Tuning the shear-thickening of suspensions through surface roughness and physico-chemical interactions

This study demonstrates that both surface roughness and physico-chemical interactions, particularly hydrogen bonding, independently govern the shear-thickening behavior of suspensions, enabling the prediction of continuous shear thickening onset and the fine-tuning of discontinuous shear thickening through particle surface engineering.

The Gist

Imagine you have a bucket of sand mixed with water. If you stir it slowly, it flows like a liquid. But if you stir it violently, it suddenly turns hard as a rock, resisting your spoon. This strange behavior is called shear thickening. It's the science behind "oobleck" (cornstarch and water), but scientists have been trying to figure out exactly why it happens and how to control it.

This paper is like a detective story where researchers investigated two main suspects: how rough the particles are and how "sticky" they are chemically.

Here is the breakdown of their findings using simple analogies:

1. The Two Suspects: Roughness and Stickiness

The researchers used tiny silica particles (think of them as microscopic marbles) suspended in a thick, oily liquid. They created four different types of these "marbles":

• Rough & Sticky: Like a piece of sandpaper covered in Velcro.
• Rough & Slippery: Like sandpaper coated in Teflon.
• Smooth & Sticky: Like a smooth marble covered in Velcro.
• Smooth & Slippery: Like a smooth Teflon marble.

They wanted to see which combination made the mixture turn into a solid the fastest and easiest.

2. The "Velcro" Effect (Chemical Stickiness)

The most surprising discovery was about stickiness.

• The Hydrophilic (Sticky) Particles: These particles have tiny chemical "hooks" (hydrogen bonds) on their surface. When the liquid is stirred fast, these hooks grab onto neighboring particles.

  • Analogy: Imagine a crowded dance floor. If everyone is wearing Velcro suits, and they start bumping into each other quickly, they get stuck together in a giant, immovable clump.
  • Result: This caused Discontinuous Shear Thickening (DST). The liquid went from flowing to rock-hard almost instantly. This is the "superpower" behavior needed for things like bulletproof vests.

• The Hydrophobic (Slippery) Particles: The researchers coated the particles to make them water-repelling and chemically "slippery" (no hooks).

  • Analogy: Now imagine the same dance floor, but everyone is wearing slick, oily suits. Even if they bump into each other fast, they just slide past one another. They never get stuck.
  • Result: The mixture never turned into a solid. It just got thicker and thicker (like honey) but never jammed. This proved that chemical stickiness is essential for the dramatic "instant solid" effect.

3. The "Bumpy Road" Effect (Surface Roughness)

Next, they looked at roughness.

• The Rough Particles: These had a bumpy, fractal surface (like a tiny coral reef).
• The Smooth Particles: These were relatively flat.

They found that roughness acts like a low-speed trigger.

• Analogy: Imagine driving a car. If the road is perfectly smooth, you can drive fast before your tires start to slip and lose control. But if the road is full of deep potholes and rocks (rough), your tires start to struggle and grip much sooner, even at lower speeds.
• Result: The rough particles caused the mixture to thicken at much lower concentrations (less sand needed) than smooth particles. The bumps on the particles made them "catch" on each other earlier, even without the chemical Velcro.

4. The "Mix and Match" Experiment

The coolest part of the study was mixing the two types. They took a bucket of "Sticky" particles and slowly replaced them with "Slippery" ones.

• The Result: They could dial the stiffness like a volume knob.
  • 100% Sticky = Instant rock-hard solid.
  • 50% Sticky / 50% Slippery = A medium-thick goo that gets stiff but doesn't lock up completely.
  • 100% Slippery = Just a thick liquid that never locks up.

This means engineers can now design materials that are "just right" for specific jobs. You don't always want a material to turn into a brick; sometimes you just want it to get a little tougher.

Why Does This Matter?

For a long time, scientists thought you needed a huge amount of particles (like 50% of the bucket being sand) to get this thickening effect. This paper shows that if you use rough, sticky particles, you can get the same effect with very little material (as low as 5-10%).

Real-world applications:

• Body Armor: You can make lighter, more flexible vests that harden only when hit by a bullet.
• Construction: You can control how concrete flows so it's easy to pour but hardens instantly when you stop moving it.
• Sports Gear: Helmets or pads that feel soft when you walk but turn hard when you crash.

The Bottom Line

To make a liquid that turns into a solid when shaken, you need two things working together:

1. Roughness to make the particles catch on each other early.
2. Chemical Stickiness (Hydrogen bonds) to lock them together tightly enough to stop the flow completely.

Without the stickiness, the particles just slide past each other. Without the roughness, you need way too much material to get the effect. By mixing them, we can now build "smart fluids" that behave exactly how we want them to.

Technical Summary

1. Problem Statement

Shear thickening (the reversible increase in viscosity under shear) is a ubiquitous phenomenon in dense suspensions, ranging from continuous shear thickening (CST) to discontinuous shear thickening (DST). While the microscopic origins of DST are widely debated, the consensus has shifted toward frictional contacts between particles. However, the relative contributions of solid friction (driven by surface roughness) versus reversible chemical adhesion (e.g., hydrogen bonding) remain poorly quantified. Furthermore, traditional models suggest DST only occurs at very high volume fractions ($\phi$) near the jamming point ($\phi_J \approx 50-60\%$). This paper addresses the need to decouple these factors to understand how surface roughness and chemistry independently and synergistically control the onset and magnitude of shear thickening, particularly at lower volume fractions.

2. Methodology

The authors employed a model system of fumed silica particles suspended in a viscous polar solvent (polypropylene glycol, PPG). They utilized a systematic experimental approach to independently tune two key particle properties while keeping the global particle size constant ($D \approx 300$ nm):

• Surface Roughness: Two distinct particle morphologies were used:
  • "Less rough": Primary nodules of size $R_u \approx 25$ nm.
  • "Rougher": Primary nodules of size $R_u \approx 10$ nm (higher specific surface area and more convoluted texture).
• Surface Chemistry: Both roughness types were tested in two chemical states:
  • Hydrophilic (HP): Native surface with hydroxyl groups capable of forming hydrogen bonds.
  • Hydrophobic (HB): Coated with silanes (alkyl chains) to eliminate hydrogen bonding and induce hydrophobicity.
• Mixtures: The authors created binary mixtures of hydrophilic and hydrophobic particles (varying the ratio $x$) to study the competition between friction and adhesion.
• Rheological Characterization: Steady-state flow curves ($\eta$ vs. $\dot{\gamma}$ or $\sigma$) were measured using a stress-controlled rheometer. A dimensionless slope parameter $\beta = d\ln(\eta)/d\ln(\sigma)$ was used to quantitatively define rheological regimes (Newtonian, CST, DST).

3. Key Contributions

• Decoupling Mechanisms: The study provides the first experimental evidence allowing the independent tuning of surface roughness and chemistry, clearly distinguishing the roles of solid friction and hydrogen bonding.
• Low-Volume Fraction DST: The authors demonstrate that DST can occur at remarkably low volume fractions ($\phi \approx 8.8\%$), significantly lower than the $\phi \gtrsim 50\%$ typically reported for spherical particles.
• Geometric Scaling Law: A simple geometric argument is proposed to predict the onset of CST based on the interpenetration of textured particles, independent of surface chemistry.
• Tunability via Blending: The paper establishes a method to finely tune the shear-thickening response (from DST to CST to shear thinning) by blending particles of opposing surface chemistries.

4. Key Results

A. Role of Surface Chemistry (Hydrophilic vs. Hydrophobic)

• Hydrophilic Particles: Both rough and less-rough hydrophilic suspensions exhibited CST and DST. The presence of surface hydroxyl groups allows for the formation of reversible hydrogen bonds under shear. These bonds enhance solid friction, enabling the transition to DST at moderate volume fractions.
• Hydrophobic Particles:
  • Less Rough: Completely suppressed shear thickening; the suspension remained shear-thinning across all tested volume fractions.
  • Rougher: Exhibited weak CST at moderate volume fractions but no DST. This indicates that while high roughness can generate friction-induced CST, hydrogen bonding is essential to trigger the extreme viscosity jump associated with DST.

B. Role of Surface Roughness

• Onset of CST: Rougher particles ($R_u = 10$ nm) triggered CST at a much lower critical volume fraction ($\phi_{CST} \approx 3.4\%$) compared to less rough particles ($\phi_{CST} \approx 5\%$).
• Mechanism: The onset of CST is driven by the geometric interpenetration of the textured nodules. The authors derived a scaling law where the critical volume fraction depends on the ratio of the primary nodule size ($R_u$) to the global particle size ($D$).
• Prediction: The theoretical estimate $\phi_{CST} \propto (R_u/D)^3$ matched experimental data well, confirming that CST is primarily a geometric/frictional phenomenon, while DST requires the additional chemical "glue."

C. Mixture Experiments

• By mixing hydrophilic and hydrophobic particles, the authors showed a smooth transition in rheological behavior.
• DST Threshold: DST was only observed when the volume fraction of hydrophilic particles exceeded a critical threshold ($\phi_{HP} \approx 6\%$).
• Screening Effect: Adding hydrophobic particles to a hydrophilic suspension "screens" the short-range attractive forces, inhibiting the formation of the frictional networks required for DST. This allows for precise control over the transition point.

5. Significance and Implications

• Theoretical Framework: The results challenge existing theoretical frameworks (e.g., Wyart & Cates) that rely on jamming of smooth spheres. The study highlights that for highly textured particles, the "jamming" or frictional transition occurs at much lower global volume fractions due to local interpenetration of surface features.
• Engineering Applications:
  • Stability: Unlike cornstarch suspensions, these fumed silica suspensions show no aging, migration, or settling, making them ideal for stable industrial fluids.
  • Design Control: The ability to tune shear thickening via surface chemistry and roughness allows for the design of shear-thickening fluids (STFs) with specific transition points. This is critical for applications like ballistic armor (where low particle loading is preferred for flexibility) and cement paste rheology (where controlling thickening prevents clogging).
  • Transparency: The refractive index match between the silica and PPG allows for optical transparency, enabling in-situ visualization of microstructure evolution during DST.

In conclusion, the paper establishes that surface roughness dictates the onset of continuous shear thickening, while reversible hydrogen bonding is the critical enabler for discontinuous shear thickening. This dual-control mechanism offers a powerful toolkit for engineering advanced non-Newtonian fluids.