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 a much higher viscosity than water.^{[2]}
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. In common parlance, however, a liquid is said to be viscous if its viscosity is substantially greater than that of water, and may be described as mobile if the viscosity is noticeably less than water. A fluid with a relatively high viscosity, such as pitch, may appear to be a solid.
Viscosity (kinematic or dynamic)  

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 
SI unit  Pa·s = (N·s)/m^{2} = kg/(s·m) 
Derivations from
other quantities 
μ = G·t 
The word "viscosity" is derived from the Latin "viscum", meaning mistletoe and also a viscous glue made from mistletoe berries.^{[3]}
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 . 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:
The proportionality factor μ in this formula is the viscosity (specifically, the dynamic viscosity) of the fluid, with units of (pascalsecond).
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 . Isaac Newton expressed the viscous forces by the differential equation
where τ = F/A, and ∂u/∂y is the local shear velocity. This formula assumes that the flow is moving along parallel lines to xaxis. Furthermore, it assumes that the yaxis, 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.
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]}
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 .
It is a convenient concept when analyzing the Reynolds number, which expresses the ratio of the inertial forces to the viscous forces:
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).
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.
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.
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 nonNewtonian fluids that significantly deviate from that law in some way or other. For example:
Shearthinning 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.
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 10^{12} 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]}
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 the 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 (nonNewtonian fluids) cannot be described by a single number. NonNewtonian 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.^{[13]}
Also used in coatings, a Stormer viscometer uses loadbased 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 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.
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)/m^{2}, 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 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.
The SI unit of kinematic viscosity is m^{2}/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.
Water at 20 °C has a kinematic viscosity of about 10^{−6 }m^{2}·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.
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
which is only slightly simpler than the equivalent equation in terms of viscosity:
where χ_{A} and χ_{B} are the mole fractions of components A and B respectively, and μ_{A} and μ_{B} are the components' pure viscosities.
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 in centistokes can be converted from SUS according to the arithmetic and the reference table provided in ASTM D 2161.^{[17]}
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 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.
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:
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.
In relation to diffusion, the kinematic viscosity provides a better understanding of the behavior of mass transport of a dilute species. 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:
for a unit area parallel to the xzplane, 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
where ⟨u_{x}⟩ is the average velocity, along the xaxis, 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:
.
Further manipulation will show,^{[22]}
where
Note, that the mean free path itself typically depends (inversely) on the density.
Sutherland's formula can be used to derive the dynamic viscosity of an ideal gas as a function of the temperature:^{[23]}
This, in turn, is equal to
where
is a constant for the gas.
in Sutherland's formula:
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)  T_{0} (K)  μ_{0} (μPa·s)  λ (μPa·s·K^{−1⁄2}) 

air  120  291.15  18.27  1.512041288 
nitrogen  111  300.55  17.81  1.406732195 
oxygen  127  292.25  20.18  1.693411300 
carbon dioxide  240  293.15  14.8  1.572085931 
carbon monoxide  118  288.15  17.2  1.428193225 
hydrogen  72  293.85  8.76  0.636236562 
ammonia  370  293.15  9.82  1.297443379 
sulfur dioxide  416  293.65  12.54  1.768466086 
helium  79.4^{[24]}  273  19^{[25]}  1.484381490 
The Chapman–Enskog equation^{[26]} may be used to estimate viscosity for a dilute gas. This equation is based on a semitheoretical 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 (ε/к) and the collision integral (ω(T*)).
with
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. Thus, in liquids:
The dynamic viscosities of liquids are typically several orders of magnitude higher than dynamic viscosities of gases.
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:
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:
where x_{X} 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:
where VBN_{Blend} is the viscosity blending number of the blend.
alternatively use the more accurate LedererRoegiers equation [1]
The viscosity of air depends mostly on the temperature. At 15 °C, the viscosity of air is 1.81×10^{−5} kg/(m·s), 18.1 μPa·s or 1.81×10^{−5} Pa·s. The kinematic viscosity at 15 °C is 1.48×10^{−5} m^{2}/s or 14.8 cSt. At 25 °C, the viscosity is 18.6 μPa·s and the kinematic viscosity 15.7 cSt.
The dynamic viscosity of water is 8.90×10^{−4} Pa·s or 8.90×10^{−3} dyn·s/cm^{2} or 0.890 cP at about 25 °C.
As a function of temperature T (in kelvins): μ = A × 10^{B/(T − C)}, where A = 2.414×10^{−5} Pa·s, B = 247.8 K, and C = 140 K.
The dynamic viscosity of liquid water at different temperatures up to the normal boiling point is listed below.
Temperature (°C)  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 
Some dynamic viscosities of Newtonian fluids are listed below:
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 
Fluid  Viscosity (Pa·s)  Viscosity (cP) 

blood (37 °C)^{[9]}  3×10^{−3} – 4×10^{−3}  3–4 
honey  2–10^{[30]}  2000–10000 
molasses  5–10  5000–10000 
molten glass  10–1000  10000–1000000 
chocolate syrup  10–25  10000–25000 
molten chocolate^{[a]}  45–130^{[31]}  45000–130000 
ketchup^{[a]}  50–100  50000–100000 
lard  ≈ 100  ≈ 100000 
peanut butter^{[a]}  ≈ 250  ≈ 250000 
shortening^{[a]}  ≈ 250  ≈ 250000 
Liquid  Viscosity (Pa·s)  Viscosity (cP) 

acetone^{[29]}  3.06×10^{−4}  0.306 
benzene^{[29]}  6.04×10^{−4}  0.604 
castor oil^{[29]}  0.985  985 
corn syrup^{[29]}  1.3806  1380.6 
ethanol^{[29]}  1.074×10^{−3}  1.074 
ethylene glycol  1.61×10^{−2}  16.1 
glycerol (at 20 °C)^{[25]}  1.2  1200 
HFO380  2.022  2022 
mercury^{[29]}  1.526×10^{−3}  1.526 
methanol^{[29]}  5.44×10^{−4}  0.544 
motor oil SAE 10 (20 °C)^{[20]}  0.065  65 
motor oil SAE 40 (20 °C)^{[20]}  0.319  319 
nitrobenzene^{[29]}  1.863×10^{−3}  1.863 
liquid nitrogen (−196 °C)  1.58×10^{−4}  0.158 
propanol^{[29]}  1.945×10^{−3}  1.945 
olive oil  0.081  81 
pitch  2.3×10^{8}  2.30×10^{11} 
sulfuric acid^{[29]}  2.42×10^{−2}  24.2 
water  8.94×10^{−4}  0.894 
Solid  Viscosity (Pa·s)  Temperature (°C) 

granite^{[8]}  3×10^{19}  6×10^{19}  25 
asthenosphere^{[32]}  7.0×10^{19}  900 
upper mantle^{[32]}  7×10^{20} – 1×10^{21}  1300–3000 
lower mantle  1×10^{21} – 2×10^{21}  3000–4000 
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:
where μ_{s} and μ_{l} are respectively the dynamic viscosity of the slurry and liquid (Pa·s), and μ_{r} 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:
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:
Further modification of this equation was proposed by Thomas^{[35]} from the fitting of empirical data:
where A = 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]}
where A = 0.68 for smooth spherical particles.
Nanofluid is a novel class of fluid, which is developed by dispersing nanosized 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]}.
Brinkman model
Brinkman modified Einstein’s model for used with average particle volume fraction up to 4%^{[38]}
Batchelor model
Batchelor reformed Einstein’s theoretical model by presenting Brownian motion effect^{[39]}.
Wang et al model
Wang et al. found a model to predict viscosity of nanofluid as follows^{[40]}.
Masoumi et al. model
Masoumi et al. suggested a new viscosity correlation by considering Brownian motion of nanoparticle in nanofluid^{[41]}.
Udawattha et al. model
Udawattha et al. modified the Masoumi et al. model. The developed model valid for suspension containing microsize particles^{[42]}.
where
Viscous flow in amorphous materials (e.g. in glasses and melts)^{[44]}^{[45]}^{[46]} is a thermally activated process:
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 Arrheniustype behavior: Q 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). Depending on this change, amorphous materials are classified as either
The fragility of amorphous materials is numerically characterized by Doremus' fragility ratio:
and strong materials have R_{D} < 2 whereas fragile materials have R_{D} ≥ 2.
The viscosity of amorphous materials is quite exactly described by a twoexponential equation:
with constants A_{1}, A_{2}, B, C and D related to thermodynamic parameters of joining bonds of an amorphous material.
Not very far from the glass transition temperature, T_{g}, this equation can be approximated by a Vogel–Fulcher–Tammann (VFT) equation.
If the temperature is significantly lower than the glass transition temperature, T ≪ T_{g}, then the twoexponential equation simplifies to an Arrheniustype equation:
with
where H_{d} is the enthalpy of formation of broken bonds (termed configurons^{[47]}) and H_{m} is the enthalpy of their motion.
When the temperature is less than the glass transition temperature, T < T_{g}, 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 ≫ T_{g}, the twoexponential equation also simplifies to an Arrheniustype equation:
with
When the temperature is higher than the glass transition temperature, T > T_{g}, the activation energy of viscosity is low because amorphous materials are melted and have most of their joining bonds broken, which facilitates flow.
In the study of turbulence in fluids, a common practical strategy for calculation is to ignore the smallscale vortices (or eddies) in the motion and to calculate a largescale motion with an eddy viscosity that characterizes the transport and dissipation of energy in the smallerscale flow (see large eddy simulation). Values of eddy viscosity used in modeling ocean circulation may be from 5×10^{4} to 1×10^{6} Pa·s depending upon the resolution of the numerical grid.
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