An experimental study of the first normal stress difference ? shear stress relationship in simple shear flow for concentrated shear thickening suspensions (original) (raw)

Normal stress differences in dense suspensions

Journal of Fluid Mechanics

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 N1N_{1}N1 , whereas positive values of N1N_{1}N1 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.

Shear stress during the flow of thixotropic and rheopex suspensions

A semi-empirical equation is derived that describes the dependence of shear stress on shear rate during the flow of a one-component thixotropic and rheopex suspension. The suspension is considered as consisting of two fractions: single grains and their dimers, and the dimerization of single grains is considered as a reaction characterized by a invariable rate constant, and the decomposition of dimers is considered as a reverse reaction with a rate constant that linearly increases with shear rate. The equation is based on the Krieger formula generalized to the case of multicomponent suspensions. The equation needs experimental verification.

Non-monotonic flow curves of shear thickening suspensions

The discontinuous shear thickening (DST) of dense suspensions is a remarkable phenomenon in which the viscosity can increase by several orders of magnitude at a critical shear rate. It has the appearance of a first order phase transition between two "states" that we have recently identified as Stokes flows with lubricated or frictional contacts, respectively. Here we extend the analogy further by means of stress-controlled simulations and show the existence of a non-monotonic steadystate flow curve analogous to a non-monotonic equation of state. While we associate DST to an S-shaped flow curve, at volume fractions above the shear jamming transition the frictional state loses flowability and the flow curve reduces to an arch, permitting the system to flow only at small stresses. Whereas a thermodynamic transition leads to phase separation in the coexistence region, we observe a uniform shear flow all along the thickening transition. A stability analysis suggests that uniform shear may be mechanically stable for the small Reynolds numbers and system sizes in a rheometer.