Loading....

# Viscosity

Origin of Viscosity and its Definition
It is a common experience of daily life that different liquids flow at different speeds. For example, water flows with greater speed than glycerol. Obviously, some sort of internal friction is operating which checks the flow of liquids and which varies from liquid to liquid. This internal friction in the case of liquids is primarily due to forces of attraction between the molecules. If we have a laminar flow of liquid in a tube, then the velocity of the layer just in touch with the side of the tube is zero and it increases as we proceed towards the center of the tube (fig). Thus, there exists what is known as the velocity gradient between different layers of the liquid.
Due to greater intermolecular attractions amongst molecules of a liquid, the molecules moving in any one layer will tend to impede the movement of the molecules in the adjacent faster-moving layer. As a result, the velocity of molecules in the faster layer decreases. Unless a force is applied along the layer in the forward direction, the velocity of the faster moving layer will go on decreasing and will ultimately prevents this decrease become zero. At this state the liquid will stop flowing. The internal friction that resists the flow of a liquid can be measured in terms of the tangential force that is needed to keep the speeds of different layers constant. This force F depends upon the following factors:
1. It is directly proportional to A, the area of contact of two adjacent layers. The larger the area of contact between the two layers, the large will be the effect of intermolecular attractions. Hence, the decrease in speeds will be more; consequently, a large force is required, to maintain constant speeds.
2. It is directly proportional to dx, the velocity difference between two adjacent layers. The larger the velocity difference, the larger will be the force required to maintain the difference a constant.
3. It is inversely proportional to dx, the distance between the two adjacent layers. The larger the distance, the lesser will be the effect of intermolecular attractions. Thus the decrease in speed will be less. Consequently, a lesser force will be required to maintain the speeds of different layers at constant values.