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The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress.[1] For liquids, it corresponds to the informal concept of "thickness"; for example, honey has higher viscosity than water.[2] Viscosity
Viscosity
is a property of the fluid which opposes the relative motion between the two surfaces of the fluid that are moving at different velocities. In simple terms, viscosity means friction between the molecules of fluid. When the fluid is forced through a tube, the particles which compose the fluid generally move more quickly near the tube's axis and more slowly near its walls; therefore some stress (such as a pressure difference between the two ends of the tube) is needed to overcome the friction between particle layers to keep the fluid moving. For a given velocity pattern, the stress required is proportional to the fluid's viscosity. 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, all fluids have positive viscosity and are technically said to be viscous or viscid. A fluid with a relatively high viscosity, such as pitch, may appear to be a solid.

Contents

1 Etymology 2 Definition

2.1 Dynamic (shear) viscosity 2.2 Kinematic viscosity 2.3 Bulk viscosity 2.4 Viscosity
Viscosity
tensor

3 Newtonian and non-Newtonian fluids 4 In solids 5 Measurement 6 Units

6.1 Dynamic viscosity, μ 6.2 Kinematic viscosity, ν 6.3 Fluidity 6.4 Non-standard units

7 Molecular origins

7.1 Gases

7.1.1 Relation to mean free path of diffusing particles 7.1.2 Effect of temperature on the viscosity of a gas 7.1.3 Viscosity
Viscosity
of a dilute gas

7.2 Liquids

7.2.1 Viscosity
Viscosity
of blends of liquids

8 Selected substances

8.1 Air 8.2 Water 8.3 Other substances

9 Slurry 10 Nanofluids 11 Amorphous materials 12 Eddy viscosity 13 See also 14 References 15 Further reading 16 External links

Etymology[edit] The word "viscosity" is derived from the Latin
Latin
"viscum", meaning mistletoe and also a viscous glue made from mistletoe berries.[3] Definition[edit] Dynamic (shear) viscosity [edit]

Laminar shear of fluid between two plates and cups. Friction
Friction
between the fluid and the moving boundaries/plates causes the fluid to shear and cut. The force required for this action is a measure of the fluid's viscosity.

In a general parallel flow (such as could occur in a straight pipe), the shear stress is proportional to the gradient of the velocity

The dynamic viscosity of a fluid expresses its resistance to shearing flows, where adjacent layers move parallel to each other with different speeds. It can be defined through the idealized situation known as a Couette flow, where a layer of fluid is trapped between two horizontal plates, one fixed and one moving horizontally at constant speed

u

displaystyle u

. This fluid has to be homogeneous in the layer and at different shear stresses. (The plates are assumed to be very large so that one need not consider what happens near their edges.) If the speed of the top plate is low enough, the fluid particles will move parallel to it, and their speed will vary linearly from zero at the bottom to u at the top. Each layer of fluid will move faster than the one just below it, and friction between them will give rise to a force resisting their relative motion. In particular, the fluid will apply on the top plate a force in the direction opposite to its motion, and an equal but opposite one to the bottom plate. An external force is therefore required in order to keep the top plate moving at constant speed. The magnitude F of this force is found to be proportional to the speed u and the area A of each plate, and inversely proportional to their separation y:

F = μ A

u y

.

displaystyle F=mu A frac u y .

The proportionality factor μ in this formula is the viscosity (specifically, the dynamic viscosity) of the fluid, with units of

P a ⋅ s

displaystyle Pacdot s

(pascal-second). The ratio u/y 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). Isaac Newton expressed the viscous forces by the differential equation

τ = μ

∂ u

∂ y

,

displaystyle tau =mu frac partial u partial y ,

where τ = F/A, and ∂u/∂y is the local shear velocity. This formula assumes that the flow is moving along parallel lines to x-axis. Furthermore, it assumes that the y-axis, perpendicular to the flow, points in the direction of maximum shear velocity. This equation can be used where the velocity does not vary linearly with y, such as in fluid flowing through a pipe. This equation is called the defining equation for shear viscosity. The viscosity is not a material constant, but a material property that depends on physical properties like temperature. The functional relationship between viscosity and other physical properties is described by a mathematical viscosity model called a constitutive equation which is usually more complex than the defining equation for viscosity. There exists many viscosity models, and based on type of development-reasoning are some viscosity models selected and presented in the article Viscosity
Viscosity
models for mixtures. Use of the Greek letter mu (μ) for the dynamic stress viscosity is common among mechanical and chemical engineers, as well as physicists.[4][5][6] However, the Greek letter eta (η) is also used by chemists, physicists, and the IUPAC.[7] Kinematic viscosity[edit] The kinematic viscosity (also called "momentum diffusivity") is the ratio of the dynamic viscosity μ to the density of the fluid ρ. It is usually denoted by the Greek letter nu (ν) and has units

m

2

/

s

displaystyle mathrm m^ 2 /s

.

ν =

μ ρ

displaystyle nu = frac mu rho

It is a convenient concept when analyzing the Reynolds number, which expresses the ratio of the inertial forces to the viscous forces:

R e

=

ρ u L

μ

=

u L

ν

,

displaystyle mathrm Re = frac rho uL mu = frac uL nu ,

where L is a typical length scale in the system, and u is the velocity of the fluid with respect to the object (m/s). Bulk viscosity[edit] Main article: Volume viscosity When a compressible fluid is compressed or expanded evenly, without shear, it may still exhibit a form of internal friction that resists its flow. These forces are related to the rate of compression or expansion by a factor called the volume viscosity, bulk viscosity or second viscosity. The bulk viscosity is important only when the fluid is being rapidly compressed or expanded, such as in sound and shock waves. Bulk viscosity explains the loss of energy in those waves, as described by Stokes' law of sound attenuation. Viscosity
Viscosity
tensor[edit] Main article: Viscous stress tensor In general, the stresses within a flow can be attributed partly to the deformation of the material from some rest state (elastic stress), and partly to the rate of change of the deformation over time (viscous stress). In a fluid, by definition, the elastic stress includes only the hydrostatic pressure. In very general terms, the fluid's viscosity is the relation between the strain rate and the viscous stress. In the Newtonian fluid model, the relationship is by definition a linear map, described by a viscosity tensor that, multiplied by the strain rate tensor (which is the gradient of the flow's velocity), gives the viscous stress tensor. The viscosity tensor has nine independent degrees of freedom in general. For isotropic Newtonian fluids, these can be reduced to two independent parameters. The most usual decomposition yields the dynamic viscosity μ and the bulk viscosity σ. Newtonian and non-Newtonian fluids[edit]

Viscosity, the slope of each line, varies among materials

Newton's law of viscosity is a constitutive equation (like Hooke's law, Fick's law, Ohm's law): it is not a fundamental law of nature but an approximation that holds in some materials and fails in others. A fluid that behaves according to Newton's law, with a viscosity μ that is independent of the stress, is said to be Newtonian. Gases, water, and many common liquids can be considered Newtonian in ordinary conditions and contexts. There are many non- Newtonian fluids that significantly deviate from that law in some way or other. 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
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.

Shear-thinning liquids are very commonly, but misleadingly, described as thixotropic. 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. In solids[edit] 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.[8] 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 1012 Pa·s). [9] 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,[10] 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.[11] 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.[12] Measurement[edit] Main article: Viscometer Viscosity
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
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.[13] 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
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. Units[edit] Dynamic viscosity, μ[edit] Both the physical unit of dynamic viscosity in SI units, the poiseuille (Pl), and cgs units, the poise (P), are named after Jean Léonard Marie Poiseuille. The poiseuille, which is rarely used, is equivalent to the pascal second (Pa·s), or (N·s)/m2, or kg/(m·s). If a fluid is placed between two plates with distance one meter, and one plate is pushed sideways with a shear stress of one pascal, and it moves at x meters per second, then it has viscosity of 1/x pascal seconds. For example, water at 20 °C has a viscosity of 1.002 mPa·s, while a typical motor oil could have a viscosity of about 250 mPa·s.[14] The units used in practice are either Pa·s and its submultiples or the cgs poise referred to below, and its submultiples. The cgs physical unit for dynamic viscosity, the poise[15] (P), is also named after Jean Poiseuille. It is more commonly expressed, particularly in ASTM
ASTM
standards, as centipoise (cP) since the latter is equal to the SI multiple millipascal seconds (mPa·s). For example, water at 20 °C has a viscosity of 1.002 mPa·s = 1.002 cP.

1 Pl = 1 Pa·s 1 P = 1 dPa·s = 0.1 Pa·s = 0.1 kg·m−1·s−1 1 cP = 1 mPa·s = 0.001 Pa·s = 0.001 N·s·m−2 = 0.001 kg·m−1·s−1.

Kinematic viscosity, ν[edit] The SI unit of kinematic viscosity is m2/s. The cgs physical unit for kinematic viscosity is the stokes (St), named after George Gabriel Stokes. It is sometimes expressed in terms of centistokes (cSt). In U.S. usage, stoke is sometimes used as the singular form.

1 St = 1 cm2·s−1 = 10−4 m2·s−1. 1 cSt = 1 mm2·s−1 = 10−6 m2·s−1.

Water
Water
at 20 °C has a kinematic viscosity of about 10−6 m2·s−1 or 1 cSt. The kinematic viscosity is sometimes referred to as diffusivity of momentum, because it is analogous to diffusivity of heat and diffusivity of mass. It is therefore used in dimensionless numbers which compare the ratio of the diffusivities. Fluidity[edit] The reciprocal of viscosity is fluidity, usually symbolized by φ = 1/μ or F = 1/μ, 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. The concept of fluidity can be used to determine the viscosity of an ideal solution. For two components A and B, the fluidity when A and B are mixed is

F ≈

χ

A

F

A

+

χ

B

F

B

,

displaystyle Fapprox chi _ mathrm A F_ mathrm A +chi _ mathrm B F_ mathrm B ,

which is only slightly simpler than the equivalent equation in terms of viscosity:

μ ≈

1

χ

A

μ

A

+

χ

B

μ

B

,

displaystyle mu approx frac 1 dfrac chi _ mathrm A mu _ mathrm A + dfrac chi _ mathrm B mu _ mathrm B ,

where χA and χB are the mole fractions of components A and B respectively, and μA and μB are the components' pure viscosities. Non-standard units[edit] The reyn is a British unit of dynamic viscosity. Viscosity index is a measure for the change of kinematic viscosity with temperature. It is used in the automotive industry to characterise lubricating oil. 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).[16] Other abbreviations such as SSU (Saybolt seconds universal) or SUV (Saybolt universal viscosity) are sometimes used. Kinematic viscosity
Kinematic viscosity
in centistokes can be converted from SUS according to the arithmetic and the reference table provided in ASTM
ASTM
D 2161.[17] Molecular origins[edit]

Pitch has a viscosity approximately 230 billion (7011229999999999999♠2.3×1011) times that of water.[18]

The viscosity of a system is determined by how 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
Time
Correlation Function expressions derived by Evans and Morriss in 1985.[19] Although these expressions are each exact, in order to calculate the viscosity of a dense fluid using these relations currently requires the use of molecular dynamics computer simulations. Gases[edit] Viscosity
Viscosity
in gases arises principally from the molecular diffusion that transports momentum between layers of flow. The kinetic theory of gases allows accurate prediction of the behavior of gaseous viscosity. Within the regime where the theory is applicable:

Viscosity
Viscosity
is independent of pressure and Viscosity
Viscosity
increases as temperature increases.[20]

James Clerk Maxwell
James Clerk Maxwell
published a famous paper in 1866 using the kinetic theory of gases to study gaseous viscosity.[21] To understand why the viscosity is independent of pressure, consider two adjacent boundary layers (A and B) moving with respect to each other. The internal friction (the viscosity) of the gas is determined by the probability a particle of layer A enters layer B with a corresponding transfer of momentum. Maxwell's calculations show that the viscosity coefficient is proportional to the density, the mean free path, and the mean velocity of the atoms. On the other hand, the mean free path is inversely proportional to the density. So an increase in density due to an increase in pressure doesn't result in any change in viscosity. Relation to mean free path of diffusing particles[edit] In relation to diffusion, the kinematic viscosity provides a better understanding of the behavior of mass transport of a dilute species. Viscosity
Viscosity
is related to shear stress and the rate of shear in a fluid, which illustrates its dependence on the mean free path, λ, of the diffusing particles. From fluid mechanics, for a Newtonian fluid, the shear stress, τ, on a unit area moving parallel to itself, is found to be proportional to the rate of change of velocity with distance perpendicular to the unit area:

τ = μ

d

u

x

d

y

displaystyle tau =mu frac mathrm d u_ x mathrm d y

for a unit area parallel to the xz-plane, moving along the x axis. We will derive this formula and show how μ is related to λ. Interpreting shear stress as the time rate of change of momentum, p, per unit area A (rate of momentum flux) of an arbitrary control surface gives

τ =

p ˙

A

=

m ˙

u

x

A

.

displaystyle tau = frac dot p A = frac dot m leftlangle u_ x rightrangle A .

where ⟨ux⟩ is the average velocity, along the x-axis, of fluid molecules hitting the unit area, with respect to the unit area and ṁ is the rate of fluid mass hitting the surface. By making simplified assumption that the velocity of the molecules depends linearly on the distance they are coming from, the mean velocity depends linearly on the mean distance:

u

x

= λ

d

u

x

d

y

displaystyle leftlangle u_ x rightrangle =lambda frac mathrm d u_ x mathrm d y

. Further manipulation will show,[22]

m ˙

= ρ

u ¯

A

τ

=

ρ

u ¯

λ

μ

d

u

x

d

y

ν =

μ ρ

=

u ¯

λ ,

displaystyle begin aligned dot m &=rho bar u A\tau &=underbrace rho bar u lambda _ mu cdot frac mathrm d u_ x mathrm d y quad Rightarrow quad nu = frac mu rho = bar u lambda ,end aligned

where

ρ is the density of the fluid, ū is the root mean square molecular speed: ū = √⟨u2⟩, λ is the mean free path, μ is the dynamic viscosity.

Note, that the mean free path itself typically depends (inversely) on the density. Effect of temperature on the viscosity of a gas[edit] Sutherland's formula can be used to derive the dynamic viscosity of an ideal gas as a function of the temperature:[23]

μ =

μ

0

T

0

+ C

T + C

(

T

T

0

)

3 2

.

displaystyle mu =mu _ 0 frac T_ 0 +C T+C left( frac T T_ 0 right)^ frac 3 2 .

This, in turn, is equal to

μ = λ

T

3 2

T + C

,

displaystyle mu =lambda frac T^ frac 3 2 T+C ,

where

λ =

μ

0

(

T

0

+ C

)

T

0

3 2

displaystyle lambda = frac mu _ 0 left(T_ 0 +Cright) T_ 0 ^ frac 3 2

is a constant for the gas. in Sutherland's formula:

μ = dynamic viscosity (Pa·s or μPa·s) at input temperature T, μ0 = reference viscosity (in the same units as μ) at reference temperature T0, T = input temperature (K), T0 = reference temperature (K), C = Sutherland's constant for the gaseous material in question.

Valid for temperatures between 0 < T < 555 K with an error due to pressure less than 10% below 3.45 MPa. According to Sutherland's formula, if the absolute temperature is less than C, the relative change in viscosity for a small change in temperature is greater than the relative change in the absolute temperature, but it is smaller when T is above C. The kinematic viscosity though always increases faster than the temperature (that is, d log(ν)/d log(T) is greater than 1). Sutherland's constant, reference values and λ values for some gases:

Gas C (K) T0 (K) μ0 (μPa·s) λ (μPa·s·K−​1⁄2)

air 120 291.15 18.27 7000151204128800000♠1.512041288

nitrogen 111 300.55 17.81 7000140673219500000♠1.406732195

oxygen 127 292.25 20.18 7000169341130000000♠1.693411300

carbon dioxide 240 293.15 14.8 7000157208593100000♠1.572085931

carbon monoxide 118 288.15 17.2 7000142819322500000♠1.428193225

hydrogen 72 293.85 8.76 6999636236562000000♠0.636236562

ammonia 370 293.15 9.82 7000129744337900000♠1.297443379

sulfur dioxide 416 293.65 12.54 7000176846608600000♠1.768466086

helium 79.4[24] 273 19[25] 7000148438149000000♠1.484381490

Viscosity
Viscosity
of a dilute gas[edit] The Chapman–Enskog equation[26] may be used to estimate viscosity for a dilute gas. This equation is based on a semi-theoretical assumption by Chapman and Enskog. The equation requires three empirically determined parameters: the collision diameter (σ), the maximum energy of attraction divided by the Boltzmann constant
Boltzmann constant
(ε/к) and the collision integral (ω(T*)).

μ

0

×

10

6

=

2.6693

M T

σ

2

ω (

T

)

,

displaystyle mu _ 0 times 10^ 6 = 2.6693 frac sqrt MT sigma ^ 2 omega (T^ * ) ,

with

T* = κT/ε is reduced temperature (dimensionless), μ0 is viscosity for dilute gas (μPa·s), M is molecular mass (g/mol), T is temperature (K), σ is the collision diameter (Å), ε/к is the maximum energy of attraction divided by the Boltzmann constant (K), ωμ is the collision integral.

Liquids[edit]

Play media

Video showing three liquids with different viscosities

Play media

Experiment showing the behaviour of a viscous fluid with blue dye for visibility.

In liquids, the additional forces between molecules become important. This leads to an additional contribution to the shear stress though the exact mechanics of this are still controversial.[citation needed] Thus, in liquids:

Viscosity
Viscosity
is independent of pressure (except at very high pressure); and Viscosity
Viscosity
tends to fall as temperature increases (for example, water viscosity goes from 1.79 cP to 0.28 cP in the temperature range from 0 °C to 100 °C); see temperature dependence of liquid viscosity for more details.

The dynamic viscosities of liquids are typically several orders of magnitude higher than dynamic viscosities of gases. Viscosity
Viscosity
of blends of liquids[edit] The viscosity of the blend of two or more liquids can be estimated using the Refutas equation.[27][28] The calculation is carried out in three steps. The first step is to calculate the viscosity blending number (VBN) (also called the viscosity blending index) of each component of the blend:

V B N

= 14.534 × ln ⁡

(

ln ⁡ ( ν + 0.8 )

)

+ 10.975

displaystyle mathrm VBN =14.534times ln big ( ln(nu +0.8) big ) +10.975,

  (1)

where ν is the kinematic viscosity in centistokes (cSt). It is important that the kinematic viscosity of each component of the blend be obtained at the same temperature. The next step is to calculate the VBN of the blend, using this equation:

V B

N

B l e n d

=

(

x

A

×

V B

N

A

)

+

(

x

B

×

V B

N

B

)

+ ⋯ +

(

x

N

×

V B

N

N

)

displaystyle mathrm VBN_ Blend =left(x_ mathrm A times mathrm VBN_ A right)+left(x_ mathrm B times mathrm VBN_ B right)+cdots +left(x_ mathrm N times mathrm VBN_ N right),

  (2)

where xX is the mass fraction of each component of the blend. Once the viscosity blending number of a blend has been calculated using equation (2), the final step is to determine the kinematic viscosity of the blend by solving equation (1) for ν:

ν = exp ⁡

(

exp ⁡

(

V B

N

B l e n d

− 10.975

14.534

)

)

− 0.8 ,

displaystyle nu =exp left(exp left( frac mathrm VBN_ Blend -10.975 14.534 right)right)-0.8,

  (3)

where VBNBlend is the viscosity blending number of the blend. alternatively use the more accurate Lederer-Roegiers equation [1]

ln ⁡

η

1 , 2

=

x

1

ln ⁡

η

1

x

1

+

x

2

α

+

α

x

2

ln ⁡

η

2

x

1

+

x

2

α

displaystyle ln eta _ 1,2 = frac x_ 1 ln eta _ 1 x_ 1 +x_ 2 alpha + frac alpha x_ 2 ln eta _ 2 x_ 1 +x_ 2 alpha

α

displaystyle alpha

is based on the difference in intermolecular cohesion energies between the liquids

η

displaystyle eta

=dynamic viscosity x[i]=mole_fraction[i]

Selected substances[edit] Air[edit]

Pressure
Pressure
dependence of the dynamic viscosity of dry air at the temperatures of 300, 400 and 500 kelvins

The viscosity of air depends mostly on the temperature. At 15 °C, the viscosity of air is 6995181000000000000♠1.81×10−5 kg/(m·s), 18.1 μPa·s or 6995181000000000000♠1.81×10−5 Pa·s. The kinematic viscosity at 15 °C is 6995148000000000000♠1.48×10−5 m2/s or 14.8 cSt. At 25 °C, the viscosity is 18.6 μPa·s and the kinematic viscosity 15.7 cSt. Water[edit]

Dynamic viscosity of water

The dynamic viscosity of water is 6996890000000000000♠8.90×10−4 Pa·s or 6997890000000000000♠8.90×10−3 dyn·s/cm2 or 0.890 cP at about 25 °C. As a function of temperature T (in kelvins): μ = A × 10B/(T − C), where A = 6995241400000000000♠2.414×10−5 Pa·s, B = 247.8 K, and C = 140 K.[citation needed] The dynamic viscosity of liquid water at different temperatures up to the normal boiling point is listed below.

Dynamic viscosity of water at various temperatures

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

10 1.308

20 1.002

30 0.7978

40 0.6531

50 0.5471

60 0.4658

70 0.4044

80 0.3550

90 0.3150

100 0.2822

Other substances[edit]

Example of the viscosity of milk and water. Liquids with higher viscosities make smaller splashes when poured at the same velocity.

Honey
Honey
being drizzled.

Peanut butter
Peanut butter
is a semi-solid and can therefore hold peaks.

Some dynamic viscosities of Newtonian fluids are listed below:

Viscosity
Viscosity
of selected gases at 100 kPa, (μPa·s)

Gas at 0 °C (273 K) at 27 °C (300 K)[29]

air 17.4 18.6

hydrogen 8.4 9.0

helium

20.0

argon

22.9

xenon 21.2 23.2

carbon dioxide

15.0

methane

11.2

ethane

9.5

Viscosity
Viscosity
of fluids with variable compositions

Fluid Viscosity
Viscosity
(Pa·s) Viscosity
Viscosity
(cP)

blood (37 °C)[9] 6997300000000000000♠3×10−3 – 6997400000000000000♠4×10−3 3–4

honey 2–10[30] 2000–7004100000000000000♠10000

molasses 5–10 5000–7004100000000000000♠10000

molten glass 10–1000 7004100000000000000♠10000–7006100000000000000♠1000000

chocolate syrup 10–25 7004100000000000000♠10000–7004250000000000000♠25000

molten chocolate[a] 45–130[31] 7004450000000000000♠45000–7005130000000000000♠130000

ketchup[a] 50–100 7004500000000000000♠50000–7005100000000000000♠100000

lard ≈ 100 ≈ 7005100000000000000♠100000

peanut butter[a] ≈ 250 ≈ 7005250000000000000♠250000

shortening[a] ≈ 250 ≈ 7005250000000000000♠250000

Viscosity
Viscosity
of liquids (at 25 °C unless otherwise specified)

Liquid Viscosity
Viscosity
(Pa·s) Viscosity
Viscosity
(cP)

acetone[29] 6996306000000000000♠3.06×10−4 0.306

benzene[29] 6996604000000000000♠6.04×10−4 0.604

castor oil[29] 0.985 985

corn syrup[29] 1.3806 1380.6

ethanol[29] 6997107400000000000♠1.074×10−3 7000107400000000000♠1.074

ethylene glycol 6998161000000000000♠1.61×10−2 7001161000000000000♠16.1

glycerol (at 20 °C)[25] 7000120000000000000♠1.2 7003120000000000000♠1200

HFO-380 7000202199999999999♠2.022 7003202200000000000♠2022

mercury[29] 6997152600000000000♠1.526×10−3 7000152600000000000♠1.526

methanol[29] 6996544000000000000♠5.44×10−4 6999544000000000000♠0.544

motor oil SAE 10 (20 °C)[20] 6998650000000000000♠0.065 7001650000000000000♠65

motor oil SAE 40 (20 °C)[20] 6999319000000000000♠0.319 7002319000000000000♠319

nitrobenzene[29] 6997186300000000000♠1.863×10−3 7000186300000000000♠1.863

liquid nitrogen (−196 °C) 6996158000000000000♠1.58×10−4 6999158000000000000♠0.158

propanol[29] 6997194500000000000♠1.945×10−3 7000194500000000000♠1.945

olive oil 6998810000000000000♠0.081 7001810000000000000♠81

pitch 7008229999999999999♠2.3×108 7011229999999999999♠2.30×1011

sulfuric acid[29] 6998242000000000000♠2.42×10−2 7001242000000000000♠24.2

water 6996894000000000000♠8.94×10−4 6999894000000000000♠0.894

Viscosity
Viscosity
of solids

Solid Viscosity
Viscosity
(Pa·s) Temperature (°C)

granite[8] 7019300000000000000♠3×1019 - 7019600000000000000♠6×1019 25

asthenosphere[32] 7019700000000000000♠7.0×1019 900

upper mantle[32] 7020700000000000000♠7×1020 – 7021100000000000000♠1×1021 1300–3000

lower mantle 7021100000000000000♠1×1021 – 7021200000000000000♠2×1021 3000–4000

^ a b c d These materials are highly non-Newtonian.

Slurry[edit]

Plot of slurry relative viscosity μr as calculated by empirical correlations from Einstein,[33] Guth and Simha,[34] Thomas,[35] and Kitano et al..[36]

The term slurry describes mixtures of a liquid and solid particles that retain some fluidity. The viscosity of slurry can be described as relative to the viscosity of the liquid phase:

μ

s

=

μ

r

μ

l

,

displaystyle mu _ mathrm s =mu _ mathrm r mu _ mathrm l ,

where μs and μl are respectively the dynamic viscosity of the slurry and liquid (Pa·s), and μr is the relative viscosity (dimensionless). Depending on the size and concentration of the solid particles, several models exist that describe the relative viscosity as a function of volume fraction φ of solid particles. In the case of extremely low concentrations of fine particles, Einstein's equation[33] may be used:

μ

r

= 1 + 2.5 φ

displaystyle mu _ mathrm r =1+2.5varphi

In the case of higher concentrations, a modified equation was proposed by Guth and Simha,[34] which takes into account interaction between the solid particles:

μ

r

= 1 + 2.5 φ + 14.1

φ

2

displaystyle mu _ mathrm r =1+2.5varphi +14.1varphi ^ 2

Further modification of this equation was proposed by Thomas[35] from the fitting of empirical data:

μ

r

= 1 + 2.5 φ + 10.05

φ

2

+ A

e

B φ

,

displaystyle mu _ mathrm r =1+2.5varphi +10.05varphi ^ 2 +Ae^ Bvarphi ,

where A = 6997272999999999999♠0.00273 and B = 16.6. In the case of high shear stress (above 1 kPa), another empirical equation was proposed by Kitano et al. for polymer melts:[36]

μ

r

=

(

1 −

φ A

)

− 2

,

displaystyle mu _ mathrm r =left(1- frac varphi A right)^ -2 ,

where A = 0.68 for smooth spherical particles. Nanofluids[edit]

Alumina-Ethylene Glycol, Particle sizes 8 and 43 nm at 10 Celsius

Magnatite-Water,Particle sizes 25nm at 20 Celsius

Nanofluid is a novel class of fluid, which is developed by dispersing nano-sized particles in base fluid.[37] Einstein model Einstein derived the applicable first theoretical formula for the estimation of viscosity values of composites or mixtures in 1906. This model developed while assuming linear viscous fluid including suspensions of rigid and spherical particles. Einstein’s model is valid for very low volume fraction.[33]

μ

n f

=

μ

b f

( 1 + 2.5 ∅ )

displaystyle mu _ nf =mu _ bf (1+2.5varnothing )

Brinkman model Brinkman modified Einstein’s model for used with average particle volume fraction up to 4%[38]

μ

n f

=

μ

b f

(

1

  ( 1 − ∅

)

2.5

)

=

μ

b f

(

1 + 2.5 ∅ + 4.375

2

+ O (

3

)

)

displaystyle mu _ nf =mu _ bf Biggl ( frac 1 (1-varnothing )^ 2.5 Biggr ) =mu _ bf Big ( 1+2.5varnothing +4.375varnothing ^ 2 +O(varnothing ^ 3 ) Big )

Batchelor model Batchelor reformed Einstein's theoretical model by presenting Brownian motion effect.[39]

μ

n f

=

μ

b f

( 1 + 2.5 ∅ + 6.5

2

)

displaystyle mu _ nf =mu _ bf (1+2.5varnothing +6.5 varnothing ^ 2 )

Wang et al. model Wang et al. found a model to predict viscosity of nanofluid as follows.[40]

μ

n f

=

μ

b f

( 1 + 7.3 ∅ + 123

2

)

displaystyle mu _ nf =mu _ bf (1+7.3varnothing +123 varnothing ^ 2 )

Masoumi et al. model Masoumi et al. suggested a new viscosity correlation by considering Brownian motion of nanoparticle in nanofluid.[41]

μ

n f

=

μ

b f

(

1 +

ρ

p

V

B

d

p

2

72 δ C

)

displaystyle mu _ nf =mu _ bf Biggl ( 1+ frac rho _ p V_ B d_ p ^ 2 72delta C Biggr )

V

B

=

18

K

B

T

π

ρ

p

d

p

3

displaystyle V_ B = sqrt frac 18K_ B T pi rho _ p d_ p ^ 3

δ =

π

d

p

3

6 ∅

3

displaystyle delta = sqrt[ 3 ] frac pi d_ p ^ 3 6varnothing

C =

( − 1.133

d

p

− 2.771 ) ∅ + ( 0.09

d

p

− 0.393 )

×

10

− 6

displaystyle C= (-1.133d_ p -2.771)varnothing +(0.09d_ p -0.393) times 10^ -6

Udawattha et al. model Udawattha et al. modified the Masoumi et al. model. The developed model valid for suspension containing micro-size particles.[42]

μ

n f

=

μ

b f

(

1 + 2.5

e

+

ρ

p

V

B

d

p

2

72 δ [ T ×

10

− 10

×

− 0.002 T − 0.284

]

)

displaystyle mu _ nf =mu _ bf Biggl ( 1+2.5varnothing _ e + frac rho _ p V_ B d_ p ^ 2 72delta [Ttimes 10^ -10 times varnothing ^ -0.002T-0.284 ] Biggr )

e

= ∅

(

1 +

h r

)

3

displaystyle varnothing _ e =varnothing Biggl ( 1+ frac h r Biggr ) ^ 3

where

μ

displaystyle mu

is the viscosity of the sample, in [Pa·s]

n f

displaystyle nf

is nanofluid

b f

displaystyle bf

is basefluid

p

displaystyle p

is particle

displaystyle varnothing

is volume fraction

ρ

displaystyle rho

is density of the sample, in [kg·m−3]

δ

displaystyle delta

is distance between two particles

V

B

displaystyle V_ B

is Brownian motion of particle

K

B

displaystyle K_ B

is the Boltzmann constant

T

displaystyle T

is Temperature of the sample, in [K]

d

p

displaystyle d_ p

is diameter of a particle

h

displaystyle h

is nanolayer thickness (1 nm)

r

displaystyle r

is radius of a particle

Amorphous materials[edit]

Common glass viscosity curves.[43]

Viscous flow in amorphous materials (e.g. in glasses and melts)[44][45][46] is a thermally activated process:

μ = A

e

Q

R T

,

displaystyle mu =Ae^ frac Q RT ,

where Q is activation energy, T is temperature, R is the molar gas constant and A is approximately a constant. The viscous flow in amorphous materials is characterized by a deviation from the Arrhenius-type behavior: Q changes from a high value QH at low temperatures (in the glassy state) to a low value QL at high temperatures (in the liquid state). Depending on this change, amorphous materials are classified as either

strong when: QH − QL < QL or fragile when: QH − QL ≥ QL.

The fragility of amorphous materials is numerically characterized by Doremus' fragility ratio:

R

D

=

Q

H

Q

L

displaystyle R_ mathrm D = frac Q_ mathrm H Q_ mathrm L

and strong materials have RD < 2 whereas fragile materials have RD ≥ 2.

Common logarithm
Common logarithm
of viscosity against temperature for B2O3, showing two regimes

The viscosity of amorphous materials is quite exactly described by a two-exponential equation:

μ =

A

1

T

(

1 +

A

2

e

B

R T

)

(

1 + C

e

D

R T

)

,

displaystyle mu =A_ 1 Tleft(1+A_ 2 e^ frac B RT right)left(1+Ce^ frac D RT right),

with constants A1, A2, B, C and D related to thermodynamic parameters of joining bonds of an amorphous material. Not very far from the glass transition temperature, Tg, this equation can be approximated by a Vogel–Fulcher–Tammann (VFT) equation. If the temperature is significantly lower than the glass transition temperature, T ≪ Tg, then the two-exponential equation simplifies to an Arrhenius-type equation:

μ =

A

L

T

e

Q

H

R T

displaystyle mu =A_ mathrm L Te^ frac Q_ mathrm H RT

with

Q

H

=

H

d

+

H

m

,

displaystyle Q_ mathrm H =H_ mathrm d +H_ mathrm m ,

where Hd is the enthalpy of formation of broken bonds (termed configurons[47]) and Hm is the enthalpy of their motion. When the temperature is less than the glass transition temperature, T < Tg, the activation energy of viscosity is high because the amorphous materials are in the glassy state and most of their joining bonds are intact. If the temperature is much higher than the glass transition temperature, T ≫ Tg, the two-exponential equation also simplifies to an Arrhenius-type equation:

μ =

A

H

T

e

Q

L

R T

,

displaystyle mu =A_ mathrm H Te^ frac Q_ mathrm L RT ,

with

Q

L

=

H

m

.

displaystyle Q_ mathrm L =H_ mathrm m .

When the temperature is higher than the glass transition temperature, T > Tg, the activation energy of viscosity is low because amorphous materials are melted and have most of their joining bonds broken, which facilitates flow. Eddy viscosity[edit] In the study of turbulence in fluids, a common practical strategy for calculation is to ignore the small-scale vortices (or eddies) in the motion and to calculate a large-scale motion with an eddy viscosity that characterizes the transport and dissipation of energy in the smaller-scale flow (see large eddy simulation). Values of eddy viscosity used in modeling ocean circulation may be from 7004500000000000000♠5×104 to 7006100000000000000♠1×106 Pa·s depending upon the resolution of the numerical grid. See also[edit]

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

References[edit]

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IUPAC
Compendium of Chemical Terminology. Oxford: Blackwell Scientific Publications. doi:10.1351/goldbook. ISBN 0-9678550-9-8.  ^ a b Kumagai, Naoichi; Sasajima, Sadao; Ito, Hidebumi (15 February 1978). "Long-term Creep of Rocks: Results with Large Specimens Obtained in about 20 Years and Those with Small Specimens in about 3 Years". Journal of the Society of Materials Science (Japan). Japan Energy
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Reviews. 43 (Supplement C): 584–598. doi:10.1016/j.rser.2014.11.020.  ^ Brinkman, H. C. (1952-04-01). "The Viscosity
Viscosity
of Concentrated Suspensions and Solutions". The Journal of Chemical Physics. 20 (4): 571–571. Bibcode:1952JChPh..20..571B. doi:10.1063/1.1700493. ISSN 0021-9606.  ^ Batchelor, G. K. (November 1977). "The effect of Brownian motion on the bulk stress in a suspension of spherical particles". Journal of Fluid Mechanics. 83 (1): 97–117. Bibcode:1977JFM....83...97B. doi:10.1017/S0022112077001062. ISSN 1469-7645.  ^ Wang, Xinwei; Xu, Xianfan; Choi, Stephen U. S. (1999). "Thermal Conductivity of Nanoparticle - Fluid Mixture". Journal of Thermophysics and Heat Transfer. 13 (4): 474–480. doi:10.2514/2.6486. ISSN 0887-8722.  ^ Masoumi, N.; Sohrabi, N.; Behzadmehr, A. (2009). "A new model for calculating the effective viscosity of nanofluids". Journal of Physics D: Applied Physics. 42 (5): 055501. Bibcode:2009JPhD...42e5501M. doi:10.1088/0022-3727/42/5/055501. ISSN 0022-3727.  ^ Udawattha, Dilan S.; Narayana, Mahinsasa; Wijayarathne, Uditha P. L. "Predicting the effective viscosity of nanofluids based on the rheology of suspensions of solid particles". Journal of King Saud University - Science. doi:10.1016/j.jksus.2017.09.016.  ^ Fluegel, Alexander. " Viscosity
Viscosity
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Further reading[edit]

Hatschek, Emil (1928). The Viscosity
Viscosity
of Liquids. New York: Van Nostrand. OCLC 53438464.  Massey, B. S.; Ward-Smith, A. J. (2011). Mechanics
Mechanics
of Fluids (9th ed.). London & New York: Spon Press. ISBN 978-0-415-60259-4. OCLC 690084654. 

External links[edit]

Look up viscosity in Wiktionary, the free dictionary.

Wikisource
Wikisource
has the text of The New Student's Reference Work article Viscosity
Viscosity
of Liquids.

Fluid properties High accuracy calculation of viscosity and other physical properties of frequent used pure liquids and gases. Gas
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Air
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measurement, viscosity units and fixpoints, glass viscosity calculation Kinematic Viscosity
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conversion between kinematic and dynamic viscosity. Physical Characteristics of Water
Water
A table of water viscosity as a function of temperature Vogel–Tammann–Fulcher Equation Parameters Calculation of temperature-dependent dynamic viscosities for some common components "Test Procedures for Testing Highway and Nonroad Engines and Omnibus Technical Amendments". United States Environmental Protection Agency Artificial viscosity

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