The science behind why ketchup refuses to budge until you shake the bottle has revolutionized everything from medicine to materials engineering.
Exploring insights from the 1st International Conference on Rheology and Modeling of Materials
Imagine trying to build a skyscraper with concrete that collapses before setting, or performing life-saving surgery with blood that wouldn't flow. Consider the challenge of creating a paint that doesn't drip from the brush yet spreads smoothly on a wall. These everyday scenarios all hinge on a fundamental scientific discipline: rheology, the study of how matter deforms and flows4 6 .
The term "rheology" comes from the Greek words "rheo" (meaning flow) and "logia" (meaning study of), and was coined by Eugene C. Bingham in 19204 5 . This science bridges the gap between the deformation of solids and the flow of liquids, helping us understand and engineer the complex behaviors of materials from molten plastics and concrete to blood and food products4 5 .
The 1st International Conference on Rheology and Modeling of Materials (ic-rmm1) recognized that to advance technology, we must fundamentally understand how materials behave under stress, heat, and deformation. This article explores the key concepts and exciting discoveries in this field that are quietly shaping the world around us.
Rheology is the branch of physics devoted to studying how matter deforms and flows in response to applied forces6 . It moves beyond simple measurements of thickness to explore how materials respond to stress, strain, or flow, revealing crucial information about their viscosity, elasticity, and plasticity6 .
Where fluid mechanics might treat a substance as simply liquid or solid, rheology acknowledges that most real-world materials exist somewhere in between. As the famous rheology aphorism goes, "Everything flows"1 . The question is how, when, and why.
Interactive visualization of different flow behaviors
To understand rheology, it helps to grasp a few key concepts that describe how materials behave:
A measure of a material's resistance to flow. High viscosity means a material flows slowly (like honey), while low viscosity means it flows easily (like water)6 .
Some materials require a minimum amount of stress to start flowing. Below this "yield stress," they behave like solids; above it, they flow like liquids. Toothpaste is a classic example—it stays on your brush until you press it, then it flows6 .
The ability of a material to return to its original shape after deformation. Highly elastic materials like rubber can withstand large deformations and return to their original state.
The property of a material to undergo permanent deformation after sufficient stress is applied. Unlike elasticity, plastic materials do not return to their original shape.
| Material | Rheological Class | Key Behavior | Everyday Example |
|---|---|---|---|
| Water, Sugar Syrup | Newtonian Fluid | Constant viscosity regardless of force | Easy pouring |
| Ketchup, Paint | Shear-Thinning | Thins when shaken or stirred | Ketchup bottle shaking |
| Cornstarch Mix | Shear-Thickening | Thickens under sudden force | Sinking on, bouncing off |
| Toothpaste, Mayonnaise | Yield-Stress Fluid | Doesn't flow until pushed | Staying on brush/sandwich |
| Rubber, Bread Dough | Viscoelastic | Flows slowly, springs back quickly | Rising dough, bouncing ball |
The field of rheology is far from static. Recent research has unveiled astonishing complexities in how materials behave, particularly through two groundbreaking approaches.
Traditional rheology relied on bulk measurements in controlled instruments called rheometers. However, a significant disconnect has emerged between how materials behave in these simple tests and how they perform in real-world applications3 .
A pioneering study published in 2025 introduces a novel approach using automatic differentiation and differentiable simulations to learn constitutive relationships from arbitrary flow measurements3 . Essentially, this method uses advanced computing to create a "digital rheometer" that can analyze fluid behavior in complex geometries, much like how fluid flows through an engine part or blood vessel3 .
This framework employs a tensor basis neural network (TBNN) that learns stress-strain relationships directly from flow data3 . The system can then identify which classical constitutive model best describes the learned behavior, effectively allowing any flow measurement—no matter how complex the geometry—to become a potential rheometer3 .
While the digital revolution advances, laboratory experiments continue to reveal surprising material behaviors. A groundbreaking 2025 study from UCSB, which won the Journal of Rheology Publication Award, tackled a long-standing industrial problem: shear banding in polymer fluids8 .
Shear banding occurs when a flowing material spontaneously separates into regions flowing at different speeds, creating an unstable, banded structure. This phenomenon can severely impact manufacturing processes involved in making plastic products like bottles, films, and fibers8 .
The UCSB team challenged a fundamental assumption in polymer flow theories: that the composition of polymer fluids remains uniform during flow. Their research demonstrated, systematically and for the first time, that flow can actually cause these materials to demix or separate, creating large spatial variations in concentration that trigger shear banding8 .
| Reagent/Instrument | Primary Function | Research Application |
|---|---|---|
| Fluorescently-Tagged Polymers | Visualize concentration in flow | Tracking flow-induced demixing 8 |
| Tensor Basis Neural Network (TBNN) | Learn stress-strain relationships | Digital rheometry from flow data 3 |
| Differentiable Fluid Solver | Compute gradients for optimization | Parameter inference in complex flows 3 |
| Rheo-Microscopy Systems | Simultaneous measurement of flow & structure | Connecting macroscopic flow to microstructure 8 |
The UCSB study on shear banding provides a perfect example of how innovative experimental design can solve long-standing rheological mysteries.
The researchers recognized that conventional stress measurements alone were insufficient to characterize the complex behavior of polymers under flow. They developed a comprehensive approach:
The team tagged polymers with fluorescent markers to enable concentration measurements within the flowing material. This chemical preparation was crucial for visualizing previously invisible phenomena8 .
They created new rheo-microscopy instrumentation with custom optics and software capable of simultaneously measuring flow, composition, and stress in complex fluids as they varied within an imposed flow device8 .
The team integrated cutting-edge experiments with polymer flow simulations based on a recent theory developed by Professor Gary Leal's group8 .
The experiment provided the first direct experimental evidence that shear banding in polymers can arise from flow-induced demixing8 . This finding fundamentally challenges established theories that assume uniform composition during flow.
When the researchers simultaneously measured the flow field and polymer concentration, they discovered that under certain flow conditions, the initially uniform polymer solution would separate into regions of higher and lower polymer concentration. This demixing created the conditions for shear banding—where adjacent layers of fluid move at different speeds8 .
The implications are profound: manufacturers can no longer assume that a polymer fluid remains uniform during processing. This discovery explains why products sometimes develop defects during manufacturing and opens new pathways for preventing these issues by controlling flow-induced demixing.
This research has direct implications for:
The tools and concepts of rheology have become indispensable across countless industries and research fields.
Rheologists employ specialized instruments and techniques to characterize material behavior:
Using cone-plate or concentric cylinder geometries, researchers can measure how materials respond to steady shearing, generating flow and viscosity curves6 .
By applying a sinusoidal stress or strain, scientists can probe the viscoelastic character of materials, distinguishing between solid-like and liquid-like behaviors6 .
Specialized methods exist for studying the mechanical properties of interfaces between different phases, crucial for understanding foams, emulsions, and thin films.
| Parameter | Description | Reveals About Material |
|---|---|---|
| Viscosity (η) | Resistance to flow | How easily it pours or spreads |
| Yield Stress | Minimum stress to start flow | Stability at rest (e.g., toothpaste on brush) |
| Storage Modulus (G') | Energy stored and recovered | Solid-like, elastic character |
| Loss Modulus (G") | Energy dissipated as heat | Liquid-like, viscous character |
| Tan δ (G"/G') | Balance between viscous & elastic | Overall solid/liquid behavior |
The practical applications of rheology span virtually every field of materials science and engineering:
Rheology helps optimize the workability of fresh concrete, ensuring it can be pumped to great heights while maintaining stability. Researchers study how recycled concrete aggregates affect rheological properties, supporting sustainable construction5 .
The entire plastics manufacturing industry relies on understanding and controlling the rheology of polymer melts during processes like injection molding and fiber spinning8 .
The science of rheology has come a long way since its formal inception in 1929 when the Society of Rheology began "flowing"1 . What began as a specialized field bridging physics and engineering has evolved into an indispensable discipline that touches nearly every aspect of modern material science.
From the groundbreaking research presented at the 1st International Conference on Rheology and Modeling of Materials to the recent award-winning work on polymer flow instabilities, rheology continues to reveal the hidden complexities of how materials deform and flow7 8 . The field is now embracing powerful new tools from machine learning and advanced computation, creating a future where we can design materials with precisely tailored flow properties for specific applications3 .
As we look toward the Society of Rheology's centennial celebration in 20291 , one thing remains certain: understanding how "everything flows" will continue to be fundamental to technological progress, from the microscopic flow of blood in our veins to the massive movement of concrete in our cities.
Emerging areas where rheology will play a crucial role:
Biotheology and medical applications
Sustainable materials development
AI-driven material design