arXiv subjects

Guillaume Ovarlez

Publications and source records attributed to Guillaume Ovarlez.

30 records · Page 2Linked to original sources

arXiv2010

Macroscopic behavior of bidisperse suspensions of noncolloidal particles in yield stress fluids

We study both experimentally and theoretically the rheological behavior of isotropic bidisperse suspensions of noncolloidal particles in yield stress fluids. We focus on materials in which noncolloidal particles interact with the suspending fluid only through hydrodynamical interactions. We observe that both the elastic modulus and yield stress of bidisperse suspensions are lower than those of monodisperse suspensions of same solid volume fraction. Moreover, we show that the dimensionless yield stress of such suspensions is linked to their dimensionless elastic modulus and to their solid volume fraction through the simple equation of Chateau et al.[J. rheol. 52, 489-506 (2008)]. We also show that the effect of the particle size heterogeneity can be described by means of a packing model developed to estimate random loose packing of assemblies of dry particles. All these observations finally allow us to propose simple closed form estimates for both the elastic modulus and the yield stress of bidisperse suspensions: while the elastic modulus is a function of the reduced volume fraction $ϕ/ϕ_m$ only, where $ϕ_m$ is the estimated random loose packing, the yield stress is a function of both the volume fraction $ϕ$ and the reduced volume fraction.

cond-mat.soft↗

arXiv2010

Homogenization approach to the behavior of suspensions of noncolloidal particles in yield stress fluids

The behavior of suspensions of rigid particles in a non-Newtonian fluid is studied in the framework of a nonlinear homogenization method. Estimates for the overall properties of the composite material are obtained. In the case of a Herschel-Bulkley suspending fluid, it is shown that the properties of a suspension with overall isotropy can be satisfactory modeled as that of a Herschel-Bulkley fluid with an exponent equal to that of the suspending fluid. Estimates for the yield stress and the consistency at large strain rate levels are proposed. These estimates compare well to both experimental data obtained by Mahaut et al [J. Rheol. 52, 287-313 (2008)] and to experimental data found in the literature.

cond-mat.soft↗

arXiv2010

Transition from a simple yield stress fluid to a thixotropic material

From MRI rheometry we show that a pure emulsion can be turned from a simple yield stress fluid to a thixotropic material by adding a small fraction of colloidal particles. The two fluids have the same behavior in the liquid regime but the loaded emulsion exhibits a critical shear rate below which no steady flows can be observed. For a stress below the yield stress, the pure emulsion abruptly stops flowing, whereas the viscosity of the loaded emulsion continuously increases in time, which leads to an apparent flow stoppage. This phenomenon can be very well represented by a model assuming a progressive increase of the number of droplet links via colloidal particles.

cond-mat.soft↗

arXiv2010

Shear induced drainage in foamy yield-stress fluids

Shear induced drainage of a foamy yield stress fluid is investigated using MRI techniques. Whereas the yield stress of the interstitial fluid stabilizes the system at rest, a fast drainage is observed when a horizontal shear is imposed. It is shown that the sheared interstitial material behaves as a viscous fluid in the direction of gravity, the effective viscosity of which is controlled by shear in transient foam films between bubbles. Results provided for several bubble sizes are not captured by the R^2 scaling classically observed for liquid flow in particulate systems, such as foams and thus constitute a remarkable demonstration of the strong coupling of drainage flow and shear induced interstitial flow. Furthermore, foam films are found to be responsible for the unexpected arrest of drainage, thus trapping irreversibly a significant amount of interstitial liquid.

cond-mat.soft↗

arXiv2010

Phenomenology and physical origin of shear-localization and shear-banding in complex fluids

We review and compare the phenomenological aspects and physical origin of shear-localization and shear-banding in various material types, namely emulsions, suspensions, colloids, granular materials and micellar systems. It appears that shear-banding, which must be distinguished from the simple effect of coexisting static-flowing regions in yield stress fluids, occurs in the form of a progressive evolution of the local viscosity towards two significantly different values in two adjoining regions of the fluids in which the stress takes slightly different values. This suggests that from a global point of view shear-banding in these systems has a common physical origin: two physical phenomena (for example, in colloids, destructuration due to flow and restructuration due to aging) are in competition and, depending on the flow conditions, one of them becomes dominant and makes the system evolve in a specific direction.

cond-mat.soft↗

arXiv2009

Yield stress and shear-banding in granular suspensions

We study the emergence of a yield stress in dense suspensions of non-Brownian particles, by combining local velocity and concentration measurements using Magnetic Resonance Imaging with macroscopic rheometric experiments. We show that the competition between gravity and viscous stresses is at the origin of the development of a yield stress in these systems at relatively low volume fractions. Moreover, it is accompanied by a shear banding phenomenon that is the signature of this competition. However, if the system is carefully density matched, no yield stress is encountered until a volume fraction of 62.7 0.3%.

cond-mat.soft↗

arXiv2008

Influence of shear stress applied during flow stoppage and rest period on the mechanical properties of thixotropic suspensions

We study the solid mechanical properties of several thixotropic suspensions as a function of the shear stress history applied during their flow stoppage and their aging in their solid state. We show that their elastic modulus and yield stress depend strongly on the shear stress applied during their solid-liquid transition (i.e., during flow stoppage) while applying the same stress only before or only after this transition may induce only second-order effects: there is negligible dependence of the mechanical properties on the preshear history and on the shear stress applied at rest. We also found that the suspensions age with a structuration rate that hardly depends on the stress history. We propose a physical sketch based on the freezing of a microstructure whose anisotropy depends on the stress applied during the liquid-solid transition to explain why the mechanical properties depend strongly on this stress. This sketch points out the role of the internal forces in the colloidal suspensions' behavior. We finally discuss briefly the macroscopic consequences of this phenomenon and show the importance of using a controlled-stress rheometer.

cond-mat.soft↗

arXiv2008

Wide-gap Couette flows of dense emulsions: Local concentration measurements, and comparison between macroscopic and local constitutive law measurements through magnetic resonance imaging

Flows of dense emulsions show many complex features among which long range nonlocal effects pose a problem for macroscopic characterization. In order to get around this problem, we study the flows of several dense emulsions in a wide-gap Couette geometry. We couple macroscopic rheometric experiments and local velocity measurements through MRI techniques. As concentration heterogeneities can be expected, we designed a method to measure the local droplet concentration in emulsions with a MRI device. In contrast to dense suspensions of rigid particles where very fast migration occurs under shear, we show that no migration takes place in dense emulsions even for strains as large as 100 000 in our systems. As a result of the absence of migration and of finite size effect, we are able to determine very precisely the local rheological behavior of several dense emulsions. As the materials are homogeneous, this behavior can also be inferred from purely macroscopic measurements. We thus suggest that properly analyzed purely macroscopic measurements in a wide-gap Couette geometry can be used as a tool to study the local constitutive laws of dense emulsions. All behaviors are basically consistent with Herschel-Bulkley laws of index 0.5, but discrepancies exist at the approach of the yield stress due to slow shear flows below the apparent yield stress in the case of a strongly adhesive emulsion. The existence of a constitutive law accounting for all flows contrasts with previous results obtained within a microchannel by Goyon et al. (2008): the use of a wide-gap Couette geometry is likely to prevent here from nonlocal finite size effects; it also contrasts with the observations of Bécu et al. (2006).

cond-mat.soft↗

arXiv2008

Yield stress and elastic modulus of suspensions of noncolloidal particles in yield stress fluids

We study experimentally the behavior of isotropic suspensions of noncolloidal particles in yield stress fluids. This problem has been poorly studied in the literature, and only on specific materials. In this paper, we manage to develop procedures and materials that allow us to focus on the purely mechanical contribution of the particles to the yield stress fluid behavior, independently of the physicochemical properties of the materials. This allows us to relate the macroscopic properties of these suspensions to the mechanical properties of the yield stress fluid and the particle volume fraction, and to provide results applicable to any noncolloidal particle in any yield stress fluid. We find that the elastic modulus-concentration relationship follows a Krieger-Dougherty law, and we show that the yield stress-concentration relationship is related to the elastic modulus-concentration relationship through a very simple law, in agreement with results from a micromechanical analysis.

cond-mat.soft↗

arXiv2005

Local determination of the constitutive law of a dense suspension of non-colloidal particles through MRI

We investigate the flowing behavior of dense suspensions of non-colloidal particles, by coupling macroscopic rheometric experiments and local velocity and concentration measurements through MRI techniques. We find that the flow is localized at low velocities, and that the material is inhomogeneous; the local laws inferred from macroscopic rheometric observations must then be reinterpreted in the light of these local observations. We show that the short time response to a velocity step allows to characterize dense suspensions locally: they have a purely viscous behavior, without any observable influence of friction. In the jammed zone, there may be a contact network, whereas in the sheared zone there are only hydrodynamic interactions: localization consists in a change in configuration at the grain scale. From the concentration and velocity profiles, we have provided for the first time local measurements of the concentration dependence of viscosity; we find a Krieger-Dougherty law $η(ϕ)=η_0(1-ϕ/0.605)^{-2}$. Shear induced migration is almost instantaneous and seems inconsistent with most observations: it would imply that the diffusion coefficients strongly depend on the concentration. We finally propose a simple constitutive law for dense suspensions, based on a purely viscous behavior, that accounts for all the macroscopic and local observations.

cond-mat.soft↗

arXiv2001

Rheology of a confined granular material

We study the rheology of a granular material slowly driven in a confined geometry. The motion is characterized by a steady sliding with a resistance force increasing with the driving velocity and the surrounding relative humidity. For lower driving velocities a transition to stick-slip motion occurs, exhibiting a blocking enhancement whith decreasing velocity. We propose a model to explain this behavior pointing out the leading role of friction properties between the grains and the container's boundary.

cond-mat↗

arXiv1999

An introduction to perturbative and non-perturbative string theory

In these lectures we give a brief introduction to perturbative and non-perturbative string theory. The outline is the following: 1. Introduction to perturbative string theory 1.1 From point particle to extended objects 1.2 Free closed and open string spectrum 1.3 Compactification on a circle and T-duality 1.4 The Superstring: type IIA and IIB 1.5 Heterotic string and orbifold compactifications 1.6 Type I string theory 1.7 Effective field theories References 2. Introduction to non-perturbative string theory 2.1 String solitons 2.2 Non-perturbative string dualities 2.3 M-theory 2.4 Effective field theories and duality tests References

hep-th↗