🔬 condensed matter

Acoustic modulation of shear thickening transition in dense adhesive suspensions

This study demonstrates that high-frequency ultrasound does not directly lower the viscosity of dense adhesive suspensions but instead delays the discontinuous shear thickening transition to higher shear rates by destabilizing fragile force networks through acoustic streaming and fluctuating hydrodynamic forces, thereby providing a mechanism to control flow instabilities in such materials.

Original authors: Aoxuan Wang, Fabrice Toussaint, Thomas Gibaud

Published 2026-05-01

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CC BY 4.0

Original authors: Aoxuan Wang, Fabrice Toussaint, Thomas Gibaud

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a bucket of wet sand mixed with water. If you stir it slowly, it flows like a liquid. But if you stir it violently, it suddenly turns into a solid block that refuses to move. This is called "shear thickening," and it's a common problem in industries that pump thick mixtures (like cement or cornstarch paste), because the pipes can get clogged or the pumps can break.

Scientists have known for a while that if you blast these thick mixtures with high-power ultrasound (sound waves too high for humans to hear), the mixture seems to get thinner and flow easier again. But nobody knew why this happened. Was the sound melting the particles? Was it breaking them apart?

This paper investigates exactly that question using a special mixture of cornstarch and water. Here is the simple breakdown of what they found:

The Big Discovery: It's Not Melting, It's Shifting

The researchers discovered that the ultrasound isn't actually making the mixture "thinner" in the way you might think. Instead, it's like moving the goalposts.

Think of the mixture as a car with a very sensitive accelerator. Normally, if you press the pedal (stir the mixture) past a certain point, the car suddenly locks its wheels and stops (it jams). The ultrasound doesn't fix the brakes; instead, it pushes that "locking point" much further down the road. You have to press the pedal much harder (stir much faster) before the mixture decides to jam.

So, the ultrasound doesn't change the nature of the mixture; it just delays the moment it turns into a solid.

The Secret Mechanism: The "Porous Sponge" Effect

To understand how this works, the authors used a clever analogy. They treated the thick, jammed cornstarch mixture like a sponge that is mostly solid but has tiny holes (pores) filled with water.

The Speed Difference: The ultrasound vibrates incredibly fast (about 15,000 times a second), while the thick mixture is slow to react. It's like shaking a heavy, wet sponge very quickly. The sponge itself (the cornstarch particles) is too heavy to move with the shake, but the water inside the tiny holes (the pores) gets squeezed back and forth violently.
The Tug-of-War: Inside this "sponge," the cornstarch particles are holding hands tightly, forming a rigid chain that makes the mixture solid. These "handshakes" are a mix of friction and sticky adhesive forces (like tiny bits of glue).
The Breaker: As the water in the pores sloshes back and forth at high speed, it creates tiny, chaotic currents. These currents push and pull on the particles, creating a "drag" force.
- Imagine the particles are holding hands in a line. The water sloshing around is like a mischievous child tugging on their arms from different angles.
- These tugs create a "noise" or instability in the chain. To keep the chain from breaking, the mixture needs to be squeezed much harder than usual.
- Because the ultrasound is constantly adding this "tugging noise," the mixture needs a much stronger stir to stay solid. If you don't stir it hard enough, the tugging wins, the chain breaks, and the mixture flows like a liquid again.

What About the "Sound Waves" Themselves?

The researchers also checked if the sound waves were creating a steady flow of water (called "acoustic streaming") that washed the particles away. They found that inside the tiny pores of the mixture, this effect was too weak to matter. The main hero was the rapid, chaotic sloshing of water inside the pores, not a steady river flowing through them.

The Takeaway

The paper concludes that ultrasound acts as a tuner for the mixture. It doesn't destroy the mixture's ability to thicken; it just makes the mixture "tougher" to jam. You have to apply more stress to get it to lock up.

This is a big deal because it gives engineers a way to control these thick, sticky mixtures in real-time. If a pipe is about to clog, you can blast it with ultrasound to "unlock" the jam and keep the flow moving, without having to stop the machine or change the chemical recipe of the mixture. It's like having a remote control that tells the mixture, "Not yet, keep flowing!"

1. Problem Statement

Dense suspensions of non-Brownian particles, such as cornstarch in water, exhibit Discontinuous Shear Thickening (DST). In this regime, the material undergoes a sudden transition from a fluid-like state to a solid-like, jammed state under high shear stress. This phenomenon is driven by the formation of frictional, load-bearing force networks and is often accompanied by large, intermittent stress fluctuations and density waves.

DST poses significant challenges in industrial processing (e.g., pumping, mixing, extrusion of cementitious slurries) because it can cause catastrophic equipment failure. While high-power ultrasound has been observed to reduce the apparent viscosity of such materials, the underlying physical mechanism remains unclear. Specifically, it is unknown whether ultrasound directly fluidizes the jammed network or alters the conditions under which the shear-thickening transition occurs.

2. Methodology

The authors investigated dense adhesive cornstarch suspensions using a custom Rheo-Ultrasound (Rheo-US) setup.

Sample: Cornstarch particles (average radius $r_{cs} \approx 7 \, \mu\text{m}$ ) dispersed in water with density matching (using CsCl) to prevent sedimentation. The study focused on a weight concentration of $c_w = 39\%$ (volume fraction $\phi \approx 44\%$ ), a regime known to exhibit DST and density-wave dynamics.
Apparatus: A stress-controlled rheometer (Anton Paar MCR 301) with a parallel-plate geometry ( $R=25$ mm, gap $h=1$ mm). The bottom plate was replaced by a piezoelectric transducer driven at its resonance frequency ( $f = 15.17$ kHz).
Protocol:
- Experiments were conducted under controlled shear rate ( $\dot{\gamma}$ ) rather than stress, to capture the intermittent stress fluctuations and viscosity bifurcation inherent to DST.
- After establishing a baseline flow curve, a constant shear rate ( $\dot{\gamma} = 90 \, \text{s}^{-1}$ ) was imposed within the DST regime.
- Ultrasound was applied with varying amplitudes ( $a_{US}$ ) from 0 to $3.76 \, \mu\text{m}$ .
- Shear stress ( $\sigma$ ) and normal force ( $F_N$ ) were recorded at 20 Hz to analyze the probability distributions of the system's mechanical states.

3. Key Contributions

The paper makes several critical conceptual and experimental contributions:

Mechanism Clarification: It demonstrates that ultrasound does not simply reduce viscosity by breaking contacts directly; rather, it shifts the critical shear rate ( $\dot{\gamma}^*$ ) required to trigger the DST transition to higher values.
Master Curve Scaling: The authors show that stress probability distributions collapse onto master curves when shear rates are normalized by an ultrasound-dependent critical shear rate ( $\dot{\gamma}^*_{US}$ ), revealing a continuous evolution toward fluid-like states without a sharp threshold.
Physical Model: They propose a model based on time-scale separation, treating the jammed suspension as an effectively immobile porous medium through which high-frequency interstitial fluid flows.
Destabilization Mechanism: The study identifies that ultrasound-induced interstitial flows generate fluctuating hydrodynamic drag forces at the pore scale. These fluctuations destabilize the fragile, adhesive force chains, requiring higher external stresses to maintain a jammed state.

4. Key Results

Shift of the DST Transition:
- In the absence of ultrasound, the system at $\dot{\gamma} = 90 \, \text{s}^{-1}$ spends most of its time in a high-stress, jammed state ( $P_+$ is maximal).
- As ultrasound amplitude ( $a_{US}$ ) increases, the probability of observing the low-stress, fluid-like state ( $P_-$ ) increases monotonically.
- The data collapses onto a single master curve when the shear rate is rescaled as $(\dot{\gamma} - \dot{\gamma}^*_{US})/\dot{\gamma}^*_{US}$ , where $\dot{\gamma}^*_{US}$ increases linearly with $a_{US}$ . This confirms that ultrasound effectively "de-thickens" the suspension by pushing the transition to higher shear rates.
Time-Scale Separation and Porous Medium Analogy:
- The structural relaxation time of the suspension ( $\sim 1$ s) is orders of magnitude longer than the ultrasonic period ( $\sim 70 \, \mu\text{s}$ ).
- Consequently, the particle network appears immobile on the timescale of the ultrasound. The system behaves as a porous medium where the interstitial fluid undergoes oscillatory squeeze flow.
Destabilization Mechanisms:
- Boundary Slip: The squeeze flow creates strong velocity gradients near the plates, promoting slip at the boundaries.
- Bulk Destabilization (Primary Mechanism): In the bulk, the interstitial flow enters the Forchheimer regime (inertial effects dominate) at higher amplitudes. This leads to spatially heterogeneous flow and velocity fluctuations ( $\Delta v$ ) within the pores.
- Force Imbalance: These velocity fluctuations generate drag force fluctuations ( $\Delta F_{drag}$ ) on individual particles. When $\Delta F_{drag}$ becomes comparable to the stabilizing adhesive and frictional forces ( $\sim 20$ nN), the linear force chains are destabilized.
- Acoustic Streaming: The authors calculated that acoustic streaming forces within the pores are negligible due to confinement (pore size $\ll$ viscous penetration depth), ruling it out as the primary driver.

5. Significance

Fundamental Physics: The work provides a consistent physical picture of "acoustic fluidization," distinguishing it from simple viscosity reduction. It establishes that ultrasound acts as a source of mechanical noise at the pore scale, renormalizing the force balance governing shear thickening.
Industrial Application: This offers a non-invasive, in-situ strategy to control flow instabilities in complex materials like cementitious slurries, paints, and food products. By tuning ultrasound parameters, operators can prevent jamming in pipes and extruders without altering the chemical composition of the suspension.
Methodological Advance: The use of controlled shear rate combined with high-frequency acoustic forcing provides a robust framework for studying the dynamics of intermittent transitions in dense soft matter.

In conclusion, the authors demonstrate that ultrasound does not merely "melt" the jammed state but continuously shifts the stability boundary of the shear-thickening transition by introducing fluctuating hydrodynamic forces that destabilize the load-bearing force networks.