Volume 50, Issue 3, May 2006
Index of content:
50(2006); http://dx.doi.org/10.1122/1.2191076View Description Hide Description
The results from the development of a novel technique for detecting and monitoring load bearing, percolating networks in a dense suspension (concentrated suspension) sheared in a narrow gap are presented. The technique employs an electrical method which allows for much higher sampling rates than with traditional mechanical measurements and can therefore reveal the occurrence of shorter lived phenomena. The suspensions under investigation were solder pastes (used in electronics manufacturing) since they inherently lend themselves to this technique. Solder pastes consist of spherical metal alloy particles suspended in an insulating non-Newtonian fluid (flux). Particles are polydisperse and commonly have diameters in the range of . Volume fraction of solid is typically 45%–50%. The particles have a native insulating oxide layer that is removed by the mechanical action of neighboring particles, allowing conduction across the particle interfaces. The manner and rate at which conduction occurs therefore provides information on the magnitude of the contact forces between the particles. Transient conduction spikes measured as current passes through the particles detect the occurrence, magnitude, and duration (lifetime) of load bearing percolating networks.
Local determination of the constitutive law of a dense suspension of noncolloidal particles through magnetic resonance imaging50(2006); http://dx.doi.org/10.1122/1.2188528View Description Hide Description
We investigate the flowing behavior of dense suspensions of noncolloidal particles, by coupling macroscopic rheometric experiments and local velocity and concentration measurements through magnetic resonance imaging (MRI) techniques. We find that the flow is localized at low velocities, and that the material is inhomogeneous; the local laws inferred from macroscopic rheometric observations must then be reinterpreted in light of these local observations. We show that the short time response to a velocity step allows dense suspensions to be characterized locally: they have a purely viscous behavior, without any observable influence of granular friction. In the “jammed” zone, there may be a contact network, whereas in the sheared zone there are only hydrodynamic interactions: localization consists of a change in configuration at the grain scale. From the concentration and velocity profiles, we provide for the first time local measurements of the concentration dependence of viscosity, and find a Krieger-Dougherty law to apply. Shear-induced migration is almost instantaneous, in contrast to most other observations, and implies that the diffusion coefficients depend strongly on the concentration. We finally propose a simple constitutive law for dense suspensions, based on a purely viscous behavior, that accounts for all the macroscopic and local observations.
The effect of the shear-thickening transition of model colloidal spheres on the sign of and on the radial pressure profile in torsional shear flows50(2006); http://dx.doi.org/10.1122/1.2188567View Description Hide Description
A novel rheometer plate was used to measure radial pressure profiles during cone-and-plate and parallel-plate shearing flows of a concentrated colloidal dispersion of poly(methyl methacryalate) spheres suspended in dioctyl phthalate. There is a long history of using suspensions of this type as a model rheological system. The measuredpressure profile can be used to calculate and , and also provides a check on the flow field in the rheometer. At shear rates just below onset of shear thickening, our measurements show that is positive as predicted by Stokesian dynamics simulations of model Brownian hard spheres, but we are unable to determine the sign of . After the onset of thickening, we find that in both flow geometries the pressure increases sharply with radial position. This is in striking contrast to the pressure profiles ordinarily observed for viscoelastic liquids (with the exception of certain liquid crystal polymers), for which the pressure decreases with radial position. Under these conditions, the apparent values of and are both negative with , as predicted by the Stokesian dynamics simulations. However, the flow in the cone-and-plate rheometer may not be viscometric after the onset of shear thickening.
Development of extrudate distortions in poly(dimethyl siloxane) and its suspensions with rigid particles50(2006); http://dx.doi.org/10.1122/1.2184312View Description Hide Description
The developments of flow instabilities and the associated surface and bulk irregularities during the extrusion of a poly(dimethyl siloxane), PDMS, melt and its suspensions with spherical hollow glass particles (10% to 40% by volume) were investigated using both capillary and rectangular slit flows.Flow instabilities were pronounced in the extrusion of unfilled PDMS as manifested by surface and bulk distortions of extrudates over a wide range of shear rates. The range of apparent shear rates, in which such flow instabilities and the associated surface/bulk distortions of the extrudates were observed, decreased upon the incorporation of rigid particles, especially as the concentration of the particles increased. Flow instabilities and the distortions of the extrudates could largely be eliminated upon the incorporation of particles at 40% by volume to the PDMS melt. The geometry of the entry region from the reservoir into the die (taper angles of 15, 45, and 75°) did not generate any differences in the flow instability behavior.
50(2006); http://dx.doi.org/10.1122/1.2184127View Description Hide Description
Elongational viscosity and birefringence of two nearly monodisperse polystyrene melts with molar mass of (PS206k) and (PS465k), respectively, were measured simultaneously by Luap et al. [Rheol. Acta.45, 83–91 (2005)]. The samples did not follow the stress optical rule (SOR). Elongational viscosity data can be modeled quantitatively by the molecular stress function (MSF) model of Wagner et al. [J. Rheol.49, 1317–1327 (2005)], which is based on the assumption of a strain-dependent tube diameter and the interchain pressure term of Marrucci and Ianniruberto [Macromolecules37, 3934–3942 (2004)], and which is modified here to account for non-Gaussian chain extension using the Padé approximation of the inverse Langevin function. The tube diameter relaxation time scales with . While the transient elongational viscosity shows a small dependence on finite extensibility, the predicted steady-state elongational viscosity is not affected by non-Gaussian effects. The power-law exponent of the relation between steady-state elongational stress and Deborah number predicted is found to be 0.59 for PS465k, confirming a previous result of 0.6 for a similar molecular mass sample [Wagner et al. (2005)]. The power-law exponent is invariant with respect to temperature but slightly dependent on molar mass, thereby increasing with decreasing molar mass. Deviations from the SOR are described quantitatively by the MSF model by taking into account finite chain extensibility, and within the experimental window investigated, deviations from the SOR are predicted to be strain rate-, temperature-, and molar mass independent, in good agreement with experimental data.
Shear-induced structure change and flow-instability in start-up Couette flow of aqueous, wormlike micelle solution50(2006); http://dx.doi.org/10.1122/1.2193742View Description Hide Description
The occurrence of shear-induced structures (SIS) of a wormlike micellar solution is investigated using an aqueous solution of cethyltrimetylammonium bromide (CTAB)/aqueous sodium salicylate (NaSal) with composition CTAB . A concentric cylinder Couette cell made of transparent glass is used to generate stepwise start-up shear flow. The shear stress and the brightness, which is an index of the transparency of the fliud, are measured simultaneously and the fluid motion is observed by adding tracer particles to the fluid. Measurements of flowbirefringence and light intensity of a transmitted laser beam passing parallel to the vorticity axis are also carried out to monitor flow-induced microstructural changes. The transient behavior of the macroscopic properties, i.e., the shear stress and the brightness, are consistent with each other and with the optical microstructural measurements, confirming that the occurrence of the transparent-to-opaque change in the wormlike micellesolution is caused by a change in its structure. The transient behavior of SIS with the time is correlated to the shear strain and divided into four regions depending on the shear rate. At the highest shear-rate range, a strong instability occurs immediately following a maximum in the shear stress. Upon cessation of the shear stress, elastic recoil is observed along with a sudden recovery to the original transparent state of the solution.
50(2006); http://dx.doi.org/10.1122/1.2186982View Description Hide Description
Soft microgelpastes are modeled as a three-dimensional system of randomly packed elastic spheres. Simulations are performed wherein the packing is subject to isochoric uniaxial extension to compute the high- and low-frequency shear moduli for various packing fractions. The simulation results agree well with the data from experiments on crosslinked polymermicrogels. The low-frequency shear modulus is found to follow the osmotic pressure (the mean normal stress) closely, especially at high packing fractions. In addition, expressions involving the radial distribution function and the pair-wise Hertzian potential have been used to reproduce the high-frequency shear modulus and the osmotic pressure. The radial distribution functions for varying packing fractions are self-similar and after suitable coordinate transformations, all the first peaks collapse onto a single curve. The shift in the first peak and the peak width are related to the packing fraction through semiempirical scaling relations. Shearing introduces systematic changes in the pastemicrostructure and the final pair correlation function has an additional dependence on the angle from the extended axis.