Why is viscosity an important property of matter

While relative viscosity can give you a general number, tools that can measure dynamic viscosity or aboslute viscosity have been refined and proven to provide more reliable and accurate measurements. This is because absolute viscosity is given instead of values that compare fluids to one another. Interested in getting more information on how a viscometer can assist with your work?

All rights reserved. What is Viscosity. Viscosity is defined as the resistance to flow! Want More Information About Viscosity?

RheoSense, Inc. VROC vs. As Table 2 shows, the more structurally complex are the molecules in a liquid and the stronger the IMFs between them, the more difficult it is for them to move past each other and the greater is the viscosity of the liquid.

As the temperature increases, the molecules move more rapidly and their kinetic energies are better able to overcome the forces that hold them together; thus, the viscosity of the liquid decreases. The various IMFs between identical molecules of a substance are examples of cohesive forces. The molecules within a liquid are surrounded by other molecules and are attracted equally in all directions by the cohesive forces within the liquid.

However, the molecules on the surface of a liquid are attracted only by about one-half as many molecules. Because of the unbalanced molecular attractions on the surface molecules, liquids contract to form a shape that minimizes the number of molecules on the surface—that is, the shape with the minimum surface area.

A small drop of liquid tends to assume a spherical shape, as shown in Figure 2 , because in a sphere, the ratio of surface area to volume is at a minimum.

Larger drops are more greatly affected by gravity, air resistance, surface interactions, and so on, and as a result, are less spherical.

Surface tension is defined as the energy required to increase the surface area of a liquid, or the force required to increase the length of a liquid surface by a given amount. This property results from the cohesive forces between molecules at the surface of a liquid, and it causes the surface of a liquid to behave like a stretched rubber membrane.

Surface tensions of several liquids are presented in Table 3. Among common liquids, water exhibits a distinctly high surface tension due to strong hydrogen bonding between its molecules. A steel needle carefully placed on water will float. Some insects, like the one shown in Figure 3 , even though they are denser than water, move on its surface because they are supported by the surface tension.

The IMFs of attraction between two different molecules are called adhesive forces. Consider what happens when water comes into contact with some surface. For example, water does not wet waxed surfaces or many plastics such as polyethylene. Water forms drops on these surfaces because the cohesive forces within the drops are greater than the adhesive forces between the water and the plastic.

Water spreads out on glass because the adhesive force between water and glass is greater than the cohesive forces within the water. When water is confined in a glass tube, its meniscus surface has a concave shape because the water wets the glass and creeps up the side of the tube.

This produces a relative motion between the different layers of the liquid. Stoke performed many experiments on the motion of small spherical bodies in different fluids and concluded that the viscous force F acting on a spherical body of radius r depends directly on. Using dimensions, the above equation can be written as.

Since the raindrops are smaller in size and their terminal velocities are small, remain suspended in air in the form of clouds. As they grow up in size, their terminal velocities increase and they start falling in the form of rain. This law explains the following:. Poiseuille analyzed the steady flow of liquid through a capillary tube.

He derived an expression for the volume of the liquid flowing per second through the capillary tube. As per the theory, the following conditions must be retained while deriving the equation. Consider a liquid flowing steadily through a horizontal capillary tube. So, equating the powers of M, L, and T on both sides, we get.

We have three unknowns a , b , and c. We have three equations, on solving, we get. Therefore, equation 7. This relation holds good for the fluids whose velocities are lesser than the critical velocity v c. The importance of viscosity can be understood from the following examples. To select a suitable lubricant, we should know its viscosity and how it varies with temperature [Note: As temperature increases, the viscosity of the liquid decreases].

Also, it helps to choose oils with low viscosity used in car engines light machinery. He used the knowledge of viscosity to determine the charge. Developed by Therithal info, Chennai. Toggle navigation BrainKart. Home Physics 11th std Viscosity. Cause of Viscosity: Consider a liquid flowing through a horizontal surface with two neighboring layers. Coefficient of Viscosity: Consider a liquid flowing steadily over a horizontal fixed layer Figure 7.

First Name. Your Response. Associative property of addition B. Commutative property of addition C. Distributive property D.