An introduction to viscosity, fluid density is the measurement used to compare the weight of the same volume of various fluids. The same volume of two different substances will likely have widely varied mass, or weight. The official definition of density is: Density = mass of a substance / volume. The density of water is used as a baseline measurement: 100 ml of pure water always weighs 100 grams, or 100 / 100 = 1. Substances with a density less than one, such as oil, will float on water, whereas substances with a greater density than water, such as honey, will sink to the bottom of a water-filled container.
Viscosity is related to density, but the two properties are not identical. Density does not change and is not dependent on temperature whereas any fluid's viscosity is thermally dependent. Fluids with high levels of viscosity are more highly affected by temperature changes. This property can be demonstrated in the classroom by taking a number of fluids and measuring their viscosity or flow rates at cool temperatures, at room temperature and at high temperature. For example, students place equal amounts of water, vegetable oil, molasses and honey into four glass containers. The students then tip the containers over and measure the time it takes for the fluids to pour completely out of the glass containers. By completing this experiment three times, with fluids at significantly different temperatures, students will discover the relationship between temperature and viscosity.
Water and oil have significantly different density and viscosity levels. This can be demonstrated through two simple experiments. When water and oil are poured into the same glass container, the oil floats on top of the water. This demonstrates the water's density is higher than oil's. However, if both liquids are poured onto a desktop, the water will spread out and spill onto the floor much faster than the oil. This demonstrates the oil possesses a higher viscosity level than water.
Depending on a fluid's viscosity, some fluids are better suited for lubrication than others. Fluids with higher viscosity levels are more resistant to breakdown and therefore are better suited for lubrication. This property can be demonstrated with an old shoe and a rubber band or strip of elastic material. Attach one end of the rubber band to the old shoe. Place a small weight in the shoe and then pull on the shoe with the rubber band until the shoe begins to move across the floor. Repeat the experiment with water, oil and honey between the bottom of the shoe and the floor in successive experiments. During each experiment, measure the amount of force needed to pull the shoe across the floor and how far the shoe will travel before the force increases significantly because the fluid's lubricative properties are no longer decreasing friction between the shoe and the floor. This experiment will demonstrate the various viscosity levels and lubrication properties of the three fluids.