initially increasing by 1–2 orders of magnitude at an intermediate temperature, before rapidly decreasing at higher temperatures. Furthermore, when subject to a contraction flow through an extruder, above the main phase transition temperature of the bilayers, the suspensions instantaneously transform into a “jammed,” glassy-state at the extruder outlet. The glassy dispersions behave like stiff gel-like materials, having visco-elastic moduli that are several orders of magnitude higher than their unextruded counterparts. We probe mechanisms leading to the formation and subsequent aging of the jammed suspensions and show that the microstructural evolution of the extruded “gels” on aging is qualitatively different from that of the unextruded suspensions.
On the appearance of vorticity and gradient shear bands in wormlike micellar solutions of different CPCl/salt systems58(2014); http://dx.doi.org/10.1122/1.4887536View Description Hide Description
Wormlike micellar salt/surfactant solutions (X-salicylate, cetylpyridinium chloride) are studied with respect to the applied shear stress, concentration, temperature, and composition of the counterions (X = lithium, sodium, potassium, magnesium, and calcium) of the salicylate salt solute to determine vorticity and gradient shear bands. A combination of rheological measurements, laser technique, video analysis, and rheo-small-angle neutron scattering allow for a detailed exploration of number and types of shear bands. Typical flow curves of the solutions show Newtonian, shear-thinning, and shear-thickening flow behavior. In the shear-thickening regime, the solutions show vorticity and gradient shear bands simultaneously, in which vorticity shear bands dominate the visual effect, while gradient shear bands always coexist and predominate the rheological response. It is shown that gradient shear bands change their phases (turbid, clear) with the same frequency as the shear rate oscillates, whereas vorticity shear bands change their phases with half the frequency of the shear rate. Furthermore, we show that with increasing molecular mass of the counterions the number of gradient shear bands increases, while the number of vorticity shear bands remains constant. The variation of temperature, shear stress, concentration, and counterions results in a predictable change in the rheological behavior and therefore allows adjustment of the number of vorticity shear bands in the shear band regime.
58(2014); http://dx.doi.org/10.1122/1.4889902View Description Hide Description
A theoretical model of magnetoviscous effect in a suspension of nonBrownian linearly magnetizable particles is suggested. A simple shear flow in the presence of an external magnetic field aligned with the velocity gradient is considered. Under the action of the applied field, the particles are supposed to form dense highly elongated droplike aggregates. Two different scenarios of the aggregates' destruction under shearing forces are considered, namely, a “bulk” destruction of aggregates into pieces and an “erosive” destruction connected to the rupture of individual particles from the aggregate surface. Both models are based on a balance of forces acting either on the whole aggregate or on individual particles. The two approaches lead to qualitatively different Mason number (Ma) behaviors of the magnetic suspensions: The suspension viscosity scales as either Ma −2/3 for the bulk destruction of aggregates or Ma −4/5 for the erosive destruction. In any case, we do not recover Bingham behavior (Ma −1) often predicted by chain models of the magneto- or electrorheology. Our theoretical results are discussed in view of comparison with existing theories and experimental results in the wide range of Mason numbers.
58(2014); http://dx.doi.org/10.1122/1.4890747View Description Hide Description
Particles suspended in a Newtonian fluid raise the viscosity and also generally give rise to a shear-rate dependent rheology. In particular, pronounced shear thickening may be observed at large solid volume fractions. In a recent article [R. Seto et al., Phys. Rev. Lett. 111, 218301 (2013)], we have considered the minimum set of components to reproduce the experimentally observed shear thickening behavior, including discontinuous shear thickening. We have found frictional contact forces to be essential and were able to reproduce the experimental behavior by a simulation including this physical ingredient along with viscous lubrication. In the present article, we thoroughly investigate the effect of friction and express it in the framework of the jamming transition. The viscosity divergence at the jamming transition has been a well known phenomenon in suspension rheology, as reflected in many empirical laws for the viscosity. Friction can affect this divergence, and in particular the jamming packing fraction is reduced if particles are frictional. Within the physical description proposed here, shear thickening is a direct consequence of this effect: As the shear rate increases, friction is increasingly incorporated as more contacts form, leading to a transition from a mostly frictionless to a mostly frictional rheology. This result is significant because it shifts the emphasis from lubrication hydrodynamics and detailed microscopic interactions to geometry and steric constraints close to the jamming transition.
58(2014); http://dx.doi.org/10.1122/1.4891247View Description Hide Description
A direct comparative study on the creep-recovery behavior of conventional magnetorheological (MR) fluids is carried out using magnetorheometry and particle-level simulations. Two particle concentrations are investigated ( and ) at two different magnetic field strengths (53 and 173 kA·m−1) in order to match the yield stresses developed in both systems for easier comparison. Simulations are mostly started with random initial structures with some additional tests of using preassembled single chains in the low concentration case. Experimental and simulation data are in good qualitative agreement. The results demonstrate three regions in the creep curves: (i) In the initial viscoelastic region, the chainlike (at ) or percolated three-dimensional network (at structures fill up the gap and the average cluster size remains constant; (ii) Above a critical strain of 0.1 (10%), in the retardation region, these structures begin to break and rearrange under shear. At large enough imposed stress values, they transform into thin sheetlike or thick lamellar structures, depending on the particle concentration; (iii) Finally in the case of larger strain values either the viscosity diverges (at low stress values) or reaches a constant low value (at high stress values), showing a clear bifurcation behavior. For stresses below the bifurcation point, the MR fluid is capable to recover the strain by a certain fraction. However, no recovery is observed for large stress values.
A fractional K-BKZ constitutive formulation for describing the nonlinear rheology of multiscale complex fluids58(2014); http://dx.doi.org/10.1122/1.4892114View Description Hide Description
The relaxation processes of a wide variety of soft materials frequently contain one or more broad regions of power-law like or stretched exponential relaxation in time and frequency. Fractional constitutive equations have been shown to be excellent models for capturing the linear viscoelastic behavior of such materials, and their relaxation modulus can be quantitatively described very generally in terms of a Mittag–Leffler function. However, these fractional constitutive models cannot describe the nonlinear behavior of such power-law materials. We use the example of Xanthan gum to show how predictions of nonlinear viscometric properties, such as shear-thinning in the viscosity and in the first normal stress coefficient, can be quantitatively described in terms a nonlinear fractional constitutive model. We adopt an integral K-BKZ framework and suitably modify it for power-law materials exhibiting Mittag–Leffler type relaxation dynamics at small strains. Only one additional parameter is needed to predict nonlinear rheology, which is introduced through an experimentally measured damping function. Empirical rules such as the Cox–Merz rule and Gleissle mirror relations are frequently used to estimate the nonlinear response of complex fluids from linear rheological data. We use the fractional model framework to assess the performance of such heuristic rules and quantify the systematic offsets, or shift factors, that can be observed between experimental data and the predicted nonlinear response. We also demonstrate how an appropriate choice of fractional constitutive model and damping function results in a nonlinear viscoelastic constitutive model that predicts a flow curve identical to the elastic Herschel-Bulkley model. This new constitutive equation satisfies the Rutgers-Delaware rule, which is appropriate for yielding materials. This K-BKZ framework can be used to generate canonical three-element mechanical models that provide nonlinear viscoelastic generalizations of other empirical inelastic models such as the Cross model. In addition to describing nonlinear viscometric responses, we are also able to provide accurate expressions for the linear viscoelastic behavior of complex materials that exhibit strongly shear-thinning Cross-type or Carreau-type flow curves. The findings in this work provide a coherent and quantitative way of translating between the linear and nonlinear rheology of multiscale materials, using a constitutive modeling approach that involves only a few material parameters.
58(2014); http://dx.doi.org/10.1122/1.4892426View Description Hide Description
The shear-thickening behavior of reversibly cross-linked guar network is studied using rheological and particle imaging velocity measurements. New evidence suggests that both shear-induced increase in crosslink density and non-Gaussian chain stretching are possible mechanisms for shear thickening. Which mechanism plays a predominant role depends on the applied shear rate and shear time. At not too much larger than , where is the network relaxation time, shear thickening is mainly caused by the increase in crosslink density. At higher shear rates, shear thickening is initiated by the increase in chain density at short times, and non-Gaussian chain stretching occurs at longer times. It is demonstrated that the linear elastic modulus measured for the shear-thickening state and its relaxation time can be used to discriminate between non-Gaussian chain stretching and shear-induced crosslinking mechanisms. The detection of a linear step strain regime where the measured modulus does not change with the strain amplitude indicates the absence of non-Gaussian chain stretch. When chains are stretched into the non-Gaussian regime, the relaxation time becomes smaller whereas relaxation time remains unchanged if only crosslink density increases. At high shear rates, flow may become unstable with bulk fracture, shear banding, and continuous flow occurring randomly as revealed by the velocity profile across the flow cell gap.
Bubble migration in two-dimensional foam sheared in a wide-gap Couette device: Effects of non-Newtonian rheology58(2014); http://dx.doi.org/10.1122/1.4892660View Description Hide Description
We report experiments on the migration of a large bubble in an otherwise monodisperse two-dimensional (2D) foam sheared in a wide-gap Couette device. The bubble migrates away from the walls toward an equilibrium position between the center of the gap and the inner cylinder. This differs from the situation in a narrow-gap Couette device, where the equilibrium position is at the center of the gap [Mohammadigoushki and Feng, Phys. Rev. Lett. 109, 084502 (2012)]. The shift in equilibrium position is attributed to the non-Newtonian rheology of the foam, which is brought out by the nonhomogeneous shearing in a wide-gap geometry. Two aspects of the rheology, shear-thinning and the first normal stress difference, are examined separately by comparing with bubble migration in a xanthan gum solution and a Boger fluid. Shear-thinning shifts the equilibrium position inward while the normal stress does the opposite. Bubble migration in the 2D foam is the outcome of the competition between the two effects.
58(2014); http://dx.doi.org/10.1122/1.4893586View Description Hide Description
In this work, we have studied the magnetorheological (MR) fluid rheology in the magnetic field parallel to the fluid vorticity. Experimentally, the MR fluid flow was realized in the Couette coaxial cylinder geometry with the magnetic field parallel to the symmetry axis. The rheological measurements were compared to those obtained in the cone-plate geometry with the magnetic field perpendicular to the lower rheometer plate. Experiments revealed a quasi-Bingham behavior in both geometries with the stress level being just a few dozens of percent smaller in the Couette cylindrical geometry at the same internal magnetic field. The unexpectedly high MR response in the magnetic field parallel to the fluid vorticity is explained by stochastic fluctuations of positions and orientations of the particle aggregates. These fluctuations are induced by magnetic interactions between them. Once misaligned from the vorticity direction, the aggregates generate a high stress independent of the shear rate, and thus assimilated to the suspension apparent (dynamic) yield stress. Quantitatively, the fluctuations of the aggregate orientation are modeled as a rotary diffusion process with a diffusion constant proportional to the mean square interaction torque. The model gives a satisfactory agreement with the experimental field dependency of the apparent yield stress and confirms the nearly quadratic concentration dependency , revealed in experiments. The practical interest of this study lies in the development of MR smart devices with the magnetic field nonperpendicular to the channel walls.