Dramatic effect of fluid chemistry on cornstarch suspensions: Linking particle interactions to macroscopic rheology

Abstract

Suspensions of cornstarch in water exhibit strong dynamic shear-thickening. We show that partly replacing water by ethanol strongly alters the suspension rheology. We perform steady and nonsteady rheology measurements combined with atomic force microscopy to investigate the role of fluid chemistry on the macroscopic rheology of the suspensions and its link with the interactions between cornstarch grains. Upon increasing the ethanol content, the suspension goes through a yield-stress fluid state and ultimately becomes a shear-thinning fluid. On the cornstarch grain scale, atomic force microscopy measurements reveal the presence of polymers on the cornstarch surface, which exhibit a cosolvency effect. At intermediate ethanol content, a maximum of polymer solubility induces high microscopic adhesion which we relate to the macroscopic yield stress.Suspensions are mixtures of undissolved particles in a liquid. They are literally found all around us: mud, paints, pastes and blood [1]. The viscosity of a dense suspension can vary by orders of magnitude in a small shear rate interval [2]. Subjected to an increasing shear rate, dense suspensions first tend to become less viscous (shear-thinning) and then more viscous (shearthickening). The viscosity of some suspensions, especially non-Brownian ones, may increase so much that they effectively become solid [3]. Although standard rheology measurements provide a great tool to study this phenomenon [e.g. 4, 5], they are mainly limited to steadystate conditions.Many studies point out that dense suspensions exhibit remarkable dynamic phenomena emerging under non-steady-shear conditions: stable holes in thin vibrated layers [6], non-monotonic settling [7], dynamic compaction front [8] or fracturing [3]. Oscillatory rheology helps to describe some of these dynamic behaviours [9], but remains limited to constant volume conditions. Dynamic shear-thickening has been widely investigated [10], but its physical origin remains an active debate. Although several parameters seem to contribute to it (e.g. particles size [11], shape [12] or roughness [13]), it has become increasingly clear that frictional and non-contact interactions between particles play a key role [14,15]. Such interactions are easily modified in numerical simulations, but present a real challenge in experiments. Consequently, only few experimental studies addressthe role of particle-particle interactions in dense suspensions rheology [e.g. 5, 16] however lacking systematic variation of these interactions. Moreover, direct measurements of these interactions in relation to the rheology are also lacking so far.Here, we directly probe the microscopic interactions between individual particles and explore their link with * adeline.pons@normalesup.org the macroscopic rheology for dense cornstarch (CS) suspensions. The archetypical suspension of CS grains in water exhibits a strong dynamic shear-thickening [3,[6][7][8]. Interestingly, Taylor [17] shows that replacing water by polypropylene glycol in CS suspensions completely su...

The role of friction in the yielding of adhesive non-Brownian suspensions

“…Cornstarch in aqueous media is a model for the rheology of purely repulsive nB suspensions, showing characteristic friction-driven shear thickening at a fixed onset stress 9,25 . When cornstarch is dispersed in non-aqueous solvents, shear thickening is no longer observed and a finite yield stress arises 26,27 . We disperse previouslyemployed 9,25 cornstarch (Sigma Aldrich) in sunflower oil (Flora) to form a model adhesive nB suspension.…”

Competing Timescales Lead to Oscillations in Shear-Thickening Suspensions

“…Since t c /t r = 5 × 10 −4 1, the strain-dependent mechanism dominates away from jamming [30]. Interestingly, t r ≈ 0.24 s is comparable to the relaxation time for cornstarch grains pushed into adhesive contact, ∼ 0.5 s, so that surface chemistry matters [36].…”

Penetration and bouncing during impact in shallow cornstarch suspensions

“…Additionally, if the layer depth is large enough to allow the impactor to reach a quasi-terminal velocity [7], then the deviation time increases, presumably due to melting of the solidified front. In this way, we connect the literature that deals with the settling of an object in a deep cornstarch layer [7,8,14] with the work concentrating on (high-speed) impact on a cornstarch layer [6,11,13,19].…”

“…Four different cornstarch volume fractions Cs were tested, namely 0.38, 0.40, 0.42, and 0.44. We initially obtained the mass fraction of the suspension, then we converted it to volume fraction taking the density of cornstarch as: Cs = 1, 542 kg/m 3 [14] and the density of water as: w = 994 kg/m 3 , measured with a density meter (DMA 35, Anton Paar). In this conversion, we have neglected the effect of the porosity of the cornstarch grains [5], which may cause the true volume fractions to be slightly different from the reported ones, but will not affect our conclusions.…”