A structural analysis of concentrated, aggregated colloids under flow (original) (raw)

Formation of a Highly Ordered Colloidal Microstructure upon Flow Cessation from High Shear Rates

Physical Review Letters, 1996

A model suspension of charged colloidal spheres exhibits anomalous modulus relaxations upon flow cessation from the shear-thickened state. Small angle neutron scattering (SANS) measurements indicate the development of a metastable crystalline state upon flow cessation from a disordered, shear-thickened state. The metastable state consists of a random stacking of tightly packed hcp planes of particles lying in the shear velocity-vorticity plane. These measurements suggest that the apparently disordered shearthickened state may contain a high degree of latent order not apparent in the SANS measurements during flow.

The rheology and microstructure of concentrated, aggregated colloids

Journal of Rheology, 1999

The rheology of concentrated, aggregated colloidal suspensions is determined through particulate simulations. Aggregating systems experience a large viscous enhancement over nonaggregating systems, this being due to the increase in the component of the viscosity arising from the repulsive colloid ͑thermodynamic͒ forces when attractive forces are present. The shear behavior of aggregating systems, for colloid volume fraction 0.47 р c р 0.57, is characterized in the steady state regime over a wide range in shear rate, and is found to be power law, shear thinning ϳ f (c)␥ Ϫ␣ , where the shear thinning index ␣ ϭ 0.84Ϯ0.01. The effect of volume fraction enters as f (c) ϭ (1Ϫ c / max) Ϫ1 , with max ϭ 0.64, the value of random close packing; similarly, the viscosity also scales with the potential well depth as a power law, of index ␣. Consequently, we are able to deduce the full constitutive relation for this power law behavior. The associated structural features which emerge as a result of the imposed shear are identified with the rheology. The shear thinning regime crosses over into a state of ordered phase flow at high shear rates likewise simulations of hard sphere fluids. We also show that the high-shear ordered configurations appear to be a function of colloid concentration, with a transition from string phase order through to layered phases as c increases.

The order–disorder transition in colloidal suspensions under shear flow

Journal of Physics: Condensed Matter, 2007

We study the order-disorder transition in colloidal suspensions under shear flow by performing Brownian dynamics simulations. We characterize the transition in terms of a statistical property of time-dependent maximum value of the structure factor. We find that its power spectrum exhibits the power-law behaviour only in the ordered phase. The power-law exponent is approximately-2 at frequencies greater than the magnitude of the shear rate, while the power spectrum exhibits the 1/f-type fluctuations in the lower frequency regime.

Shear thickening in colloidal dispersions

Physics Today, 2009

The popular interest in cornstarch and water mixtures known as "oobleck" after the complex fluid in one of Dr. Seuss's classic children's books arises from their transition from fluid-like to solid-like behavior when stressed. The viscous liquid that emerges from a roughly 2-to-1 (by volume) combination of starch to water can be poured into one's hand. When squeezed, the liquid morphs into a doughy paste that can be formed into shapes, only to "melt" into a puddle when the applied stress is relieved. Internet videos show people running across a large pool of the stuff, only to sink once they stop in place, and "monsters" that grow out of the mixture when it's acoustically vibrated (for an example, see the online version of this article). Shear-thickening fluids certainly entertain and spark our curiosity, but their effect can also vex industrial processes by fouling pipes and spraying equipment, for instance. And yet, when engineered into composite materials, STFs can be controlled and harnessed for such exotic applications as shockabsorptive skis and the soft body armor discussed in box 1. Engineers and colloid scientists have wrestled with the scientific and practical problems of shear-thickening colloidal dispersions-typically composed of condensed polymers , metals, or oxides suspended in a liquid-for more than a century. More recently, the physics community has explored the highly nonlinear materials in the context of jamming 1 (see the article by Anita Mehta, Gary Barker, and Jean-Marc Luck in PHYSICS TODAY, May 2009, page 40) and the more general study of colloids as model systems for understanding soft condensed matter. Hard-sphere colloids are the "hydrogen atom" of colloidal dispersions. Because of their greater size and interaction times compared with atomic and molecular systems, colloidal dispersions are often well suited for optical microscopy and scattering experiments using light, x rays, and neutrons. That makes the dispersions, beyond their own intrinsic technological importance, ideal models for exploring equilibrium and near-equilibrium phenomena of interest in atomic and molecular physics-for example, phase behavior and "dynamical arrest," in which particles stop moving collectively at the glass transition. The relevance of colloids to atomic and molecular systems breaks down, though, for highly nonequilibrium phenomena. Indeed, shear thickening in strongly flowing colloidal dispersions may be among the most spectacular, and elucidating, examples of the differences between the systems.

Shear-Induced Order in Suspensions of Hard Spheres

Physical Review Letters, 1988

Suspensions in a liquid of "nearly hard" colloidal spheres were subjected to steady and oscillatory shear flows and studied by light scattering. Samples which exhibit a fluidlike ordering of the particles in equilibrium were induced to a solidlike order by oscillatory shear of strain amplitude =l. In steady shear flow the suspensions showed evidence of "string" structures similar to but less complete than those found in computer simulations of simple liquids.

Shear Thickening in Concentrated Soft Sphere Colloidal Suspensions: A Shear Induced Phase Transition

model of shear thickening in dense suspensions of Brownian soft sphere colloidal particles is established. It suggests that shear thickening in soft sphere suspensions can be interpreted as a shear induced phase transition. Based on a Landau model of the coagulation transition of stabilized colloidal particles, taking the coupling between order parameter fluctuations and the local strain-field into account, the model suggests the occurrence of clusters of coagulated particles (subcritical bubbles) by applying a continuous shear perturbation.The critical shear stress of shear thickening in soft sphere suspensions is derived while reversible shear thickening and irreversible shear thickening have the same origin. The comparison of the theory with an experimental investigation of electrically stabilized colloidal suspensions confirms the presented approach.

Structural changes and orientaional order in a sheared colloidal suspension

Physical Review Letters, 1992

Small-angle neutron scattering experiments were carried out on a charged-stabilized dense colloidal suspension to observe changes in lattice structure as a function of shear rate. It is shown that (I) the lattice will evolve from a crystalline state in equilibrium to a polycrystalline state if subjected to a very low shear; and (2) as the shear is increased, partial order is reestablished with the apparent appearance of sliding layer flow. The transitions are reversible. The first transition occurs when the dynamic yield stress is exceeded; the second is discontinuous and is associated with an anomalous flow region in which the stress decreases with increasing shear rate.

Oscillatory shear-induced 3D crystalline order in colloidal hard-sphere fluids

Soft Matter, 2012

The non-equilibrium phase behavior of a colloidal hard-sphere fluid under oscillatory shear was investigated in real-space with experiments on poly(methyl methacrylate) (PMMA) colloidal suspensions and Brownian Dynamics computer simulations. All the samples in both experiments and the simulation are below the coexistence density of hard-sphere freezing, so the shear-induced crystals are out-of-equilibrium and melt after cessation of the shear. The physics is therefore fundamentally different from shear-induced crystallization in jammed or glassy systems. Although the computer simulations neglect hydrodynamic interactions and impose a linear flow, the results are in good agreement with the experiments. Depending on the amplitude and frequency of the oscillation, four regimes with different structures, hereafter referred to as phases, were identified: an oscillating twinned face-centered-cubic (fcc) phase, a sliding layer phase, a string phase and a phase that has not been reported previously in experiments, which we identify as tilted layers. This phase consists of lanes of particles that order in a hexagonal-like array (in the gradient-vorticity plane) which has lines of particles under an angle with the horizontal. Phases similar to the sliding layers, string phase and tilted layer phase were reported in Brownian and Molecular Dynamics simulations (systematically called string formation) but the validity of these simulations has been questioned. We demonstrate the experimental existence of these string-like phases and elucidate their structural differences in real-space.

Dynamics of colloidal crystals in shear flow

Soft Matter, 2009

We investigate particle dynamics in nearly hard sphere colloidal crystals submitted to a steady shear flow. Both the fluctuations of single colloids and the collective motion of crystalline layers as a whole are studied by using a home-built counter rotating shear cell in combination with confocal microscopy. Firstly, our real space observations confirm the global structure and orientation as well as the collective zigzag motion as found by early scattering experiments. Secondly, dynamic processes accompanying the shear melting transition are followed on the particle level. Local rearrangements in the crystal are seen to occur more frequently with increasing shear rate. This shear-enhanced particle mobility is quantified by measuring the random particle displacements from time-tracked particle coordinates. We find that shear induced melting takes place when these random displacements reach 12% of the particle separation, reminiscent of the Lindemann criterion for melting in equilibrium systems. In addition, a dynamic criterion for melting, based on the relative importance of the long time self diffusion compared to the short time self diffusion, is discussed.