Hervé Tabuteau

Wall slip and yielding in pasty materials

We carried out systematic rheometrical tests under controlled stress with smooth and rough parallel disks, along with magnetic resonance imaging (MRI) tests in coaxial cylinder geometry, with foam and a model concentrated emulsion. At low shear stress wall slip appears to occur but the bulk fluid remains static, as proved by the fact that in this regime the apparent shear rate obtained for a given shear stress is inversely proportional to the gap between the disks. At high shear stress data with different surface types and gaps coincide, suggesting that wall slip is negligible in this regime. In parallel, MRI results show that, in contrast with the apparent, simple, yielding behavior observed in usual rheometry, there is an abrupt transition from a finite shear rate to a static one at critical stress. This critical shear rate precisely corresponds to the transition between the two regimes of slip. This suggests that different flow regimes occur with these materials: (1) at low stress, with smooth surfaces a layer of a different material is sheared along the solid surfaces whereas the rest of the fluid does not flow; with rough surfaces there is no flow; (2) beyond a critical stress, for both surface types, the bulk fluid starts to flow but the shear is localized in a thin layer; then the thickness of this layer increases when stress is applied; (3) for both surface types homogeneous flow is obtained only beyond slightly larger stress, which is associated to a critical, apparent shear rate.We carried out systematic rheometrical tests under controlled stress with smooth and rough parallel disks, along with magnetic resonance imaging (MRI) tests in coaxial cylinder geometry, with foam and a model concentrated emulsion. At low shear stress wall slip appears to occur but the bulk fluid remains static, as proved by the fact that in this regime the apparent shear rate obtained for a given shear stress is inversely proportional to the gap between the disks. At high shear stress data with different surface types and gaps coincide, suggesting that wall slip is negligible in this regime. In parallel, MRI results show that, in contrast with the apparent, simple, yielding behavior observed in usual rheometry, there is an abrupt transition from a finite shear rate to a static one at critical stress. This critical shear rate precisely corresponds to the transition between the two regimes of slip. This suggests that different flow regimes occur with these materials: (1) at low stress, with smooth surfaces...

Drag force on a sphere in steady motion through a yield- stress fluid

We have studied the motion of spheres falling through yield-stress Carbopol gels. We measured the velocity of the falling sphere as a function of time and sphere density. Reproducible results were obtained when the experimental fluids were carefully prepared and homogenized. Three regimes of motion were observed. Spheres of high enough density reached a constant terminal velocity, as in Newtonian fluids. Below a critical density, the sphere came to a complete stop, while in an intermediate regime, the sphere continued to move but with a velocity which steadily decreased with time. We have also carefully characterized the rheological behavior of the fluids. The flow regimes observed for the falling sphere are analogous to those observed in creep tests for different applied stress levels. The yielding criterion and the drag force on the sphere obtained from our data are in excellent agreement with the longstanding but previously unconfirmed theoretical predictions of Beris et al. [J. Fluid Mech. 158, 219–24...

Linear viscoelasticity of entangled wormlike micelles bridged by telechelic polymers: An experimental model for a double transient network

We survey the linear viscoelasticity of a new type of transient network: bridged wormlike micelles, whose structure has been characterized recently by Ramos and Ligoure [Macromolecules 40, 1248–1251 (2007)]. This composite material is obtained by adding telechelic copolymers (water-soluble chains with hydrophobic stickers at each extremity) to a solution of entangled wormlike micelles (WM). For comparison, naked WM and WM decorated by amphiphilic copolymers are also investigated. While these latter systems exhibit almost a same single ideal Maxwell behavior, solutions of bridged WM can be described as two Maxwell fluids component blends, characterized by two markedly different characteristic times, τfast and τslow, and two elastic moduli, Gfast and Gslow, with Gfast⪢Gslow. We show that the slow mode is related to the viscoelasticity of the transient network of entangled WM, and the fast mode to the network of telechelic active chains (i.e., chains that do not form loops but bridge two micelles). The dependence of the viscoelasticity with the surfactant concentration, ϕ, and the sticker-to-surfactant molar ratio, β, is discussed. In particular, we show that Gfast is proportional to the number of active chains in the material, ϕβ. Simple theoretical expectations then allow evaluation of the bridges/loops ratio for the telechelic polymers.We survey the linear viscoelasticity of a new type of transient network: bridged wormlike micelles, whose structure has been characterized recently by Ramos and Ligoure [Macromolecules 40, 1248–1251 (2007)]. This composite material is obtained by adding telechelic copolymers (water-soluble chains with hydrophobic stickers at each extremity) to a solution of entangled wormlike micelles (WM). For comparison, naked WM and WM decorated by amphiphilic copolymers are also investigated. While these latter systems exhibit almost a same single ideal Maxwell behavior, solutions of bridged WM can be described as two Maxwell fluids component blends, characterized by two markedly different characteristic times, τfast and τslow, and two elastic moduli, Gfast and Gslow, with Gfast⪢Gslow. We show that the slow mode is related to the viscoelasticity of the transient network of entangled WM, and the fast mode to the network of telechelic active chains (i.e., chains that do not form loops but bridge two micelles). The depen...

Drag on a sphere moving through an aging system

We have investigated the drag on a sphere falling through a clay suspension that has a yield stress and exhibits rheological aging. The drag force increases with both speed and the rest time between the preparation of the system and the start of the experiment, but there exists a non-zero minimum speed below which steady motion is not possible. We find that only a very thin layer of material around the sphere is fluidized when it moves, while the rest of the suspension is deformed elastically. This is in marked contrast to what is found for yield-stress fluids that do not age.