arXiv subjects

Sebastien Manneville

Publications and source records attributed to Sebastien Manneville.

26 records · Page 2Linked to original sources

arXiv2009

Influence of boundary conditions on yielding in a soft glassy material

The yielding behavior of a sheared Laponite suspension is investigated within a 1 mm gap under two different boundary conditions. No-slip conditions, ensured by using rough walls, lead to shear localization as already reported in various soft glassy materials. When apparent wall slip is allowed using a smooth geometry, the sample is shown to break up into macroscopic solid pieces that get slowly eroded by the surrounding fluidized material up to the point where the whole sample is fluid. Such a drastic effect of boundary conditions on yielding suggests the existence of some macroscopic characteristic length that could be connected to cooperativity effects in jammed materials under shear.

cond-mat.soft↗

arXiv2006

Comment on ``Large Slip of Aqueous Liquid Flow over a Nanoengineered Superhydrophobic Surface'' by C-H Choi and C Kim

In a recent Letter (Phys. Rev. Lett. vol 96, 066001 (2006), ref [1]), Choi and Kim reported slip lengths of a few tens of microns for water on nanoengineered superhydrophobic surfaces, on the basis of rheometry (cone-and-plate) measurements. We show that the experimental uncertainty in the experiment of Ref. [1], expressed in term of slip lengths, lies in the range 20 - 100 micrometers, which is precisely the order of magnitude of the reported slip lengths. Moreover we point out a systematic bias expected on the superhydrophobic surfaces. We thus infer that it is not possible to draw out any conclusion concerning the existence of huge slip lengths in the system studied by Choi and Kim.

cond-mat.soft↗

arXiv2004

Signature of elasticity in the Faraday instability

We investigate the onset of the Faraday instability in a vertically vibrated wormlike micelle solution. In this strongly viscoelastic fluid, the critical acceleration and wavenumber are shown to present oscillations as a function of driving frequency and fluid height. This effect, unseen neither in simple fluids nor in previous experiments on polymeric fluids, is interpreted in terms of standing elastic waves between the disturbed surface and the container bottom. It is shown that the model of S. Kumar [Phys. Rev. E, {\bf 65}, 026305 (2002)] for a viscoelastic fluid accounts qualitatively for our experimental observations. Explanations for quantitative discrepancies are proposed, such as the influence of the nonlinear rheological behaviour of this complex fluid.

cond-mat.soft↗

arXiv2003

A spatio-temporal study of rheo-oscillations in a sheared lamellar phase using ultrasound

We present an experimental study of the flow dynamics of a lamellar phase sheared in the Couette geometry. High-frequency ultrasonic pulses at 36 MHz are used to measure time-resolved velocity profiles. Oscillations of the viscosity occur in the vicinity of a shear-induced transition between a high-viscosity disordered fluid and a low-viscosity ordered fluid. The phase coexistence shows up as shear bands on the velocity profiles. We show that the dynamics of the rheological data result from two different processes: (i) fluctuations of slip velocities at the two walls and (ii) flow dynamics in the bulk of the lamellar phase. The bulk dynamics are shown to be related to the displacement of the interface between the two differently sheared regions in the gap of the Couette cell. Two different dynamical regimes are investigated under applied shear stress: one of small amplitude oscillations of the viscosity ($δη/η\simeq 3$%) and one of large oscillations ($δη/η\simeq 25$%). A phenomenological model is proposed that may account for the observed spatio-temporal dynamics

cond-mat.soft↗

arXiv2002

Velocity profiles in shear-banding wormlike micelles

Using Dynamic Light Scattering in heterodyne mode, we measure velocity profiles in a much studied system of wormlike micelles (CPCl/NaSal) known to exhibit both shear-banding and stress plateau behavior. Our data provide evidence for the simplest shear-banding scenario, according to which the effective viscosity drop in the system is due to the nucleation and growth of a highly sheared band in the gap, whose thickness linearly increases with the imposed shear rate. We discuss various details of the velocity profiles in all the regions of the flow curve and emphasize on the complex, non-Newtonian nature of the flow in the highly sheared band.

cond-mat.soft↗

arXiv2002

An optical fiber based interferometer to measure velocity profiles in sheared complex fluids

We describe an optical fiber based interferometer to measure velocity profiles in sheared complex fluids using Dynamic Light Scattering (DLS). After a review of the theoretical problem of DLS under shear, a detailed description of the setup is given. We outline the various experimental difficulties induced by refraction when using a Couette cell. We also show that homodyne DLS is not well suited to measure quantitative velocity profiles in narrow-gap Couette geometries. On the other hand, the heterodyne technique allows us to determine the velocity field inside the gap of a Couette cell. All the technical features of the setup, namely its spatial resolution ($\approx 50$--$100 μ$m) and its temporal resolution ($\approx 1$ s per point, $\approx 1$ min per profile) are discussed, as well as the calibration procedure with a Newtonian fluid. As briefly shown on oil-in-water emulsions, such a setup permits one to record both velocity profiles and rheological data simultaneously

cond-mat.soft↗

arXiv2002

Towards local rheology of emulsions under Couette flow using Dynamic Light Scattering

We present local velocity measurements in emulsions under shear using heterodyne Dynamic Light Scattering. Two emulsions are studied: a dilute system of volume fraction $ϕ=20$ % and a concentrated system with $ϕ=75$ %. Velocity profiles in both systems clearly show the presence of wall slip. We investigate the evolution of slip velocities as a function of shear stress and discuss the validity of the corrections for wall slip classically used in rheology. Focussing on the bulk flow, we show that the dilute system is Newtonian and that the concentrated emulsion is shear-thinning. In the latter case, the curvature of the velocity profiles is compatible with a shear-thinning exponent of 0.4 consistent with global rheological data. However, even if individual profiles can be accounted for by a power-law fluid (with or without a yield stress), we could not find a fixed set of parameters that would fit the whole range of applied shear rates. Our data thus raise the question of the definition of a global flow curve for such a concentrated system. These results show that local measurements are a crucial complement to standard rheological tools. They are discussed in light of recent works on soft glassy materials.

cond-mat.soft↗

arXiv1995

Secondary Instabilities of Surface Waves on Viscous Fluids in the Faraday Instability

Secondary instabilities of Faraday waves show three regimes: (1) As seen previously, low-viscosity (nu) fluids destabilize first into squares. At higher driving accelerations a, squares show low-frequency modulations corresponding to the motion of phase defects, while theory predicts a stationary transverse amplitude modulation (TAM). (2) High-nu fluids destabilize first to stripes. Stripes then show an oscillatory TAM whose frequency is incommensurate with the driving frequency. At higher a, the TAM undergoes a phase instability. At still higher a, edge dislocations form and fluid droplets are ejected. (3) Intermediate-nu fluids show a complex coexistence of squares and stripes, as well as stationary and oscillatory TAM instabilities of the stripes.

patt-sol↗