Shear-thickening and entropy-driven reentrance

Inverse transitions and disappearance of the λ-line in the asymmetric random-field Ising and Blume-Capel models

We report on reentrance in the random-field Ising and Blume-Capel models, induced by an asymmetric bimodal random-field distribution. The conventional continuous line of transitions between the paramagnetic and ferromagnetic phases, the λ-line, is wiped away by the asymmetry. The phase diagram, then, consists of only first-order transition lines that always end at ordered critical points. We find that, while for symmetric random-field distributions there is no reentrance, the asymmetry in the random-field results in a range of temperatures for which magnetization shows reentrance. While this does not give rise to an inverse transition in the Ising model, for the Blume-Capel model, however, there is a line of first-order inverse phase transitions that ends at an inverse-ordered critical point. We show that the location of the inverse transitions can be inferred from the ground-state phase diagram of the model.

Controlling the shear thickening behavior of suspensions by changing the surface properties of dispersed microspheres

To investigate the effect of the surface properties of dispersed particles on the shear thickening behavior of their corresponding suspensions and further control this characteristic, three kinds of suspensions were prepared by mixing SiO₂, SiO₂-NH₂, and SiO₂-COOH microspheres with a poly(ethylene glycol) fluid medium, and their rheological behavior was analyzed carefully. Compared to the SiO₂ microsphere suspension, the SiO₂-NH₂ and SiO₂-COOH microsphere suspensions show a weaker thickening behavior and a greater critical shear rate due to the aggregation tendency caused primarily by the organic chains. Moreover, the rheological behavior of the three suspensions display different dependencies on the pH value, which is comprehensively determined by the interaction between the microspheres and the medium. Moreover, the critical shear stress of suspensions with different pH values could be predicted by the Wagner model, which basically proves that the interaction between the particles significantly influences the beginning of thickening. The thickening degree could be interpreted by friction theory. The critical volume fraction corresponding to the onset of discontinuous shear thickening is determined by the friction coefficient between the particles, which is greatly affected by the pH value.

Discontinuous shear thickening of frictional hard-sphere suspensions

Discontinuous shear thickening (DST) observed in many dense athermal suspensions has proven difficult to understand and to reproduce by numerical simulation. By introducing a numerical scheme including both relevant hydrodynamic interactions and granularlike contacts, we show that contact friction is essential for having DST. Above a critical volume fraction, we observe the existence of two states: a low viscosity, contactless (hence, frictionless) state, and a high viscosity frictional shear jammed state. These two states are separated by a critical shear stress, associated with a critical shear rate where DST occurs. The shear jammed state is reminiscent of the jamming phase of granular matter. Continuous shear thickening is seen as a lower volume fraction vestige of the jamming transition.

Shear-induced structures and thickening in fumed silica slurries

Chemical mechanical polishing (CMP) is an essential technology used in the semiconductor industry to polish and planarize a variety of materials for the fabrication of microelectronic devices (e.g., computer chips). During the high shear (~1,000,000 s(-1)) CMP process, it is hypothesized that individual slurry particles are driven together to form large agglomerates (≥0.5 μm), triggering a shear thickening effect. These shear-induced agglomerates are believed to cause defects during polishing. In this study, we examined the shear thickening of a 25 wt % fumed silica slurry with 0.17 M added KCl using in situ small-angle light scattering during rheological characterization (rheo-SALS). The salt-adjusted slurry displays a ~3-fold increase in viscosity at a critical shear rate of 20,000 s(-1) during a stepped shear rate ramp from 100 to 25,000 s(-1). As the shear rate is reduced back to 100 s(-1), the slurry displays irreversible thickening behavior with a final viscosity that is 100-times greater than the initial viscosity. Corresponding rheo-SALS images indicate the formation of micrometer scale structures (2-3 μm) that directly correlate with the discontinuous and irreversible shear thickening behavior of the fumed silica slurry; these micrometer scale structures are 10-times the nominal particle diameter (~0.2 μm). The scattering patterns from the 25 wt % slurry were corroborated through rheo-SALS examination of 27 and 29 wt % slurries (C(KCl) = 0.1 M). All slurries, regardless of ionic strength and solids loading, display scattering patterns that are directly associated with the observed thickening behavior. Scattering was only observable during and after thickening (i.e., no scattering was detected in the absence of thickening). This work serves as the first in situ observation of micrometer scale structures within the fumed silica CMP slurry while under shear.

Polydirectional stability of granular matter

We investigate jammed granular matter in a slowly rotating drum partially filled with granular material and find a state of polydirectional stability. In this state, the material responds elastically to small stresses in a wide angular interval while it responds by plastic deformation when subjected to small stresses outside this interval of directions. We describe the evolution of the granulate by means of a rate equation and find quantitative agreement with the experiment. The state of polydirectional stability complements the fragile state, where the material responds elastically to small applied stresses only in a certain direction but even very small stresses in any other direction would lead to plastic deformations. Similar to fragile matter, polydirectionally stable matter is created in a dynamic process by self-organization.

Focused force transmission through an aqueous suspension of granules

We investigate force transmission through a layer of shear-thickening fluid, here a concentrated aqueous cornstarch suspension. When a solid body is pushed through this complex fluid and approaches its containing wall, a hardened volume of the suspension is observed that adds to the leading side of the body. This volume leads to an imprint on the wall which is made of molding clay. By studying the geometry of the hardened volume, inferred by the imprint shapes, we find that its geometry is determined by the size and speed of the body. By characterizing the response of the clay to deformation we show that the force transmitted through the suspension to the wall is localized. We also study other aspects of this dynamical hardening of the suspension, such as the effect of the substrate and body shape, and its relaxation as the imposed straining is stopped.

Incorporating friction and collective shear moves into a lattice gas

Here we examine a lattice-gas model that has been extended to include collective shear moves and friction interactions between particles. A shearlike field is applied by periodically translating entire subsections of the lattice with respect to one another, as opposed to biasing the individual movements of particles. Friction is introduced by forming a network of temporary bonds between particles that prevent particles from moving along with the flow. The extent of the network is controlled by the sticking parameter, P stk. We find that there are two distinct phases in the model: an isotropic phase that exhibits only small fluctuations in local density, and a striped phase that features one or more large clusters of particles that span the system. We examine the transition between these two phases using the radial distribution function. By introducing a measure of viscosity, we examine the relationship between the viscosity and shear rate for many different densities and values of P stk, and pursue an analogy to colloidal shear-thickening systems.

Generality of shear thickening in dense suspensions

Suspensions are of wide interest and form the basis for many smart fluids. For most suspensions, the viscosity decreases with increasing shear rate, that is, they shear thin. Few are reported to do the opposite, that is, shear thicken, despite the longstanding expectation that shear thickening is a generic type of suspension behaviour. Here we resolve this apparent contradiction. We demonstrate that shear thickening can be masked by a yield stress and can be recovered when the yield stress is decreased below a threshold. We show the generality of this argument and quantify the threshold in rheology experiments where we control yield stresses arising from a variety of sources, such as attractions from particle surface interactions, induced dipoles from applied electric and magnetic fields, as well as confinement of hard particles at high packing fractions. These findings open up possibilities for the design of smart suspensions that combine shear thickening with electro- or magnetorheological response.

Dynamic jamming point for shear thickening suspensions

We report on rheometry measurements to characterize the critical behavior in two model shear thickening suspensions: cornstarch in water and glass spheres in oil. The slope of the shear thickening part of the viscosity curve is found to increase dramatically with packing fraction and diverge at a critical packing fraction phi(c). The magnitude of the viscosity and the yield stress are also found to have scalings that diverge at phi(c). We observe shear thickening as long as the yield stress is less than the stress at the viscosity maximum. Above this point the suspensions transition to purely shear thinning. Based on these data we present a dynamic jamming phase diagram for suspensions and show that a limiting case of shear thickening corresponds to a jammed state.

Out-of-equilibrium phase re-entrance(s) in long-range interacting systems

Systems with long-range interactions display a short-time relaxation toward quasistationary states (QSSs) whose lifetime increases with system size. The application of Lynden-Bell's theory of "violent relaxation" to the Hamiltonian Mean Field model leads to the prediction of out-of-equilibrium first- and second-order phase transitions between homogeneous (zero magnetization) and inhomogeneous (nonzero magnetization) QSSs, as well as an interesting phenomenon of phase re-entrances. We compare these theoretical predictions with direct N -body numerical simulations. We confirm the existence of phase re-entrance in the typical parameter range predicted from Lynden-Bell's theory, but also show that the picture is more complicated than initially thought. In particular, we exhibit the existence of secondary re-entrant phases: we find unmagnetized states in the theoretically magnetized region as well as persisting magnetized states in the theoretically unmagnetized region. We also report the existence of a region with negative specific heats for QSSs both in the numerical and analytical caloric curves.

Asymmetric exclusion processes with constrained dynamics

Asymmetric exclusion processes with locally reversible kinetic constraints are introduced to investigate the effect of nonconservative driving forces in athermal systems. At high density they generally exhibit rheological-like behavior, negative differential resistance, two-step structural relaxation, dynamical heterogeneity and, possibly, a jamming transition driven by the external field.

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.