Yielding and Flow in Adhesive and Nonadhesive Concentrated Emulsions

Abstract

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...

Jamming in Sheared Foams and Emulsions, Explained by Critical Instability of the Films between Neighboring Bubbles and Drops

“…Non-homogeneous flow, often discussed in terms of "shear banding" or "jammingunjamming transitions", attracted researchers' attention in several areas, because it appears as a generic phenomenon in various systems, such as glassy and granular materials, concentrated suspensions, foams, emulsions, and micellar solutions [1][2][3][4][5][6][7][8][9][10]. This phenomenon is still poorly understood and appropriate theoretical modeling, beyond the phenomenological description, is missing.…”

“…Note that no quantitative agreement is expected for these data: For the large bubbles studied in [8], gravity is expected to accelerate film thinning, thus leading to higher experimental critical rates. On the other side, the extremely small size of the emulsion drops studied in [10] corresponds to very fast film thinning, see Eq.…”

“…Also, the high capillary pressure of these small drops and the fact that Φ ≈ 0.73 was around the sphere close-packing mean that the drops could behave like solid spheres before jamming in [10], i.e. Eq.…”

Spontaneous formation of permanent shear bands in a mesoscopic model of flowing disordered matter

“…The second class of materials is characterized by flow curves that exhibit a minimum, indicating an instability of homogeneous flow in the region where dσ /dγ < 0, and leading to heterogeneous flow in the form of shear bands 1 . These two classes have been distinguished in several recent works as "simple" fluids without shear banding behaviour and "thixotropic" fluids, that phase separate in a solid and a fluid phase [2][3][4][5] .…”

Slow dynamics, aging, and glassy rheology in soft and living matter

“…Owing to their larger sizes and slower relaxation times when compared to, say, atomic fluids, the structure, dynamics and mechanical response of soft materials can be studied experimentally by common laboratory techniques such as microscopy, light scattering and rheology. The interaction between the individual constituents of soft materials can be tuned easily [17] and this feature is exploited for example in the verification of the predictions of the MCT for repulsive and attractive colloidal glasses [18,19]. For a more complete discussion, we would like to refer the readers to a recent review article by Cipelletti and Ramos [20].…”