Exploring materials that challenge our understanding of solids and liquids
Discover MoreWhat do ketchup, blood, printer ink, and molten plastic have in common? At first glance, these substances seem utterly unrelated. Yet they all belong to a fascinating category of materials that challenge our conventional understanding of solids and liquids. These are rheologically complex fluids—materials that flow like liquids but can behave like solids under certain conditions, substances with memory that remember how they've been treated, and materials whose properties change dramatically based on how fast you push or pull them.
The science of rheology—the study of how matter flows and deforms—has revealed that these complex fluids are everywhere in our daily lives and technology. From the paint that doesn't drip from the brush to the yogurt that feels thick on the spoon but smooth in the mouth, from the 3D-printed biomedical implants to the clot-busting drugs that save lives—understanding and designing with complex fluids is transforming technology, medicine, and manufacturing.
Everyday examples that exhibit fascinating rheological behavior
From manufacturing to 3D printing, complex fluids enable advanced processes and products.
Understanding blood flow and developing targeted drug delivery systems.
Creating appealing textures and optimizing food processing techniques.
What makes a fluid "complex"? The answer lies in its internal architecture. While simple fluids like water consist of identical molecules, complex fluids contain mesoscopic structures—organizational features at scales between individual molecules and the bulk material. These may include polymer chains, colloidal particles, droplets, bubbles, or micelles that constantly rearrange under flow 1 5 .
This microstructure evolution occurs on time scales comparable to the flow itself, creating a dynamic feedback loop: flow affects structure, and structure affects flow. This coupling leads to the rich, often surprising behaviors that make these materials both challenging to understand and technologically valuable 1 .
Complex fluids occupy the fascinating middle ground between solids and liquids, exhibiting both viscous and elastic characteristics simultaneously—a property known as viscoelasticity 4 9 .
Imagine a material that responds like a spring (elastic solid) when quickly jerked but flows like honey (viscous fluid) when slowly pushed. This dual nature means these materials have fading memory—they remember how they've been deformed, but gradually forget over time 9 . This behavior is often modeled using combinations of springs (representing elasticity) and dashpots (representing viscosity), such as in the Maxwell and Kelvin-Voigt models 9 .
Spring and dashpot in series
Spring and dashpot in parallel
Perhaps the most dramatic feature of complex fluids is how their viscosity can change under different flow conditions:
Some complex fluids exhibit thixotropy—their viscosity depends not just on the current flow conditions but on the entire history of deformation. Thixotropic materials become less viscous over time when subjected to constant stress, then gradually recover their original consistency when the stress is removed 3 9 . This explains why paint stirs easily but thickens again after sitting.
| Material Type | Everyday Examples | Key Rheological Features | Applications |
|---|---|---|---|
| Polymer Solutions & Melts | Shampoo, molten plastic | Viscoelastic, shear thinning | 3D printing, drug delivery |
| Colloidal Suspensions | Paint, ink | Yield stress, thixotropy | Coatings, ceramics |
| Emulsions | Mayonnaise, milk | Shear thinning, yield stress | Food products, cosmetics |
| Surfactant Systems | Soap solutions, fabric softener | Viscoelastic, thixotropic | Cleaning products, personal care |
| Biological Fluids | Blood, synovial fluid | Shear thinning, viscoelastic | Medical diagnostics, therapeutics |
For decades, most theories assumed that polymeric fluids remained homogeneous during flow. However, recent award-winning research has overturned this assumption, demonstrating that these fluids can undergo flow-induced "demixing"—separating into regions with different concentrations and shear rates, a phenomenon known as shear banding .
This discovery emerged from innovative experiments combining rheometry, particle tracking velocimetry, and rheo-fluorescence to simultaneously measure mechanical stress, velocity, and concentration profiles. The findings represent a paradigm shift in how we understand polymeric flows, with potentially far-reaching implications for polymer processing and applications .
This research represents a major step forward and is significant because it shows that the long-held assumption of macroscopic homogeneity, which is the basis of most theories of polymeric flows, is generally incorrect.
— Professor L. Gary Leal
Studying complex fluids presents unique challenges, as their behavior depends on multiple variables and can be easily disrupted by measurement techniques themselves. Traditional rheometers sometimes produce artifacts like wall slip and shear banding, where the sample separates into regions flowing at different rates 5 .
Advanced techniques like Doppler Optical Coherence Tomography (D-OCT) now enable researchers to peer inside opaque complex fluids and measure velocity profiles with micrometer-scale resolution 5 . This has opened new windows into understanding how these materials behave under flow in contexts ranging from capillary tubes to industrial processing equipment.
Microfluidics—the science of manipulating fluids in miniaturized channels—has emerged as a powerful platform for characterizing and utilizing complex fluids. The intrinsic advantages of microfluidics include precision control of flow geometry, access to unique flow regimes, small sample volumes, and parallel analysis capabilities 8 .
These benefits are particularly valuable for biomedical applications, where sample volumes may be limited. For instance, researchers have used viscoelastic non-Newtonian fluids to improve single-cell analysis efficiency from 5% to 40% by enhancing cell-bead encapsulation in droplets—a crucial advancement for cancer diagnostics 7 .
Exact manipulation of flow geometry and conditions
Minimal material requirements for analysis
Enhanced diagnostics and single-cell analysis
Parallel analysis capabilities
The groundbreaking experiment that demonstrated flow-concentration coupling and shear banding employed a sophisticated combination of techniques :
This experimental design allowed, for the first time, direct correlation of local concentration variations with flow behavior and stress response.
The experiments revealed that under certain flow conditions, the initially homogeneous polymer solutions spontaneously separated into distinct bands with different concentrations and shear rates . This shear banding phenomenon occurred due to the coupling between flow strength and polymer concentration—regions with higher concentration experience different flow behaviors than dilute regions.
The significance of these findings cannot be overstated. As Professor L. Gary Leal noted, "This research represents a major step forward and is significant because it shows that the long-held assumption of macroscopic homogeneity, which is the basis of most theories of polymeric flows, is generally incorrect" .
Visual representation of concentration bands forming during shear banding
| Parameter | Measurement Technique | Key Finding | Theoretical Significance |
|---|---|---|---|
| Shear Stress | Rheometry | Non-monotonic relationship with shear rate in certain regimes | Explains mechanical instability leading to banding |
| Velocity Profile | Particle Tracking Velocimetry | Discontinuous profile with distinct bands | Direct evidence of heterogeneous flow |
| Concentration Distribution | Rheo-fluorescence | Correlation between concentration and local shear rate | Demonstrates flow-concentration coupling |
| Polymer Architecture | Chemical synthesis | Controlled by researchers to test specific hypotheses | Enables isolation of microstructural effects |
| Flow Condition | Concentration Distribution | Velocity Profile | Resulting Material Behavior |
|---|---|---|---|
| Low Shear Rates | Homogeneous | Linear | Newtonian-like flow |
| Critical Shear Rate | Beginning of heterogeneity | Slight deviation from linear | Onset of flow instability |
| Intermediate Shear Rates | Distinct bands forming | Discontinuous profile | Established shear banding |
| High Shear Rates | Complex heterogeneous pattern | Strongly nonlinear | Fully developed heterogeneous flow |
Understanding and designing with complex fluids requires specialized equipment and materials. Here are some essential components of the modern complex fluids researcher's toolkit:
| Tool/Material | Primary Function | Specific Applications | Key Features |
|---|---|---|---|
| Rheometers | Measure mechanical response under deformation | Characterizing viscosity, viscoelasticity, yield stress | Can operate in controlled stress or controlled rate modes 7 9 |
| Model Polymer Solutions | Well-characterized complex fluids for fundamental studies | Testing theoretical predictions, understanding microstructure-property relationships | Customizable molecular weight, architecture, and concentration |
| Microfluidic Devices | Manipulate small fluid volumes in controlled geometries | Single-cell analysis, point-of-care diagnostics, high-throughput screening | Precise geometrical control, small sample volumes, automation capability 7 8 |
| Fluorescent Tracers | Visualize flow and concentration fields | Particle tracking velocimetry, concentration measurements | Chemical compatibility with sample, appropriate excitation/emission properties |
| Colloidal Suspensions | Model systems with tunable interactions | Studying phase transitions, glass formation, shear thickening | Controllable particle size, shape, and surface chemistry 5 |
| Doppler Optical Coherence Tomography | Non-invasive velocity and structure measurement | Velocimetry in opaque fluids, boundary layer studies | Micrometer resolution, high frequency measurement capability 5 |
Advanced instruments for characterizing flow behavior and material properties under various conditions.
Well-characterized model systems for fundamental research and testing theoretical predictions.
Computational tools for data analysis, simulation, and modeling of complex fluid behavior.
The science of rheologically complex fluids has come a long way from its beginnings, but the most exciting developments likely lie ahead. As Professor Dimitris Vlassopoulos notes, research in this area provides "a framework to design new experiments and eventually construct operating diagrams for flowing polymer solutions" . This represents a shift from simply understanding these materials to truly engineering their behavior.
Future directions in the field include developing more sophisticated data-driven methods like Rheology-Informed Neural Networks (RhINNs) to solve complex constitutive equations 3 , creating advanced measurement techniques with even higher spatial and temporal resolution, and designing next-generation functional materials that harness the unique properties of complex fluids in predictive ways.
From personalized medical treatments based on a patient's specific blood rheology to smart manufacturing processes that adapt material properties in real-time, and from sustainable alternatives to plastic packaging to revolutionary food textures that enhance experience while reducing waste—the potential applications are as vast as they are transformative.
The study of complex fluids reminds us that the boundaries between scientific disciplines are often as fluid as the materials themselves, requiring collaboration between physicists, chemists, biologists, engineers, and mathematicians. As we continue to unravel the mysteries of how these materials flow and deform, we open new possibilities for technological innovation that is both extremely soft and extremely powerful.
Advanced modeling of complex fluid behavior
Targeted drug delivery and diagnostics
Eco-friendly alternatives to current materials
Adaptive processes using complex fluids