Rheology and contact lifetimes in dense granular flows

We study the rheology and distribution of interparticle contact lifetimes for gravity-driven, dense granular flows of noncohesive particles down an inclined plane using large-scale, three dimensional, granular dynamics simulations. Rather than observing a large number of long-lived contacts as might be expected for dense flows, brief binary collisions predominate. In the hard-particle limit, the rheology conforms to Bagnold scaling, where the shear stress is quadratic in the strain rate. As the particles are made softer, however, we find significant deviations from Bagnold rheology; the material flows more like a viscous fluid. We attribute this change in the collective rheology of the material to subtle changes in the contact lifetime distribution involving the increasing lifetime and number of the long-lived contacts in the softer particle systems.     

Rheological properties of dense granular flows

Recent progresses in understanding the behavior of dense granular flows are presented. After presenting a bulk rheology of granular materials, I focus on the new developments to account for non-local effects, and on ongoing research concerning the surface rheology and the evolution of mechanical properties for heterogeneous systems.     

Unifying suspension and granular rheology

Using an original pressure-imposed shear cell, we study the rheology of dense suspensions. We show that they exhibit a viscoplastic behavior similarly to granular media successfully described by a frictional rheology and fully characterized by the evolution of the friction coefficient μ and the volume fraction ? with a dimensionless viscous number I(v). Dense suspension and granular media are thus unified under a common framework. These results are shown to be compatible with classical empirical models of suspension rheology and provide a clear determination of constitutive laws close to the jamming transition.     

SiO2 Reinforced EVM/TPU Blends: Bound Rubber and Rheology Behavior

Silica is shown to be a more active filler than carbon black for the reinforcement of EVM/TPU blends. The static rheology of EVM/TPU blends is considerably influenced by a tightly bound rubber structure developing between the TPU and silica that was identified by a tetrahydrofuran extraction test and SEM observations. Dynamic rheology behavior corroborated the development of bound rubber and indicated the existence of the Payne effect because of filler-filler networks in the blends. The silica-TPU interaction is thermally unstable at 190°C and mechanically unstable at a shear frequency of 1 Hz.     

Microrheology, stress fluctuations, and active behavior of living cells

We report the first measurements of the intrinsic strain fluctuations of living cells using a recently developed tracer correlation technique along with a theoretical framework for interpreting such data in heterogeneous media with nonthermal driving. The fluctuations' spatial and temporal correlations indicate that the cytoskeleton can be treated as a course-grained continuum with power-law rheology, driven by a spatially random stress tensor field. Combined with recent cell rheology results, our data imply that intracellular stress fluctuations have a nearly 1/omega2 power spectrum, as expected for a continuum with a slowly evolving internal prestress.     

Rheology and Dispersion Behavior of Ethylene‐Vinyl Acetate Copolymers/TiO2 Masterbatches Prepared by Melt‐Compounding

The rheology and dispersion behavior of ethylene‐vinyl acetate (EVA) copolymer/TiO₂ masterbatches prepared by melt‐compounding were investigated. The pure EVA exhibits obviously pseudoplastic behavior and the apparent viscosity decreases remarkably at experimental temperatures, especially in the range of 100–500 s^?1. The EVA/TiO₂ masterbatches exhibit similar shear rheology behavior with pure EVA and the apparent viscosities are obviously higher than that of pure EVA when the TiO₂ content is above 10 wt.%. Field‐emission scanning electron microscopy (FE‐SEM) and energy dispersive x‐ray spectroscopy (EDX) show that relatively low TiO₂ loading and moderate shear rate are helpful for the improvement of dispersion behavior of TiO₂ nanoparticles; moreover, the dispersion behavior of TiO₂ greatly influences the melt viscosity. The extensional rheology of pure EVA decreases with increasing extension rate, especially at low melt temperatures. EVA/TiO₂ masterbatches have similar extensional rheology behavior as pure EVA and the TiO₂ loading has almost no influence on the extensional viscosity of materbatches.     

Age-dependent transient shear banding in soft glasses

We study numerically the formation of long-lived transient shear bands during shear startup within two models of soft glasses (a simple fluidity model and an adapted "soft glassy rheology" model). The degree and duration of banding depends strongly on the applied shear rate, and on sample age before shearing. In both models the ultimate steady flow state is homogeneous at all shear rates, consistent with the underlying constitutive curve being monotonic. However, particularly in the soft glassy rheology case, the transient bands can be extremely long lived. The banding instability is neither "purely viscous" nor "purely elastic" in origin, but is closely associated with stress overshoot in startup flow.     

Slow granular flows: The dominant role of tiny fluctuations

What mechanism governs slow flows of granular media? Microscopically, the grains experience enduring frictional contacts in these flows. However, a straightforward translation to a macroscopic frictional rheology, where the shear stresses are proportional to the normal stresses with a rate-independent friction coefficient, fails to capture important aspects of slow granular flows. There is now overwhelming evidence that agitations, tiny fluctuations of the grain positions, associated with large fluctuation of their contact forces, play a central role for slow granular flows. These agitations are generated in flowing regions, but travel deep inside the quiescent zones, and lead to a nonlocal rheology.     

Shear thickening of cornstarch suspensions as a reentrant jamming transition

We study the rheology of cornstarch suspensions, a non-Brownian particle system that exhibits shear thickening. From magnetic resonance imaging velocimetry and classical rheology it follows that as a function of the applied stress the suspension is first solid (yield stress), then liquid, and then solid again when it shear thickens. For the onset of thickening we find that the smaller the gap of the shear cell, the lower the shear rate at which thickening occurs. Shear thickening can then be interpreted as the consequence of dilatancy: the system under flow wants to dilate but instead undergoes a jamming transition because it is confined, as confirmed by measurement of the dilation of the suspension as a function of the shear rate.     

Effect of pH and Protein Concentration on Rheological and Structural Behavior of Temperature-induced Bovine Serum Albumin Gels

The rheology of a temperature-induced protein bovine serum albumin gel is shown to strongly depend on the solution pH and protein concentration. Small-angle neutron scattering studies showed the presence of a fractal structure of the gels, resembling the aggregation of protein molecules and causing a three-dimensional network kind of arrangement. The fractal dimensions were observed to be constant and independent of the variation of pH and the protein concentration. The results of rheology and scattering experiments are correlated in terms of pH-dependent flexibility of flocs in the gels and hindrance to flow with concentration, while the structure of such flocs remains similar irrespective of the solution conditions.     

Surface rheology of two-dimensional percolating networks: Langmuir films of polymer pancakes

We report surface rheological measurements on Langmuir films of submicron-sized polymer coils (pancakes). The dynamics of the sol phase is found to be governed by free volume as in the simplest percolation problem. The observed rheology of the percolated phase (gel-like) is compatible with predictions for compressed arrangements of highly deformed soft particles interacting through their contact interfaces.     

Activation energy of shear transformation zones: a key for understanding rheology of glasses and liquids

Key manifestations of the glassy and liquid states, such as viscous flow and structural relaxation, occur spatial and temporal heterogeneously, within highly localized rare events, termed shear transformation zones. Characterization of these basic entities with respect to thermal activation and mechanical response is vital for understanding the rheology of glasses across length scales. This is achieved in classical molecular dynamics computer simulations on the model glass, CuTi, by determining the activation energy barrier and plastic yield strain of individual shear transformation zones as a function of size and external stress loading. Sizes of approximately equal to 140 atoms are identified to be especially energetically favorable with an activation energy barrier of approximately equal to 0.35 eV. Using these parameters, a rheology model is proposed to quantitatively explain viscosity.     

Vesicle migration and spatial organization driven by flow line curvature

Cross-streamline migration of deformable entities is essential in many problems such as industrial particulate flows, DNA sorting, and blood rheology. Using two-dimensional numerical experiments, we have discovered that vesicles suspended in a flow with curved flow lines migrate towards regions of high flowline curvature, which are regions of high shear rates. The migration velocity of a vesicle is found to be a universal function of the normal stress difference and the flow curvature. This finding quantitatively demonstrates a direct coupling between a microscopic quantity (migration) and a macroscopic one (normal stress difference). Furthermore, simulations with multiple vesicles revealed a self-organization, which corresponds to segregation, in a rim closer to the inner cylinder, resulting from a subtle interaction among vesicles. Such segregation effects could have a significant impact on the rheology of vesicle flows.     

BaSO4 nanorods produced in polymer-modified bicontinuous microemulsions

The influence of the water soluble polymer poly(ethylene glycol) (PEG) on structure formation in the quasiternary system sodium dodecylsulfate (SDS)/pentanol-xylene/water was checked by means of conductometry, rheology, and micro differential calorimetry. The polymer induces the formation of an isotropic phase channel between the o/w and w/o microemulsion. The transition from the normal as well as from the inverse micellar to the bicontinuous phase range can be detected by conductometry, rheology as well as micro-DSC. As a result of polymer–surfactant interactions, the spontaneous curvature of the surfactant film is changed and a sponge phase is formed. The bicontinuous phase is characterized by a moderate shear viscosity, a Newtonian flow behaviour, and the disappearence of interphasal water in the heating curve of the micro-DSC. When the polymer-modified bicontinuous phase is used as a template phase for the nanoparticle formation, spherical BaSO₄ nanoparticles were formed. During the following solvent evaporation process the primarily formed spherical nanoparticles aggregate to nanorods and triangular structures due to the non-restriction of the bicontinuous template phase in longitudinal direction.     

MRI investigation of granular interface rheology using a new cylinder shear apparatus

The rheology of granular materials near an interface is investigated through proton magnetic resonance imaging. A new cylinder shear apparatus has been inserted in the magnetic resonance imaging device, which allows the control of the radial confining pressure exerted by the outer wall on the grains and the measurement of the torque on the inner shearing cylinder. A multi-layer velocimetry sequence has been developed for the simultaneous measurement of velocity profiles in different sample zones, while the measurement of the solid fraction profile is based on static imaging of the sample. This study describes the influence of the roughness of the shearing interface and of the transverse confining walls on the granular interface rheology.     

Studying the Avalanching Behaviour of a Powder in a Rotating Disc

The use of a rotating disc to study the avalanching behaviour of a powder is discussed. It is shown that the strange attractor plotted in discreet time maps summarizes useful information on the rheological behaviour of powders and powder mixtures. In particular it is shown that the avalanching behaviour is related to the particle size distribution of the powder and that one can study the changes in rheological behaviour as another powder is mixed with it. The strange attractor patterns generated are dependent upon the environmental conditions under which the experiments are carried out. For this reason the measurements are referred to as an assessment of the holistic powder rheology. The potential use of the disc to study the holistic rheology of powder systems is outlined.     

Controlling Necking in Extrusion Film Casting Using Polymer Nanocomposites

The research described was concerned with the effect of layered-silicate-based organically modified nanoclay fillers on controlling the extent of necking in a polymer melt extrusion film casting (EFC) process. We show that a linear polythylene resin (such as a linear low-density polyethylene—LLDPE) filled with a very low percentage of well-dispersed (or intercalated) nanoclay displays an enhanced resistance to the necking phenomenon. In general, melt-compounded nanoclay-filled LLDPE resin formulations displayed a higher final film width (less necking), thus a lower final film thickness (greater draw down for the same draw ratio), and cooled down faster when compared to the base LLDPE resin. Incorporation of nanoclay filler in the mainly linear chain LLDPE resin led to significant modification of the melt rheological properties that, in turn, affected the melt processability of these formulations. Primarily, the intercalated nanoclay-filled LLDPE formulations displayed the presence of strain-hardening in unaxial extensional rheology. Additionally, the presence of well-dispersed nanoclay in the LLDPE resin led to a display of prominent extrudate swell indicating the presence of melt elasticity in such formulations. The presence of melt elasticity, as shown by shear rheology and strain-hardening, observed by uniaxial extensional rheology, contributed to the LLDPE nanoclay formulations displaying an enhanced resistance to necking for these films. It can be concluded that linear chain polymers susceptible to necking in an EFC process can be made more resistant to such necking by using nanoclay fillers at very low levels of loading.     

Properties and Morphology of Poly(Lactic Acid)/Calcium Carbonate Whiskers Composites Prepared by a Vane Mixer based on an Extensional Flow Field

Binary composites of poly(lactic acid) (PLA)/calcium carbonate whiskers (CCW) with different weight fractions were prepared with a vane mixer based on extensional rheology. The mechanical properties, thermostability, crystallization behavior, rheology behaviors and micromorphology of the composites were analysed to study the effect of the CCW fibers on the composite's properties; a pure PLA sample was also prepared for comparison. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) revealed that the CCW fibers had excellent compatibility with the PLA matrix and the CCW fibers were dispersed and distributed evenly in the PLA matrix under the action of the extensional flow field produced by the vane mixer. Differential scanning calorimetric (DSC) analysis showed that introducing a vane mixer into the PLA processing could increase the degree of crystallization (χ_c) of the composites significantly, and moderate CCW fibers adding could further increase its χ_c value. Thermogravimetric analysis (TGA) revealed that adding the CCW fibers reduced the thermostability of the composites. The G′, G″, η* and the torque, T_N, of the composites, obtained from rheology analyses, declined obviously, because of the hydrolysis and chains scission induced by residual water and fatty acid when the CCW content less than 4%. Tensile tests proved that filling moderate amounts of CCW fibers into PLA could increase its tensile strength and strain at break, increasing by 5% and 29.6%, respectively.