Why Soft Materials Remember Their Past
A glimpse into the mysterious world of rheology reveals why your hand lotion, the asphalt you drive on, and the gel in your smartwatch behave the way they do.
Have you ever wondered why a bottle of hand lotion sometimes becomes runny after sitting on the shelf, or why a fresh blob of hair gel holds its shape so perfectly? These everyday mysteries are governed by the science of rheology—the study of how materials flow and deform. Soft materials like gels, foams, and pastes are everywhere, from the cosmetics we use to the roads we drive on. Yet, despite their ubiquity, their behavior has long puzzled scientists. Recent breakthroughs are now uncovering the hidden memories and complex inner lives of these squishy substances, revealing that they are far more complex and interesting than they appear.
Soft matter refers to a vast family of materials—including polymers, gels, foams, and lotions—that can be easily molded and shaped. What sets them apart is their structure: they are built from larger building blocks than atoms or simple molecules, which gives them unique properties 6 .
They exist in a delicate state between solid and liquid, a characteristic that defines them as "soft glassy materials" 5 . A soft material can be poured like a liquid but also hold its shape like a solid.
Soft matter is highly responsive. Mild external influences—a little stress, a change in temperature—can produce dramatic effects . Their complex behaviors emerge from the interactions of their constituent parts, meaning you can't always predict how the material will behave just by studying its individual molecules .
Soft materials dramatically change behavior with small external influences like stress or temperature changes.
Complex behaviors emerge from interactions between constituent parts, not predictable from individual molecules.
Soft materials retain mechanical memories of how they were processed, affecting their long-term behavior.
For a long time, the prevailing hypothesis was that "double yielding" meant a material needed to yield twice to fully fluidize. However, researchers at the University of Illinois Urbana-Champaign have now shown this behavior is more nuanced 1 .
"Some of it will stretch and, if you let it go, some of it will recoil—it'll spring back a little, but it won't spring back all the way. That little bit of recoil is what we call the recoverable part, and the fact that it doesn't come all the way back is the unrecoverable part."
The researchers placed the soft gel material between two plates in an instrument called a rheometer. They then applied a carefully controlled and increasing shearing force (strain).
As the strain increased, they meticulously measured the material's response, observing the two characteristic overshoots in energy dissipation.
Crucially, the team tracked how much of the applied deformation was recoverable (elastic, like a spring) and how much was unrecoverable (viscous, like honey). This allowed them to distinguish between different types of internal processes.
The experiment yielded clear and transformative results. The two overshoots were not the same; they represented two independent physical processes.
| Overshoot Peak | Underlying Process | Recoverability | Significance |
|---|---|---|---|
| First, Smaller Peak | Material Softening | Reversible | The material's elastic structure softens and rearranges, but can potentially recover. It has not yet truly yielded. |
| Second, Larger Peak | True Yielding | Irreversible | This marks the permanent, irreversible transition to a fluid-like state. The material begins to flow. |
This discovery resolved the long-standing puzzle: the material isn't yielding twice. Instead, it first undergoes a reversible softening before committing to an irreversible flow 1 . The team further validated their findings by developing a predictive model that accurately captures this two-stage behavior, providing a new, more accurate description of how soft materials behave under the large, rapid deformations common in industrial processing 1 .
To conduct such sophisticated research, scientists rely on a suite of specialized tools and materials. The following table details some of the key reagents and instruments essential to the field of soft matter rheology.
| Tool/Material | Function in Research |
|---|---|
| Rheometer | The workhorse instrument for applying controlled stresses or strains to a material and measuring its resulting deformation, directly quantifying properties like stiffness and flow. |
| Model Soft Material (e.g., Carbopol) | Well-characterized gels like Carbopol serve as standard testbeds for fundamental studies, allowing researchers to compare results and validate new theories 1 . |
| Recovery Rheology Protocols | A novel methodological approach that separates deformation into recoverable and unrecoverable components, providing deeper insight into a material's internal structure 1 . |
| Block Copolymers | These soft, chain-like molecules naturally self-assemble into orderly nanostructures, acting as scaffolds for creating structured porous materials for advanced applications 2 . |
| 3D Printing Inks | Specially formulated inks containing copolymers and nanoparticles allow for the fabrication of soft materials with complex, hierarchically ordered structures impossible to achieve with traditional methods 2 . |
The essential instrument for measuring how materials deform and flow under stress, providing quantitative data on viscosity, elasticity, and yield stress.
A commonly used model system in rheology studies, providing a consistent and well-characterized soft material for experimental validation.
The implications of these discoveries extend far beyond the lab. The finding that soft materials hold onto "mechanical memories" is another revolutionary insight. Research from MIT has shown that common products like hair gel and shaving cream retain a memory of how they were mixed—including the direction and duration—for days, long after manufacturers assumed these stresses had dissipated 5 . This residual stress is why different batches of the same product can behave differently and why materials can degrade over time 5 .
Creating longer-lasting, more predictable products with controlled flow properties and stability.
Developing more durable asphalt and construction materials with improved mechanical properties.
Advanced materials for additive manufacturing with complex, hierarchically ordered structures 2 .
This new understanding empowers scientists and engineers to design better materials. By using protocols like the one developed at MIT to measure residual stress, manufacturers can create products with "shorter-term memory," leading to longer-lasting cosmetics, more durable asphalt, and more predictable food products 5 .
Furthermore, the field of soft matter is incredibly interdisciplinary, with connections to cosmology and mathematics . For instance, the way defects form in liquid crystals during a phase transition is analogous to the formation of cosmic strings in the early universe . This "universality of phenomena" means that studying a simple gel can sometimes provide insights into the fundamental laws of the cosmos.
The humble world of squishy materials is finally yielding its secrets. Through innovative experiments, scientists are learning that soft matter is not passive; it has a memory and a complex internal life that dictates its behavior. The once-misunderstood phenomenon of "double yielding" is now revealed as a two-stage process of softening and flow, and the hidden "mechanical memories" in our everyday products are now measurable. As we continue to unravel the rheology of soft materials, we pave the way for a future where products are more reliable, roads are more durable, and the boundaries between biology and electronics become seamlessly, and softly, intertwined.