Index of content:
Volume 49, Issue 4, July 2005
49(2005); http://dx.doi.org/10.1122/1.1940640View Description Hide Description
The history of the concentric cylinders apparatus for measuring the shear viscosity of liquids, and its attribution to Maurice Couette, have been explored. Examination of the Nineteenth Century literature has revealed that the concept goes back to Stokes and later Margules, the design and execution of the apparatus, apparently independently, to Perry, Couette, Mallock, and Schwedoff. Mallock’s and Schwedoff’s measurements were the most accurate and were within 1% of the viscosities derived from Poiseuille’s measurements on the basis of no slip at the tube walls and cylinder surfaces.Measurement of fluid viscosity was closely linked to the adoption of the no-slip boundary condition at solid-fluid interfaces.
49(2005); http://dx.doi.org/10.1122/1.1917841View Description Hide Description
In order to test the adaptability of a homogenization scheme originally developed for dry granular media, a micromechanical constitutive law for three-dimensional monodisperse wet foam is developed. The advantage of the homogenization scheme is that it provides a link between the discrete bubble-scale microstructure and the continuum quantities of stress and deformation. Furthermore, the resulting constitutive law is in terms of physical material properties (e.g., fluid viscosity, surface tension, and bubble radius) rather than the more phenomenological parameters, such as power-law indices, etc. Hence, predictions of the continuum model can be, and are, compared directly to experiments and simulations. Many of the general features of foam behavior are reproduced, including small-strain elasticity, dependence on gas fraction, and Bingham plastic type deformation rate dependency. However, the current micromechanical continuum model substantially overestimates the static shear modulus and yield stress of foams in comparison to simulations and experiments. This discrepancy is due to the adoption of a mean-field assumption for the deformation in the current model. It is then argued that future micromechanical continuum models for foams cannot neglect the nonaffine motions associated with bubble rearrangements. Finally, for both foams and granular materials, our study indicates that the uniform deformation assumption is only valid near the critical packing of monodisperse spherical particles.
49(2005); http://dx.doi.org/10.1122/1.1940641View Description Hide Description
This work describes a detailed study of the rheopectic effect in the flow of highly concentrated emulsions at low stresses. Experiments with the shear rate sweep demonstrated that the upward and downward branches of the flow curves coincide above some specific shear rate value. The upward experiments show the existence of the Newtonian part on the flow curve in the low-shear-rate domain, while the effect of yielding is observed in the downward curve. Restoration of the initial structure (and properties) after cessation of loading occurs very quickly. This allows associating the rheopexy with elastic deformations and the relaxation process (observed in frequency dependence of dynamic moduli) with characteristic times of about 0.03 s. Transient processes proceed in the range of shear deformation of the order of several units. Some quantitative measures of the rheopectic behavior are proposed and discussed. Normal stresses are constant in the low shear stress domain, but decrease sharply above the range of rheopectic behavior. The mechanisms of the observed effects are discussed. Temperature is not an important factor in the rheopectic behavior studied in this work.
Characterization of the viscoelastic behavior of complex fluids using the piezoelastic axial vibrator49(2005); http://dx.doi.org/10.1122/1.1917843View Description Hide Description
The piezoelectric axial vibrator (PAV) is a squeeze-flow rheometer working at frequencies between 1 and . It can be used to measure the storage modulus and the loss modulus of complex fluids in this frequency range. Using polymer solutions with known and it is shown that the PAV gives reliable mechanical spectra for frequencies between 10 and . The measurements done with the PAV are combined with a conventional mechanical rheometer and a set of torsional resonators (, 25, and ) to obtain and between and . Using this combination we present the first analysis of the viscoelasticity of an aqueous suspension of thermosensitive latex particles in this range of frequency. It is demonstrated that the combination of the three devices gives the entire mechanical spectra without resort to the time-temperature superposition principle.
49(2005); http://dx.doi.org/10.1122/1.1917846View Description Hide Description
We describe the use of cleated surfaces on parallel disk tools to quantitatively measure the rheological properties of diverse slip-prone fluids and soft materials. Densely packed protrusions ( cross section of length, apart) penetrate the slip layer, preventing significant flow between cleats. This creates a no-slip boundary below their tips, which serves as the sample gap boundary, in direct analogy to the parallel-plate geometry. This “cleat” geometry suppresses slip without application of significant normal force, imposes well-defined shear to enable absolute measurements and is compatible with small sample volumes. The geometry is validated in steady and oscillatory shear using a series of materials not prone to slip (Newtonian oils and an entangled polymer melt). The advantage of cleated tools over other slip-prevention methods is demonstrated using increasingly challenging materials—an emulsion (mayonnaise), a suspension (peanut butter), and a biological tissue (porcine vitreous humor).
49(2005); http://dx.doi.org/10.1122/1.1940638View Description Hide Description
We describe the linear viscoelastic response of dispersions of droplets of two biphenylcarbonitriles that exhibit both isotropic and liquid crystalline phases, -pentyl-4-biphenylcarbonitrile (5CB) and -octyl-4-biphenylcarbonitrile (8CB), in polydimethylsiloxane(PDMS). The Palierne emulsion model agrees well with the storage modulus data for 8CB in both isotropic and nematic phases, but the model overpredicts the magnitude of the interfacial relaxation in the 5CB dispersion, where the volume fraction of the dispersed phase required to fit the data is substantially smaller than the actual value. Similarly, the interfacial tension of 5CB against PDMS deduced from the Palierne fit shows much less temperature dependence than values measured with pendant drop tensiometry. The 8CB droplets are small, with a narrow size distribution, while the 5CB droplets are considerably larger and have a high degree of polydispersity. The basic mechanics of the Palierne theory appear to be applicable to nematic liquids, at least under conditions where the entropic elasticity is weak relative to the interfacial stress; the data suggest, however, that the theory is not applicable to smectic systems, and it needs to be used with considerable care with large droplets accompanied by a large degree of polydispersity.
49(2005); http://dx.doi.org/10.1122/1.1940639View Description Hide Description
A dispersion of -octyl-4-biphenylcarbonitrile (8CB) in a polydimethylsiloxane(PDMS) matrix exhibits a rheological response characteristic of a fractalgel when prepared at a temperature at which 8CB exists in the smectic phase. The dispersion remains gel-like throughout the smectic and nematic phases of 8CB, but collapses irreversibly to a fluid-like dispersed droplet morphology at the nematic-isotropic transition. The system appears to be kinetically trapped in one or the other morphology because of a large energy barrier between the bicontinuous gel, which is the stable morphology in the smectic regime, and the dispersed droplet, which is the stable morphology in the nematic regime; the energy barrier results from the homeotropic orientation of 8CB at the PDMS interface. The system thus provides an unusual opportunity to probe the consequences of very different length scales in the minor component in both the smectic and nematic liquid crystalline regimes. One such consequence is a controllable thousandfold change in the storage modulus of the blend.
Morphology development of a polystyrene/polymethylmethacrylate blend during start-up of uniaxial elongational flow49(2005); http://dx.doi.org/10.1122/1.1940642View Description Hide Description
The morphology development of a polystyrene/polymethylmethacrylate blend has been investigated during uniaxial elongational flow. For this purpose linear conservative dichroismmeasurements are used to gather time-resolved information and scanning electron microscopy is performed on quenched samples. With the former technique the evolution of the dropletdeformation in a semiconcentrated blend is studied in real time, whereas the latter technique was used for imaging individual droplets after the elongation. It is shown that the dropletdeformation is affine for the conditions under investigation and that it is possible to predict the evolution of the dichroism by existing theories and the assumption of affine deformation. It has also been verified whether rheological measurements can be used to deduce morphological information during elongational flow, as this method has been succesfully applied previously in simple shear flow.
On the gap error in parallel plate rheometry that arises from the presence of air when zeroing the gap49(2005); http://dx.doi.org/10.1122/1.1942501View Description Hide Description
When zeroing the gap between two parallel plates, the squeeze flow of air results in a detectable normal force at a finite height that is misinterpreted in rheometry as an indicator for the zero gap position. This leads to a substantial gap error using well-aligned parallel plates that is accurately predicted using Stefan’s equation for squeeze flow. Minimizing or accounting for this error enables accurate rheological measurements to be performed in torsional flow using the parallel plate geometry at narrow gap heights, thus allowing shear rates of over to be achieved.