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Have you ever wondered why honey is thicker than water? Or why different liquids have different consistency?
For example, just pour some water and honey into different tumblers simultaneously. Did you notice any difference in the time taken by each liquid to reach the respective tumblers? Did honey take more time to reach the tumbler than water or vice versa?
This is because of an internal resistance between the molecules that prevent a liquid from moving. This thickness or friction of liquids is referred to as the viscosity of a liquid. So, we can say that honey is more viscous than water. Now, let us study the topic in detail.
Viscosity
As you have seen, more viscous fluid like honey tends to flow slowly, and less viscous fluid, for example, water, flows freely. Viscosity can be simply explained as the resistance caused due to friction between adjacent layers of a liquid that are in relative motion. So, the definition of viscosity would be a fluid’s resistance to flow. The SI unit of viscosity is Pascal-second(Pa.s) and the CGS unit is Poise(P).
They are related as
1 Pa.s=10 poise
Coefficient of viscosity formula
The coefficient of viscosity in mathematical terms can be represented as:
η = F . d / A . v
where,
F = is the amount of force required to maintain velocity gradient in between two consecutive layers of liquid per unit area.
A = area
d = distance between two consecutive layers of liquid present over each other
v = velocity
Generally, the viscosity of gas is less than that of liquid.
Units of coefficient of viscosity
SI Unit: The SI unit of the coefficient of viscosity “η” is Ns.m-2 or Pa.s, that is, Newton-second per square metre or Pascal-seconds.
CGS Unit: The centimetre-gram-second unit of the coefficient of viscosity “η” is Poise, or we can also say dyne-sec / cm2.
1 poise = 0.1 Pa.s
MKS Unit: The metre-kilogram-second unit of the coefficient of viscosity
“η” is kg m-1s-1
Dimension of coefficient of viscosity:
The formula of coefficient of viscosity is written as η = F . d / A . v = M L T-2. L / (L². L T-1) after solving we get, η = M L-1 T-1 which is equal to kg m-1s-1Factors affecting viscosity
Pressure and temperature are the major factors that affect viscosity. On increasing pressure, the viscosity of liquid molecules increases due to an increase in the resistance to the flow caused by increased interaction between the molecules. In contrast, the viscosity of gas molecules decreases with increasing pressure.
The viscosity of a liquid decreases with increasing temperature. For gases, it increases with increasing temperature because upon heating, liquids flow more freely and gases flow slowly. Still, the amount of matter is the same in both liquids and gases.
Law of viscosity
It states that the shear stress is directly proportional to the rate of shear strain. The rate of shear strain, in other words, implies velocity gradient.
Shear stress – the force induced to cause slippage between adjacent layers.
Shear strain – the ratio of displacement to an object’s original dimension due to stress. It is denoted by displacement along a layer divided by the height from the base.
Viscosity of water
The viscosity of water at room temperature (250C) is 0.89m Pa.s (Milli Pascal seconds). As mentioned earlier, the temperature directly influences the viscosity of a fluid, and hence at different temperatures, liquids show different values for viscosity. As temperature increases, viscosity decreases and vice versa.
Viscosity of air
The viscosity of air is highly dependent on temperature. The viscosity of air at 25 degrees Celsius is 18.5 μPa s (micro Pascal seconds), which is roughly 50 times smaller than the viscosity of water at the same temperature.
Conclusion
Viscosity can be defined as the internal resistance faced by a liquid or fluid substance, measured in terms of the shear stress of the fluid. When the viscosity of a fluid is absolutely zero, the liquid matter is considered the ideal non-viscous fluid. Due to the internal friction between the fluid layers as they slip past one another when fluid flows, the viscosity gauges resistance.
When there is an increase in temperature, the viscosity of liquids decreases. In contrast, the viscosity of gases increases with the rise in temperature. There is always some confusion between the two types of viscosity measures. Sometimes it is considered to be the same. A viscometer helps get a more accurate measure of viscosity.
