Structuring dynamics in attractive gels

Colloidal suspensions exhibit a wide variety of rheological behaviours when the strength or the range of the interaction changes or when the concentration or temperature is varied. We are interested in colloidal gels made of carbon black particles in a mineral oil. When carbon black gels are sheared within a narrow gap (smaller than about 500 micron), the particles do not remain homogeneously distributed but arrange into highly anisotropic structures perpendicular to the shear direction. A similar behavior has been reported in other attractive systems (flocculated emulsions, carbon nanotube suspensions and clay gels). We investigate the structuring dynamics experimentally through simultaneous rheological measurements and optical observations and by varying the particle concentration, the gap width and the shear rate. We also try to elucidate the physical mechanism underlying such structuring through molecular dynamics simulations.

Past research activities

Effect of acoustic radiation pressure on fluid interfaces

    with J.-P. Delville and R. Wunenburger

Faraday instability in complex fluids

    with P. Ballesta

Heterodyne dynamic light scattering coupled to rheometry

    with A. Colin, B. Pouligny and J.-B. Salmon

Ultrasonic harmonic imaging and acousto-photonic imaging

    with R. Roy and R. Cleveland

Ultrafast imaging of fluid flows using ultrasound

    with M. Fink and L. Sandrin

Sound-vorticity interaction

    with M. Fink, A. Maurel, P. Petitjeans, C. Prada and M. Tanter

Wavelet analysis of turbulent signals

    with A. Arneodo and J.-F. Muzy

Elasticity of DNA molecules

    with F. Caron, D. Chatenay, P. Cluzel and J.-L. Viovy

Faraday instability

    with J. Bechhoefer and V. Ego

Yielding of soft jammed materials

Soft jammed materials present an amorphous solidlike structure and, generally, glassy and aging properties. When stressed above a critical "yield" stress, these materials may flow like viscous liquids. We study this stress-induced solid-to-fluid transition in various jammed materials by focusing on the flow close to the yield stress. We have shown that both interparticle attraction (in concentrated emulsions) and boundary conditions (in thixotropic Laponite suspensions) may have a drastic influence on yielding. Moreover, in non-thixotropic simple yield stress fluids such as Carbopol dispersions, transient shear localization is observed before homogeneous flow is reached over time scales that diverge as a power-law with the applied shear rate. Under imposed shear stress, the fluidization time in Carbopol and in colloidal gels made of carbon black particles suggest an analogy with ductile and brittle fracture respectively. These results point to non-universal yielding processes that depend on the microscopic details of the jammed material.

Shear banding in complex fluids

Simple shear may induce strong structural changes in complex fluids: disorder-to-order transition in colloidal suspensions or in multilamellar vesicles, isotropic-to-nematic transition in wormlike micelles, etc. During such transitions, inhomogeneous flows are expected even if the Reynolds number is very low. In particular, the shear banding theory predicts that a stress plateau in the constitutive curve is associated to flows where the fluid separates into macroscopic bands of widely different viscosities. The presence of such shear bands indicate the coexistence of the shear-induced structure with the original structure. By measuring the velocity profiles during rheological experiments through dynamic light scattering or ultrasonic velocimetry, we have evidenced shear banding in various systems such as lamellar phases, wormlike micelles and copolymers. In most cases, our measurements also reveal a complex spatiotemporal behaviour of shear-banded flows characterized by instabilities of the interface between two shear bands, wall slip dynamics and/or three-dimensional flow.

Ultrasonic velocimetry in soft materials

We have developed a velocimetry technique based on the transmission of 36 MHz ultrasonic pulses through the fluid. When the material scatters ultrasound (if needed, the fluid may be seeded with particles of diameter 5-10 microns), the backscattered signal yields an ultrasonic speckle that reflects the scatterer distribution along the ultrasonic beam. By cross-correlating this speckle signal over various pulses, one recovers a velocity profile in 0.02 to 2 seconds with a spatial resolution of 40 microns. The temporal resolution of this technique allows us to record animations of the flow field over several hours. Our velocimetry setup is associated to a standard rheometer that allows the global rheological data in the Couette geometry (shear rate, shear stress, viscosity) to be recorded simultaneously to velocity profiles.