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A Glossary of Rheological Terms used in this Book
A GLOSSARY OF RHEOLOGICAL TERMS USED IN THIS BOOK
This glossary is based mainly on the British Standard BS 5168:1975. It differs from the British Standard in that it is not intended to be comprehensive but limited to the terms which are most relevant to the present book. (See also, H.A. Barnes, J.F. Hutton and K. Walters, An Introduction to Rheology, Elsevier, 1989.) Aggregation
A process in suspension mechanics where small particles are brought together. In the extreme case, individual small particles attach to each other to form larger particles. (See §6.5.)
Antithixotropy
A n increase of the apparent viscosity under constant shear stress or shear rate, followed by a gradual recovery when the stress or shear rate is removed. The effect is time dependent. (Also called 'negative thixotropy' and 'rheopexyO. (See p. 8.)
Apparent viscosity
The shear stress divided by rate of shear when this quotient is dependent on rate of shear. Also called viscosity and shear viscosity.
Boger fluids
Constant-viscosity elastic liquids. Such fluids are usually prepared by dissolving a high molecular weight polymer at low concentration in a high-viscosity Newtonian liquid thereby masking the shear-rate dependent viscosity contribution of the polymer.
Capillary number
The product of the viscosity and a characteristic velocity divided by the surface tension. It expresses a ratio of viscous forces to surface tension forces. (See p. 9 and §6.2.)
Carbopol
A trade name for a family of polymers; polyacrylates and cross-linked polyacrylates and polyacrylic acid, which find a major use as thickeners. A broad spectrum of rheological properties can be developed with these polymers by variation of pH and/or ionic strength of the solution. Carbopol solutions form relatively inelastic shear-thinning fluids.
150
Carboxymethylcellulose (CMC)
A high molecular weight polymer derived from cellulose which is water soluble. C M C solutions at moderate concentration form relatively inelastic shear-thinning solutions.
Creeping flow
Flow in the limit of very low Reynolds number where viscous forces dominate.
Couette flow (circular)
Simple shear flow in the annulus between two co-axial cylinders in relative rotation.
Deborah number
The ratio of a characteristic (relaxation) time of a material to a characteristic time of the relevant deformation process. (See p. 9.)
Decalin
A ten carbon organic solvent used in the construction of polymer solutions.
Die swell
A post-extrusion swelling. (See §2.3.1.)
Drag reduction
Reduction in frictional losses in turbulent flow as a result of the addition of a few p p m of a high molecular weight soluble polymer to the flowing fluid. (See §4.7.)
Elastic liquid
A liquid showing elastic as well as viscous properties (see elastico-viscous and viscoelastic).
Elastico-viscous
A descriptive term for a liquid having both viscous and elastic properties.
Elongational viscosity
Synonym of extensional viscosity.
Extensional viscosity
The extensional (tensile) stress divided by the rate of extension. Also called elongational viscosity and Trouton viscosity.
Laminar flow
Flow without turbulence.
Lip vortex
The vortex formed near the re-entrant corner in axisymmetric or planar contraction flows. (See §3.2 and §3.3.)
Melt fracture
The gross irregular distortion of a polymer extrudate after passing through a die. (See §2.3.2.)
Navier-Stokes equations The equations governing the motion of a Newtonian fluid. Newtonian fluid model
A model characterized by a constant value for the quotient of the shear stress divided by the rate of shear in a simple shear flow and with zero normal stress differences.
Non-Newtonian fluid
A n y fluid whose behaviour is not characterized by the Navier-Stokes equations.
151
Normal force
In rheology, a force acting at right angles to an applied shear stress.
Normal stress
The component of stress at right angles to the area considered.
Normal stress difference
The difference between normal stress components.
Overshoot
The transient rise of a stress or velocity above their equilibrium values at the start up of flow.
Poiseuille flow
Laminar flow in a pipe of circular cross section under a constant pressure gradient.
Polyacrylamide
A water-soluble high molecular weight polymer. The molecular weight of typical polyacrylamides is of the order of 1 0 6, most polyacrylamides being in the form of a linear polymer chain. Polyacrylamide solutions form highly viscoelastic fluids (see also Separan).
Polyacrylic acid
A cationic water soluble polymer available in a large molecular weight range.
Polydimethylsiloxane
A silicone polymer available over a broad molecular weight range in both branched and linear forms.
Polyethylene oxide
A high molecular weight water-soluble polymer produced by the heterogeneous catalytic polymerisation of ethylene oxide. Polyethylene oxides are available in a broad range of molecular weights ranging from about 1 0 5 to over 5 x 1 0 6. Polyethylene oxide solutions form highly viscoelastic fluids.
Polyisobutylene
A colourless and odourless polymer constructed from isobutylene monomer. The molecular weight of the final polymer is dependent upon the catalyst used in the polymerisation reaction. Polyisobutylene polymers are soluble in hydrocarbons and chlorinated hydrocarbons. High molecular weight polyisobutylenes are solid at room temperature.
Polymer solution
A polymer molecule solute dissolved in a low molecular weight solvent. Both organic-based and water-based solvents are of interest in non-Newtonian fluid mechanics.
Polyox
A trade name for polyethylene oxide.
Relaxation time
The time taken for the shear stress of a fluid that obeys the Maxwell model to reduce to 1/e of its original equilibrium value on the cessation of steady shear flow.
152
Reynolds number
The product of a typical apparatus length and a typical fluid speed divided by the kinematic viscosity of the fluid. It expresses the ratio of the inertia forces to the viscous forces.
Rheology
The science of the deformation and flow of matter.
Rheometer
A n instrument for measuring rheological properties.
Schlieren photography
A flow visualization technique which relies on a refractive index variation in the flow field. (See p. 101.)
Secondary flow
The components of flow in a plane orthogonal to the main direction of flow.
Separan
A trade name for polyacrylamide.
Sharkskin
A n ordered surface irregularity observed in extrusion of molten polymers. The amplitude of the distortions on the surface is generally small compared to the gross extrudate distortion referred to as melt fracture. (See §2.3.2.)
Shear stress
The component of stress parallel to (tangential to) the area considered.
Shear-thickening
The increase of viscosity with increasing rate of shear in a steady shear flow.
Shear-thinning
The reduction of viscosity with increasing rate of shear in a steady shear flow.
Shear viscosity
Synonym of apparent viscosity.
Silicone, silicone oil, silicone g u m
Generic terms used to refer to polydimethylsiloxane polymers.
Simple shear
A shear caused by the parallel relative displacement of parallel planes (see viscometric flow).
Steady flow
A flow in which the velocity at every point does not vary with time.
Stress
A force per unit area.
Stress tensor
A matrix of the shear stress and normal stress components representing the state of stress at a point in a body.
Tack
Stickiness. (See p. 30.)
Taylor cells
The secondary flow consisting of ring-like cell vortices associated with an instability in circular Couette flow. (See p. 120.)
153
Tension-thickening
A n increase in extensional viscosity with increasing rate of strain in a steady extensional flow.
Tension-thinning
A decrease in extensional viscosity with increasing rate of strain in a steady extensional flow.
Thixotropy
A decrease of the apparent viscosity under constant shear stress or shear rate, followed by a gradual recovery when the stress or shear rate is removed. The effect is time dependent.
Trouton ratio
The ratio of extensional to shear viscosities.
Turbulence
A condition of flow in which the velocity components show random variation.
Velocity gradient
The derivative of the velocity of a fluid element with respect to a space coordinate.
Viscoelastic(ity)
Having both viscous and elastic properties. This term is sometimes restricted to solids.
Viscometric flow
A laminar flow which is equivalent to a steady simple-shear flow. Such a flow is determined by a m a x i m u m of three material functions: the viscosity function and two normal stress functions.
Viscosity
(1) Qualitatively, the property of a material to resist deformation increasingly with increasing rate of deformation. (2) Quantitatively, a measure of this property, defined as the shear stress divided by the rate of shear in steady simple-shear flow. Often used synonymously with apparent viscosity.
Vortex enhancement
Growth in size of the vortex in contraction flows, normally resulting from fluid elasticity. (See, for example, p. 38.)
Weissenberg effect
A n effect found in non-Newtonian fluids manifested, for example, in the climbing of the fluid up a rotating rod dipped into it. A normal stress effect. (See §2.2.)
Weiss enberg number
The product of the relaxation time, or some other characteristic time of a material, and the rate of shear of the flow. (See p. 9.)
Xanthan g u m
A high molecular weight polysaccharide polymer which was the first fermented biopolymer product to be used commercially. The rigid backbone structure gives xanthan g u m a rod-like structure in solution.
154
This glossary is based mainly on the British Standard BS 5168:1975. It differs from the British Standard in that it is not intended to be comprehensive but limited to the terms which are most relevant to the present book. (See also, H.A. Barnes, J.F. Hutton and K. Walters, An Introduction to Rheology, Elsevier, 1989.) Aggregation
A process in suspension mechanics where small particles are brought together. In the extreme case, individual small particles attach to each other to form larger particles. (See §6.5.)
Antithixotropy
A n increase of the apparent viscosity under constant shear stress or shear rate, followed by a gradual recovery when the stress or shear rate is removed. The effect is time dependent. (Also called 'negative thixotropy' and 'rheopexyO. (See p. 8.)
Apparent viscosity
The shear stress divided by rate of shear when this quotient is dependent on rate of shear. Also called viscosity and shear viscosity.
Boger fluids
Constant-viscosity elastic liquids. Such fluids are usually prepared by dissolving a high molecular weight polymer at low concentration in a high-viscosity Newtonian liquid thereby masking the shear-rate dependent viscosity contribution of the polymer.
Capillary number
The product of the viscosity and a characteristic velocity divided by the surface tension. It expresses a ratio of viscous forces to surface tension forces. (See p. 9 and §6.2.)
Carbopol
A trade name for a family of polymers; polyacrylates and cross-linked polyacrylates and polyacrylic acid, which find a major use as thickeners. A broad spectrum of rheological properties can be developed with these polymers by variation of pH and/or ionic strength of the solution. Carbopol solutions form relatively inelastic shear-thinning fluids.
150
Carboxymethylcellulose (CMC)
A high molecular weight polymer derived from cellulose which is water soluble. C M C solutions at moderate concentration form relatively inelastic shear-thinning solutions.
Creeping flow
Flow in the limit of very low Reynolds number where viscous forces dominate.
Couette flow (circular)
Simple shear flow in the annulus between two co-axial cylinders in relative rotation.
Deborah number
The ratio of a characteristic (relaxation) time of a material to a characteristic time of the relevant deformation process. (See p. 9.)
Decalin
A ten carbon organic solvent used in the construction of polymer solutions.
Die swell
A post-extrusion swelling. (See §2.3.1.)
Drag reduction
Reduction in frictional losses in turbulent flow as a result of the addition of a few p p m of a high molecular weight soluble polymer to the flowing fluid. (See §4.7.)
Elastic liquid
A liquid showing elastic as well as viscous properties (see elastico-viscous and viscoelastic).
Elastico-viscous
A descriptive term for a liquid having both viscous and elastic properties.
Elongational viscosity
Synonym of extensional viscosity.
Extensional viscosity
The extensional (tensile) stress divided by the rate of extension. Also called elongational viscosity and Trouton viscosity.
Laminar flow
Flow without turbulence.
Lip vortex
The vortex formed near the re-entrant corner in axisymmetric or planar contraction flows. (See §3.2 and §3.3.)
Melt fracture
The gross irregular distortion of a polymer extrudate after passing through a die. (See §2.3.2.)
Navier-Stokes equations The equations governing the motion of a Newtonian fluid. Newtonian fluid model
A model characterized by a constant value for the quotient of the shear stress divided by the rate of shear in a simple shear flow and with zero normal stress differences.
Non-Newtonian fluid
A n y fluid whose behaviour is not characterized by the Navier-Stokes equations.
151
Normal force
In rheology, a force acting at right angles to an applied shear stress.
Normal stress
The component of stress at right angles to the area considered.
Normal stress difference
The difference between normal stress components.
Overshoot
The transient rise of a stress or velocity above their equilibrium values at the start up of flow.
Poiseuille flow
Laminar flow in a pipe of circular cross section under a constant pressure gradient.
Polyacrylamide
A water-soluble high molecular weight polymer. The molecular weight of typical polyacrylamides is of the order of 1 0 6, most polyacrylamides being in the form of a linear polymer chain. Polyacrylamide solutions form highly viscoelastic fluids (see also Separan).
Polyacrylic acid
A cationic water soluble polymer available in a large molecular weight range.
Polydimethylsiloxane
A silicone polymer available over a broad molecular weight range in both branched and linear forms.
Polyethylene oxide
A high molecular weight water-soluble polymer produced by the heterogeneous catalytic polymerisation of ethylene oxide. Polyethylene oxides are available in a broad range of molecular weights ranging from about 1 0 5 to over 5 x 1 0 6. Polyethylene oxide solutions form highly viscoelastic fluids.
Polyisobutylene
A colourless and odourless polymer constructed from isobutylene monomer. The molecular weight of the final polymer is dependent upon the catalyst used in the polymerisation reaction. Polyisobutylene polymers are soluble in hydrocarbons and chlorinated hydrocarbons. High molecular weight polyisobutylenes are solid at room temperature.
Polymer solution
A polymer molecule solute dissolved in a low molecular weight solvent. Both organic-based and water-based solvents are of interest in non-Newtonian fluid mechanics.
Polyox
A trade name for polyethylene oxide.
Relaxation time
The time taken for the shear stress of a fluid that obeys the Maxwell model to reduce to 1/e of its original equilibrium value on the cessation of steady shear flow.
152
Reynolds number
The product of a typical apparatus length and a typical fluid speed divided by the kinematic viscosity of the fluid. It expresses the ratio of the inertia forces to the viscous forces.
Rheology
The science of the deformation and flow of matter.
Rheometer
A n instrument for measuring rheological properties.
Schlieren photography
A flow visualization technique which relies on a refractive index variation in the flow field. (See p. 101.)
Secondary flow
The components of flow in a plane orthogonal to the main direction of flow.
Separan
A trade name for polyacrylamide.
Sharkskin
A n ordered surface irregularity observed in extrusion of molten polymers. The amplitude of the distortions on the surface is generally small compared to the gross extrudate distortion referred to as melt fracture. (See §2.3.2.)
Shear stress
The component of stress parallel to (tangential to) the area considered.
Shear-thickening
The increase of viscosity with increasing rate of shear in a steady shear flow.
Shear-thinning
The reduction of viscosity with increasing rate of shear in a steady shear flow.
Shear viscosity
Synonym of apparent viscosity.
Silicone, silicone oil, silicone g u m
Generic terms used to refer to polydimethylsiloxane polymers.
Simple shear
A shear caused by the parallel relative displacement of parallel planes (see viscometric flow).
Steady flow
A flow in which the velocity at every point does not vary with time.
Stress
A force per unit area.
Stress tensor
A matrix of the shear stress and normal stress components representing the state of stress at a point in a body.
Tack
Stickiness. (See p. 30.)
Taylor cells
The secondary flow consisting of ring-like cell vortices associated with an instability in circular Couette flow. (See p. 120.)
153
Tension-thickening
A n increase in extensional viscosity with increasing rate of strain in a steady extensional flow.
Tension-thinning
A decrease in extensional viscosity with increasing rate of strain in a steady extensional flow.
Thixotropy
A decrease of the apparent viscosity under constant shear stress or shear rate, followed by a gradual recovery when the stress or shear rate is removed. The effect is time dependent.
Trouton ratio
The ratio of extensional to shear viscosities.
Turbulence
A condition of flow in which the velocity components show random variation.
Velocity gradient
The derivative of the velocity of a fluid element with respect to a space coordinate.
Viscoelastic(ity)
Having both viscous and elastic properties. This term is sometimes restricted to solids.
Viscometric flow
A laminar flow which is equivalent to a steady simple-shear flow. Such a flow is determined by a m a x i m u m of three material functions: the viscosity function and two normal stress functions.
Viscosity
(1) Qualitatively, the property of a material to resist deformation increasingly with increasing rate of deformation. (2) Quantitatively, a measure of this property, defined as the shear stress divided by the rate of shear in steady simple-shear flow. Often used synonymously with apparent viscosity.
Vortex enhancement
Growth in size of the vortex in contraction flows, normally resulting from fluid elasticity. (See, for example, p. 38.)
Weissenberg effect
A n effect found in non-Newtonian fluids manifested, for example, in the climbing of the fluid up a rotating rod dipped into it. A normal stress effect. (See §2.2.)
Weiss enberg number
The product of the relaxation time, or some other characteristic time of a material, and the rate of shear of the flow. (See p. 9.)
Xanthan g u m
A high molecular weight polysaccharide polymer which was the first fermented biopolymer product to be used commercially. The rigid backbone structure gives xanthan g u m a rod-like structure in solution.
154