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Viscosity | |
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A simulation of liquids with different viscosities. The liquid on the right has higher viscosity than the liquid on the left. | |

Common symbols | η , μ |

Derivations from other quantities | μ = G · t |

Continuum mechanics | ||||
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Laws
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The **viscosity** of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.^{ [1] }

In physics, a **fluid** is a substance that continually deforms (flows) under an applied shear stress, or external force. Fluids are a phase of matter and include liquids, gases and plasmas. They are substances with zero shear modulus, or, in simpler terms, substances which cannot resist any shear force applied to them.

In fluid dynamics, **drag** is a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers or a fluid and a solid surface. Unlike other resistive forces, such as dry friction, which are nearly independent of velocity, drag forces depend on velocity. Drag force is proportional to the velocity for a laminar flow and the squared velocity for a turbulent flow. Even though the ultimate cause of a drag is viscous friction, the turbulent drag is independent of viscosity.

In cooking, a **syrup** or **sirup** is a condiment that is a thick, viscous liquid consisting primarily of a solution of sugar in water, containing a large amount of dissolved sugars but showing little tendency to deposit crystals. Its consistency is similar to that of molasses. The viscosity arises from the multiple hydrogen bonds between the dissolved sugar, which has many hydroxyl (OH) groups.

- Etymology
- Definition
- Simple definition
- General definition
- Dynamic and kinematic viscosity
- Momentum transport
- Newtonian and non-Newtonian fluids
- In solids
- Measurement
- Units
- Molecular origins
- Pure gases
- Pure liquids
- Mixtures and blends
- Solutions and suspensions
- Amorphous materials
- Eddy viscosity
- Selected substances
- Water
- Air
- Other common substances
- Order of magnitude estimates
- See also
- References
- Footnotes
- Citations
- Sources
- External links

Viscosity can be conceptualized as quantifying the frictional force that arises between adjacent layers of fluid that are in relative motion. For instance, when a fluid is forced through a tube, it flows more quickly near the tube's axis than near its walls. In such a case, experiments show that some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow through the tube. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion: the strength of this force is proportional to the viscosity.

**Friction** is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:

**Pressure** is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the pressure relative to the ambient pressure.

A fluid that has no resistance to shear stress is known as an *ideal* or *inviscid* fluid. Zero viscosity is observed only at very low temperatures in superfluids. Otherwise, the second law of thermodynamics requires all fluids to have positive viscosity;^{ [2] }^{ [3] } such fluids are technically said to be viscous or viscid. A fluid with a high viscosity, such as pitch, may appear to be a solid.

In physics, **cryogenics** is the production and behaviour of materials at very low temperatures.

**Superfluidity** is the characteristic property of a fluid with zero viscosity which therefore flows without loss of kinetic energy. When stirred, a superfluid forms cellular vortices that continue to rotate indefinitely. Superfluidity occurs in two isotopes of helium when they are liquefied by cooling to cryogenic temperatures. It is also a property of various other exotic states of matter theorized to exist in astrophysics, high-energy physics, and theories of quantum gravity. Superfluidity is often coincidental with Bose–Einstein condensation, but neither phenomenon is directly related to the other; not all Bose-Einstein condensates can be regarded as superfluids, and not all superfluids are Bose–Einstein condensates. The semiphenomenological theory of superfluidity was developed by Lev Landau.

The **second law of thermodynamics** states that the total entropy of an isolated system can never decrease over time. The total entropy of a system and its surroundings can remain constant in ideal cases where the system is in thermodynamic equilibrium, or is undergoing a (fictive) reversible process. In all processes that occur, including spontaneous processes, the total entropy of the system and its surroundings increases and the process is irreversible in the thermodynamic sense. The increase in entropy accounts for the irreversibility of natural processes, and the asymmetry between future and past.

The word "viscosity" is derived from the Latin "* viscum*", meaning mistletoe and also a viscous glue made from mistletoe berries.

**Latin** is a classical language belonging to the Italic branch of the Indo-European languages. The Latin alphabet is derived from the Etruscan and Greek alphabets and ultimately from the Phoenician alphabet.

**Mistletoe** is the English common name for most obligate hemiparasitic plants in the order Santalales. They are attached to their host tree or shrub by a structure called the haustorium, through which they extract water and nutrients from the host plant. Their parasitic lifestyle has led to some dramatic changes in their metabolism.

In materials science and engineering, one is often interested in understanding the forces, or stresses, involved in the deformation of a material. For instance, if the material were a simple spring, the answer would be given by Hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium. Stresses which can be attributed to the deformation of a material from some rest state are called elastic stresses. In other materials, stresses are present which can be attributed to the rate of change of the deformation over time. These are called viscous stresses. For instance, in a fluid such as water the stresses which arise from shearing the fluid do not depend on the *distance* the fluid has been sheared; rather, they depend on how *quickly* the shearing occurs.

The interdisciplinary field of **materials science**, also commonly termed **materials science and engineering**, is the design and discovery of new materials, particularly solids. The intellectual origins of materials science stem from the Enlightenment, when researchers began to use analytical thinking from chemistry, physics, and engineering to understand ancient, phenomenological observations in metallurgy and mineralogy. Materials science still incorporates elements of physics, chemistry, and engineering. As such, the field was long considered by academic institutions as a sub-field of these related fields. Beginning in the 1940s, materials science began to be more widely recognized as a specific and distinct field of science and engineering, and major technical universities around the world created dedicated schools of the study, within either the Science or Engineering schools, hence the naming.

**Engineering** is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more specialized fields of engineering, each with a more specific emphasis on particular areas of applied mathematics, applied science, and types of application. See glossary of engineering.

In continuum mechanics, **stress** is a physical quantity that expresses the internal forces that neighbouring particles of a continuous material exert on each other, while strain is the measure of the deformation of the material which is not a physical quantity. For example, when a solid vertical bar is supporting an overhead weight, each particle in the bar pushes on the particles immediately below it. When a liquid is in a closed container under pressure, each particle gets pushed against by all the surrounding particles. The container walls and the pressure-inducing surface push against them in (Newtonian) reaction. These macroscopic forces are actually the net result of a very large number of intermolecular forces and collisions between the particles in those molecules. Stress is frequently represented by a lowercase Greek letter sigma (σ).

Viscosity is the material property which relates the viscous stresses in a material to the rate of change of a deformation (the strain rate). Although it applies to general flows, it is easy to visualize and define in a simple shearing flow, such as a planar Couette flow.

In fluid dynamics, **Couette flow** is the flow of a viscous fluid in the space between two surfaces, one of which is moving tangentially relative to the other. The configuration often takes the form of two parallel plates or the gap between two concentric cylinders. The flow is driven by virtue of viscous drag force acting on the fluid, but may additionally be motivated by an applied pressure gradient in the flow direction. The Couette configuration models certain practical problems, like flow in lightly loaded journal bearings, and is often employed in viscometry and to demonstrate approximations of reversibility. This type of flow is named in honor of Maurice Couette, a Professor of Physics at the French University of Angers in the late 19th century.

In the Couette flow, a fluid is trapped between two infinitely large plates, one fixed and one in parallel motion at constant speed (see illustration to the right). If the speed of the top plate is low enough (to avoid turbulence), then in steady state the fluid particles move parallel to it, and their speed varies from at the bottom to at the top.^{ [5] } Each layer of fluid moves faster than the one just below it, and friction between them gives rise to a force resisting their relative motion. In particular, the fluid applies on the top plate a force in the direction opposite to its motion, and an equal but opposite force on the bottom plate. An external force is therefore required in order to keep the top plate moving at constant speed.

In many fluids, the flow velocity is observed to vary linearly from zero at the bottom to at the top. Moreover, the magnitude of the force acting on the top plate is found to be proportional to the speed and the area of each plate, and inversely proportional to their separation :

The proportionality factor is the viscosity of the fluid, with units of (pascal-second). The ratio is called the *rate of shear deformation* or * shear velocity *, and is the derivative of the fluid speed in the direction perpendicular to the plates (see illustrations to the right). If the velocity does not vary linearly with , then the appropriate generalization is

where , and is the local shear velocity. This expression is referred to as Newton's law of viscosity. In shearing flows with planar symmetry, it is what *defines*. It is a special case of the general definition of viscosity (see below), which can be expressed in coordinate-free form.

Use of the Greek letter mu () for the viscosity is common among mechanical and chemical engineers, as well as physicists.^{ [6] }^{ [7] }^{ [8] } However, the Greek letter eta () is also used by chemists, physicists, and the IUPAC.^{ [9] } The viscosity is sometimes also referred to as the *shear viscosity*. However, at least one author discourages the use of this terminology, noting that can appear in nonshearing flows in addition to shearing flows.^{ [10] }

In very general terms, the viscous stresses in a fluid are defined as those resulting from the relative velocity of different fluid particles. As such, the viscous stresses must depend on spatial gradients of the flow velocity. If the velocity gradients are small, then to a first approximation the viscous stresses depend only on the first derivatives of the velocity.^{ [11] } (For Newtonian fluids, this is also a linear dependence.) In Cartesian coordinates, the general relationship can then be written as

where is a viscosity tensor that maps the strain rate tensor onto the viscous stress tensor .^{ [12] } Since the indices in this expression can vary from 1 to 3, there are 81 "viscosity coefficients" in total. However, due to spatial symmetries these coefficients are not all independent. For instance, for isotropic Newtonian fluids, the 81 coefficients can be reduced to 2 independent parameters. The most usual decomposition yields the standard (scalar) viscosity and the bulk viscosity :

where is the unit tensor, and the dagger denotes the transpose.^{ [10] }^{ [13] } This equation can be thought of as a generalized form of Newton's law of viscosity.

The bulk viscosity (also called volume viscosity) expresses a type of internal friction that resists the shearless compression or expansion of a fluid. Knowledge of is frequently not necessary in fluid dynamics problems. For example, an incompressible fluid satisfies and so the term containing drops out. Moreover, is often assumed to be negligible for gases since it is in a monoatomic ideal gas.^{ [10] } One situation in which can be important is the calculation of energy loss in sound and shock waves, described by Stokes' law of sound attenuation, since these phenomena involve rapid expansions and compressions.

It is worth emphasizing that the above expressions are not fundamental laws of nature, but rather definitions of viscosity. As such, their utility for any given material, as well as means for measuring or calculating the viscosity, must be established using separate means.

In fluid dynamics, it is common to work in terms of the *kinematic viscosity* (also called "momentum diffusivity"), defined as the ratio of the viscosity μ to the density of the fluid ρ. It is usually denoted by the Greek letter nu (ν) and has units :

- .

Consistent with this nomenclature, the viscosity is frequently called the *dynamic viscosity* or *absolute viscosity*, and has units force × time/area.

Transport theory provides an alternate interpretation of viscosity in terms of momentum transport: viscosity is the material property which characterizes momentum transport within a fluid, just as thermal conductivity characterizes heat transport, and (mass) diffusivity characterizes mass transport.^{ [14] } To see this, note that in Newton's law of viscosity, , the shear stress has units equivalent to a momentum flux, i.e. momentum per unit time per unit area. Thus, can be interpreted as specifying the flow of momentum in the direction from one fluid layer to the next. Per Newton's law of viscosity, this momentum flow occurs across a velocity gradient, and the magnitude of the corresponding momentum flux is determined by the viscosity.

The analogy with heat and mass transfer can be made explicit. Just as heat flows from high temperature to low temperature and mass flows from high density to low density, momentum flows from high velocity to low velocity. These behaviors are all described by compact expressions, called constitutive relations, whose one-dimensional forms are given here:

where is the density, and are the mass and heat fluxes, and and are the mass diffusivity and thermal conductivity.^{ [15] } The fact that mass, momentum, and energy (heat) transport are among the most relevant processes in continuum mechanics is not a coincidence: these are among the few physical quantities that are conserved at the microscopic level in interparticle collisions. Thus, rather than being dictated by the fast and complex microscopic interaction timescale, their dynamics occurs on macroscopic timescales, as described by the various equations of transport theory and hydrodynamics.

Newton's law of viscosity is not a fundamental law of nature, but rather a constitutive equation (like Hooke's law, Fick's law, and Ohm's law) which serves to define the viscosity . Its form is motivated by experiments which show that for a wide range of fluids, is independent of strain rate. Such fluids are called Newtonian. Gases, water, and many common liquids can be considered Newtonian in ordinary conditions and contexts. However, there are many non-Newtonian fluids that significantly deviate from this behavior. For example:

- Shear-thickening liquids, whose viscosity increases with the rate of shear strain.
- Shear-thinning liquids, whose viscosity decreases with the rate of shear strain.
- Thixotropic liquids, that become less viscous over time when shaken, agitated, or otherwise stressed.
- Rheopectic (dilatant) liquids, that become more viscous over time when shaken, agitated, or otherwise stressed.
- Bingham plastics that behave as a solid at low stresses but flow as a viscous fluid at high stresses.

Trouton's ratio is the ratio of extensional viscosity to shear viscosity. For a Newtonian fluid, the Trouton ratio is 3.^{ [16] }^{ [17] } Shear-thinning liquids are very commonly, but misleadingly, described as thixotropic.^{ [18] }

Even for a Newtonian fluid, the viscosity usually depends on its composition and temperature. For gases and other compressible fluids, it depends on temperature and varies very slowly with pressure. The viscosity of some fluids may depend on other factors. A magnetorheological fluid, for example, becomes thicker when subjected to a magnetic field, possibly to the point of behaving like a solid.

The viscous forces that arise during fluid flow must not be confused with the elastic forces that arise in a solid in response to shear, compression or extension stresses. While in the latter the stress is proportional to the *amount* of shear deformation, in a fluid it is proportional to the *rate* of deformation over time. (For this reason, Maxwell used the term *fugitive elasticity* for fluid viscosity.)

However, many liquids (including water) will briefly react like elastic solids when subjected to sudden stress. Conversely, many "solids" (even granite) will flow like liquids, albeit very slowly, even under arbitrarily small stress.^{ [19] } Such materials are therefore best described as possessing both elasticity (reaction to deformation) and viscosity (reaction to rate of deformation); that is, being viscoelastic.

Indeed, some authors have claimed that amorphous solids, such as glass and many polymers, are actually liquids with a very high viscosity (greater than 10^{12} Pa·s).^{ [20] }^{ [21] } However, other authors dispute this hypothesis, claiming instead that there is some threshold for the stress, below which most solids will not flow at all,^{ [22] } and that alleged instances of glass flow in window panes of old buildings are due to the crude manufacturing process of older eras rather than to the viscosity of glass.^{ [23] }

Viscoelastic solids may exhibit both shear viscosity and bulk viscosity. The extensional viscosity is a linear combination of the shear and bulk viscosities that describes the reaction of a solid elastic material to elongation. It is widely used for characterizing polymers.

In geology, earth materials that exhibit viscous deformation at least three orders of magnitude greater than their elastic deformation are sometimes called rheids.^{ [24] }

Viscosity is measured with various types of viscometers and rheometers. A rheometer is used for those fluids that cannot be defined by a single value of viscosity and therefore require more parameters to be set and measured than is the case for a viscometer. Close temperature control of the fluid is essential to acquire accurate measurements, particularly in materials like lubricants, whose viscosity can double with a change of only 5 °C.

For some fluids, the viscosity is constant over a wide range of shear rates (Newtonian fluids). The fluids without a constant viscosity (non-Newtonian fluids) cannot be described by a single number. Non-Newtonian fluids exhibit a variety of different correlations between shear stress and shear rate.

One of the most common instruments for measuring kinematic viscosity is the glass capillary viscometer.

In coating industries, viscosity may be measured with a cup in which the efflux time is measured. There are several sorts of cup – such as the Zahn cup and the Ford viscosity cup – with the usage of each type varying mainly according to the industry. The efflux time can also be converted to kinematic viscosities (centistokes, cSt) through the conversion equations.

Also used in coatings, a Stormer viscometer uses load-based rotation in order to determine viscosity. The viscosity is reported in Krebs units (KU), which are unique to Stormer viscometers.

Vibrating viscometers can also be used to measure viscosity. Resonant, or vibrational viscometers work by creating shear waves within the liquid. In this method, the sensor is submerged in the fluid and is made to resonate at a specific frequency. As the surface of the sensor shears through the liquid, energy is lost due to its viscosity. This dissipated energy is then measured and converted into a viscosity reading. A higher viscosity causes a greater loss of energy.^{[ citation needed ]}

*Extensional viscosity* can be measured with various rheometers that apply extensional stress.

Volume viscosity can be measured with an acoustic rheometer.

Apparent viscosity is a calculation derived from tests performed on drilling fluid used in oil or gas well development. These calculations and tests help engineers develop and maintain the properties of the drilling fluid to the specifications required.

The SI unit of dynamic viscosity is the pascal-second (Pa·s), or equivalently kilogram per meter per second (kg·m^{−1}·s^{−1}). The CGS unit is called the poise (P),^{ [25] } named after Jean Léonard Marie Poiseuille. It is commonly expressed, particularly in ASTM standards, as *centipoise* (cP) since the latter is equal to the SI multiple millipascal seconds (mPa·s).

The SI unit of kinematic viscosity is square meter per second (m^{2}/s), whereas the CGS unit for kinematic viscosity is the **stokes** (St), named after Sir George Gabriel Stokes.^{ [26] } In U.S. usage, *stoke* is sometimes used as the singular form. The submultiple *centistokes* (cSt) is often used instead.

The reciprocal of viscosity is *fluidity*, usually symbolized by or , depending on the convention used, measured in *reciprocal poise* (P^{−1}, or cm·s·g ^{−1}), sometimes called the *rhe*. Fluidity is seldom used in engineering practice.

Nonstandard units include the reyn, a British unit of dynamic viscosity.^{[ citation needed ]} In the automotive industry the viscosity index is used to describe the change of viscosity with temperature.

At one time the petroleum industry relied on measuring kinematic viscosity by means of the Saybolt viscometer, and expressing kinematic viscosity in units of Saybolt universal seconds (SUS).^{ [27] } Other abbreviations such as SSU (*Saybolt seconds universal*) or SUV (*Saybolt universal viscosity*) are sometimes used. Kinematic viscosity in centistokes can be converted from SUS according to the arithmetic and the reference table provided in ASTM D 2161.

In general, the viscosity of a system depends in detail on how the molecules constituting the system interact. There are no simple but correct expressions for the viscosity of a fluid. The simplest exact expressions are the Green–Kubo relations for the linear shear viscosity or the *transient time correlation function* expressions derived by Evans and Morriss in 1988.^{ [28] } Although these expressions are each exact, calculating the viscosity of a dense fluid using these relations currently requires the use of molecular dynamics computer simulations. On the other hand, much more progress can be made for a dilute gas. Even elementary assumptions about how gas molecules move and interact lead to a basic understanding of the molecular origins of viscosity. More sophisticated treatments can be constructed by systematically coarse-graining the equations of motion of the gas molecules. An example of such a treatment is Chapman–Enskog theory, which derives expressions for the viscosity of a dilute gas from the Boltzmann equation.^{ [29] }

Momentum transport in gases is generally mediated by discrete molecular collisions, and in liquids by attractive forces which bind molecules close together.^{ [14] } Because of this, the dynamic viscosities of liquids are typically much larger than those of gases.

Elementary calculation of viscosity for a dilute gas Consider a dilute gas moving parallel to the -axis with velocity that depends only on the coordinate. To simplify the discussion, the gas is assumed to have uniform temperature and density.

Under these assumptions, the velocity of a molecule passing through is equal to whatever velocity that molecule had when its mean free path began. Because is typically small compared with macroscopic scales, the average velocity of such a molecule has the form

- ,

where is a numerical constant on the order of . (Some authors estimate ;

^{ [14] }^{ [30] }on the other hand, a more careful calculation for rigid elastic spheres gives .) Now, because*half*the molecules on either side are moving towards , and doing so on average with*half*the average moleculer speed , the momentum flux from either side isThe

*net*momentum flux at is the difference of the two:According to the definition of viscosity, this momentum flux should be equal to , which leads to

Viscosity in gases arises principally from the molecular diffusion that transports momentum between layers of flow. An elementary calculation for a dilute gas at temperature and density gives

where is the Boltzmann constant, the molecular mass, and a numerical constant on the order of . The quantity , the mean free path, measures the average distance a molecule travels between collisions. Even without *a priori* knowledge of , this expression has interesting implications. In particular, since is typically inversely proportional to density and increases with temperature, itself should increase with temperature and be *independent* of density at fixed temperature. In fact, both of these predictions persist in more sophisticated treatments, and accurately describe experimental observations. Note that this behavior runs counter to common intuition regarding liquids, for which viscosity typically *decreases* with temperature.^{ [14] }^{ [30] }

For rigid elastic spheres of diameter , can be computed, giving

In this case is independent of temperature, so . For more complicated molecular models, however, depends on temperature in a non-trivial way, and simple kinetic arguments as used here are inadequate. More fundamentally, the notion of a mean free path becomes imprecise for particles that interact over a finite range, which limits the usefulness of the concept for describing real-world gases.^{ [31] }

A technique developed by Sydney Chapman and David Enskog in the early 1900s allows a more refined calculation of .^{ [29] } It is based on the Boltzmann equation, which provides a systematic statistical description of a dilute gas in terms of intermolecular interactions.^{ [32] } As such, their technique allows accurate calculation of for more realistic molecular models, such as those incorporating intermolecular attraction rather than just hard-core repulsion.

It turns out that a more realistic modeling of interactions is essential for accurate prediction of the temperature dependence of , which experiments show increases more rapidly than the trend predicted for rigid elastic spheres.^{ [14] } Indeed, the Chapman–Enskog analysis shows that the predicted temperature dependence can be tuned by varying the parameters in various molecular models. A simple example is the Sutherland model,^{ [lower-alpha 1] } which describes rigid elastic spheres with *weak* mutual attraction. In such a case, the attractive force can be treated perturbatively, which leads to a particularly simple expression for :

where is independent of temperature, being determined only by the parameters of the intermolecular attraction. To connect with experiment, it is convenient to rewrite as

where is the viscosity at temperature .^{ [33] } If is known from experiments at and at least one other temperature, then can be calculated. It turns out that expressions for obtained in this way are accurate for a number of gases over a sizable range of temperatures. On the other hand, Chapman & Cowling 1970 argue that this success does not imply that molecules actually interact according to the Sutherland model. Rather, they interpret the prediction for as a simple interpolation which is valid for some gases over fixed ranges of temperature, but otherwise does not provide a picture of intermolecular interactions which is fundamentally correct and general. Slightly more sophisticated models, such as the Lennard-Jones potential, may provide a better picture, but only at the cost of a more opaque dependence on temperature. In some systems the assumption of spherical symmetry must be abandoned as well, as is the case for vapors with highly polar molecules like H_{2}O.^{ [34] }^{ [35] }

In the kinetic-molecular picture, a non-zero bulk viscosity arises in gases whenever there are non-negligible relaxational timescales governing the exchange of energy between the translational energy of molecules and their internal energy, e.g. rotational and vibrational. As such, the bulk viscosity is for a monatomic ideal gas, in which the internal energy of molecules in negligible, but is nonzero for a gas like carbon dioxide, whose molecules possess both rotational and vibrational energy.^{ [36] }^{ [37] }

In contrast with gases, there is no simple yet accurate picture for the molecular origins of viscosity in liquids.

At the simplest level of description, the relative motion of adjacent layers in a liquid is opposed primarily by attractive molecular forces acting across the layer boundary. In this picture, one (correctly) expects viscosity to decrease with increasing temperature. This is because increasing temperature increases the random thermal motion of the molecules, which makes it easier for them to overcome their attractive interactions.^{ [38] }

Building on this visualization, a simple theory can be constructed in analogy with the discrete structure of a solid: groups of molecules in a liquid are visualized as forming "cages" which surround and enclose single molecules.^{ [39] } These cages can be occupied or unoccupied, and stronger molecular attraction corresponds to stronger cages. Due to random thermal motion, a molecule "hops" between cages at a rate which varies inversely with the strength of molecular attractions. In equilibrium these "hops" are not biased in any direction. On the other hand, in order for two adjacent layers to move relative to each other, the "hops" must be biased in the direction of the relative motion. The force required to sustain this directed motion can be estimated for a given shear rate, leading to

**(1)**

where is the Avogadro constant, is the Planck constant, is the volume of a mole of liquid, and is the normal boiling point. This result has the same form as the widespread and accurate empirical relation

**(2)**

where and are constants fit from data.^{ [39] }^{ [40] } On the other hand, several authors express caution with respect to this model. Errors as large as 30% can be encountered using equation (** 1 **), compared with fitting equation (** 2 **) to experimental data.^{ [39] } More fundamentally, the physical assumptions underlying equation (** 1 **) have been criticized.^{ [41] } It has also been argued that the exponential dependence in equation (** 1 **) does not necessarily describe experimental observations more accurately than simpler, non-exponential expressions.^{ [42] }^{ [43] }

In light of these shortcomings, the development of a less ad-hoc model is a matter of practical interest. Foregoing simplicity in favor of precision, it is possible to write rigorous expressions for viscosity starting from the fundamental equations of motion for molecules. A classic example of this approach is Irving-Kirkwood theory.^{ [44] } On the other hand, such expressions are given as averages over multiparticle correlation functions and are therefore difficult to apply in practice.

In general, empirically derived expressions (based on existing viscosity measurements) appear to be the only consistently reliable means of calculating viscosity in liquids.^{ [45] }

The same molecular-kinetic picture of a single component gas can also be applied to a gaseous mixture. For instance, in the Chapman-Enskog approach the viscosity of a binary mixture of gases can be written in terms of the individual component viscosities , their respective volume fractions, and the intermolecular interactions.^{ [29] } As for the single-component gas, the dependence of on the parameters of the intermolecular interactions enters through various collisional integrals which may not be expressible in terms of elementary functions. To obtain usable expressions for which reasonably match experimental data, the collisional integrals typically must be evaluated using some combination of analytic calculation and empirical fitting. An example of such a procedure is the Sutherland approach for the single-component gas, discussed above.

As for pure liquids, the viscosity of a blend of liquids is difficult to predict from molecular principles. One method is to extend the molecular "cage" theory presented above for a pure liquid. This can be done with varying levels of sophistication. One useful expression resulting from such an analysis is the Lederer-Roegiers equation for a binary mixture:

where is an empirical parameter, and and are the respective mole fractions and viscosities of the component liquids.^{ [46] }

Since blending is an important process in the lubricating and oil industries, a variety of empirical and propriety equations exist for predicting the viscosity of a blend, besides those stemming directly from molecular theory.^{ [46] }

Depending on the solute and range of concentration, an aqueous electrolyte solution can have either a larger or smaller viscosity compared with pure water at the same temperature and pressure. For instance, a 20% saline (sodium chloride) solution has viscosity over 1.5 times that of pure water, whereas a 20% potassium iodide solution has viscosity about 0.91 times that of pure water.

An idealized model of dilute electrolytic solutions leads to the following prediction for the viscosity of a solution:^{ [47] }

where is the viscosity of the solvent, is the concentration, and is a positive constant which depends on both solvent and solute properties. However, this expression is only valid for very dilute solutions, having less than 0.1 mol/L.^{ [48] } For higher concentrations, additional terms are necessary which account for higher-order molecular correlations:

where and are fit from data. In particular, a negative value of is able to account for the decrease in viscosity observed in some solutions. Estimated values of these constants are shown below for sodium chloride and potassium iodide at temperature 25 °C (mol = mole, L = liter).^{ [47] }

Solute | (mol^{-1/2} L^{1/2}) | (mol^{−1} L) | (mol^{−2} L^{2}) |
---|---|---|---|

Sodium chloride (NaCl) | 0.0062 | 0.0793 | 0.0080 |

Potassium iodide (KI) | 0.0047 | −0.0755 | 0.0000 |

In a suspension of solid particles (e.g. micron-size spheres suspended in oil), an effective viscosity can be defined in terms of stress and strain components which are averaged over a volume large compared with the distance between the suspended particles, but small with respect to macroscopic dimensions.^{ [49] } Such suspensions generally exhibit non-Newtonian behavior. However, for dilute systems in steady flows, the behavior is Newtonian and expressions for can be derived directly from the particle dynamics. In a very dilute system, with volume fraction , interactions between the suspended particles can be ignored. In such a case one can explicitly calculate the flow field around each particle independently, and combine the results to obtain . For spheres, this results in the Einstein equation:

where is the viscosity of the suspending liquid. The linear dependence on is a direct consequence of neglecting interparticle interactions; in general, one will have:

where the coefficient may depend on the particle shape (e.g. spheres, rods, disks).^{ [50] } Experimental determination of the precise value of is difficult, however: even the prediction for spheres has not been conclusively validated, with various experiments finding values in the range . This deficiency has been attributed to difficulty in controlling experimental conditions.^{ [51] }

In denser suspensions, acquires a nonlinear dependence on , which indicates the importance of interparticle interactions. Various analytical and semi-empirical schemes exist for capturing this regime. At the most basic level, a term quadratic in is added to :

and the coefficient is fit from experimental data or approximated from the microscopic theory. In general, however, one should be cautious in applying such simple formulas since non-Newtonian behavior appears in dense suspensions ( for spheres),^{ [51] } or in suspensions of elongated or flexible particles.^{ [49] }

There is a distinction between a suspension of solid particles, described above, and an emulsion. The latter is a suspension of tiny droplets, which themselves may exhibit internal circulation. The presence of internal circulation can noticeably decrease the observed effective viscosity, and different theoretical or semi-empirical models must be used.^{ [52] }

In the high and low temperature limits, viscous flow in amorphous materials (e.g. in glasses and melts)^{ [54] }^{ [55] }^{ [56] } has the Arrhenius form:

where Q is a relevant activation energy, given in terms of molecular parameters; T is temperature; R is the molar gas constant; and A is approximately a constant. The activation energy Q takes a different value depending on whether the high or low temperature limit is being considered: it changes from a high value *Q*_{H} at low temperatures (in the glassy state) to a low value *Q*_{L} at high temperatures (in the liquid state).

For intermediate temperatures, varies nontrivially with temperature and the simple Arrhenius form fails. On the other hand, the two-exponential equation

where , , , are all constants, provides a good fit to experimental data over the entire range of temperatures, while at the same time reducing to the correct Arrhenius form in the low and high temperature limits. Besides being a convenient fit to data, the expression can also be derived from various theoretical models of amorphous materials at the atomic level.^{ [55] }

In the study of turbulence in fluids, a common practical strategy is to ignore the small-scale vortices (or eddies) in the motion and to calculate a large-scale motion with an *effective* viscosity, called the "eddy viscosity", which characterizes the transport and dissipation of energy in the smaller-scale flow (see large eddy simulation).^{ [57] }^{ [58] } In contrast to the viscosity of the fluid itself, which must be positive by the second law of thermodynamics, the eddy viscosity can be negative.^{ [59] }^{ [60] }

Observed values of viscosity vary over several orders of magnitude, even for common substances (see the order of magnitude table below). For instance, a 70% sucrose (sugar) solution has a viscosity over 400 times that of water, and 26000 times that of air.^{ [62] } More dramatically, pitch has been estimated to have a viscosity 230 billion times that of water.^{ [61] }

The dynamic viscosity of water is about 0.89 mPa·s at room temperature (25 °C). As a function of temperature in kelvins, the viscosity can be estimated using the semi-empirical relation

where *A* = 0.02939 mPa·s, *B* = 507.88 K, and *C* = 149.3 K.^{ [63] } Experimentally determined values of the viscosity are also given in the table below.

Temperature (°C) | Viscosity (mPa·s) |
---|---|

10 | 1.3059 |

20 | 1.0016 |

30 | 0.79722 |

50 | 0.54652 |

70 | 0.40355 |

90 | 0.31417 |

Under standard atmospheric conditions (25 °C and pressure of 1 bar), the dynamic viscosity of air is 18.5 μPa·s, roughly 50 times smaller than the viscosity of water at the same temperature. Except at very high pressure, the viscosity of air depends mostly on the temperature.

Substance | Viscosity (mPa·s) | Temperature (°C) | Ref. |
---|---|---|---|

Benzene | 0.604 | 25 | ^{ [62] } |

Water | 1.0016 | 20 | |

Mercury | 1.526 | 25 | |

Whole milk | 2.12 | 20 | ^{ [64] } |

Olive oil | 56.2 | 26 | |

Honey | 2000-10000 | 20 | ^{ [65] } |

Ketchup ^{ [lower-alpha 2] } | 5000-20000 | 25 | ^{ [66] } |

Peanut butter ^{ [lower-alpha 2] } | 10^{4}-10^{6} | ^{ [67] } | |

Pitch | 2.3×10^{11} | 10-30 (variable) | ^{ [61] } |

The following table illustrates the range of viscosity values observed in common substances. Unless otherwise noted, a temperature of 25 °C and a pressure of 1 atmosphere are assumed. Certain substances of variable composition or with non-Newtonian behavior are not assigned precise values, since in these cases viscosity depends on additional factors besides temperature and pressure.

Factor (Pa·s) | Description | Examples | Values (Pa·s) | Ref. |
---|---|---|---|---|

10^{−6} | Lower range of gaseous viscosity | Butane | 7.49 × 10^{−6} | ^{ [68] } |

Hydrogen | 8.8 × 10^{−6} | ^{ [69] } | ||

10^{−5} | Upper range of gaseous viscosity | Krypton | 2.538 × 10^{−5} | ^{ [70] } |

Neon | 3.175 × 10^{−5} | |||

10^{−4} | Lower range of liquid viscosity | Pentane | 2.24 × 10^{−4} | ^{ [62] } |

Gasoline | 6 × 10^{−4} | |||

Water | 8.90 × 10^{−4} | ^{ [62] } | ||

10^{−3} | Typical range for small-molecule Newtonian liquids | Ethanol | 1.074 × 10^{−3} | |

Mercury | 1.526 × 10^{−3} | |||

Whole milk (20 °C) | 2.12 × 10^{−3} | ^{ [64] } | ||

Blood | 4 × 10^{−3} | |||

10^{−2} – 10^{0} | Oils and long-chain hydrocarbons | Linseed oil | 0.028 | |

Olive oil | 0.084 | ^{ [64] } | ||

SAE 10 Motor oil | 0.085 to 0.14 | |||

Castor oil | 0.1 | |||

SAE 20 Motor oil | 0.14 to 0.42 | |||

SAE 30 Motor oil | 0.42 to 0.65 | |||

SAE 40 Motor oil | 0.65 to 0.90 | |||

Glycerine | 1.5 | |||

Pancake syrup | 2.5 | |||

10^{1} – 10^{3} | Pastes, gels, and other semisolids (generally non-Newtonian) | Ketchup | ≈ 10^{1} | ^{ [66] } |

Mustard | ||||

Sour cream | ≈ 10^{2} | |||

Peanut butter | ^{ [67] } | |||

Lard | ≈ 10^{3} | |||

≈10^{8} | Viscoelastic polymers | Pitch | ^{ [61] } | |

≈10^{21} | Certain solids under a viscoelastic description | Mantle (geology) |

- Dashpot
- Deborah number
- Dilatant
- Herschel–Bulkley fluid
- Hyperviscosity syndrome
- Intrinsic viscosity
- Inviscid flow
- Joback method (estimation of liquid viscosity from molecular structure)
- Kaye effect
- Microviscosity
- Morton number
- Quasi-solid
- Rheology
- Stokes flow
- Superfluid helium-4
- Viscoplasticity
- Viscosity models for mixtures

**Rheology** 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. Rheology is the science of deformation and flow within a material. It is a branch of physics which deals with the deformation and flow of materials, both solids and liquids.

A **viscometer** is an instrument used to measure the viscosity of a fluid. For liquids with viscosities which vary with flow conditions, an instrument called a rheometer is used. Thus, a rheometer can be considered as a special type of viscometer. Viscometers only measure under one flow condition.

In 1851, George Gabriel Stokes derived an expression, now known as **Stokes' law**, for the frictional force – also called drag force – exerted on spherical objects with very small Reynolds numbers in a viscous fluid. Stokes' law is derived by solving the Stokes flow limit for small Reynolds numbers of the Navier–Stokes equations.

A **Newtonian fluid** is a fluid in which the viscous stresses arising from its flow, at every point, are linearly correlated to the local strain rate—the rate of change of its deformation over time. That is equivalent to saying those forces are proportional to the rates of change of the fluid's velocity vector as one moves away from the point in question in various directions.

A **shear stress**, often denoted by **τ**, is the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section of the material. Normal stress, on the other hand, arises from the force vector component perpendicular to the material cross section on which it acts.

A **power-law fluid**, or the **Ostwald–de Waele relationship**, is a type of generalized Newtonian fluid for which the shear stress, *τ*, is given by

A **Bingham plastic** is a viscoplastic material that behaves as a rigid body at low stresses but flows as a viscous fluid at high stress. It is named after Eugene C. Bingham who proposed its mathematical form.

**Hemorheology**, also spelled **haemorheology**, or **blood rheology**, is the study of flow properties of blood and its elements of plasma and cells. Proper tissue perfusion can occur only when blood's rheological properties are within certain levels. Alterations of these properties play significant roles in disease processes. Blood viscosity is determined by plasma viscosity, hematocrit and mechanical properties of red blood cells. Red blood cells have unique mechanical behavior, which can be discussed under the terms erythrocyte deformability and erythrocyte aggregation. Because of that, blood behaves as a non-Newtonian fluid. As such, the viscosity of blood varies with shear rate. Blood becomes less viscous at high shear rates like those experienced with increased flow such as during exercise or in peak-systole. Therefore, blood is a shear-thinning fluid. Contrarily, blood viscosity increases when shear rate goes down with increased vessel diameters or with low flow, such as downstream from an obstruction or in diastole. Blood viscosity also increases with increases in red cell aggregability.

**Darcy's law** is an equation that describes the flow of a fluid through a porous medium. The law was formulated by Henry Darcy based on results of experiments on the flow of water through beds of sand, forming the basis of hydrogeology, a branch of earth sciences.

A **cross fluid** is a type of generalized Newtonian fluid whose viscosity depends upon shear rate according to the following equation:

**Carreau fluid** is a type of generalized Newtonian fluid where viscosity, , depends upon the shear rate, , by the following equation:

Viscosity depends strongly on temperature. In liquids it usually decreases with temperature, whereas in gases it increases. This article discusses several models of this dependence, ranging from rigorous first-principles calculations for monatomic gases, to empirical correlations for liquids.

**Fluid mechanics** is the branch of physics concerned with the mechanics of fluids and the forces on them. It has applications in a wide range of disciplines, including mechanical, civil, chemical and biomedical engineering, geophysics, oceanography, meteorology, astrophysics, and biology.

The **Bagnold number** (**Ba**) is the ratio of grain collision stresses to viscous fluid stresses in a granular flow with interstitial Newtonian fluid, first identified by Ralph Alger Bagnold.

The intent of this article is to highlight the important points of the **derivation of the Navier–Stokes equations** as well as its application and formulation for different families of fluids.

**Volume viscosity** is a material property relevant for characterizing fluid flow. Common symbols are or . It has dimensions, and the corresponding SI unit is the pascal-second (Pa·s).

The **Herschel–Bulkley fluid** is a generalized model of a non-Newtonian fluid, in which the strain experienced by the fluid is related to the stress in a complicated, non-linear way. Three parameters characterize this relationship: the consistency *k*, the flow index *n*, and the yield shear stress . The consistency is a simple constant of proportionality, while the flow index measures the degree to which the fluid is shear-thinning or shear-thickening. Ordinary paint is one example of a shear-thinning fluid, while oobleck provides one realization of a shear-thickening fluid. Finally, the yield stress quantifies the amount of stress that the fluid may experience before it yields and begins to flow.

In fluid dynamics, **hydrodynamic stability** is the field which analyses the stability and the onset of instability of fluid flows. The study of hydrodynamic stability aims to find out if a given flow is stable or unstable, and if so, how these instabilities will cause the development of turbulence. The foundations of hydrodynamic stability, both theoretical and experimental, were laid most notably by Helmholtz, Kelvin, Rayleigh and Reynolds during the nineteenth century. These foundations have given many useful tools to study hydrodynamic stability. These include Reynolds number, the Euler equations, and the Navier–Stokes equations. When studying flow stability it is useful to understand more simplistic systems, e.g. incompressible and inviscid fluids which can then be developed further onto more complex flows. Since the 1980s, more computational methods are being used to model and analyse the more complex flows.

The **viscous stress tensor** is a tensor used in continuum mechanics to model the part of the stress at a point within some material that can be attributed to the strain rate, the rate at which it is deforming around that point.

- ↑ The discussion which follows draws from Chapman & Cowling 1970, pp. 232-237
- 1 2 These materials are highly non-Newtonian.

- ↑ Symon 1971.
- ↑ Balescu 1975, p. 428–429.
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- 1 2 3 Chapman & Cowling 1970.
- 1 2 Bellac, Mortessagne & Batrouni 2004.
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- ↑ Cercignani 1975.
- ↑ Sutherland 1893, pp. 507–531.
- ↑ Bird, Stewart & Lightfoot 2007, p. 25-27.
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- ↑ Chapman & Cowling 1970, pp. 197, 214-216.
- ↑ Cramer 2012, p. 066102-2.
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- 1 2 Zhmud 2014, p. 22.
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- ↑ Fluegel, Alexander. "Viscosity calculation of glasses". Glassproperties.com. Retrieved 2010-09-14.
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- 1 2 Ojovan, Travis & Hand 2007, p. 415107.
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- ↑ Bird, Stewart & Lightfoot 2007, p. 163.
- ↑ Lesieur 2012, pp. 2-.
- ↑ Sivashinsky & Yakhot 1985, p. 1040.
- ↑ Xie & Levchenko 2019, p. 045434.
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- 1 2 3 4 5 Rumble 2018.
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- 1 2 Citerne, Carreau & Moan 2001, pp. 86–96.
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- ↑ Assael et al.
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