ABSTRACT
Dense non-Brownian suspensions exhibit a spectacular and abrupt drop in viscosity under change of shear direction, as revealed by shear inversions (reversals) or orthogonal superposition. Here, we introduce an experimental setup to systematically explore their response to shear rotations, where one suddenly rotates the principal axes of shear by an angle θ, and measure the shear stresses with a biaxial force sensor. Our measurements confirm the genericness of the transient decrease of the resistance to shear under unsteady conditions. Moreover, the orthogonal shear stress, which vanishes in steady state, takes non-negligible values with a rich θ dependence, changing qualitatively with solid volume fraction Ï and resulting in a force that tends to reduce or enhance the direction of flow for small or large Ï. These experimental findings are confirmed and rationalized by particle-based numerical simulations and a recently proposed constitutive model. We show that the rotation angle dependence of the orthogonal stress results from a Ï-dependent interplay between hydrodynamic and contact stresses.
ABSTRACT
This paper focuses on shear-thinning in non-Brownian suspensions. In particular, it proposes a quantitative experimental validation of the model proposed by Lobry et al. [J. Fluid Mech., 2019, 860, 682-710] that links viscosity to microscopic friction between particles and, in particular, shear-thinning to load-dependent friction coefficient. To this aim, Atomic Force Microscopy (AFM) is used to measure the pairwise friction coefficient of polystyrene particles (40 µm in diameter), immersed in a Newtonian liquid, for different normal loads ranging from 10 to 1000 nN. It is shown that the inter-particle friction coefficient decreases with the load, contrarily to what is expected for macroscopic contacting bodies. The experimental friction law is then introduced into the viscosity model proposed by Lobry et al. and the results are compared to the viscosity of suspensions made of the same particles dispersed in the same liquid as those used for AFM measurements. The very good agreement between the measured viscosity values and those predicted by the model of Lobry et al. with the friction coefficient measured by AFM as input data shows the relevance of the scenario proposed by Lobry et al. and highlights the close links between the microscopic friction properties of the particles and the macroscopic rheological behavior of suspensions.
ABSTRACT
Dense suspensions display complex flow properties, intermediate between solid and liquid. When sheared, a suspension self-organizes and forms particle clusters that are likely to percolate, possibly leading to significant changes in the overall behavior. Some theoretical conjectures on percolation in suspensions were proposed by de Gennes some 35 years ago. Although still used, they have not received any validations so far. In this Rapid Communication, we use three-dimensional detailed numerical simulations to understand the formation of percolation clusters and assess de Gennes conjectures. We found that sheared noncolloidal suspensions do show percolation clusters occurring at a critical volume fraction in the range 0.3-0.4 depending on the system size. Percolation clusters are roughly linear, extremely transient, and involve a limited number of particles. We have computed critical exponents and found that clusters can be described reasonably well by standard isotropic percolation theory. The only disagreement with de Gennes concerns the role of percolation clusters on rheology which is found to be weak. Our results eventually validate de Gennes conjectures and demonstrate the relevance of percolation concepts in suspension physics.
ABSTRACT
The shear-induced microstructure in a semidilute noncolloidal suspension is studied. A high-resolution pair distribution function in the plane of shear is experimentally determined. It is shown to be anisotropic, with a depleted direction close to the velocity axis in the recession quadrant. The influence of roughness on the interaction between particles is quantitatively evidenced. The experimental results compare well with a model from particle pair trajectories.
ABSTRACT
In this paper, we report experimental evidence of Lorenz chaos for the Quincke rotor dynamics. We study the angular motion of an insulating cylinder immersed in slightly conducting oil and submitted to a direct current electric field. The simple equations which describe the dynamics of the rotor are shown to be equivalent to the Lorenz equations. In particular, we observe two bifurcations in our experimental system. Above a critical value of the electric field, the cylinder rotates at a constant rate. At a second bifurcation, the system becomes chaotic. The characteristic shape of the experimental first return map provides strong evidence for Lorenz-type chaos.
ABSTRACT
In a recent paper, we have shown experimental results of ultrasound wave velocity and attenuation coefficient measurement in macroscopic suspensions. The experimental method was shortly described. This spectroscopy method allows measurement over a broadband frequency spectrum in a strong absorbing and high dispersive medium. The originality of the method lies in the automatic signal processing that suppress the ambiguity in the determination of the signal phase. The present paper deals with the detailed description of the signal processing that we use.
