Volume 55, Issue 2, March 2011
Index of content:
55(2011); http://dx.doi.org/10.1122/1.3526349View Description Hide Description
We study the local flowproperties of various materials in a vane-in-cup geometry. We use magnetic resonance imaging techniques to measurevelocities and particle concentrations in flowing Newtonian fluid, yield stress fluid, and in a concentrated suspension of noncolloidal particles in a yield stress fluid. In the Newtonian fluid, we observe that the -averaged strain rate component decreases as the inverse squared radius in the gap, in agreement with a Couette analogy. This allows direct comparison (without end-effect corrections) of the resistances to shear in vane and Couette geometries. Here, the mean shear stress in the vane-in-cup geometry is slightly lower than in a Couette cell of same dimensions, and a little higher than when the vane is embedded in an infinite medium. We also observe that the flow enters deeply the region between the blades, leading to significant extensional flow. In the yield stress fluid, in contrast with the usually accepted picture based on simulation results from the literature, we find that the layer of material that is sheared near the blades at low velocity is not cylindrical. There is thus a significant extensional component of shear that should be taken into account in the analysis. Finally and surprisingly, in the suspension, we observe that a thin non-cylindrical slip layer made of the pure interstitial yield stress fluid appears quickly at the interface between the sheared material and the material that moves as a rigid body between the blades. This feature can be attributed to the non-symmetric trajectories of the noncolloidal particles around the edges of the blades. This new important observation is in sharp contradiction with the common belief that the vane tool prevents slippage and may preclude the use of the vane tool for studying the flows of pasty materials with large particles.
55(2011); http://dx.doi.org/10.1122/1.3528042View Description Hide Description
Accurately controlled step planar elongation flow generated by a two-dimensional squeezing flow cell was applied to wormlike micellar solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) in water. Opaque regions were observed in the center plane and at the corners of the flow field during squeeze flow. The opaque region in the center plane is caused by planar elongation flow, resulting in elongation-induced structure (EIS), while the corner opaque regions are mainly induced by shear, generating shear-induced structure (SIS). SIS at the corners appeared earlier than EIS in the center plane in the squeezing process, but the strain required to generate EIS is considerably smaller than strain for SIS. In the case of high elongation rate and large strain, flow fluctuations appeared after EIS and SIS occurred. The appearance of both EIS and SIS are related to strain, the Weissenberg number, the Deborah number, and the concentration ratio of CTAB and NaSal in the solution but independent of the initial structure of the micelles in solution.
55(2011); http://dx.doi.org/10.1122/1.3528651View Description Hide Description
Room temperature ionic liquids (ILs) are a class of complex fluids with valuable physical properties. Therefore, the investigation of dynamics of IL is vital for the fundamental understanding of their physical nature as well as for the successful engineering design in their applications. We succeeded in performing the first in-depth rheological studies of two ionic liquids of different nature in a wide frequency and temperature range (up to the glass transition and beyond), utilizing advanced techniques with an oversampling procedure and with adjustment of the instrument compliance. The important physical parameters of IL, such as Vogel temperatures, fragility, fractional free volume at the glass transition temperature, and volume expansion coefficient, were determined by rheology. Deviation from the Vogel–Fulcher–Tammann behavior is detected in the vicinity of the glass transition. The master curves of dynamic moduli for both IL show systematic deviations from Maxwell behavior in the low and high frequency domains. The calculated spectra exhibit a broad relaxation time distribution, which can be approximated by two non-Maxwellian modes. We attribute the fast mode to the motion of single ions, while the slow mode might reflect the cooperative motion of ions. Our results are supported by recent dielectric and optical studies, theoretical models, and simulations, which reported dynamical heterogeneities in IL.
Transient solutions of nonlinear dynamics in film blowing process accompanied by on-line crystallization55(2011); http://dx.doi.org/10.1122/1.3532091View Description Hide Description
The nonlinear dynamics in film blowing process is investigated in this study solving the governing equations of the system, which include the dynamics of crystallization occurring on the film, defined over the entire distance from the die exit to the nip roll in a single region for transient (and steady-state) solutions. The present study does not assume a priori the bubble radius at freezeline height to have the zero slope with respect to the axial coordinate as the boundary condition of the governing equations of the system. Instead, the governing equations yield this result as part of the transient solution of the partial differential equations. Aside from this, the transient solutions reported in this study also reveal some fundamental breakthroughs over previous results even during the severe periodic oscillation of instability called draw resonance: For example, the oscillatory temporal curves of the bubble radius produced by simulation during the draw resonance instability accurately exhibit the skewed characteristics, and the inflection points in the curves, and also agree well with the minima points as observed in experimental data. Additionally, there is a notable improvement on the stability diagrams by the new model, eliminating the fictitious stability region predicted by the previous simulation model.
55(2011); http://dx.doi.org/10.1122/1.3532979View Description Hide Description
Soft materials, such as gels and colloidalglasses, often exhibit different rheological properties at bulk and microscopic scales as a result of their complex microstructure. This phenomenon has recently been demonstrated for a gel-forming aqueous dispersion of Laponite® clay [Oppong et al.Phys. Rev. E78, 021405 (2008)]. For this material, microrheology reveals a significantly weaker gel and a longer gelation time than bulk measurements. By performing multiple particle tracking microrheology experiments with different probe sizes, we show that length-scale–dependent rheology is a general feature of Laponite®gels. Small changes in probe size are accompanied by order of magnitude differences in the observed rheological properties and gelation time. The probe dynamics also exhibit size-dependent spatial heterogeneities that help to elucidate a microstructural length scale in the system. Through analytical theory and Brownian dynamics simulations, we find that the correlations described by previous authors between successive displacements of individual probes are more directly a result of materialelasticity than of microstructural confinement. The apparent gelation times of dispersions with different Laponite® concentrations exhibit a self-similar dependence on probe size, suggesting a superposition of Laponite® concentration and probe size. From these observations, we propose an accordant description of the microstructural evolution of the gel.
A study of the relationship between the rheo-dielectric effect and the elasticity of viscoelastic materials55(2011); http://dx.doi.org/10.1122/1.3539654View Description Hide Description
Dielectrostriction is a rheo-dielectric phenomenon that relates the variation of dielectric properties of a material to deformation. For an initially isotropic material, two independent material parameters, called the strain-dielectric coefficients, and , are required to describe dielectrostriction in terms of strain. Deformation affects a material’s dielectric properties in two ways: (a) by introducing anisotropy in the material, which is characterized by , and (b) by changing the volume density of the polarizable species, which is associated with . Purely viscous fluids remain isotropic during any flow-induced deformation, and therefore the coefficient is always zero. In this paper, the dielectrostriction effect is studied in viscoelastic materials that are formulated to possess different degrees of elasticity. A special planar capacitance sensor rosette was employed to measure the coefficients and for these viscoelastic materials. The relationship between the material’s elasticity and the coefficient is discussed, together with some potential applications based on this relationship in the conclusion.
55(2011); http://dx.doi.org/10.1122/1.3538092View Description Hide Description
In this study we explored the rheological characteristics of Carbopol C934 gel (polyacrylic acid) containing SUS304 spherical nanoparticles of 100 nm as a simulant of gel propellants containing metal fuels. In comparison with the pure Carbopol gel, the SUS nanoparticle filled Carbopol gel exhibited stronger shear thinning and higher yield stress. As the concentration of nanoparticles increased yield stress increased, but viscosity and storage modulus increased first and then decreased abruptly beyond the critical limit. Also as the concentration of nanoparticles increased there was a transition in materialcharacteristics from the ductile type to the brittle type, which means that highly filled Carbopol gels lost the structure almost instantaneously as the imposed stress was larger than the yield stress, while Carbopol gels of low particle loading sustained the structure even after the imposed stress was larger than the yield stress. The cryogenic scanning electron microscopy analysis revealed that the network structure changed abruptly when the rheological properties changed abruptly. The change in gel structure is attributed to the nanoparticles that compete with Carbopol chains in forming networks. The abrupt change in gel structure with the addition of particles beyond the critical limit should be an exclusive phenomenon of nanoparticles.
Effects of molecular weight and volume fraction on rheological properties of PDMS-grafted alumina in PDMS melts55(2011); http://dx.doi.org/10.1122/1.3539999View Description Hide Description
To enhance their properties, melt processed polymers are commonly filled with colloidal particles. Dispersing particles homogeneously in a melt is generally difficult, particularly with dense inorganic oxides that generate strong van der Waals attractions. These attractions can be modulated by introducing repulsive forces through surface modifications such as polymer grafting. Indeed, the relative viscosity of 430 nm alumina particles stabilized by end-tethered poly(dimethylsiloxane) (PDMS) in PDMS melts decreases with increasing graft density and molecular weight (50 or 31.4 kg/mol) as expected, but also with increasing molecular weight of the melt in the range of 4.7–41.1 kg/mol. This is surprising, as well established theory predicts the grafted layer to be more swollen and, therefore, a better stabilizer in lower molecular weight melts. The answer is found in other studies showing that PDMS melts adsorb on alumina, providing a steric barrier that increases with melt molecular weight. A simple two-parameter correlation fits the shear-rate dependent viscosities with a relative high shear viscosity and a structural relaxation time that capture the hydrodynamic volume and the interparticle forces, respectively. The volume fraction dependence of the high shear viscosity indicates the degree of permanent clustering. The measuredrelaxation time can be correlated reasonably well with a characteristic relaxation time estimated by balancing the viscous forces in the gap against the van der Waals forces, both of which depend on a characteristic separation reflecting the thickness of the combined adsorbed and grafted layer.
Smooth full field reconstruction of velocity and its gradients from noisy scattered velocimetry data in a cross-slot flow55(2011); http://dx.doi.org/10.1122/1.3539986View Description Hide Description
We present a method combining generalized Tikhonov regularization with a finite element approximation for reconstructing smooth velocity and velocity gradient fields from spatially scattered and noisy velocity data in a two-dimensional complex flow domain. Synthetic velocity data for a cross-slot geometry are generated using the Oldroyd-B solution, subsequently perturbed by random noise. Performances of diverse finite element continuity-regularization criterion combinations are tested against noise-free data, while the optimum regularization parameter is determined using generalized cross-validation. The best performance is achieved for the velocity field and its gradients simultaneously by continuous Hermite finite elements and minimization of a norm of the velocity’s third derivative. The standard regularization criterion based on the second derivative is shown to lead to systematic distortions in boundary regions, allowing therefore a lower reduction in the statistical error. Furthermore, optical fields are calculated by applying a differential constitutive equation directly to the reconstructed flowkinematics; high quality velocity gradient fields are shown to be an essential prerequisite for their reliable prediction. Overall, the method is expedient to implement and does not require boundary conditions.
55(2011); http://dx.doi.org/10.1122/1.3544590View Description Hide Description
Block copolymers based on acrylate matrices are expected to be good candidates for pressure sensitive adhesives formulations because (i) all-acrylic block copolymers are known to be stable over a wide temperature range and are more resistant to UV radiation than polydiene copolymers and (ii) the soft block exhibits an elasticity that is lower than that of polydiene and closer to the Dalhquist criterion (tacky properties). This last property is due to a higher molar mass between entanglements. The block segregation is the key parameter for these kinds of systems because it induces a solid-like behavior useful for adhesion applications. In this work, we have explored this point by synthesizing styrene/-butyl acrylate (nBA) and methyl methacrylate/nBA block copolymers by nitroxide-mediated controlled radical polymerization.Atomic force microscopy and small-angle neutron scattering analysis have demonstrated the strong connection between the physico-chemical properties (molar mass, nature of the blocks, , dispersity, etc.), the microstructure, and the rheological properties of these block copolymers. Furthermore, the rheological behavior expected for good pressure sensitive adhesives performances is obtained under specific conditions. Finally, we will discuss the adherence performances of model formulations derived from these block copolymers.
55(2011); http://dx.doi.org/10.1122/1.3545844View Description Hide Description
A phenomenological model for flow-enhanced nucleation in crystallizingpolymers is developed and validated by short-term shear experiments [Housmans et al., Macromolecules42, 5728–5740 (2009); Hristova et al., Proceedings of the 228th ACS National Meeting, Philadelphia (2004)]. The model extends earlier work on flow-induced oriented crystallization [Custódio et al., Macromol. Theory Simul.18, 469–494 (2009); Peters et al., Macromol. Symp.185, 277–292 (2002); Peters, PolymerCrystallization: Observations, Concepts and Interpretations, edited by G. Reiter and J.-U. Sommer (Springer, Berlin, 2003), pp. 312–324; Zuidema, Ph.D. thesis, Technische Universiteit Eindhoven (2000); Zuidema et al., Macromol. Theory Simul.10, 447–460 (2001)] to flow-enhanced pointlike nucleation, which can lead to number densities of spherulites multiplied by orders of magnitude. Excellent agreement between simulations and experimental data is obtained, for a range of rates and durations of shearing, with only two adjustable parameters: a prefactor to the creation rate of flow-induced nucleation precursors and a parameter that governs their influence on the relaxation dynamics of the high-molecular weight (HMW) fraction of the melt. The two main conclusions of this paper are, first, that the creation of flow-induced precursors is driven by the average stretch, not by the average orientation, of the primitive paths of chains in the HMW tail of the molecular weight distribution, and second, that nucleation of these precursors is impeded by flow.
A sequence of physical processes determined and quantified in LAOS: Application to a yield stress fluid55(2011); http://dx.doi.org/10.1122/1.3544591View Description Hide Description
Recently, large-amplitude oscillatory shear has been studied in great detail with emphasis on its impact on the material response. Here we present a conceptually different, robust methodology based on viewing the stress waveforms as representing a sequence of physical processes. This novel approach provides the viscous and elastic contributions while overcoming the problems with infinite series encountered by Fourier transformation. Application to a soft colloidal star glass leads to the unambiguous determination and quantification of rate-dependent static and dynamic yield stresses, the rationalization of the response to strain sweeps and the post-yield regime by introducing the apparent cage modulus, and a connection to the steady-shear stress, all from a single-amplitude experiment. We propose that this approach is generic, but focus in this contribution only on a yield stressmaterial which exhibits repeating cycles of (i) elastic extension, (ii) yielding, (iii) flow, and (iv) reformation. We show that this approach is qualitatively consistent with the Fourier–Chebyshev analysis.