🔬 condensed matter

The Role of Hydrogen Bridging Bonds in the Shear-Thickening and Jamming of Dense Suspensions

This study demonstrates that the shear-thickening and jamming behavior of dense cornstarch suspensions can be systematically controlled by tuning solvent molecular structure to modulate hydrogen bridging bonds, revealing a sharp transition from strong shear thickening in water and ethylene glycol to shear thinning in longer-chain diols.

Original authors: Hojin Kim, Samantha M. Livermore, Yongjin Shin, Heinrich M. Jaeger

Published 2026-05-15

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Hojin Kim, Samantha M. Livermore, Yongjin Shin, Heinrich M. Jaeger

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. If you stir it slowly, it flows like a liquid. But if you hit it hard or stir it violently, it suddenly turns into a solid block that you can't move. This strange behavior is called shear thickening, and it's what happens in dense suspensions like cornstarch mixed with water (often called "oobleck").

This paper investigates why this happens and, more importantly, how we can control it by changing the liquid we mix the cornstarch with. The researchers discovered that tiny chemical "handshakes" between the liquid molecules and the cornstarch particles are the secret sauce.

Here is the story of their discovery, broken down into simple concepts:

1. The Two Faces of the Liquid

The researchers used cornstarch particles as the "solid" part. For the "liquid" part, they didn't just use water. They used a family of liquids called diols (alcohols with two ends). Think of these molecules like tiny dumbbells with sticky hands (hydroxyl groups) on both ends.

They tested four liquids:

Water: The shortest molecule.
Ethylene Glycol: A slightly longer molecule (2 carbon atoms).
Propanediol: A medium-length molecule (3 carbon atoms).
Butanediol: The longest molecule in the group (4 carbon atoms).

2. The "Velcro" Effect

The core idea is about Hydrogen Bonding. Imagine the cornstarch particles are covered in Velcro hooks. The liquid molecules are the fuzzy loops.

At rest (Low Stress): The liquid molecules wrap around the cornstarch like a protective blanket. This keeps the particles apart, preventing them from sticking together. It's like a force field that keeps the sand grains from clumping.
Under stress (High Stress): When you stir or pull the mixture hard, you squeeze the particles together so tightly that the liquid molecules get squished out of the way. Suddenly, the "hooks" on one particle can grab the "loops" on the neighbor. This creates a molecular bridge.

These bridges act like molecular Velcro. When they form, the particles lock together, creating friction. This friction is what turns the liquid into a solid (jamming).

3. The Twist: Length Matters

The researchers expected that since all these liquids had "sticky hands" on both ends, they would all behave similarly. They were wrong. The length of the molecule changed everything.

Short Molecules (Water & Ethylene Glycol): These molecules are short and stiff. They are great at forming those protective blankets at rest. But when squeezed, they snap into place quickly to form strong bridges.
- Result: The mixture gets very thick and hard very quickly when you stir it. It behaves like a classic "oobleck."
Long Molecules (Propanediol & Butanediol): Here is the surprise. These longer molecules are so flexible that they prefer to hold hands with each other instead of reaching out to the cornstarch.
- The Analogy: Imagine a group of people at a party. The short people (Water) are busy shaking hands with the host (Cornstarch). The tall people (Long Diols) are so busy hugging each other in the corner (forming "dimers" or rings) that they ignore the host.
- Result: Because they are busy hugging each other, they don't form a good protective blanket around the cornstarch. The particles clump together even before you start stirring. When you do stir, the bridges that do form are long, floppy, and weak. Instead of turning into a hard solid, the mixture just flows like a thick, gooey paste that gets thinner as you stir it (shear thinning).

4. The "Pull Test" (Tearing the Solid)

To see how strong the "solid" state really was, the researchers didn't just stir the mixture; they tried to pull it apart. They dipped a rod into the mixture and yanked it up fast.

With Water/Short Liquids: The mixture turned into a hard, brittle solid. When they pulled, it snapped like a piece of dry clay. It fractured cleanly.
With Long Liquids: The mixture didn't snap. Instead, it stretched out like taffy or chewing gum. It formed a long, thin neck before finally breaking. It was much softer and more flexible.

The Big Picture

The paper concludes that the behavior of these suspensions isn't just about the solid particles; it's about the dance between the particles and the liquid.

If the liquid molecules are short and eager to bond with the particles, you get a hard, brittle solid when you stress it (Shear Thickening).
If the liquid molecules are long and prefer to bond with each other, you get a soft, stretchy, gooey mess (Shear Thinning).

By simply changing the length of the carbon chain in the solvent by just one or two atoms, the researchers could switch the material's personality from a "hard rock" to "soft taffy." This proves that the microscopic shape and bonding habits of the liquid are just as important as the solid particles themselves in determining how the mixture flows.

Technical Summary: The Role of Hydrogen Bridging Bonds in the Shear-Thickening and Jamming of Dense Suspensions

Problem Statement
Dense suspensions exhibit strong shear thickening and shear jamming (SJ) when particles are forced into contact under high shear stress, transitioning from lubricated to frictional interactions. While particle shape and roughness are known to control these frictional interactions, the role of surface chemistry and solvent interactions remains a critical, yet less systematically explored, factor. Specifically, it is established that solvent molecules can form "bridges" between adjacent particles via hydrogen bonding (H-bonding) when shear closes the interparticle gap, thereby enhancing effective friction. However, prior studies have not systematically investigated how the molecular size and conformation of the solvent molecule—specifically the length of the carbon backbone in diol solvents—affect the transition from shear thickening to shear thinning and the mechanical properties of the resulting shear-jammed state.

Methodology
The authors investigated this size dependence using cornstarch (CS) suspensions in four distinct solvents: water (W), ethylene glycol (EG, 2 carbons), 1,3-propanediol (PD, 3 carbons), and 1,4-butanediol (BD, 4 carbons). This series was chosen to maintain comparable H-bonding capability (two hydroxyl groups per molecule) while varying the backbone length and dielectric constant.

The study employed a multi-modal approach:

Shear Rheology: Stress-controlled measurements were performed on an Anton Paar MCR 301 rheometer to determine viscosity ( $\eta$ ) as a function of shear stress ( $\tau$ ) up to 100 Pa. Samples were tested at various particle weight fractions ( $\phi_W$ ) within one hour of preparation to avoid aging effects from particle swelling.
Density Functional Theory (DFT) Simulations: Calculations using PBE and B3LYP functionals (Quantum ESPRESSO) were conducted to analyze the thermodynamic stability of solvent molecular conformations (linear, ring, and dimer). Vibrational free energies were computed to understand how solvent-solvent interactions (self-association) compete with solvent-particle interactions.
Fixed-Rate Pull Tests: To probe the shear-jammed regime at stresses far exceeding rheometer limits (tens of kPa), a uniaxial tensile test was performed. A submerged aluminum rod was pulled vertically at fixed speeds (1–50 mm/s) while force and high-speed video were recorded to observe fracture modes (brittle vs. ductile) and the onset of shear jamming.

Key Results

Transition from Shear Thickening to Shear Thinning:
- Water and EG: Suspensions in water and ethylene glycol exhibited strong shear thickening, transitioning to discontinuous shear thickening (DST) at high particle fractions.
- PD and BD: A sudden transition occurred when moving to propanediol (3 carbons) and butanediol (4 carbons). These suspensions displayed shear thinning behavior with a yield stress, lacking the shear thickening regime observed in shorter solvents.
- Convergence at High Stress: Despite the low-stress differences, at very high particle fractions ( $\phi_W = 55\%$ ) and high stresses, the flow curves of all four suspensions began to converge, suggesting that frictional particle-particle contacts eventually dominate regardless of the solvent.
DFT Insights on Molecular Conformations:
- DFT simulations revealed that while ring and dimer configurations are thermodynamically favored over linear ones for all diols, the energy penalty for forming dimers is significantly lower for PD and BD compared to EG.
- Consequently, PD and BD molecules are more likely to exist as dimers (self-associated via H-bonds) in the bulk. This self-association reduces the number of free H-bond donors/acceptors available to interact with the cornstarch surface, weakening the solvation layer and interparticle repulsion. This leads to the observed shear thinning and yield stress behavior.
Shear Jamming and Mechanical Failure:
- Brittle vs. Ductile Failure: Pull tests revealed distinct mechanical behaviors in the shear-jammed state. Water and EG suspensions failed via brittle fracture (sharp force drop, jagged cracks). In contrast, PD suspensions exhibited extensive ductile necking and sustained larger tensile strains before failure.
- Role of Bridging: The authors attribute the ductility in PD/BD systems to weaker, longer, and more flexible H-bond bridges between particles. While water and EG form strong, short bridges leading to rigid, brittle jammed solids, the reduced solvation and longer bridges in PD/BD result in a softer, less brittle jammed state.

Significance and Claims
The paper claims to demonstrate that the macroscopic rheological and mechanical failure properties of dense suspensions can be tuned by the molecular structure of the solvent. Specifically, the study establishes that:

Solvent-Solvent vs. Solvent-Particle Competition: The competition between solvent self-association (dimerization) and solvent-particle H-bonding dictates the strength of the solvation layer and the nature of interparticle friction.
Dual Role of H-Bonding: H-bonds act as a dual mechanism: they provide steric repulsion at low stress (preventing aggregation) but enable frictional bridging at high stress (inducing thickening/jamming).
Tailoring Jamming Behavior: By increasing the solvent backbone length, one can systematically shift the system from a regime of strong shear thickening and brittle jamming to shear thinning and ductile jamming. This provides a chemical handle to tailor the stress response and failure modes of dense suspensions without altering particle shape or roughness.

The authors conclude that small changes in solvent-particle and solvent-solvent interactions profoundly affect the stress response of dense suspensions, offering a pathway to control shear thickening and jamming through molecular design.