viscosity

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Viscosity

The viscosity of a fluid is an internal measure of its resistance to continuous deformation caused by sliding or shearing forces. It is commonly described as a resistance to pouring of a liquid, such as the viscosity different grades of automobile oil. Thus, it can be described as a measure of fluid friction.

Imagine a fluid between two flat plates; one plate is stationary and the other is being moved by a force at a constant velocity parallel to the first plate. The applied force per unit area of the plate is called the shear stress. The applied shear stress keeps the plate in motion and, when the plate velocity is steady, this shear stress is in equilibrium with the frictional and drag forces within the fluid.

All fluids, except superfluids (those with a complete absence of viscosity), contain some factor that pose a resistance to shear stress. Those fluids that have no resistance to shear stress are called ideal fluids.

The shear stress is proportional to the speed of the plate and inversely proportional to the distance between the plates. The proportionality factor between the shear stress and the velocity difference between the plates is defined as the coefficient of viscosity or simply the viscosity of the fluid. Thick fluids such as tar or honey have a high viscosity; thin fluids such as water or alcohol have a low viscosity.

In general, viscosity is a function of temperature and pressure; however, in some fluids viscosity is dependent on the rate of shear and time. When brushed on (sheared) quickly, fluids such as paint have a low viscosity and flow easily. After paint is applied, only the slow and steady pull of its weight causes it to flow; at this slow shear rate the viscosity of paint is high and it resists the tendency to flow or sag. Fluids that behave in this manner are called non-Newtonian fluids. Other examples are liquid plastics and mud. For gases and non-polymeric liquids like water, viscosity is independent of the fluids shear stress and history. These are called Newtonian fluids. In the case of gases, the viscosity increases with temperature because of the increased molecular activity at higher temperatures. Liquids, conversely, generally show decreasing viscosity with increasing temperature.

When large intermolecular forces are present, a liquid will tend to be thick or highly viscous. For example, glycerin is thick because it has a high capacity to form hydrogen bonds with itself. Liquids that have complex molecular structure viscosity will also tend to be higher because the molecules can become entangled.

Any material that flows will exhibits viscosity. The unit of measure typically used for viscosity is poise (P or Po) or centipoise (cP or cPo) (where one centipoise equals 100 poises; and one poise is also equal to 0.1 pascal-second). The unit of poise was defined around 1924, being named for French physician and physiologist Jean Louis Marie Poiseuille (17991869), who formulated mathematical expressions for the flow rate of fluids in circular tubes. The viscosity equation known as the Hagen-Poiseuille equation is named after him and Gotthilf Heinrich Ludwig Hagen (17971884).

Viscosity is determined by applying a continuous force in order to push one layer of a fluid so that it moves faster than another fluid layer. The magnitude of this constant force, which is often measured in dynes per square centimeter, is determined as that amount necessary to maintain a difference of velocity of one centimeter per second between two fluid layers that are one centimeter apart. The viscosity measurement for water is defined as 1 centipoise (0.0100 poises) at room temperature of 68° F (20° C). However, waters viscosity at its boiling point of 212° F (100° C) drops to 0.0038 poises. Oils are thicker, having values between 100 and 100,000 centipoises. High molecular weight polymers can have viscosity values in the millions of centipoises range.

Liquids can be classified by the type of flow behavior they exhibit. In Newtonian fluids, the shear stress is in direct proportion to the shear rate. This proportion means that the viscosity measurement will remain constant even after several viscosity measurements. Examples of Newtonian fluids include water, glycerin, and light oils. In non-Newtonian fluids, the viscosity measurements change depending on shear stress. Shear thinning fluids exhibit a reduced viscosity value as the shear rate is increased. Shear thickening fluids have higher viscosity values as shear rate is increased.

The viscosity of certain systems is time-dependent. These systems are called thixotropic, and are characterized by a difference in viscosity, depending on the speed at which the shear force is applied. An example of this type of system is ketchup. When left in a bottle undisturbed, ketchup remains thick. When enough force is put on the system, it immediately thins. Ketchup has a viscosity of about 50,000 to 70,000 centipoises.

Viscosity is an important quality control characteristic for a variety of products. It is used to ensure that such products as shampoos, hand creams, paints, and inks are stable. Additionally, it is used to measure the molecular weight of polymers and determine when a polymerization reaction is complete.

Lubricants are important in industrial and commercial applications because of their ability to flow easily under large changes in temperatures. Products, such as silicone oils, help to lubricate machinery in a wide variety of uses. They are also applied in various areas of the world, from those with extreme cold temperatures to those with extreme hot climates. The flow of liquids in pipes, the performance of oil-lubricated bearings in engines or oil-filled automotive shock absorbers, and the air resistance on a moving car or airplane are all dependent on the viscosity of the fluids involved.

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Viscosity

The viscosity of a fluid is a measure of its resistance to continuous deformation caused by sliding or shearing forces. Imagine a fluid between two flat plates; one plate is stationary and the other is being moved by a force at a constant velocity parallel to the first plate. The applied force per unit area of the plate is called the shear stress. The applied shear stress keeps the plate in motion and, when the plate velocity is steady, this shear stress is in equilibrium with the frictional and drag forces within the fluid. The shear stress is proportional to the speed of the plate and inversely proportional to the distance between the plates. The proportionality factor between the shear stress and the velocity difference between the plates is defined as the coefficient of viscosity or simply the viscosity of the fluid. Thick fluids such as tar or honey have a high viscosity; thin fluids such as water or alcohol have a low viscosity.

In general, viscosity is a function of temperature and pressure ; however, in some fluids viscosity is dependent on the rate of shear and time. When brushed on (sheared) quickly, fluids such as paint have a low viscosity and flow easily. After paint is applied, only the slow and steady pull of its weight causes it to flow; at this slow shear rate the viscosity of paint is high and it resists the tendency to flow or sag. Fluids that behave in this manner are called non-Newtonian fluids. Other examples are liquid plastics and mud. For gases and non-polymeric liquids like water, viscosity is independent of the fluid's shear stress and history. These are called Newtonian fluids. In the case of gases, the viscosity increases with temperature because of the increased molecular activity at


higher temperatures. Liquids, conversely, generally show decreasing viscosity with increasing temperature.

The flow of liquids in pipes, the performance of oillubricated bearings in engines or oil-filled automotive shock absorbers, and the air resistance on a moving car or airplane are all dependent on the viscosity of the fluids involved.

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viscosity The internal resistance of a substance to flow when a shear stress is applied. Quantitatively defined, it is the ratio of the shear stress to the strain rate, in units of pascal seconds (1 Pa s = 10 poise). Resistance to flow is caused essentially by molecular or ionic cohesion. In magmas, molecular cohesion can be very high, especially if the silica content is high as in rhyolite magmas, and a yield strength must be overcome before the magma can flow. The presence of solid crystals increases the effective internal cohesion, and dissolved gas reduces it. In general, basaltic magmas have lower viscosities than rhyolite magmas.

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vis·cos·i·ty / ˌviˈskäsitē/ • n. (pl. -ties) the state of being thick, sticky, and semifluid in consistency, due to internal friction. ∎  a quantity expressing the magnitude of such friction, as measured by the force per unit area resisting a flow in which parallel layers unit distance apart have unit speed relative to one another.

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viscosity Resistance to flow of a fluid because of internal friction. The more viscous the fluid, the slower it flows. Viscosity is large for liquids and extremely small for gases. It is measured in the SI unit of pascal seconds.

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viscosity Of liquids, resistance to flow.

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