Tunable shear thickening in active non-Brownian suspensions

Using particle-based simulations, this study demonstrates that self-propulsion in active, non-Brownian suspensions can counteract friction-mediated shear thickening to induce a tunable "dethickening" effect, which follows a universal scaling framework based on dimensionless active stress.

The Gist

Imagine you are stirring a thick pot of oatmeal or a bucket of wet sand. Usually, the faster you stir, the harder it gets to move. The mixture suddenly turns into a solid-like block that resists your spoon. This is called shear thickening. It's a bit like a crowd of people in a hallway: if they walk slowly, they can slip past each other easily. But if they all rush at once, they bump into each other, get stuck, and the whole group jams up.

In the world of physics, this "jamming" happens because the tiny particles inside the liquid start rubbing against each other (friction) when pushed hard enough.

The Problem: When Things Get Too Sticky

In industries like food processing or manufacturing, this sudden thickening is a nightmare. It can clog pipes, stop machines, and make it impossible to pump materials. Scientists have been trying to find a "magic switch" to stop this jamming without changing the recipe of the liquid itself.

The Solution: Giving Particles a "Kick"

This paper introduces a clever new idea: What if the particles could move on their own?

The researchers used computer simulations to create a liquid filled with tiny, non-smooth grains. But instead of just sitting there waiting to be pushed, they gave these grains a "self-propulsion" ability—like tiny robots or bacteria that can swim or wiggle around on their own.

Think of it like this:

• Normal Liquid: Imagine a crowded dance floor where everyone is standing still. If you push the crowd (apply stress), they bump into each other and freeze.
• Active Liquid: Now, imagine everyone on that dance floor is also doing a little jig or hopping around randomly. Even if you push the crowd, their constant hopping keeps them from locking arms and freezing solid. They stay loose and fluid.

How It Works: The "Dethickening" Effect

The study found that by turning up the "energy" of these self-moving particles, they could reverse the thickening.

1. The Jam: When you push a normal thick liquid hard, the particles form a rigid network of "force chains" (like a human pyramid) that locks everything in place.
2. The Fix: When the particles are "active" (self-moving), their random hopping breaks up these pyramids before they can form. It's like having a bunch of hyperactive kids running through a line of people holding hands; the line can't stay connected.
3. The Result: The liquid stays runny and easy to stir, even when you push it very hard. The researchers call this "dethickening."

Tuning the Flow

The best part is that this effect is tunable.

• If the particles are only slightly active, they might not do much.
• If they are very active, the liquid becomes very fluid.
• The scientists found a way to measure this "activity" and predict exactly how runny the liquid will be. It's like having a dimmer switch for the thickness of your liquid.

Why This Matters

This discovery is exciting because it offers a new way to control thick liquids without adding chemicals or changing the ingredients.

• Real-world analogy: Imagine a traffic jam. Usually, you can't fix it by just honking (pushing harder). But if every car had a little engine that made them wiggle and change lanes randomly (activity), the traffic jam might dissolve, and cars could keep moving smoothly even during rush hour.

The Big Picture

The researchers showed that this "self-moving" trick works just like other methods scientists have tried, such as shaking the liquid or hitting it with sound waves. They proved that all these different methods follow the same underlying rules.

In short: By giving the tiny particles inside a thick liquid a little bit of "life" and movement, we can stop them from getting stuck together. This turns a stubborn, solid-like block back into a smooth, flowing liquid, giving engineers a powerful new tool to control how materials move.

Technical Summary

Here is a detailed technical summary of the paper "Tunable shear thickening in active non-Brownian suspensions" by Bhanu Prasad Bhowmik and Christopher Ness.

1. Problem Statement

Dense suspensions of non-Brownian particles often exhibit shear thickening, a phenomenon where viscosity increases with flow rate or applied stress. In many industrial applications (e.g., food processing, concrete pumping), this behavior leads to operational challenges like pipe blockages and flow stagnation.

• Mechanism: Shear thickening arises when applied stress overcomes repulsive forces between particles, allowing them to form frictional contacts and force chains. At high volume fractions ($\phi$), this can lead to Discontinuous Shear Thickening (DST) or shear jamming.
• Current Solutions: Traditional methods to control this involve changing the chemical formulation (e.g., adding surfactants) or applying external mechanical perturbations like Orthogonal Superposition (OSP) flow, vibrations, or acoustic noise. These methods disrupt force chains but require external energy input or compositional changes.
• Research Gap: The authors investigate whether self-propulsion (activity) inherent to the particles (mimicking active matter like bacteria or synthetic microswimmers) can serve as an intrinsic, tunable mechanism to suppress shear thickening without altering the suspension's chemical composition.

2. Methodology

The study employs particle-based simulations using a variant of the Discrete Element Method (DEM) to model a dense suspension of $N=10^3$ non-Brownian particles.

• Particle Model:
  • Geometry: Bidisperse mixture of spheres with radii $a$ and $1.4a$.
  • Interactions:
    • Repulsive: Coulombic repulsion prevents direct contact at low stress.
    • Hydrodynamic: Short-range lubrication forces model viscous drag and torque in the suspending fluid.
    • Frictional: When particles overcome repulsion ($\delta_{ij} < 0$), history-dependent Hookean contact forces (normal and tangential) with a friction coefficient $\mu_p = 1$ are activated.
  • Activity: Particles are modeled as run-and-tumble active agents. Each particle experiences a constant self-propulsion force $f_a$ with a random orientation vector $\mathbf{e}$. The orientation persists for a time $\tau_p$ before re-randomizing.
• Simulation Setup:
  • Shear flow is imposed via Lees-Edwards boundary conditions.
  • Dimensionless Control Parameters:
    • $\sigma^* = \sigma_{xy} a^2 / f_r$: Ratio of hydrodynamic shear stress to repulsive stress.
    • $\sigma^*_a = f_a / (6\pi\eta_f \dot{\gamma} a^2)$: Ratio of active stress to hydrodynamic stress.
  • The persistence time is fixed such that $\tau_p \dot{\gamma} \sim 1$ to mimic the saturation effects seen in OSP experiments.

3. Key Contributions

1. Demonstration of "Dethickening" via Activity: The paper establishes that increasing particle self-propulsion can actively reduce the viscosity of a shear-thickening suspension, effectively reversing the thickening process at high stresses.
2. Microstructural Mechanism: The authors identify that self-propulsion introduces isotropic dynamics that compete with the flow-driven formation of anisotropic force chains. This activity disrupts the frictional contact network, reducing the number of frictional contacts ($n_{fc}$) even under high shear stress.
3. Universal Scaling Framework: The study demonstrates that the rheology of active suspensions obeys the same universal scaling framework previously proposed for mechanically perturbed (OSP) and acoustically perturbed suspensions. This suggests a unified physical description for tunable shear thickening across different disturbance types.
4. Phase Diagram Modification: The authors map how activity alters the $\sigma^* - \phi$ phase diagram, showing that self-propulsion shrinks the regions of Discontinuous Shear Thickening (DST) and Shear Jamming.

4. Key Results

• Viscosity Behavior:
  • Passive Case ($\sigma^*_a = 0$): As $\sigma^*$ increases, viscosity rises smoothly and then jumps (DST) as frictional contacts proliferate.
  • Active Case: At a fixed high $\sigma^*$, increasing $\sigma^*_a$ initially causes a sharp drop in viscosity ("dethickening"). However, at very high $\sigma^*_a$, viscosity begins to rise again due to activity-induced diffusion and inertial effects.
• Microstructure Analysis:
  • The mean number of frictional contacts ($n_{fc}$) decreases monotonically with increasing $\sigma^*_a$.
  • Visualizations of force chains show that while high stress creates a spanning, anisotropic network in passive systems, high activity breaks these chains, resulting in isolated, pairwise contacts and a fluid-like state.
• Scaling Collapse:
  • By defining a scaling variable $\tilde{x}_a = f(\sigma^*) C(\phi) g(\sigma^*_a) / (\phi_0 - \phi)$, the authors successfully collapse data from various volume fractions and stress levels onto a single master curve.
  • The scaling function $F(\tilde{x}_a)$ exhibits two power-law regimes:
    • Exponent $\approx -2$: Associated with isotropic jamming (lubrication dominated).
    • Exponent $\approx -1.48$: Associated with friction-dominated jamming.
• Phase Diagram Shifts:
  • Increasing $\sigma^*_a$ pushes the boundary of the shear jamming region toward higher volume fractions ($\phi$).
  • The DST region also shrinks and shifts toward the random close packing limit ($\phi_0$), though it does not vanish entirely within the simulated parameter range (unlike some OSP results where it might vanish).

5. Significance and Implications

• Fundamental Physics: The work bridges the gap between active matter physics and granular rheology. It proves that internal activity can act as a "tunable knob" to control macroscopic flow properties, offering a new paradigm for understanding how non-equilibrium dynamics influence jamming transitions.
• Universal Description: The successful application of the OSP scaling framework to active systems suggests that the mechanism of "dethickening" is universal: any perturbation that introduces sufficient isotropic motion to break force chains (whether external vibration, acoustic noise, or internal self-propulsion) follows the same rheological laws.
• Industrial Application: This research points toward the potential of designing "smart" suspensions where viscosity can be dynamically controlled by the activity of the particles (e.g., using light-activated or chemically fueled swimmers) without adding external mechanical devices or chemical additives. This could prevent blockages in pipelines or optimize mixing processes in real-time.
• Future Directions: The authors note that while they fixed the persistence time, varying $\tau_p$ could further optimize the phase diagram, potentially eliminating DST entirely. This opens avenues for future research into the interplay between activity timescales and shear rates.