Fluid, both liquid and gas substances of different types, have different viscosity levels. Viscosity is a friction between the molecules that make up a fluid. So, the molecules make up fluid friction when the fluid flows. In liquids, the viscosity is caused by cohesion forces (pulling forces between similar molecules). Whereas in gas substances, the viscosity is caused by collisions between molecules.

Liquid fluid is easier to flow, for example, water. Conversely, thick fluid is more difficult to flow, for example, lubricant. You can prove by pouring water and lubricant on an inclined surface. Water flows faster than oil. The level of viscosity of fluid also depends on temperature. The higher the temperature of the liquid, the less viscous the liquid is. For example, when a mother fry fish in the kitchen, cooking oil which initially thickens becomes more liquid when heated. Conversely, the higher the temperature of a gas substance, the thicker the gas substance is.

Coefficient of Viscosity

Viscosity 1The symbol η symbolizes the viscosity of the fluid (read: eta). η = coefficient of viscosity. So, the viscosity level of liquid is expressed by the viscosity coefficient of the fluid.

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A thin fluid layer is placed between 2 plates. Cohesion is an attractive force between similar molecules, whereas adhesion is a force of attraction between different molecules. Adhesion forces work between the plate and the fluid layer attached to the plate (fluid molecules and plate molecules attract each other). Whereas, cohesion forces work between liquid membranes (fluid molecules attract each other).

Viscosity 2At first, the plate and fluid layer are at rest (figure 1). Thereafter, the plate on the top is pulled to the right (figure 2). The plates on the bottom are not pulled (the bottom plate is at rest). The magnitude of the pull force is arranged such that the plate at the top shifts to the right at a constant speed (v constant). Because there is an adhesion force acting between the edge of the plate and the part of the fluid attached to the plate, the fluid below the plate also shifts to the right. Because there is a cohesive force between fluid molecules, the fluid that moves to the right pulls the fluid that is at the bottom.

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The part of the fluid that is on the top pulls the fluid at the bottom to shift to the right,

whereas the part of the fluid at the bottom holds the fluid above, so the rate of the fluid varies. The part of the fluid that is at the top moves with a larger (v) rate, the fluid at the bottom moves with a smaller v, and so on. So, the lower down v, the smaller. In other words, the velocity of the fluid layer changes regularly from top to bottom as far as l.

Changes in the velocity of the fluid layer (v) divided by the distance of the change (l) = v / l. V / l is known as the speed gradient. Plates that are on the top can move because there is a pull force (F). For certain fluids, the amount of attraction required is directly proportional to the area of the fluid attached to the plate (A), the rate of the fluid (v) and inversely proportional to the distance l. Mathematically, it can be written as follows:

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Viscosity 3

Previously, it was explained that a thinner fluid is usually easier to flow, whereas a thicker fluid is more difficult to flow. The coefficient of viscosity expresses the level of viscosity of the fluid. If the fluid is thicker, the tensile force needed is also greater. In this case, the tensile force is directly proportional to the coefficient viscosity. Mathematically can be written as follows:

Viscosity 4

η = coefficient of viscosity, F = force, l = distance, A = surface area, v = speed, ∝ = comparison

The International System Unit (SI) for coefficient of viscosity is Ns/m2 = Pa.s (pascal second). The CGS unit (centimeters gram second) for the coefficient viscosity is dyn.s/cm2 = poise (P). Viscosity is also often expressed in centipoise (cP). 1 cP = 1/100 P. The poise unit is used in honor of a French scientist, Jean Louis Marie Poiseuille (read: pwa-zoo-yuh).

1 poise = 1 dyn. s/cm2 = 10-1 N.s/m2

Viscosity 5

Viscosity 6

Viscosity 7

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