Normal stress differences in dense suspensions

The presence and the microscopic origin of normal stress differences in dense suspensions under simple shear flows are investigated by means of inertialess particle dynamics simulations, taking into account hydrodynamic lubrication and frictional contact forces. The synergic action of hydrodynamic and contact forces between the suspended particles is found to be the origin of negative contributions to the first normal stress difference $N_1$, whereas positive values of $N_1$ observed at higher volume fractions near jamming are due to effects that cannot be accounted for in the hard-sphere limit. Furthermore, we found that the stress anisotropy induced by the planarity of the simple shear flow vanishes as the volume fraction approaches the jamming point for frictionless particles, while it remains finite for the case of frictional particles.Comment: 14 pages, 9 figure

Microstructure and thickening of dense suspensions under extensional and shear flows

Dense suspensions are non-Newtonian fluids which exhibit strong shear thickening and normal stress differences. Using numerical simulation of extensional and shear flows, we investigate how rheological properties are determined by the microstructure which is built under flows and by the interactions between particles. By imposing extensional and shear flows, we can assess the degree of flow-type dependence in regimes below and above thickening. Even when the flow-type dependence is hindered, nondissipative responses, such as normal stress differences, are present and characterise the non-Newtonian behaviour of dense suspensions.Comment: 11 pages, 6 figure

How a colloidal paste flows – scaling behaviors in dispersions of aggregated particles under mechanical stress –

We have developed a novel computational scheme that allows direct numerical simulation of the mechanical behavior of sticky granular matter under stress. We present here the general method, with particular emphasis on the particle features at the nanometric scale. It is demonstrated that, although sticky granular material is quite complex and is a good example of a challenging computational problem (it is a dynamical problem, with irreversibility, self-organization and dissipation), its main features may be reproduced on the basis of rather simple numerical model, and a small number of physical parameters. This allows precise analysis of the possible deformation processes in soft materials submitted to mechanical stress. This results in direct relationship between the macroscopic rheology of these pastes and local interactions between the particles

Crystallization kinetics of binary colloidal monolayers

Experiments and simulations are used to study the kinetics of crystal growth in a mixture of magnetic and nonmagnetic particles suspended in ferrofluid. The growth process is quantified using both a bond order parameter and a mean domain size parameter. The largest single crystals obtained in experiments consist of approximately 1000 particles and form if the area fraction is held between 65-70% and the field strength is kept in the range of 8.5-10.5 Oe. Simulations indicate that much larger single crystals containing as many as 5000 particles can be obtained in impurity-free conditions within a few hours. If our simulations are modified to include impurity concentrations as small as 1-2%, then the results agree quantitatively with the experiments. These findings provide an important step toward developing strategies for growing single crystals that are large enough to enable follow-on investigations across many subdisciplines in condensed matter physics.Comment: 10 pages, 11 figure

Competing Lengthscales in Colloidal Gelation with Non-Sticky Particles

Colloidal gels are widely applied in industry due to their rheological character -- no flow takes place below the yield stress. Such property enables gels to maintain uniform distribution in practical formulations; otherwise, solid components may quickly sediment without the support of gel matrix. Compared with pure gels of sticky colloids, therefore, the composites of gel and non-sticky inclusions are more commonly encountered in reality. Through numerical simulations, we investigate the gelation process in such binary composites. We find that the non-sticky particles not only confine gelation in the form of an effective volume fraction, but also introduce another lengthscale that competes with the size of growing clusters in gel. The ratio of two key lengthscales in general controls the two effects. Using different gel models, we verify such scenario within a wide range of parameter space, suggesting a potential universality in all classes of colloidal composites

Shear jamming and fragility of suspensions in a continuum model with elastic constraints

Under an applied traction, highly concentrated suspensions of solid particles in fluids can turn from a state in which they flow to a state in which they counteract the traction as an elastic solid: a shear-jammed state. Remarkably, the suspension can turn back to the flowing state simply by inverting the traction. A tensorial model is presented and tested in paradigmatic cases. We show that, to reproduce the phenomenology of shear jamming in generic geometries, it is necessary to link this effect to the elastic response supported by the suspension microstructure rather than to a divergence of the viscosity.Comment: Updated figures and included Supplemental materia

A theoretical framework for steady-state rheometry in generic flow conditions

open2siopenGiusteri, Giulio G.; Seto, RyoheiGiusteri, Giulio G.; Seto, Ryohe