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Journal of RheologyVolume 42, Issue 6 (November-December 1998) |
Contents
Rheological Phenomena Occurring During the Shearing Flow of Mayonnaise J. A. Goshawk* and D. M. Binding† D. B. Kell and R. Goodacre *Present address: Westlakes Scientific Consulting † Corresponding authorAbstractA rheometrical study reveals that mayonnaise exhibits apparent wall slip in both small amplitude oscillatory shear flow and torsional flow. Whereas in the case of small amplitude oscillatory shear classical techniques are applicable for extracting bulk material properties, this is not the case for torsional flow. Progressively increasing the shear stress in torsional flow manifests another flow phenomenon that dominates the slip process. It is argued that "particles" within the mayonnaise migrate radially towards the centre of the geometry. Spectroscopic techniques are employed to confirm that, following an experiment, material at the rim of the geometry differs from that towards the centre. Experiments performed in a cone-and-plate geometry yield results that are qualitatively similar to those obtained in torsional flow tests thus indicating that the migration is not due solely to the radial shear-rate gradient that is present in torsional flow. Some Mathematical Issues In the Modeling of Flow Phenomena of Polymeric Liquid Crystals M. Carme Calderer and Bagisa Mukherjee Department of Mathematics and Center for Materials Physics AbstractWe consider shear flow of polymeric liquid crystals for large and small values of the Ericksen number e. The model consists of governing equations for the velocity field v, the pressure p, the director n, and the order parameter s. The constitutive functions for the Leslie coefficients, ai, derived from the molecular theory of Doi play a crucial role in the modeling. One of the goals of the analysis is to examine the role of s in describing singularities as well as in obtaining new regimes which cannot be predicted by the previous Leslie-Ericksen model. In particular, solutions are obtained that correspond to domain structures parallel to the flow. The domains are separated by singular lines across which the director experiences jumps of ±45 degrees. Such flows correspond to regimes with large values of e. Another type of configurations analyzed in the article correspond to periodic stripes parallel to the flow and such that the optic fields vary along the direction perpendicular to the plane of shear. Such configurations occur in the limit of small e.Influence of Elastic Properties on Drop Deformation and Breakup in Shear Flow F. Mighri, P. J. Carreau* and A. Ajji** Centre de Recherche Appliquée sur les Polymères, CRASP ** Industrial Materials Institute, National Research Council Canada *Corresponding author AbstractWe report in this paper experimental results on the deformation and the critical breakup conditions of a single drop in a medium under simple shear flow. The role played by both drop and matrix elasticities is quantified by using constant viscosity elastic (Boger) fluids. The experiments were conducted using two transparent parallel disks mounted on a R-18 Weissenberg rheogoniometer. The critical shear rate was determined by imposing successive small changes in shear rate from low to higher values until the drop breakup was observed. The results show remarkable differences in the mode of deformation and breakup for Newtonian and elastic fluid systems. It is also found that the drop resistance to deformation and breakup increases with increasing elasticity ratio. The contribution of the drop and matrix elasticities is quantified by using an empirical relation established between the drop deformation and the capillary number, Ca. The critical breakup conditions, such as a dimensionless breakup time, t*b, and a critical capillary number, Cac, are determined as a function of the drop/matrix elasticity ratio, k' . The values of Cac and t*b are found to increase with increasing k'. Yield Stress Measurements of Aqueous Foams in the Dry Limit B. S. Gardiner, B. Z. Dlugogorski*, and G.
J. Jameson R. P. Chhabra *Corresponding author E-mail: cgbzd@alinga.newcastle.edu.au AbstractThis paper reports measurements of yield stress of aqueous foams approaching the dry foam limit using a pendulum device. Traditionally, the vane rheometer has been used to measure the yield stress in liquids that exhibit wall slip. However, using the simple and inexpensive pendulum technique, shear rates many orders of magnitudes lower can be achieved. The pendulum was used to observe the change in yield stress for the foam as the gas fraction and bubble size increased. The local gas fraction in the foam was found by measuring the sonic velocity, and the bubble size was determined photographically. Strong support is found for the existence of a true yield stress in aqueous foams at the dry foam limit. Yield stress results, once scaled by s/<R>, agree well with data from previous studies.A Model for Slip at Polymer/Solid Interfaces Alexander L. Yarin1,2 and Michael D. Graham2 1 Faculty of Mechanical EngineeringTechnion-Israel Institute of Technology, Haifa 32000, Israel 2 Department of Chemical Engineering and Rheology Research CenterUniversity of Wisconsin-Madison, Madison, Wisconsin 53706-1691 USA AbstractA steady-state dependence of slip velocity on shear stress at polymer/solid surfaces is derived from a molecular model that includes effects of drag on polymer chains, disentanglement, and detachment and reattachment of chains from the solid surface. The dependence includes two turning points and thus a region where slip velocity is a multivalued function of shear stress. This behavior is consistent with experimental observations of abrupt and hysteretic transitions between regimes of small and large slip. Parameters involved in the model are estimated and a reasonable comparison with experiment is obtained. Non-Linear Shear and Extensional Rheology of Long-Chain Randomly Branched Polybutadiene Leo J. Kasehagen† and
Christopher W. Macosko* †Present address: Elf Atochem N.A., Inc. AbstractWe present non-linear shear and uniaxial extensional measurements on a series of polybutadienes with varying amounts of long-chain, random branching. Start-up of steady shear experiments are used to evaluate the damping function of the melts. The damping function is found to show a trend toward decreased dependency on strain with increasing branching content. Interior chains, which are believed to be responsible for changing the damping function, are calculated to comprise less than 3 wt% of the melt. Extensional measurements are used to investigate the role of branching in strain hardening. We show that samples with increased branch contents do show larger deviations of the transient Trouton ratio from the linear viscoelastic limit of three. However, we also show that the extensional data can be fit using parameters determined solely by the shear measurements. Furthermore, we show that the changes in the damping function seen in shear have little impact on extensional behavior. The extensional behavior of the melt is found to be most affected by changes in the relaxation spectra which can result from both branching and increases in the high end of the molecular weight distribution. This statement runs contrary to the often expressed view that strain hardening behavior in extension is exclusively produced by branching. Shear Thickening As A Consequence of An Acoustic Resonance in Sheared Colloidal Crystals Joachim Kaldasch, Jozua Laven*, and Hans N.
Stein *Corresponding Author AbstractA model is presented that predicts the critical shear rate of shear thickening of soft sphere colloidal suspensions. It is based on the idea that shear in a colloidal crystal leads to a periodic variation of the elastic modulus with time. At a specific shear rate an acoustic resonance occurs which leads to an increase of the viscosity. Good agreement with experimental results could be obtained by fitting the single parameter of the model. The Effect of Molecular Mass and Temperature on Slip of Polystyrene Melts at Low Stress Levels Michael E. Mackay* and David J. Henson *Corresponding author E-mail: m.mackay@cheque.uq.oz.au AbstractThe slip of monodisperse polystyrene melts next to a solid, inhomogeneous, metal substrate (stainless steel) is measured at small stress levels for a variety of temperatures. A critical stress, below which no-slip occurs, is not seen and the polystyrene melts used here slip at all stress levels. The slip velocity is quantified by the slip length (b, equal to the slip velocity divided by the shear rate) and friction coefficient (k, equal to the slip velocity divided by the shear stress). The slip length shows complicated dependence with both the molecular mass and temperature, however, when converted to the friction coefficient a master curve with molecular mass results for temperatures above 170 °C. The data are compared to contemporary theories for slip. It is concluded that none of the present theories accurately represent the data and that the number of adsorbed molecules are in a dynamic equilibrium which affects the slip behavior. Analysis of the force on the adsorbed molecules during shear demonstrates this may cause adhesive failure which contributes to the slip. Thixotropic Behavior of Clay Dispersions: Combinations of Scattering and Rheometric Techniques Frédéric Pignon, Albert Magnin, and
Jean-Michel Piau *Université Joseph Fourier Grenoble I, AbstractThe thixotropic behavior of a colloidal dispersion of clay consisting of disc-shaped particles was studied by means of a combination of rheometric measurements, static light scattering and small-angle neutron scattering At rest, the structure of the gel consists of dense micrometer-sized aggregates assembled into a fractal mass of dimension D. Under shear, in the case of volume fractions with (1 £ D £ 1.2) at rest, a butterfly-type light scattering pattern is observed. This is attributed to the formation of rollers within the dispersion, which align themselves on average perpendicular to the direction of shearing. This produces a fall in resistance to flow and in viscosity. The influence of shear rate on this disaggregation process was studied and linked to the rheometric measurements. In shear flow conditions, the fall in viscosity is due to orientation and disaggregation processes occurring at length scales of the order of a micrometer. During recovery, the two time scales identified correspond respectively to a rapid relaxation of the particle orientations and a slow aggregation process. Thixotropic behavior is identified as being mainly a reversible aggregation process, for which large length scales of the order of a micrometer associated with a fractal arrangement play a decisive role.Deformation and Break-Up Mechanisms of Single Drops during Shear Vassilios T. Tsakalos, Patrick Navard, and
Edith Peuvrel-Disdier* *Corresponding author AbstractThe behavior of single drops of isotropic hydroxypropylcellulose solutions immersed in a polydimethylsiloxane matrix subjected to a constant shear rate was investigated using a rheo-optical technique. Our main results concern the non-stationary deformation and break-up mechanisms of drops characterized by large capillary numbers. The deformation of threads follows a pseudo-affine deformation for Ca/Cacrit larger than 2.5 and above a certain strain. The end pinching mechanism occurs at a specific scaled strain, which includes the initial drop size and the applied shear rate. Capillary instabilities develop when the thread diameter reaches a critical value, dr, inversely proportional to the applied shear rate and independent of the initial drop diameter. The time necessary for the total rupture of a thread depends on the initial drop diameter, the applied shear rate and the critical thread diameter, dr. The droplet size resulting from the final rupture of a thread is found to be half that of the critical size, below which a drop deforms into a stable ellipsoid. Shear-Induced Particle Migration in a Polydisperse Concentrated Suspension Anat Shauly, Amir Wachs, and Avinoam Nir AbstractThe shear-induced particle migration in a polydisperse concentrated suspension is described using migration potentials for the various particle size fractions. The model is applied to solve the flow patterns and the particle concentration distributions in bidisperse suspensions in various viscometric flows. For the case of a continuous particle size distribution a formulation of this model in terms of concentration distribution moments is presented. The model predicts the total migration of the particles in the various flow devices and the size segregation of the various fractions. The calculations capture well the few experimental observations reported so far. It is found that in all calculated cases the total energy dissipation rate and, therefore, the power required to drive the flow were diminished considerably at steady state. Linear Viscoelastic Properties of Sucrose Ester Stabilized Oil-In-Water Emulsions A. Guerrero1, P. Partal2*, and C. Gallegos2 1 Departamento de Ingeniería Química, Universidad de SevillaC/. P. García González s/n, 41012 Sevilla, Spain 2 Departamento de Ingeniería Química, Universidad de HuelvaEscuela Politécnica Superior, Ctra. de Palos de la Frontera s/n, 21819, Huelva, Spain *Corresponding author E-mail: partal@uhu.es AbstractThis paper deals with the influence of composition (sucrose ester, SE, and oil, O, concentrations) and temperature on the linear viscoelasticity properties of highly concentrated oil-in-water (o/w) emulsions, using a hydrophilic sucrose palmitate as emulsifier. Different oil-in-water emulsions were prepared using a sucrose palmitate (SE = 1-5% wt.) with hydrophilic-lipophilic balance of 15, sunflower oil (O = 60-80% wt.) and water. Oscillatory measurements were carried out in the linear viscoelasticity region. The relaxation time spectra of the emulsions were calculated using regularization techniques and were fitted to an empirical model proposed by Madiedo (1996). The results obtained are explained on the basis of the relationship among linear viscoelasticity properties, droplet size distribution and characteristics of the continuous phase. Thus, an increase in sucrose palmitate concentration produces higher values of the linear viscoelasticity functions and a broader plateau region in the relaxation time spectrum, which suggests a certain enhancement of the elastic network. NMRI Investigation of Sedimentation of Concentrated Suspensions in Non-Newtonian Fluids Serge Bobroff and Ronald J. Phillips AbstractNuclear magnetic resonance imaging (NMRI) is used to study rates of sedimentation in several non-Newtonian fluids. It is shown that either shear-thinning or elasticity in a fluid can result in a time-dependent sedimentation rate, as measured by the motion of the suspension-supernatant interface. Sedimentation is most rapid at the beginning of the process, and grows steadily slower in time. Two-dimensional images of vertical sections of suspension show clearly the development of a non-homogeneous microstructure during sedimentation in a viscoelastic fluid. Elongated columns of particles form in the direction of gravity, and these columns are separated by comparably sized regions of pure fluid. Such structures are not present in sedimentation in Newtonian fluids, where the suspension microstructure is homogeneous for the duration of the process. Wall Slip of A Polydimethylsiloxane Extruded through A Slit Die with Rough Steel Surfaces: Micrometric Measurement at the Wall with Fluorescent Labelled Chains Frédéric Legrand and Jean Michel Piau Hubert Hervet AbstractUpstream instability and wall slip of a high molecular weight polydimethylsiloxane (PDMS) have been studied in a rough stainless steel slit die. The velocity field at a micrometric scale close to wall is determined with a new fluorescence technique. A film of a mixture of PDMS and small fluorescent labelled chains (PDMS-NBD) is deposited on the steel surface prior to the high pressure flow. During the flow, the fluorescence of a small area in the middle of the surface is excited with an argon laser line and measured with a phototube. The signal decays towards an asymptotic low value, due to stray light and thermal current. The slip velocity is inferred from a comparison between the experimental decrease and a theoretical model, which takes diffusion effects into account. It is shown that the upstream instability induces a transverse oscillating velocity in the channel. A precise determination of the residual fluorescence after the wall slip, with known diffusion coefficient and surface roughness, shows that the wall slip occurs in a plane localised at less than 1 µm from the ridges of the surface roughness. Rheology of Suspensions of Weakly Attractive Particles: Approach to Gelation C. J. Rueb and C. F. Zukoski AbstractThe flow properties of suspensions of weakly attractive particles are investigated as a function of volume fraction and strength of interparticle attraction. The suspensions are composed of uniform silica spheres with covalently bound octadecyl chains with diameters 100-125 nm suspended in decalin and tetradecane. At elevated temperatures, the particles interact like hard spheres. As the temperature is lowered, the suspensions gel. At the gel point, storage and loss moduli are power law functions of strain frequency providing evidence that cluster relaxation rates are much slower than the lowest strain frequency used. At volume fractions below the gel point (temperatures above the gel point), suspensions shear thin in much the same way as hard spheres. These studies suggest that the effect of volume fraction and attractive strength can be correlated when the normalized zero shear rate viscosity, h0/h0HS is written as a function of j/j G where h0HS is the high temperature (hard sphere), zero shear rate viscosity of a suspension at volume fraction j and j G is the volume fraction where, at the same strength of attraction, the suspensions gel. At j /j G £ 0.95, h0/h0HS £ 5 suggesting suspension viscosities are weak function of attractions up to the gel point where relaxation times rapidly increase. |
Please e-mail suggestions and comments to albertco@umche.maine.edu. Updated 25 January 2004 |