Rheology (/riːˈɒlədʒi/; from Greek ῥέω rhéō, "flow" and -λoγία, -logia, "study of") is the study of the flow of matter, primarily in a liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force.[1] It applies to substances which have a complex microstructure, such as muds, sludges, suspensions, polymers and other glass formers (e.g., silicates), as well as many foods and additives, bodily fluids (e.g., blood) and other biological materials or other materials which belong to the class of soft matter.
Newtonian fluids can be characterized by a single coefficient of viscosity for a specific temperature. Although this viscosity will change with temperature, it does not change with the strain rate. Only a small group of fluids exhibit such constant viscosity. The large class of fluids whose viscosity changes with the strain rate (the relative velocity of flow) are called non-Newtonian fluids.
Rheology generally accounts for the behavior of non-Newtonian fluids, by characterizing the minimum number of functions that are needed to relate stresses with rate of change of strains or strain rates. For example, ketchup can have its viscosity reduced by shaking (or other forms of mechanical agitation, where the relative movement of different layers in the material actually causes the reduction in viscosity) but water cannot. Ketchup is a shear thinning material, as an increase in relative velocity caused a reduction in viscosity, while some other non-Newtonian materials show the opposite behavior: viscosity going up with relative deformation, which are called shear thickening or dilatant materials. Since Sir Isaac Newton originated the concept of viscosity, the study of liquids with strain rate dependent viscosity is also often called Non-Newtonian fluid mechanics.[1]
The term rheology was coined by Eugene C. Bingham, a professor at Lafayette College, in 1920, from a suggestion by a colleague, Markus Reiner.[2][3] The term was inspired by the aphorism of Simplicius (often attributed to Heraclitus), panta rhei, "everything flows"[4][5]
The experimental characterization of a material's rheological behaviour is known as rheometry, although the term rheology is frequently used synonymously with rheometry, particularly by experimentalists. Theoretical aspects of rheology are the relation of the flow/deformation behaviour of material and its internal structure (e.g., the orientation and elongation of polymer molecules), and the flow/deformation behaviour of materials that cannot be described by classical fluid mechanics or elasticity.
Contents
1 Scope
2 Rheologist
3 Viscoelasticity
4 Applications
4.1 Materials science
4.1.1 Polymers
4.1.2 Biopolymers
4.1.3 Sol-gel
4.2 Geophysics
4.3 Physiology
4.4 Food rheology
4.5 Concrete rheology
5 Measurement
6 Dimensionless numbers
6.1 Deborah number
6.2 Reynolds number
7 See also
8 References
9 External links
Scope
In practice, rheology is principally concerned with extending continuum mechanics to characterize flow of materials, that exhibits a combination of elastic, viscous and plastic behavior by properly combining elasticity and (Newtonian) fluid mechanics. It is also concerned with establishing predictions for mechanical behavior (on the continuum mechanical scale) based on the micro- or nanostructure of the material, e.g. the molecular size and architecture of polymers in solution or the particle size distribution in a solid suspension. Materials with the characteristics of a fluid will flow when subjected to a stress which is defined as the force per area. There are different sorts of stress (e.g. shear, torsional, etc.) and materials can respond differently for different stresses. Much of theoretical rheology is concerned with associating external forces and torques with internal stresses and internal strain gradients and velocities.[1][6][7][8]
Continuum mechanics
The study of the physics of continuous materials Solid mechanics
The study of the physics of continuous materials with a defined rest shape. Elasticity
Describes materials that return to their rest shape after applied stresses are removed.
Plasticity
Describes materials that permanently deform after a sufficient applied stress. Rheology
The study of materials with both solid and fluid characteristics.
Fluid mechanics
The study of the physics of continuous materials which deform when subjected to a force. Non-Newtonian fluids do not undergo strain rates proportional to the applied shear stress.
Newtonian fluids undergo strain rates proportional to the applied shear stress.
Rheology unites the seemingly unrelated fields of plasticity and non-Newtonian fluid dynamics by recognizing that materials undergoing these types of deformation are unable to support a stress (particularly a shear stress, since it is easier to analyze shear deformation) in static equilibrium. In this sense, a solid undergoing plastic deformation is a fluid, although no viscosity coefficient is associated with this flow. Granular rheology refers to the continuum mechanical description of granular materials.
One of the major tasks of rheology is to empirically establish the relationships between deformations (or rates of deformation) and stresses, by adequate measurements, although a number of theoretical developments (such as assuring frame invariants) are also required before using the empirical data. These experimental techniques are known as rheometry and are concerned with the determination with well-defined rheological material functions. Such relationships are then amenable to mathematical treatment by the established methods of continuum mechanics.
The characterization of flow or deformation originating from a simple shear stress field is called shear rheometry (or shear rheology). The study of extensional flows is called extensional rheology. Shear flows are much easier to study and thus much more experimental data are available for shear flows than for extensional flows.