Rheology (Pronounced /Riˈɒlədʒi/) is the Study of the Flow Of | Rheology | Fluid Mechanics

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  Rheology Rheology (pronounced /riˈɒlədʒi/) is the study of the flow of matter: mainly liquids but also soft solids or solids under conditions in which they flow rather than deform elastically[1]. It applies to substances which have a complex structure, including muds, sludges, suspensions, polymers, many foods, bodily fluids, and other biological materials. The flow of these substances cannot be characterized by a single value of viscosity (at a fixed temperature)[2] - instead the viscosity chan
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  Rheology Rheology (pronounced /riˈɒlədʒi/   ) is the study of the flow of matter: mainlyliquids but also soft solids or solids under conditions in which they flow rather thandeform elastically [1] . It applies to substances which have a complex structure,includingmuds,sludges, suspensions, polymers, many foods,bodily fluids, and other biological materials. The flow of these substances cannot be characterized by asingle value of viscosity(at a fixed temperature) [2] - instead the viscosity changes dueto other factors. For exampleketchup can have its viscosity reduced by shaking, but water cannot. SinceIsaac Newtonsrcinated the concept of viscosity, the study of variable viscosity liquids is also often called  Non-Newtonian fluid mechanics . [1] Theterm rheology was coined by Eugene C. Bingham, a professor atLafayette College, in 1920, from a suggestion by a colleague,Markus Reiner. [3] The term was inspiredby the quotation mistakenly attributed to  Heraclitus, (actually coming from thewritings of Simplicius   )  panta rei , everything flows . The experimental characterisationof a material's rheological behavior is known as rheometry , although the term rheology isfrequently used synonymously with rheometry, particularly by experimentalists.Theoretical aspects of rheology are the relation of the flow/deformation behavior of material and its internal structure (e.g., the orientation and elongation of polymer molecules), and the flow/deformation behavior of materials that cannot be described byclassical fluid mechanics or elasticity. Scope In practice, rheology is principally concerned with extending the classical disciplines of elasticity and ( Newtonian   )fluid mechanicsto materials whose mechanical behavior cannot be described with the classical theories. It is also concerned with establishing predictions for mechanical behavior (on the continuum mechanical scale) based on themicro- or nanostructure of the material, e.g. themolecular  size and architecture of   polymersin solution or the particle size distribution in a solid suspension. Materials flowwhen subjected to astress, that is a force per area. There are different sorts of stress [4]  andmaterials can respond in various ways, so much of theoretical rheology is concerned withforces and stresses. [1] Rheology unites the seemingly unrelated fields of   plasticityandnon-Newtonian fluidsby recognizing that both these types of materials are unable to support ashear stress in static equilibrium. In this sense, a plastic solid is a fluid.Granular rheology refers to the continuum mechanical description of granular materials.One of the tasks of rheology is to empirically establish the relationships betweendeformations and stresses, respectively their derivativesby adequate measurements. These experimental techniques are known asrheometry and are concerned with the determination with well-defined rheological material functions . Such relationships arethen amenable to mathematical treatment by the established methods of  continuummechanics.  The characterisation of flow or deformation srcinating from a simple shear stress field iscalled shear rheometry (or shear rheology   ). The study of extensional flows is calledextensional rheology.Shear flows are much easier to study and thus much moreexperimental data are available for shear flows than for extensional flows. Rheologist A rheologist is aninterdisciplinary scientistwho studies the flow of complex liquids or  the deformation of soft solids. It is not taken as a primary degree subject, and there is nogeneral qualification. He or she will usually have a primary qualification in one of severalfields: mathematics, the physical sciences [5] , engineering [6] ,medicine, or certaintechnologies, notablymaterialsor food. A small amount of rheology may be given during the first degree, but the professional will extend this knowledge during postgraduateresearch or by attending short courses and by joining one of the professional associations Applications Rheology has applications inengineering, geophysics, physiology and  pharmaceutics. In engineering , it affects the production and use of   polymeric materials, but  plasticity  theory has been similarly important for the design of  metal forming processes. Many industrially important substances such as concrete,  paint and chocolatehave complex flow characteristics. Geophysics includes the flow of lava,but in addition measures the flow of solidEarthmaterials over long time scales: those that display viscous behavior,e.g. granite  [7] , are known asrheids. In physiology , many bodily fluids are have complexcompositions and thus flow characteristics. In particular there is a specialist study of  bloodflow calledhemorheology.The term biorheologyis used for the wider field of  study of the flow properties of biological fluids. Elasticity, viscosity, solid- and liquid-like behavior,plasticity One generally associates liquids with viscous behavior (a thick  oil is a viscous liquid) andsolids with elastic behavior (an elastic string is an elastic solid). A more general point of view is to consider the material behavior at short times (relative to the duration of theexperiment/application of interest) and at long times.Liquid and solid character are relevant at long timesWe consider the application of a constant stress (a so-called creep experiment  ): ã if the material, after some deformation, eventually resists further deformation, it isconsidered a solid ã if, by contrast, the material flows indefinitely, it is considered a liquid  By contrast, elastic and viscous (or intermediate, viscoelastic) behavior is relevant at short times ( transient behavior  )We again consider the application of a constant stress: ã if the material deformation strain increases linearly with increasing applied stress ,then the material is purely elastic ã if the material deformation rate increases linearly with increasing applied stress,then the material is purely viscous ã if neither the deformation strain, nor its derivative with time ( rate ) follows theapplied stress, then the material is viscoelasticPlasticityis equivalent to the existence of a  yield stress  A material that behaves as a solid under low applied stresses may start to flow above acertain level of stress, called the  yield stress of the material. The term  plastic solid  isoften used when this plasticity threshold is rather high, while  yield stress fluid  is usedwhen the threshold stress is rather low. There is no fundamental difference, however, between both concepts. Newtonian fluid From Wikipedia, the free encyclopedia Jump to:navigation, search A Newtonian fluid (named for Isaac Newton   ) is afluidwhosestressversus rate of  strain curve is linear and passes through thesrcin. The constant of proportionality is known astheviscosity. Definition A simple equation to describe Newtonian fluid behaviour iswhere τ is the shear stress exerted by the fluid ( drag ) [Pa] μ is the fluid viscosity - a constant of proportionality [Pa·s]is the velocity gradient perpendicular to the direction of shear [s −1 ]In common terms, this means the fluid continues to flow, regardless of the forces actingon it. For example, water is Newtonian, because it continues to exemplify fluid propertiesno matter how fast it is stirred or mixed. Contrast this with a non-Newtonian fluid, in which stirring can leave a hole behind (that gradually fills up over time - this behaviour is seen in materials such as pudding, starch in water (oobleck    ), or, to a less rigorous  extent, sand), or cause the fluid to become thinner, the drop in viscosity causing it to flowmore (this is seen in non-drip paints, which brush on easily but become more viscouswhen on walls).For a Newtonian fluid, the viscosity, by definition, depends only ontemperature and  pressure(and also the chemical composition of the fluid if the fluid is not a puresubstance), not on the forces acting upon it.If the fluid isincompressibleand viscosity is constant across the fluid, the equationgoverning the shear stress, in theCartesian coordinate system, iswith comoving stress tensor (also written as )where, by the convention of tensor notation, τ ij is the shear stress on the i th face of a fluid element in the  j th direction u i is the velocity in the i th direction  x  j is the  j th direction coordinateIf a fluid does not obey this relation, it is termed anon-Newtonian fluid,of which there are several types, including polymer  solutions, molten polymers, many solid suspensions and most highly viscous fluids. Non-Newtonian fluid A non-Newtonian fluid is afluidwhose flow properties are not described by a singleconstant value of viscosity. Many  polymer solutions and molten polymers are non-  Newtonian fluids, as are many commonly found substances such asketchup,starch  suspensions,  paint,  bloodandshampoo. In a Newtonian fluid, the relation between the shear stressand thestrain rateis linear (and if one were to plot this relationship, it would  pass through thesrcin   ), the constant of proportionality being the coefficient of  viscosity.  In a non-Newtonian fluid, the relation between the shear stressand the strain rate is nonlinear, and can even be time-dependent. Therefore a constant coefficient of viscositycannot be defined. A ratio between shear stress and rate of strain (or shear-dependentviscosity) can be defined, this concept being more useful for fluids without time-dependent behavior.Although the concept of viscosity is commonly used to characterize a material, it can beinadequate to describe the mechanical behavior of a substance, particularly non- Newtonian fluids. They are best studied through several other rheological  properties which relate the relations between the stress and strain rate tensors under many different flow conditions, such asoscillatoryshear, or extensional flow which are measured usingdifferent devices or rheometers. The properties are better studied usingtensor -valuedconstitutive equations, which are common in the field of continuum mechanics.
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