Volume 47, Issue 5, September 2003
Index of content:
Rheological study of the stabilization of magnetizable colloidal suspensions by addition of silica nanoparticles47(2003); http://dx.doi.org/10.1122/1.1595094View Description Hide Description
An experimental investigation is described on the stability of magnetorheological fluids (MRFs) consisting of ironsuspensions in silicone oil with a thixotropic agent (silicananoparticles) as stabilizer. The rheological properties were investigated using a commercial rheometer with a parallel-plate measuring cell. Several kinds of experiments were performed in steady-state, oscillatory, and transient regimes. The effects of the volume fraction of magnetic particles, the concentration of silica,magnetic flux density, and waiting time after preshear on the rheology of the MRFs were considered. Steady-state measurements demonstrated that our systems only display plastic behavior, for which a yield stress, is appreciable, for the highest iron concentrations and/or magnetic fields. The yield stress was found to be independent of the magnetic flux density when the concentration of silica particles was large enough This is a manifestation of the entrapment of iron particles in the silica gel. The adhesion of silica on iron particles by acid-base (proton donation) reactions between both colloids in apolar media is also investigated as another mechanism that hinders the aggregation among iron particles under the external field action. For the same reasons, ceased to scale as or to increase with iron volume fraction, for such a threshold silica concentration. Oscillometric determinations were performed at a frequency of 1 Hz, and the complex viscosity was found to increase with due to structure formation as a result of magnetic particle–particle interactions. In agreement with steady-state results, if the concentration of silica is sufficiently large, the complex viscosity reaches high values, but independent of magnetic flux density. Creep-recovery experiments are particularly sensitive to MRF stability, because the interplay between iron and silica concentrations, magnetic flux density, and waiting time after preshear, led to a broad range of behaviors, ranging from liquid-like to almost elastic solid.
47(2003); http://dx.doi.org/10.1122/1.1595097View Description Hide Description
We report an experimental and computational investigation of λ-phage DNA molecules that stretch and orient in an evaporating droplet resting on an adsorbing 3-aminopropyltriethoxysilane (APTES)-coated surface, which has potential applications for high-throughput gene analysis [J. Jing et al., Proc. Nat. Acad. Sci. USA 95, 8046–8051 (1999)]. Using Brownian dynamics simulations and the microscopic flow field from a lubricationanalysis, we predict that the degree of stretch obtainable by this method is substantially less than can be obtained by deposition from a simple uniform shear flow. This prediction is confirmed by detailed comparisons of the degree of stretch, orientation, and molecular configurations measured directly on DNA molecules deposited onto an APTES-coated surface. Statistical analysis reveals that the inefficiency of stretching in the drying droplet results from the presence of a velocity component normal to the surface, which reduces the time available for the chain to unravel sequentially as it adsorbs to the surface.
47(2003); http://dx.doi.org/10.1122/1.1603237View Description Hide Description
The rheological behavior of concentrated aqueous suspensions of nearly monodisperse submicron, spherical silica particles was studied in Couette and vane geometries. End corrections and wall depletion effects were found to be important. The apparent yield stress and shear viscosity were investigated in the light of interactions between the charged silica particles. The effects of as well as the addition of electrolyte were examined. The suspensions in de-ionized water without addition of acid, base, or electrolyte gave the largest apparent yield stress and shear viscosity, while the addition of base, acid, and KCl resulted in significant decreases of the apparent yield stress as well as of the viscosity. These effects have been interpreted in light of the DLVO theory and the compression of the double layer around the solid particles (Debye length).
47(2003); http://dx.doi.org/10.1122/1.1603238View Description Hide Description
In previous publications it has been shown that the flow-morphology relationships that apply to two-phase polymer blends or solutions, and to immiscible emulsions, also apply to two-phase aqueous solutions of mixed biopolymers, (referred to here as “biopolymer blends”). The relationships in question are classical theories of single drop deformation in shear flow,fibril breakup upon cessation of steady shear flow, and the Palierne model. In this paper, we report the experimental investigation of string phase formation in steady shear as another structural phenomenon which has been observed in polymer blends or polymer solutions, but has not been previously reported for biopolymer blends. The rheological and morphological behavior of gelatin-dextran mixtures was analyzed using conventional rotational rheometry and shear-microscopy techniques. It was found that the development of a string phase depends on the applied shear rate, the viscosity ratio and the phase volume of the minor phase. A rheological model by Kume and Hashimoto (1995) predicting string phase morphology was successfully applied for viscosity ratios not too dissimilar from unity. The application of a further model by Jeon and Hobbie (2001), however, gave ambiguous results with regard to the prediction of string phase formation.
Microscopic theory of linear, entangled polymer chains under rapid deformation including chain stretch and convective constraint release47(2003); http://dx.doi.org/10.1122/1.1595099View Description Hide Description
A refined version of the Doi and Edwards tube model for entangled polymer liquids is presented. The model is intended to cover linear chains in the full range of deformation rates from linear to strongly nonlinear flows. The effects of reptation, chain stretch, and convective constraint release are derived from a microscopic stochastic partial differential equation that describes the dynamics of the chain contour down to the length scale of the tube diameter. Contour length fluctuations are also included in an approximate manner. Predictions of mechanical stresses as well as the single chain structure factor under flow are shown. A comparison with experimental data is made in which all model parameters are fixed at universal values or are obtained from linear oscillatory shear measurements. With no parameter modification the model produces good agreement over a wide range of rheological data for entangled polymer solutions, including both nonlinear shear and extension.
Biaxial damping function of entangled monodisperse polystyrene melts: Comparison with the Mead–Larson–Doi model47(2003); http://dx.doi.org/10.1122/1.1595096View Description Hide Description
Stress relaxationmeasurements for large step strains were performed in shear and biaxial extension using an entangled monodisperse polystyrene melt. The shear and biaxial damping functions were obtained, which agreed very well with our previous data for other monodisperse polystyrene melts. The applicability of the Mead, Larson and Doi (MLD) model to the nonlinear damping behavior was investigated based on these experimental data as well as on the previous data of the uniaxial damping function. The experimental data of shear and uniaxial damping functions were larger than the MLD prediction and were closer to the prediction of the Doi–Edwards model with the independent alignment approximation. On the other hand, the experimental biaxial damping function agreed very well with the MLD prediction. The convective constraint release (CCR) accelerates the orientation relaxation when the stretched chain contracts and the CCR gives stronger strain dependence for the damping functions in shear, uniaxial and biaxial extensions. It is suggested that the orientation relaxation time around the contraction time predicted by the MLD model may be too short, at least in shear deformation.
47(2003); http://dx.doi.org/10.1122/1.1595098View Description Hide Description
We carried out systematic rheometrical tests under controlled stress with smooth and rough parallel disks, along with magnetic resonance imaging(MRI) tests in coaxial cylinder geometry, with foam and a model concentrated emulsion. At low shear stress wall slip appears to occur but the bulk fluid remains static, as proved by the fact that in this regime the apparent shear rate obtained for a given shear stress is inversely proportional to the gap between the disks. At high shear stress data with different surface types and gaps coincide, suggesting that wall slip is negligible in this regime. In parallel, MRI results show that, in contrast with the apparent, simple, yielding behavior observed in usual rheometry, there is an abrupt transition from a finite shear rate to a static one at critical stress. This critical shear rate precisely corresponds to the transition between the two regimes of slip. This suggests that different flow regimes occur with these materials: (1) at low stress, with smooth surfaces a layer of a different material is sheared along the solid surfaces whereas the rest of the fluid does not flow; with rough surfaces there is no flow; (2) beyond a critical stress, for both surface types, the bulk fluid starts to flow but the shear is localized in a thin layer; then the thickness of this layer increases when stress is applied; (3) for both surface types homogeneous flow is obtained only beyond slightly larger stress, which is associated to a critical, apparent shear rate.
47(2003); http://dx.doi.org/10.1122/1.1603242View Description Hide Description
A filament stretching rheometer is used to follow the evolution in the tensile force and the flow induced birefringence of a wormlike micellesolution experiencing a uniaxial elongation flow. The experiments are performed using a series of wormlike micellesolutions of cetyltrimethylammonium bromide and sodium salicylate in de-ionized water. The linear viscoelastic shear rheology of the wormlike micellesolutions is well described by an upper convected Maxwell model with a single relaxation time. In transient homogeneous uniaxial extension, the wormlike micellesolutions demonstrate significant strain hardening and a failure of the stress-optical, however, no stress-conformation hysteresis is observed. A quantitative fit to the extensional rheology of each of the wormlike micellesolution tested is achieved with a FENE–PM model having as few as two relaxation modes. At a critical stress, nearly independent of strain rate, the wormlike micellesolutions filaments are found to fail through a dramatic rupture near the axial midplane. This filament failure is not the result of elastocapillary thinning as is commonly observed in the filament stretching of weakly strain hardening polymer solutions. Instead, the filament failure might stem from the local scission of individual wormlike micelle chains. The energy of wormlike micelle chain scission is calculated to be roughly for all the solutions tested, nearly independent of both the imposed extension rate and the concentration of the surfactant and the salt.
Monolithic rheometer plate fabricated using silicon micromachining technology and containing miniature pressure sensors for and measurements47(2003); http://dx.doi.org/10.1122/1.1595095View Description Hide Description
An attachment suitable for any standard torsional rheometer is described; it extends the capability of the rheometer, thereby enabling simultaneous measurement of the first and second normal stress differences. The attachment is a monolithic rheometer plate 25 mm in diameter that contains eight miniature capacitive pressuresensors, each with a pressure-sensing area less than or equal to 1 mm2. It is found that when the sensor plate is adapted to an ARES rheometer, highly accurate values of and can be measured at room temperature for a standard elasticpolymer solution (SRM-1490 from the National Institute of Standards and Technology). Compared to standard measurement methods for use of novel sensor plates of this type should reduce the axial compliance and allow smaller sample sizes to be studied, since no spring-type normal force transducers are required. In addition, a rheometer adapted with the sensor plate, unlike a conventional rheometer, can be used to measure and with a single sample.
47(2003); http://dx.doi.org/10.1122/1.1603239View Description Hide Description
The start-up, reversal, and cessation of capillary Poiseuille flows of uniaxial isothermal incompressible discotic nematic liquid crystals are characterized using analytical, computational, and scaling methods, for low (linear regime) and high (nonlinear regime) pressuredrops. In the linear regime, transient flows provide information on the main viscoelastic materialproperties, including the steady shear Miesowicz’ viscosity, the transient reorientation viscosity, as well as the viscosity reduction due to backflow. It is shown that cessation of weak flow provides a way to measurepressuredrops. Transient flows in the nonlinear regime involve orientation-dependent material functions. The flow rate in start-up is characterized by an overshoot and strain scaling, similar to stress overshoot in simple shear. Recoil after cessation of flow is shown to be a direct function of stored Frank elastic energy. Scaling and computation show that the maximum recoil volume is shown to be close to where R is the capillary radius. Scaling and computation show that flow reversal under large pressuredrops is characterized by the formation of a sharp cusp at a time equal to the orientation time scale of the material.
47(2003); http://dx.doi.org/10.1122/1.1603240View Description Hide Description
We report a study on the deformation and breakup of drops in an impulsively started shear flow under Stokes flow conditions using boundary-integral simulations and video-microscopy experiments. Two independent techniques are used for determining the physical parameters of the system from the combined use of numerical simulations and experiments. Accurate breakup criteria (critical capillary numbers) are presented for a range of viscosity ratios. The time required for breakup events has a broad minimum corresponding to moderate shear rates. The size distribution of droplets produced by breakup events is shown to scale with the critical size drop for breakup in shear. A simplified model, based on this finding, is developed for the size distribution in a sheared emulsion. According to the model, the drop size distribution in a given emulsion depends only on the average initial drop size and the shear rate.
Observations of immiscible polymer blend electrorheological fluids with a confocal scanning laser microscope47(2003); http://dx.doi.org/10.1122/1.1603241View Description Hide Description
By using a new system that combines a confocal scanning laser microscope and a rheometer, we observed structural changes and measured shear stress simultaneously in immiscible polymer blends, while they were being subjected to shear flow and an electric field. In three different blends various structural changes were observed, which were all related to the electrorheological effect. These blends were composed of liquid crystalline polymers(LCPs) and oils. In one blend we observed a change from a LCP droplet-dispersed structure to an oil-dispersed one, i.e., a phase inversion, while in another blend there was a change from a LCP droplet-dispersed structure to a network structure. After applying shear flow an oil layer was three-dimensionally observed in the third blend, which had an oil-dispersed structure. In addition, it was found that the layer disappeared under no shear and the annihilation process was accelerated by the application of an electric field.