Amorphous glassy materials of diverse nature-concentrated emulsions, granular materials, pastes, molecular glasses-display complex flow properties, intermediate between solid and liquid, which are at the root of their use in many applications. A general feature of such systems, well documented yet not really understood, is the strongly nonlinear nature of the flow rule relating stresses and strain rates. Here we use a microfluidic velocimetry technique to characterize the flow of thin layers of concentrated emulsions, confined in gaps of different thicknesses by surfaces of different roughnesses. We find evidence for finite-size effects in the flow behaviour and the absence of an intrinsic local flow rule. In contrast to the classical nonlinearities of the rheological behaviour of amorphous materials, we show that a rather simple non-local flow rule can account for all the velocity profiles. This non-locality of the dynamics is quantified by a length, characteristic of cooperativity within the flow at these scales, that is unobservable in the liquid state (lower emulsion concentrations) and that increases with concentration in the jammed state. Beyond its practical importance for applications involving thin layers (for example, coatings), these non-locality and cooperativity effects have parallels in the behaviour of other glassy, jammed and granular systems, suggesting a possible fundamental universality.

A kinetic model for the elastoplastic dynamics of a jammed material is proposed, which takes the form of a nonlocal--Boltzmann-like--kinetic equation for the stress distribution function. Coarse graining this equation yields a nonlocal constitutive law for the flow, exhibiting as a key dynamic quantity the local rate of plastic events. This quantity, interpreted as a local fluidity, is spatially correlated with a correlation length diverging in the quasistatic limit, i.e., close to yielding. In line with recent experimental and numerical observations, we predict finite size effects in the flow behavior, as well as the absence of an intrinsic local flow curve.

International audienceIn this paper, we investigate the rheological behavior of jammed emulsions in microchannels on the basis of microvelocimetry techniques. We demonstrate that velocity profiles in this confined geometry cannot be accounted for by the bulk - Herschel-Bulkley - rheological flow curve measured independently in a rheometer. A strong dependence of the flow behavior on the confinement, pressure drop and surface roughness is evidenced, which cannot be described by classical rheological descriptions. We show that these behaviors can be rationalized on the basis of a non local rheological model, introducing the notion of local fluidity as a key rheological quantity. The model reproduces the experimental velocity profiles for any confinements and any surface nature. The non-locality is quantified by a length, z, characterizing the flow cooperativity of jammed emulsions, and typically of the order of several emulsion droplet diameters. We study the influence of volume fraction, droplet diameter, and emulsions polydispersity on this length

Motivated by its importance for microfluidic applications, we study the stability of jets formed by pressure-driven concentric biphasic flows in cylindrical capillaries. The specificity of this variant of the classical Rayleigh-Plateau instability is the role of the geometry which imposes confinement and Poiseuille flow profiles. We experimentally evidence a transition between situations where the flow takes the form of a jet and regimes where drops are produced. We describe this as the transition from convective to absolute instability, within a simple linear analysis using lubrication theory for flows at low Reynolds number, and reach remarkable agreement with the data.

The nonlinear rheological response of soft glassy materials is addressed experimentally by focusing on concentrated emulsions where interdroplet attraction is tuned through varying the surfactant content. Velocity profiles are recorded using ultrasonic velocimetry simultaneously to global rheological data in the Couette geometry. Our data show that non-adhesive and adhesive emulsions have radically different flow behaviors in the vicinity of yielding: while the flow remains homogeneous in the non-adhesive emulsion and the Herschel-Bulkley model for a yield stress fluid describes the data very accurately, the adhesive system displays shear localization and does not follow a simple constitutive equation, suggesting that the mechanisms involved in yielding transitions are not universal.PACS numbers: 83.60. La, 83.80.Iz, 83.50.Rp, 83.60.Rs The term "jamming" describes different ways by which a system of particles loses its ability to flow: increasing the volume fraction, lowering the temperature, or releasing some external stress [1,2]. It occurs in a wide variety of materials known as "soft glassy materials," ranging from polymers and colloids to granular assemblies [3,4] (for a recent review see Ref.[5]). The response of such systems to an external shear stress is characterized by two regimes: for stresses below the yield stress σ 0 they remain jammed and respond elastically, whereas for stresses above σ 0 they flow as liquids [6].A first way to investigate this stress-induced solid-fluid transition (hereafter referred to as the yielding transition) is to perform oscillatory shear experiments and to measure the viscoelastic moduli of the system during frequency or stress/strain sweeps. Very useful information on the yielding behavior can be gained from such measurements e.g. estimations of σ 0 [4,7], and when coupled to dynamic light scattering, the number of local rearrangements within the material [8,9].Another way to study the yielding transition is to probe the sample response to steady shear and to focus on the flow behavior deep into the nonlinear regime. Such experiments, which are the subject of the present contribution, have already attracted a lot of attention during the last decade. In particular magnetic resonance imaging (MRI) has shown that various colloidal suspensions and emulsions cannot flow at a uniform shear rate smaller than some critical valueγ c in the vicinity of the yield stress: under applied shear rate a "liquid" zone sheared at a rate larger thanγ c coexists with a jammed, solid-like region which disappears as the shear rate is increased [10]. Similar shear localization (or shear banding) had already been observed in thixotropic suspensions [11] and was confirmed very recently using MRI in a concentrated hard-sphere colloidal system [12]. This picture also emerges from molecular dynamics simulations of model glasses [13] and athermal systems [14].However at this stage it is not clear whether all systems that are jammed at rest display shear localization as they go through the yieldin...

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.PACS numbers: 83.80.Qr, 47.50.+d, 83.85.Ei Understanding the correlation between mechanical and structural response in non-Newtonian fluids submitted to high deformation rates is crucial on both fondamental and technological grounds [1]. Among the variety of complex fluids investigated in recent years, a wide class exhibits flow-structure coupling that leads to a strong shear-thinning behavior: along the steady-state flow curve (shear stress σ vs. shear rateγ), a drop of up to three orders of magnitude in the effective viscosity η = σ/γ is observed in a very narrow stress range leading to a stress plateau (for a review, see for instance Refs. [1,2]). In correlation with this stress plateau, bands of different micro-structures and normal to the velocity gradient appear. Such bands correspond to a new shearinduced structure (SIS), whose low viscosity is in general supposed to be responsible for the shear-thinning. This so-called shear-banding behavior has been observed in both ordered mesophases (lamellar, hexagonal, cubic) [3] and transient gels [4].A particularly well-documented example is the group of wormlike micellar systems of self-assembled surfactant molecules [5,6]. They consist of very long cylindrical aggregates whose configurations mimic polymer solutions. However their dynamics is strongly modified by the equilibrium character of the chains, which enables them to break and recombine [7]. Generically, one starts from an isotropic viscoelastic solution of these micelles above the semidilute regime, which behaves like a Maxwell fluid at low shear rates. Upon increasingγ and entering the nonlinear regime, the onset of the stress plateau for a critical shear rateγ 1 is associated with the nucleation and growth of highly birefringent bands, suggesting strong alignment of the micelles along the velocity direction [5,6]. As the shear rate is further increased aboveγ 1 , the new organization progressively fills the gap at almost constant stress, up to a second critical shear rateγ 2 . Aboveγ 2 , the system enters a second regime of apparently homogeneous structure, with a second branch of increasing stress. The flow curve of Fig. 1 is typical of micellar systems like that investigated in the present work. Such a stress plateau has been reported for concentrations close to the equilibrium isotropic-nematic (I-N) transition, where coupling between the order parameter an...

The process by which sheared suspensions go through a dramatic change in viscosity is known as discontinuous shear thickening. Although well-characterized on the macroscale, the microscopic mechanisms at play in this transition are still poorly understood. Here, by developing new experimental procedures based on quartz-tuning fork atomic force microscopy, we measure the pairwise frictional profile between approaching pairs of polyvinyl chloride and cornstarch particles in solvent. We report a clear transition from a low-friction regime, where pairs of particles support a finite normal load, while interacting purely hydrodynamically, to a high-friction regime characterized by hard repulsive contact between the particles and sliding friction. Critically, we show that the normal stress needed to enter the frictional regime at nanoscale matches the critical stress at which shear thickening occurs for macroscopic suspensions. Our experiments bridge nano and macroscales and provide long needed demonstration of the role of frictional forces in discontinuous shear thickening.

In this work, the flow of immiscible fluids in microchannels is studied. Flow pattern diagrams obtained in microfluidic chips are presented. Monodisperse droplets or parallel flows are obtained depending on the flow rate values of the aqueous phase and the oil phase. Transition from droplet regime to parallel flows cannot be described in terms of capillary numbers. Using confocal microscopy and high speed imaging, it was shown that droplets are formed through a blocking-pinching mechanism ruled by flow rate conservation. Conditions for parallel flow stability are quantified.