Impact of Attractive Interactions on the Rheology of Dense Athermal Particles

Abstract

Using numerical simulations, the rheological response of an athermal assembly of soft particles with tunable attractive interactions is studied in the vicinity of jamming. At small attractions, a fragile solid develops and a finite yield stress is measured. Moreover, the measured flow curves have unstable regimes, which lead to persistent shearbanding. These features are rationalized by establishing a link between the rheology and the inter-particle connectivity, which also provides a minimal model to describe the flow curves.PACS numbers: 83.10. Rs,83.60.Wc,66.20.Cy In nature, soft disordered solids occur in different forms (eg. gels, emulsions, colloids, foams, grains etc) across a wide range of packing fractions φ, which is made possible by the tuning of particle interactions. The flow properties of these soft materials have been harnessed for various applications, e.g. in the food or the chemical industry. Thus, understanding the role of particle interactions and the corresponding mechanisms which lead to observed rheological behaviour is an important recurrent theme.For non-Brownian suspensions of frictionless repulsive spheres, it is observed that ramping up the packing fraction results in the occurrence of jamming at φ = φ J [1, 2]. The rheological signature of the onset of jamming is the development of a finite yield stress at φ J [3]. For Brownian suspensions of such particles, it has been shown that a yield stress exists at φ < φ J , due to the presence of thermal vibrations [4]. A similar systematic investigation of how the jamming paradigm is changing upon the introduction of attractive particle interactions is still missing. It is known that at smaller φ, such systems do exhibit finite yield stress [5,9,19,24]; but, a quantitative bridge with the jamming scenario needs to be developed.Such studies are also needed since shear-banding, the phenomenon of spatially inhomogeneous flows observed in many soft yield-stress fluids [6, 7], has often been attributed to attractive interactions [8,9]. In general, persistent occurrence of shear-bands has been linked to nonmontonic constitutive laws leading to flow instabilities [6, 10] (e.g. in micelles [11]). It is not known how interparticle attractions could result in such instabilities.Conceptually, one can imagine the steady flowing state to be a regime where there is a continuous competition between the rupturing induced by shear and processes that try to restore local structure. Theoretical models suggest that non-monotonic flow curves can occur due to long-lived local fluidizations when the post-rupture restructuring takes a very long time [12][13][14]. However, experiments and numerical simulations have shown that for φ > φ J , no such instabilities occur in the flow curve for either repulsive or attractive systems [15][16][17][18]. The question now arises whether for φ < φ J , a short-ranged attraction which introduces a new lengthscale for structure formation leads to longer restoration timescales and if this is indeed the origin of a shear banding ...

Shear-Induced Heterogeneity in Associating Polymer Gels: Role of Network Structure and Dilatancy

“…[37] for a more quantitative measure through the structure factor), and (iv) both bands are appreciably flowing. These observations are in contrast to the constitutive-instability (dσ xz =d_ γ < 0) mechanism [17] (that has been invoked to explain the recent finding of shear bands in attractive, dense athermal (non-Brownian) particles [44]) wherein the stress and shear rates of the bands remain constant and the width of the bands increases linearly with increasing shear [17,27]. Rather, these observations are consistent with a flow instability triggered by a strong coupling between shear and concentration [20].…”

Universal rescaling of flow curves for yield-stress fluids close to jamming

“…Effects of particle friction and attraction will change the local dynamics and bulk rheology and have been studied, e.g., in Refs. [73,74].…”

Arrest stress of uniformly sheared wet granular matter

“…We expect that in this range the repulsive forces gives rise to an additional contribution to the stress, as observed in [17].…”

“…This is guaranteed by enforcing n > 1. To be consistent, we choose a quadratic profile, n = 2, which is the most trivial choice given by the Bagnold scaling [17,29].…”

“…Moreover, in a recent study, the difference of the static and dynamic yield stress is pinned down to the nonmonotonicity of the flow curves. Furthermore, it was shown that the nonmonotonicity of the flow curves indicates a shear banding instability [17].…”