Can Someone Please Explain Viscosity To Me?

Answer 1

You already got some good answers, but here is a synthesis of the good points from them:

1) viscosity is the resistance to shear deformation and thus flow, which is a shear of different layers of the fluid.

2) The ease or difficulty is due to the intermolecular forces.

3) All real materials are viscous. Even gases have viscosity as evidenced by the friction between the aeroplane and the air.

4) Asphalt at room temperature is a highly viscous liquid and flows over time.

5) Glass is supercooled liquid with a very high viscosity and thus appears to be a solid.

Pl. feel free to ask with specific doubts.

Answer 2

Viscosity is a measure of the resistance of a fluid to deform under shear stress. It is commonly perceived as "thickness", or resistance to pouring. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Thus, water is "thin", having a lower viscosity, while vegetable oil is "thick" having a higher viscosity. All real fluids (except superfluids) have some resistance to shear stress, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid (Symon 1971).

Answer 3

viscosity is the degree of how freely something flows...fluids with a high viscosity are more solid than those with a low level of viscosity. hope this helps

Answer 4

its basically just the thickness of a liquid. ex: grape jelly has a high viscosity. water has a low viscosity

Answer 5

this term is used for fluids only (liquids and gases)
it is the 'density' or hardness or the abiility to flow of a material.

eg: honey in natural state and temperature flows slowly as compared to say water or orange junice, thus honey is more viscous than orange juice.

this phenomenon is due to the stronger interlocking between the molecules of a material which retards the swift motion

Answer 6

Viscosity is a measure of the resistance of a fluid to deform under shear stress. It is commonly perceived as "thickness", or resistance to pouring. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Thus, water is "thin", having a lower viscosity, while vegetable oil is "thick" having a higher viscosity. All real fluids (except superfluids) have some resistance to shear stress, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid [edit] Newton's theory

Laminar shear of fluid between two plates. Friction between the fluid and the moving boundaries causes the fluid to shear. The force required for this action is a measure of the fluid's viscosity. This type of flow is known as a Couette flow.
Laminar shear, the non-linear gradient, is a result of the geometry the fluid is flowing through (e.g. a pipe).In general, in any flow, layers move at different velocities and the fluid's viscosity arises from the shear stress between the layers that ultimately opposes any applied force.

Isaac Newton postulated that, for straight, parallel and uniform flow, the shear stress, τ, between layers is proportional to the velocity gradient, ∂u/∂y, in the direction perpendicular to the layers, in other words, the relative motion of the layers.

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Here, the constant μ is known as the coefficient of viscosity, the viscosity, or the dynamic viscosity. Many fluids, such as water and most gases, satisfy Newton's criterion and are known as Newtonian fluids. Non-Newtonian fluids exhibit a more complicated relationship between shear stress and velocity gradient than simple linearity.

The relationship between the shear stress and the velocity gradient can also be obtained by considering two plates closely spaced apart at a distance y, and separated by a homogeneous substance. Assuming that the plates are very large, with a large area A, such that edge effects may be ignored, and that the lower plate is fixed, let a force F be applied to the upper plate. If this force causes the substance between the plates to undergo shear flow (as opposed to just shearing elastically until the shear stress in the substance balances the applied force), the substance is called a fluid. The applied force is proportional to the area and velocity of the plate and inversely proportional to the distance between the plates. Combining these three relations results in the equation F = μ(Au/y), where μ is the proportionality factor called the absolute viscosity (with units Pa·s = kg/(m·s) or slugs/(ft·s)). The equation can be expressed in terms of shear stress; τ = F/A = μ(u/y). u/y is the rate of shear deformation and can be written as a shear velocity, du/dy. Hence, through this method, the relation between the shear stress and the velocity gradient can be obtained.

In many situations, we are concerned with the ratio of the viscous force to the inertial force, the latter characterised by the fluid density ρ. This ratio is characterised by the kinematic viscosity, defined as follows:

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James Clerk Maxwell called viscosity fugitive elasticity because of the analogy that elastic deformation opposes shear stress in solids, while in viscous fluids, shear stress is opposed by rate of deformation.

Viscosity's Measurement
Viscosity is measured with various types of viscometer, typically at 25 °C (standard state). For some fluids, it is a constant over a wide range of shear rates. The fluids without a constant viscosity are called Non-Newtonian fluids.

In paint industries, viscosity is commonly measured with a Zahn cup, in which the efflux time is determined and given to customers. The efflux time can also be converted to kinematic viscosities (cSt) through the conversion equations.

Also used in paint, a Stormer viscometer uses load-based rotation in order to determine viscosity. It uses units, Krebs units (KU), unique to this viscometer.


Units
Viscosity (dynamic viscosity): μ
The SI physical unit of dynamic viscosity (Greek symbol: μ) is the pascal-second (Pa·s), which is identical to 1 kg·m−1·s−1. In France there have been some attempts to establish the poiseuille (Pl) as a name for the Pa·s but without international success. Care must be taken in not confusing the poiseuille with the poise named after the same person!

The cgs physical unit for dynamic viscosity is the poise (P) named after Jean Louis Marie Poiseuille. It is more commonly expressed, particularly in ASTM standards, as centipoise (cP). The centipoise is commonly used because water has a viscosity of 1.0020 cP (at 20 °C; the closeness to one is a convenient coincidence).

1 poise = 100 centipoise = 1 g·cm−1·s−1 = 0.1 Pa·s. Definition
1 centipoise = 0.001 Pa·s.

[edit] Kinematic viscosity: ν = μ / ρ
Kinematic viscosity (Greek symbol: ν) has SI units (m2·s−1). The cgs physical unit for kinematic viscosity is the stokes (abbreviated S or St), named after George Gabriel Stokes . It is sometimes expressed in terms of centistokes (cS or cSt). In U.S. usage, stoke is sometimes used as the singular form.

1 stokes = 100 centistokes = 1 cm²·s−1 = 0.0001 m²·s−1.
Conversion between kinematic and dynamic viscosity, then, is given by νρ = μ. For example, if ν=1 St and ρ=1000 kg/m3 then

μ=νρ=0.1 kg·m−1s−1 = 0.1 Pa·s. [1]
for a plot of kinematic viscosity as a function of absolute temperature see James Ierardi's Fire Protection Engineering Site


Molecular origins
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. Although these expressions are each exact in order to calculate the viscosity of a dense fluid, using these relations requires the use of molecular dynamics computer simulation.


Gases
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 behaviour of gaseous viscosity, in particular that, within the regime where the theory is applicable:

Viscosity is independent of pressure; and
Viscosity increases as temperature increases.

Liquids
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 is independent of pressure (except at very high pressure); and
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 of materials
The viscosity of air and water are by far the two most important materials for aviation aerodynamics and shipping fluid dynamics. Temperature plays the main role in determining viscosity.

Viscosity of air
The viscosity of air depends mostly on the temperature. At 15.0 °C, the viscosity of air is 1.78 × 10−5 kg/(m·s). You can get the viscosity of air as a function of altitude from the eXtreme High Altitude Calculator


Viscosity of water
The viscosity of water is 8.90 × 10-4 Pa·s or 8.90 × 10-3 dyn·s/cm2at about 25