# Find out more about Viscosity of Liquids

### The Point of the Experiment

An important property of drilling fluid, also known as mud, is viscosity. Inour experiments on the viscosity of liquids, we compare the viscosity of different liquids by measuring how quickly pebbles sink through them. These pebbles stand in for the rock cuttings that have to be carried to the surface by the drilling fluid.

In Drilling Fluid: Lifeblood of the Well, we follow drilling fluid as it is pumped down into the borehole through the hollow drill pipe. It emerges at the tip, then travels back to the surface in the annulus—the space between the outside of the drill pipe and the walls of the bore hole—carrying rock cuttings with it. The drilling fluid has to rise faster than the rock cuttings can sink through it if the cuttings are to make it the surface. A more viscous fluid slows the descent of the pebbles. On the other hand, a fluid with higher viscosity requires more pump pressure to keep it moving.

### The Activity

The pebbles used in this experiment should be fairly uniform in shape and size. We chose coarse aquarium gravel from a pet store. You will need graduated cylinder or other tall containers to hold the liquids and an accurate stopwatch to time the descent of the pebbles.

Corn syrup has a much higher viscosity than water so doing the experiment with these two liquids is likely to produce unambiguous results.

### Discussion and Further Explorations

Students may come up with other ways to compare the viscosity of liquids. For example, instead of timing how long it takes for an object to sink to the bottom of a container, you could use an object that floats on the liquid, such as a piece of wood. Using a stick or metal rod, push the object to the bottom of the container. Then let go and time how long it takes to float to the surface.

Another option is to put a few drops of liquid on a horizontal surface and then tilt the surface. The liquid will begin to flow downhill. The more viscous the liquid, the more slowly it will move.

Try this experiment with different liquids such as honey, glycerin, and oil.

Try a single liquid at different temperatures and compare the results. Most liquids are less viscous at higher temperatures. This is a problem in some situations.One example is an automobile engine, which is cool or cold when the car is turned off but gets very hot inside when it has been running for a while. The oil used to lubricate the engine has to maintain viscosity at high temperatures but not get so thick at low temperatures that it interferes with the operation of the engine. Multigrade oils include additives that help them retain a relatively uniform viscosity over a range of temperatures.

### Viscosity and Density

Density is mass per unit of volume. Corn syrup is denser than water. One liter of water has a mass of 1,000 g or 1 kg. One liter of corn syrup has a mass of about 1,380 g. It may seem that heavy, dense liquids are also more viscous, but this is not necessarily the case. Corn oil is less dense than water—about 930 g/L—but has a higher viscosity.

The "Buoyancy" experiment explores the density of liquids. It might be good to work on that one in conjunction with this exploration of viscosity.