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The viscosity of a is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, has a higher viscosity than .

(1971). 9780201073928, Addison-Wesley.

Viscosity can be conceptualized as quantifying the 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 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.

A fluid that has no resistance to shear stress is known as an ideal or inviscid fluid. Zero viscosity is observed only at in . Otherwise, the second law of thermodynamics requires all fluids to have positive viscosity; such fluids are technically said to be viscous or viscid. A fluid with a relatively high viscosity, such as pitch, may appear to be a .

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


Simple definition
In materials science and , 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 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.

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 .

In the Couette flow, a fluid is trapped between two infinitely large plates, one fixed and one in parallel motion at constant speed u (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 0 at the bottom to u at the top.

(2019). 9780521515993, Cambridge University Press. .
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 u at the top. Moreover, the magnitude F of the force acting on the top plate is found to be proportional to the speed u and the area A of each plate, and inversely proportional to their separation y:

F=\mu A \frac{u}{y}.
The proportionality factor \mu is the viscosity of the fluid, with units of \text{Pa} \cdot \text{s} (pascal-). The ratio u/y is called the rate of shear deformation or , and is the of the fluid speed in the direction to the plates (see illustrations to the right). If the velocity does not vary linearly with y, then the appropriate generalization is
\tau=\mu \frac{\partial u}{\partial y},
where \tau = F / A, and \partial u / \partial y 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 \mu. 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 (\mu) for the viscosity is common among mechanical and chemical engineers, as well as physicists.

(1998). 9780070625372, McGraw-Hill.
(2019). 9780071226219, McGraw-Hill.
(2019). 9780471457282, Wiley.
However, the Greek letter eta (\eta) is also used by chemists, physicists, and the .
(1997). 9780967855097, Blackwell Scientific Publications.
The viscosity \mu is sometimes also referred to as the shear viscosity. However, at least one author discourages the use of this terminology, noting that \mu can appear in nonshearing flows in addition to shearing flows.

General definition
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. (For Newtonian fluids, this is also a linear dependence.) In Cartesian coordinates, the general relationship can then be written as

\tau_{ij} = \sum_k \sum_l \mu_{ijkl} \frac{\partial v_k}{\partial r_l},

where \mu_{ijkl} is a viscosity tensor that maps the strain rate tensor \partial v_k / \partial r_l onto the viscous stress tensor \tau_{ij}.Bird, Steward, & Lightfoot, p. 18 (Note that this source uses a alternate sign convention, which has been reversed here.) Since the indices in this expression can vary from 1 to 3, there are 81 "viscosity coefficients" \mu_{ijkl} in total. However, due to spatial symmetries these coefficients are not all independent. For instance, for Newtonian fluids, the 81 coefficients can be reduced to 2 independent parameters. The most usual decomposition yields the standard (scalar) viscosity \mu and the \kappa:

\mathbf{\tau} = \mu \left\nabla - \left(\frac{2}{3} \mu - \kappa \right) (\nabla \cdot \mathbf{v}) \mathbf{\delta}, where \mathbf{\delta} is the unit tensor, and the dagger \dagger denotes the .Bird, Steward, & Lightfoot, p. 19Landau & Lifshitz p. 45 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 \kappa is frequently not necessary in fluid dynamics problems. For example, an incompressible fluid satisfies \nabla \cdot \mathbf{v} = 0 and so the term containing \kappa drops out. Moreover, \kappa is often assumed to be negligible for gases since it is 0 in a . One situation in which \kappa can be important is the calculation of energy loss in and , 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.

Dynamic and kinematic viscosity
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 to the of the fluid . It is usually denoted by the Greek letter nu () and has units \mathrm{(length)^2/time}:

\nu = \frac{\mu}{\rho}.

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

Momentum transport
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 transport, and (mass) characterizes mass transport. To see this, note that in Newton's law of viscosity, \tau = \mu (\partial u / \partial y), the shear stress \tau has units equivalent to a momentum , i.e. momentum per unit time per unit area. Thus, \tau can be interpreted as specifying the flow of momentum in the y 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:

\begin{align} \mathbf{J} &= -D \frac{\partial \rho}{\partial x} \qquad \; \; \; \, \text{(Fick's law of diffusion)} \\ \mathbf{q} &= -k_t \frac{\partial T}{\partial x} \qquad \; \; \, \text{(Fourier's law of heat conduction)} \\ \tau &= \mu \frac{\partial u}{\partial y} \qquad \qquad \text{(Newton's law of viscosity)} \\ \end{align} where \rho is the density, \mathbf{J} and \mathbf{q} are the mass and heat fluxes, and D and k_t are the mass diffusivity and thermal conductivity.
(1999). 9780201380279, Addison Wesley. .

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.

Newtonian and non-Newtonian fluids
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 \mu. Its form is motivated by experiments which show that for a wide range of fluids, \mu is independent of strain rate. Such fluids are called . , , 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:

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

The Trouton ratio or Trouton's ratio is the ratio of extensional viscosity to . For a Newtonian fluid, the Trouton ratio is 3.

Shear-thinning liquids are very commonly, but misleadingly, described as thixotropic.

(2019). 9780521515993, Cambridge University Press. .

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 , possibly to the point of behaving like a solid.

In solids
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 ) will flow like liquids, albeit very slowly, even under arbitrarily small stress. Such materials are therefore best described as possessing both elasticity (reaction to deformation) and viscosity (reaction to rate of deformation); that is, being .

Indeed, some authors have claimed that , such as and many , are actually liquids with a very high viscosity (greater than 1012 Pa·s). 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, 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.

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 , earth materials that exhibit viscous deformation at least three orders of magnitude greater than their elastic deformation are sometimes called .

Viscosity is measured with various types of and . 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 (). 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 industries, viscosity may be measured with a cup in which the is measured. There are several sorts of cup – such as the 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.

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

can be measured with an acoustic rheometer.

Apparent viscosity is a calculation derived from tests performed on 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 per per second (kg·m−1·s−1). The unit is called the poise
(2019). 9780967855097
(P), named after Jean Léonard Marie Poiseuille. It is commonly expressed, particularly in 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 (m2/s), whereas the CGS unit for kinematic viscosity is the stokes (St), named after Sir George Gabriel Stokes. 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 \phi = 1 / \mu or F = 1 / \mu, depending on the convention used, measured in reciprocal poise (P−1, or ··−1), sometimes called the rhe. Fluidity is seldom used in practice.

Nonstandard units include the , a British unit of dynamic viscosity. In the automotive industry the 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).ASTM D 2161 (2005) "Standard Practice for Conversion of Kinematic Viscosity to Saybolt Universal Viscosity or to Saybolt Furol Viscosity", p. 1 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 D 2161.

Molecular origins
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 1985. 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.

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

Consider a dilute gas moving parallel to the x-axis with velocity u(y) that depends only on the y coordinate. To simplify the discussion, the gas is assumed to have uniform temperature and density.

Under these assumptions, the x velocity of a molecule passing through y = 0 is equal to whatever velocity that molecule had when its mean free path \lambda began. Because \lambda is typically small compared with macroscopic scales, the average x velocity of such a molecule has the form

u(0) \pm \alpha \lambda \frac{d u}{d y}(0),

where \alpha is a numerical constant on the order of 1. (Some authors estimate \alpha = 2/3; on the other hand, a more careful calculation for rigid elastic spheres gives \alpha \simeq 0.998.) Now, because half the molecules on either side are moving towards y=0, and doing so on average with half the average moleculer speed (8 k_\text{B} T/\pi m)^{1/2}, the momentum flux from either side is

\frac{1}{4} \rho \cdot \sqrt{\frac{8 k_\text{B} T}{\pi m}} \cdot \left(u(0) \pm \alpha \lambda \frac{d u}{d y}(0)\right). The net momentum flux at y=0 is the difference of the two:
-\frac{1}{2} \rho \cdot \sqrt{\frac{8 k_\text{B} T}{\pi m}} \cdot \alpha \lambda \frac{d u}{d y}(0).

According to the definition of viscosity, this momentum flux should be equal to -\mu \frac{d u}{d y}(0), which leads to

\mu = \alpha \rho \lambda \sqrt{\frac{2 k_\text{B} T}{\pi m}}.

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 T and density \rho gives

\mu = \alpha\rho\lambda\sqrt{\frac{2k_\text{B} T}{\pi m}},

where k_\text{B} is the Boltzmann constant, m the molecular mass, and \alpha a numerical constant on the order of 1. The quantity \lambda, the mean free path, measures the average distance a molecule travels between collisions. Even without a priori knowledge of \alpha, this expression has interesting implications. In particular, since \lambda is typically inversely proportional to density and increases with temperature, \mu 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.

For rigid elastic spheres of diameter \sigma, \lambda can be computed, giving

\mu = \frac{\alpha}{\pi^{3/2}} \frac{\sqrt{k_\text{B} m T}}{\sigma^2}.

In this case \lambda is independent of temperature, so \mu \propto T^{1/2}. For more complicated molecular models, however, \lambda 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.Chapman & Cowling, p. 103

Chapman–Enskog theory
A technique developed by Sydney Chapman and in the early 1900s allows a more refined calculation of \mu. It is based on the Boltzmann equation, which provides a systematic statistical description of a dilute gas in terms of intermolecular interactions. As such, their technique allows accurate calculation of \mu 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 \mu, which experiments show increases more rapidly than the T^{1/2} trend predicted for rigid elastic spheres. 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,The discussion which follows draws from Chapman & Cowling, pp. 232-237. 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 \mu:

\mu = \frac{5}{16 \sigma^2} \left(\frac{k_\text{B} m T}{\pi}\right)^{1/2} \left(1 + \frac{S}{T} \right)^{-1}, where S is independent of temperature, being determined only by the parameters of the intermolecular attraction. To connect with experiment, it is convenient to rewrite as

\mu = \mu_0 \left(\frac{T}{T_0}\right)^{3/2} \frac{T_0 + S}{T + S}, where \mu_0 is the viscosity at temperature T_0. If \mu is known from experiments at T = T_0 and at least one other temperature, then S can be calculated. It turns out that expressions for \mu obtained in this way are accurate for a number of gases over a sizable range of temperatures. On the other hand, Chapman and Cowling argue that this success does not imply that molecules actually interact according to the Sutherland model. Rather, they interpret the prediction for \mu 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 like H2O.Bird, Steward, & Lightfoot, p. 25-27Chapman & Cowling, pp. 235 - 237

Bulk viscosity
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 . As such, the bulk viscosity is 0 for a monatomic ideal gas, in which the internal energy of molecules in negligible, but is nonzero for a gas like , whose molecules possess both rotational and vibrational energy.Chapman & Cowling (1970), pp. 197, 214-216

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.

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.Bird, Steward, & Lightfoot, pp. 29-31 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

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

where A and B are constants fit from data.Reid & Sherwood, pp. 203-204 One the other hand, several authors express caution with respect to this model. Errors as large as 30% can be encountered using equation (), compared with fitting equation () to experimental data. More fundamentally, the physical assumptions underlying equation () have been extensively criticized. It has also been argued that the exponential dependence in equation () does not necessarily describe experimental observations more accurately than simpler, non-exponential expressions.Hildebrand p. 37

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. 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.Reid & Sherwood, pp. 206-209

Mixtures, blends, and suspensions

Gaseous mixtures
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 \mu_{\text{mix}} of a binary mixture of gases can be written in terms of the individual component viscosities \mu_{1,2}, their respective volume fractions, and the intermolecular interactions.Chapman & Cowling (1970) As for the single-component gas, the dependence of \mu_{\text{mix}} 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 \mu_{\text{mix}} 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.

Blends of liquids
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:

\mu_{\text{blend}} = \frac{x_1}{x_1 + \alpha x_2} \ln \mu_1 + \frac{\alpha x_2}{x_1 + \alpha x_2} \ln \mu_2,

where \alpha is an empirical parameter, and x_{1,2} and \mu_{1,2} are the respective and viscosities of the component liquids.

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.

In a suspension of solid particles (e.g. -size spheres suspended in oil), an effective viscosity \mu_{\text{eff}} 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.Bird, Steward, & Lightfoot pp. 31-33 Such suspensions generally exhibit non-Newtonian behavior. However, for dilute systems in steady flows, the behavior is Newtonian and expressions for \mu_{\text{eff}} can be derived directly from the particle dynamics. In a very dilute system, with volume fraction \phi \lesssim 0.02, 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 \mu_{\text{eff}}. For spheres, this results in the Einstein equation:

\mu_{\text{eff}} = \mu_0 \left(1 + \frac{5}{2} \phi \right),

where \mu_0 is the viscosity of the suspending liquid. The linear dependence on \phi is a direct consequence of neglecting interparticle interactions; in general, one will have:

\mu_{\text{eff}} = \mu_0 \left(1 + B \phi \right),

where the coefficient B may depend on the particle shape (e.g. spheres, rods, disks).Bird, Steward, & Lightfoot p. 32 Experimental determination of the precise value of B is difficult, however: even the prediction B = 5/2 for spheres has not been conclusively validated, with various experiments finding values in the range 1.5 \lesssim B \lesssim 5. This deficiency has been attributed to difficulty in controlling experimental conditions.

In denser suspensions, \mu_{\text{eff}} acquires a nonlinear dependence on \phi, 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 \phi is added to \mu_{\text{eff}}:

\mu_{\text{eff}} = \mu_0 \left(1 + B \phi + B_1 \phi^2 \right),

and the coefficient B_1 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 (\phi \gtrsim 0.25 for spheres), or in suspensions of elongated or flexible particles.

There is a distinction between a suspension of solid particles, described above, and an . 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.Bird, Steward, & Lightfoot p. 33

Amorphous materials
In the high and low temperature limits, viscous flow in (e.g. in and melts) has the Arrhenius form:

\mu = A e^\frac{Q}{RT},

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

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

\mu = AT \exp\left(\frac{B}{RT}\right) \left,

where A, B, C, D 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.

Eddy viscosity
In the study of in , a common practical strategy is to ignore the small-scale (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 in the smaller-scale flow (see large eddy simulation).Bird, Steward, & Lightfoot, p. 163
(2012). 9789400905337, Springer Science & Business Media. .
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.

Selected substances
Observed values of viscosity vary over several orders of magnitude, even for common substances. For instance, a 70% sucrose (sugar) solution has a viscosity over 400 times that of water, and 26000 times that of air.
(2019). 9781138561632, CRC Press.
More dramatically, pitch has been estimated to have a viscosity 230 billion times that of water.

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

\mu = A \times 10^{B/(T - C)},

where = , = 247.8 K, and = 140 K.

Experimentally determined values of the viscosity at various temperatures are given below.

+Viscosity of water
at various temperatures

Under standard atmospheric conditions (25 °C and pressure of 1 bar), the 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.

Other common substances
\approx 2000-1000020
\approx 5000-2000025
\approx 104-106
Pitch 10-30 (variable)

See also

Further reading

Undergraduate-level texts
  • . A standard, modern reference.

  • . A brief, elementary treatment.
  • . An advanced treatment.

Graduate-level texts
  • . A classic reference.

  • .

  • . A very advanced but classic text on the theory of transport processes in gases.

  • (2019). 9780521515993, Cambridge University Press. .
    Focuses on non-Newtonian phenomenology.
  • .

External links

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