Definition of Viscosity: The viscosity of viscosity of a fluid a fluid is a measure of its resistance its resistance to gradual deformation by shear by shear stress or tensile tensile stress. For stress. For liquids, it corresponds to the informal notion of "thickness".
Viscosity is a property arising from friction from friction between neighboring particles in a fluid that are moving at different velocities. different velocities. When When the fluid is forced through a tube, the particles which comprise the fluid generally move faster near the tube's axis and more slowly near its walls: therefore some stress, some stress, (such (such as a pressure a pressure difference between the two ends of the tube), is needed to overcome the friction between particle layers and keep the fluid moving. For the same velocity pattern, the stress required is proportional to the fluid's viscosity. A liquid's viscosity viscosity depends depends on the size size and shape shape of its particles particles and the attractions between them. A fluid that has has no resistance resistance to shear shear stress is known as an ideal at very low fluid or fluid or inviscid fluid. fluid. Zero viscosity is observed only at very temperatures, in temperatures, in superfluids. superfluids. Otherwise, Otherwise, all fluids have positive viscosity. If the viscosity is very high, for instance in pitch, in pitch, the the fluid will appear to be a solid in the short term. A liquid whose viscosity is less than that of water is sometimes known as a mobile liquid, mobile liquid, while a substance with a viscosity substantially greater than water is called a viscous liquid. viscous liquid.
Definition of Dynamic (shear) viscosity: The dynamic (shear) viscosity of viscosity of a fluid expresses its resistance 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 a Couette flow, where flow, where a layer of fluid is trapped between two horizontal plates, one fixed and one moving horizontally at constant speed . (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 small enough, the fluid particles will move parallel to it, and their speed will vary linearly from zero at the bottom to 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.
Definition of Kinematic viscosity: The kinematic viscosity is the ratio of the dynamic viscosity μ to the density of the fluid ρ.
Unit of Dynamic Viscosity: The SI physical unit of dynamic viscosity is the pascal-second (Pa·s), (equivalent to (N·s)/m2, or kg/(m·s)). If a fluid with a viscosity of one Pa·s is placed between two plates, and one plate is pushed sideways with a shear stress of one pascal, it moves a distance equal to the thickness of the layer between the plates in one second.
Unit of Kinematic Viscosity: The SI unit of kinematic viscosity is m2/s.
Viscosity in Fluids: Most common fluids, called Newtonian fluids (yes, another thing named after that Newton), have a constant viscosity. There is a greater resistance as you increase the force, but it's a constant proportional increase. In short, a Newtonian fluid keeps acting like a fluid, no matter how much force is put into it. In contrast, the viscosity of non-Newtonian fluids is not constant, but rather varies greatly depending on the force applied. A classic example of a non-Newtonian is Oobleck, which exhibits solid-like behavior when a large amount of force is used on it. Another type of non-Newtonian fluid are known as magneto rheological fluids, which respond to magnetic fields by becoming nearly solid but reverting to their fluid state when removed from the magnetic field.
Fluidity: 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 (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 and , 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 is the mole fraction of component a and b respectively, and μ a and μ b are the components' pure viscosities.
Non-standard units: 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 to characterize lubricating oil in the automotive industry. 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). Other abbreviations such as SSU (Saybolt Seconds Universal ) or SUV (Saybolt Universal Viscosity ) are sometimes used. Kinematic viscosity in centistoke can be converted from SUS according to the arithmetic and the reference table provided in ASTM D 2161.
Viscosity of selected substances: Air : Pressure dependence of the dynamic viscosity of dry air at the temperatures of 300, 400 and 500 K. 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 m2/s or 14.8 cSt. At 25 °C, the viscosity is 18.6 μPa.s and the kinematic viscosity 15.7 cSt. One can get the viscosity of air as a function of temperature from the Gas Viscosity Calculator.
Water : Dynamic viscosity of water: The dynamic viscosity of water is 8.90 × 10−4 Pa·s or 8.90 × 10−3 dyn·s/cm2 or 0.890 cP at about 25 °C. Water has a viscosity of 0.0091 poise at 25 °C, or 1 centipoise at 20 °C. As a function of temperature T (K): (Pa·s) = A × 10B/(T −C ) where A=2.414 × 10−5 Pa·s ; B = 247.8 K ; and C = 140 K. Viscosity of liquid water at different temperatures up to the normal boiling point is listed below:
Temperature
Viscosity
[°C]
[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
The Physical Basis of Viscosity:
Viscosity is a measure of the ease with which molecules move past one another. It depends on the attractive force between the molecules. It depends on whether there are structural features which may cause neighboring molecules to become "entangled". Viscosity d e c r e a s e s with increasing temperature - the increasing kinetic energy overcomes the attractive forces and molecules can more easily move past each other.
What factors can affect viscosity? · temperature · concentration · attractive force · particle size
1-Temperature: LIQUIDS AND GASES REACT OPPOSITELY TO CHANGES IN TEMPERATURE. In liquids, when the temperature increases (energy added) the particles move faster and begin to move away from each other. Because the particles are moving around more they can flow more; their viscosity is lower. In gases, the particles are far apart so when energy is added the particles move faster and collide with each other more often causing an increase in viscosity
2-Concentration: · Concentration is the amount of substance that is dissolved in a specific volume. · An increase in concentration will usually result in an increase in viscosity.
3-Attractive Force: · Particles of the same substance have an attractive force on one another. · Some substances have a strong attraction while some substances have a weaker attraction. · The stronger the attraction of particles, the higher the viscosity.
4-Particle Size: · The size of the particles of a substance will greatly affect its viscosity. · Small particles can move more easily past each other and can therefore flow faster, meaning they have a lower viscosity. · Large particles would mean a higher viscosity.
MEASURING VISCOSITY: Measuring viscosity is an effective way to know the state (properties of matter) or fluidity of a liquid or gas. It plays an important role in the quality control and in various research and development stages of a wide range of industries, including Food, Chemical, Pharmaceutical, Petrochemical, Cosmetics, Paint, Ink, Coatings, Oil and Automotives. For example, the viscosity of a liquid is an important parameter for designing the piping in a plant or transporting crude oil or chemical agent through a pipeline. In the electronic engineering industry, photresist fluid is used in the production processes of the printed circuit board, cathode-ray tube, and flat liquid crystal display. Controlling the viscosity of photoresist fluid is a crucial factor to determine the qualities, performance, and yields of finished products. Among those industries, it has been also recognized that controlling optimum viscosity reduces production costs. Viscous fluid is divided into two broad categories: ■Newtonian fluid, subject to Newton’s law of viscosity, viscosity is constant regardless of the flow (movement). ■Non-Newtonian fluid, which is not subject to Newton’s law of viscosity, viscosity changes according to the flow (movement). Note: subject to a constant temperature. Vibro Viscometer: The figure 1 shows the basic mechanism of one of the vibro 0viscometers, A&D’s Sine-wave Vibro Viscometer. When the spring plates are vibrated with a uniform frequency, the amplitude varies in response to the amount of frictional force produced by the viscidity between the sensor plates and the sample. To produce uniform amplitude, the vibro viscometer controls the electrical current that drives the vibration of the spring plates.
Figure 1 Rotational Viscometer: A motorized cylindrical rotor is inserted into a sample and rotated at a constant speed. The rotational viscometer employs the measurement method that assumes viscosity is directly proportional to a running torque required to produce a steady rotating motion. As shown in figure 2, when the rotation becomes steady, the running torque caused by the viscosity and the twist of the spring is balanced. The twist angle of the spring is proportional to the viscosity of the sample, and an index of this is displayed on the scale. Some devices display the digital value of the viscosity coefficient converted from running torque. The method shown in figure 2, the single cylindrical rotational viscometer.
Figure 2
Capillary Viscometer: In a capillary viscometer the laminar flow of liquid flows through a cylindrical capillary tube. You determine viscosity by measuring the flow rate of the fluid flowing through the capillary tube and the pressure differential between both ends of the capillary tube. This measurement method is based on the laws of physics; therefore, this is called the absolute measurement of viscosity. There is an another type of capillary viscometer, made of glass, is shown in figure 3. Although using this capillary tube is not easy, it has a simple principle and structure. Due to the simplicity of the principle, it has been used for a long time and has been greatly improved over the years. This capillary viscometer can obtain kinetic viscosity by measuring the time it takes for a certain amount of sample to flow by free-fall through the capillary tube.
Refernces :
1. http://en.wikipedia.org/wiki/Viscosity 2. http://physics.about.com/od/physicsutoz/g/Viscosity.htm 3. http://www.mikeblaber.org/oldwine/chm1045/notes/Forces/Liqui ds/Forces03.htm 4.
http://www.qclabequipment.com/VISCOSITY.html
