Tunable shear thickening in active non-Brownian suspensions

Abstract

We study tunable shear thickening in active suspensions of non-Brownian, repulsive, frictional grains using particle-based simulation, finding that activity augments the rheology beyond the friction-mediated shear thickening paradigm. Specifically, increasing particle self-propulsion drives a viscosity-reducing `dethickening' of the system at large stress, where the material would otherwise be in a thickened, highly viscous state. Self-propulsion introduces additional isotropic dynamics to the particles, which compete with the flow-driven formation of frictional contacts. The degree of dethickening can thus be tuned by varying a suitably-defined dimensionless active stress that quantifies this competition. Recognising the parallels between self-propulsion and other contemporary routes to dethickening, we demonstrate that our data obey a recently proposed scaling framework, supporting a universal description of the tunable rheology of dense suspensions.

Figures (4)

• Figure 1: Relative viscosity of an active shear thickening suspension. Shown are the variations of relative viscosity $\eta_r$ as functions of (a) the dimensionless applied stress $\sigma^{*} = \sigma_{xy}a^2/f^r$ for fixed active stress $\sigma^*_a = 0$ and (b) the dimensionless active stress $\sigma_a^{*} = f^a/6\pi\eta_f\dot{\gamma}a^2$ for a range of volume fractions $\phi$ and for fixed applied stress $\sigma^*=7.29$. The legend in (a) applies also to (b).
• Figure 2: Microstructure of an active shear thickening suspension. Shown are the variations of the mean number of frictional contacts $n_\mathrm{fc}$ as a function of (a) the dimensionless stress $\sigma^*$ and (b) the dimensionless active stress $\sigma_a^*$. (c) Shows an example force chain network taken from a typical configuration with $\phi = 0.56$ in the steady state with $\sigma^* = \infty$ and $\sigma_a^* = 0$. (d) and (e) show the same as (c), but now with $\sigma^* = 0.02$, $\sigma_a^* = 0$, and $\sigma^* = 7.29$, $\sigma_a^* = 198.95$ respectively.
• Figure 3: Scaling of the rheology data for active shear thickening suspensions. Shown in (a) is a collapse of the data from Figs. \ref{['fig1']}(a) and (b) for a range of $\phi$, $\sigma^*$ and $\sigma^*_a$, onto a single master curve with the scaling variable for our active system being $\tilde{x}^a = f(\sigma^*)C(\phi)g({\sigma_a}^*)/(\phi_0 - \phi)$. (b) Two power law regimes, with exponents $-2$ associated with isotropic jamming and exponent $-1.48$ (inset) associated with friction-dominated jamming. (c) Variation with volume fraction $\phi$ of the anisotropy factor $C(\phi)$; Sketches of the functional forms (d) $f(\sigma^*)$ and (e) $g(\sigma_a^*)$ used in the scaling variable for the data collapse.
• Figure 4: Tuning of the shear jamming and discontinuous shear thickening (DST) regions due to active stress. (a) Isotropic jamming (red), shear jamming (green), and DST (gray) regions in the $\sigma^* - \phi$ phase diagram in the absence of activity, with $\sigma_a^* = 0$. (b) Altered shear jamming and DST regions under a large active stress, $\sigma_a^* = 150$.