What Are Rheological Materials?

Rheological materials are substances whose mechanical response to applied stress or strain cannot be described by simple linear models of either ideal viscous flow or ideal elastic deformation, but instead exhibit behavior that combines both, or that changes with the rate, duration, or history of loading. The term encompasses non-Newtonian fluids, viscoelastic solids and gels, thixotropic pastes, and materials that transition between solid-like and liquid-like states depending on conditions. Rheology, the parent discipline, draws on continuum mechanics, polymer physics, and colloidal science to describe and predict these behaviors, and its study of rheological materials has direct consequences for manufacturing, formulation, and structural design across many industries.

Examples of rheological materials are widespread in both natural and engineered systems: blood and synovial fluid, polymer melts, cement paste, lubricating grease, food emulsions, printing inks, and electronic encapsulants all exhibit non-Newtonian rheology. Understanding and controlling these properties is essential for designing manufacturing processes that involve pumping, mixing, coating, or deposition of such materials.

Material Behavior and Classification

The fundamental distinction in rheological materials is between Newtonian and non-Newtonian behavior. In a Newtonian fluid, shear stress is directly proportional to shear rate, and the proportionality constant is the viscosity. In non-Newtonian materials, this relationship is nonlinear or time-dependent. Shear-thinning (pseudoplastic) materials, including many polymer solutions and blood, decrease in viscosity as shear rate increases, facilitating flow under high-rate conditions like pumping while returning to higher viscosity at rest. Shear-thickening (dilatant) materials show the opposite response, stiffening under rapid deformation. Thixotropic materials are shear-thinning but require time to rebuild structure after a shear event; this behavior governs the workability of drilling muds, concrete, and paint. Viscoelastic materials exhibit both an elastic storage modulus (G′), representing energy stored and returned, and a viscous loss modulus (G″), representing energy dissipated; the ratio G″/G′ (tan δ) characterizes the balance between liquid-like and solid-like character. An IntechOpen review of viscoelastic material properties covers the theoretical framework for these moduli and their measurement in polymer and soft matter systems.

Measurement and Rheometry

Rheological properties are measured with instruments called rheometers, which apply controlled stress or strain to a sample and record the resulting deformation. Rotational rheometers use geometries such as parallel plates, cone-and-plate, and concentric cylinders (Couette flow) to generate well-defined shear fields; oscillatory tests within the linear viscoelastic regime yield G′ and G″ as functions of frequency and temperature without disturbing the material's equilibrium structure. Capillary rheometers and slit dies are used for polymer melts at the high shear rates representative of extrusion and injection molding. Accurate rheometry requires calibration against reference materials of known properties; NIST provides standard reference materials for non-Newtonian fluids, including certified polymer solutions that support viscosity and viscoelastic measurement validation across laboratories. Time-temperature superposition is a frequently used technique for extending the accessible frequency range: because many polymers obey a master curve when data collected at different temperatures are shifted along the frequency axis, a single material can be characterized across many decades of effective frequency.

Constitutive Models and Simulation

Engineering use of rheological materials requires mathematical models that relate stress to strain history. The Upper Convected Maxwell (UCM) model, the Oldroyd-B model, and the Giesekus model are widely used for polymer solution and melt simulation, each capturing different aspects of chain relaxation and nonlinear response. Power-law and Carreau-Yasuda models describe shear-thinning viscosity curves with a small number of fitted parameters, making them practical for computational fluid dynamics simulations of injection molding, coating, and extrusion. The TA Instruments rheology theory resource provides an applied treatment of how these constitutive frameworks translate into instrument measurements and industrial process design.

Applications

Rheological materials appear in a wide range of engineering and scientific fields, including:

• Polymer processing, including extrusion, injection molding, and film blowing
• Pharmaceutical formulation, where tablet binders, gels, and suspensions must meet viscosity and flow specifications
• Food science and manufacturing, governing texture, mouthfeel, and processing behavior of emulsions and pastes
• Electronic assembly, including solder paste, underfill encapsulants, and thermal interface materials
• Civil and construction engineering, in cement and concrete mixture design and fresh-state workability