Rheology Of Non-colloidal Suspensions Research Articles

Rheology of periodically sheared suspensions undergoing reversible-irreversible transition.

The rheology of noncolloidal suspensions under cyclic shear is studied numerically. The main findings are a strain amplitude (γ_{0}) dependent response in the shear stress and second normal stress difference (N_{2}). Specifically, we find a reduced viscosity, an enhanced intracycle shear thinning, the onset of a finite N_{2}, and its frequency doubling, all near a critical strain amplitude γ_{c} that scales with the volume fraction ϕ as γ_{c}∼ϕ^{-2}. These rheological changes also signify a reversible-irreversible transition (RIT), dividing stroboscopic particle dynamics into a reversible absorbing phase (for γ_{0}<γ_{c}) and a persistently diffusing phase (for γ_{0}>γ_{c}). We explain the results based on two flow-induced mechanisms and elucidate their connection in the context of RIT through the underlying microstructure, which tends toward hyperuniformity near γ_{0}=γ_{c}. Overall, we expect this correspondence between rheology and emergent dynamics to hold in a wide range of settings where structural organizations are dominated by volume exclusions.

Oscillatory strain with superposed steady shearing in noncolloidal suspensions

The rheology of noncolloidal suspensions in superposed simple shearing and oscillatory shearing was explored. With a Newtonian matrix fluid, one would expect that G′ would be zero in an oscillatory flow, but this was not found; the action of Coulomb friction between the particles appears to cause an increment of G′ at lower frequencies. To understand this frictional effect, measurements of small and medium strain oscillatory flows, up to 10% strain magnitude, were made. The matrix fluid was 12 Pa s silicone oil, and the polystyrene spheres were on average 40.3 μm in diameter. Hysteresis during tests with varying strain amplitudes was more dominant in the storage modulus than in the loss modulus, and, at a 50% volume fraction, the effect was severe. Because of the observed tendency to hysteresis, the oscillatory flow was then combined with a parallel steady shear flow to try to control or eliminate hysteresis. The hysteresis appears to be a frictional effect, and it was reduced under superposed shearing. The effect of variable oscillatory shear stress and steady shear stress was studied, and a model was proposed for the superposed storage modulus, loss modulus, and shear viscosity responses. Frictional effects are considered in the proposed model, and one observes a generally satisfactory fit to the experimental data. From the model, the average friction coefficient is shown to be less at higher frequencies due to higher relative rubbing speeds and better lubrication between the particles. Clearly, suspension rheology is dominated by friction and is essentially a study in tribology.

Review: Rheology of noncolloidal suspensions with non-Newtonian matrices

This review deals with non-Brownian (noncolloidal) suspension rheology; experimental and computational works are compared where possible. The matrix fluids are non-Newtonian, and the rigid particles have an aspect ratio close to one. Volume fractions of 0.5 and below are considered. Shearing and extensional flows are discussed; the former are fairly well understood but the latter are not prominent in the literature. Unsteady and oscillatory flows are surveyed. A comparison of Newtonian and viscoelastic suspension rheology is made, and some aspects of finding constitutive models for these suspensions are discussed. While progress has been made, it appears that satisfying agreement between computation and experiment is rare. More attention to rheological and frictional modeling is needed, and improved computational methods need to be developed.

Rheology of non-colloidal suspensions with corn syrup matrices

We explore the rheology of corn syrup and non-colloidal suspensions of spheres in matrices based on corn syrup. We also comment on the effect of sphere roughness on suspension rheology. Two sphere materials, polystyrene (PS) and polymethyl methacrylate (PMMA), were used. The spheres were of 40 μm nominal diameter. Roughness ratios (average roughness/sphere radius) of 0.15 to 5 % were explored. The viscosity increase with the corn syrup matrix was very large and appears to be due to drying of the corn syrup at the free surfaces of the sample. A corn syrup/glycerine/polyacrylamide Boger fluid was also used. The increases of viscosity with the Boger fluid matrix were again large with the roughened spheres. This appears to be due to the deformation of the long polyacrylamide molecules in the narrow channels between the roughened spheres.

Shear-induced diffusion and rheology of noncolloidal suspensions: Time scales and particle displacements

The shear-induced self-diffusion and rheology of concentrated suspensions of noncolloidal hard spheres have been studied experimentally. The combined results provide an interesting physical picture. The projection of the trajectories of individual particles on the vorticity (z)–velocity (x) plane were determined through particle tracking. The particle trajectories turned out to be very useful for gaining qualitative insight into the microscopic particle motion. However, the technique is less suitable to obtain quantitative information. For a quantitative analysis of the particle displacements we measured the evolution of the ensemble averaged displacements as a function of time. The statistical analysis revealed two diffusion regimes, where 〈ΔzΔz〉∼γ̇Δt. For large strain values (γ̇Δt&amp;gt;1) long-time self-diffusion was observed. The associated diffusion coefficient D̂∞ is in excellent agreement with literature data on shear-induced self-diffusion. On very short times (γ̇Δt≪1) a novel diffusive regime was discovered, characterized by a diffusion coefficient D̂0, which is significantly smaller than D̂∞ and grows monotonically with φ. D̂0 is detected on time scales on which the particle configuration is not changed significantly and thus it must represent the fluctuating motion of particles in the “cage” formed by their nearest neighbors. Finally, the rheology was studied with steady shear and oscillatory rheometry. The dynamic measurements in a controlled stress rheometer revealed that the viscoelastic response of the suspension is determined mainly by the amplitude of deformation. At small strain amplitudes γ0&amp;lt;1, the response is linear and a dynamic viscosity η′ is found, which is in excellent agreement with the high frequency limit η∞′ as reported in literature for colloidal hard sphere suspensions. Around γ0=1 the “cage” around a particle is deformed and a shear-induced microstructure is built. This leads to O(a) displacements of the particles and the viscoelastic response becomes strongly nonharmonic. Although the effect persists at large amplitudes, it becomes relatively small for γ0≫1. The microstructure is rearranged immediately after flow reversal and remains unchanged for the larger part of the period of oscillation. As a result a pseudolinear viscoelastic regime is found with a viscosity close to steady shear viscosity. Experiments show a correlation between the time scales controlling the D̂0/D̂∞ diffusive behavior and the ones controlling the shear-induced changes in particle configuration as probed by the rheological measurements.

Effects of Liquid Polarity on Rheology of Noncolloidal Suspensions

This paper presents the effects of liquid polarity on the rheology of suspensions with particle sizes ranging from 2 to 150 μm. We have found that liquid polarity has a substantial effect on the non‐Newtonian behavior of suspensions of graphite particles, but only a slight effect on that of polystyrene suspensions. This may be attributed to the greater interparticular van der Waals‐London dispersion force for graphite than for polystyrene. The suspensions of polystyrene spheres become Newtonian at very high shear rates when the interparticular van der Waals force is negligible as compared to the hydrodynamic force. This high shear Newtonian limit of relative viscosity is in excellent agreement with the value predicted by the rigid sphere model for colloidal suspensions.