THEORY

The distinctive mechanical characteristic of a fluid is that it deforms continuously under the action of a shear stress (Whittaker 14). The coefficient of viscosity is the ratio of the shear stress to the rate of change of the shear strain (Serway 411). How viscous one fluid is as compared to another can best be understood by describing an experiment often performed to measure the coefficient of viscosity. Consider the motion of a fluid between two parallel plates shown in Figure II-1. The bottom plate is stationary.

FIGURE II-1. A layer of liquid between two solid surfaces in which the lower surface is fixed and the upper surface moves to the right with a velocity, v.

Safety in the Lab

     Wear goggles at all times. Use gloves when handling the glassware. Read the Material Safety Data Sheets for all  chemicals being used.

Material Safety Data Sheets  are found at 

http://hazard.com/

Couette 

Flow

Explained

The top plate moves to the right under the action of an applied external force, F as shown in Figure II-1. If the fluid between the two plates is dishwashing soap, it is easy to slide the top plate over the bottom plate. If the fluid between the plates were changed to something like honey or tar, the task of sliding the top plate over the bottom plate would be more difficult.

    Note that in Figure II-1, the velocity of successive layers increases linearly from 0 to v as one moves from a layer adjacent to the fixed plates to a layer adjacent to the moving plate (Serway 411). Linear momentum of a molecule is the product of its mass and its velocity (Serway 210). The viscosity of a fluid arises from the transport of linear momentum by molecules moving continuously between the layers of fluid. (Atkins 818-823). Figure II-2 is an illustration of what is happening inside the fluid.

FIGURE II-2. When molecules with different linear momenta (represented by the lengths of the arrows) migrate into each others' regions they contribute to the viscosity of the fluid

Tips for Experimentation:

1. Use Antifreeze! It's green color would make it easier to observe the antifreeze as it passes the measuring lines when the viscometers are submerged in the water bath.

2. When viscometers are inserted into the ring stands, make sure that the viscometers are upright. If the viscometer is inclined, the gravity effects more points along the tube. This will shorten efflux times.