SCIENCE WORKSHOP

Recreational science

Solid

or viscous liquid?

Analyse

• If we pour water, oil or honey on a surface, what happens? You can see that their behaviour is quite different: water, when poured, spreads more than oil and honey. This is because water is less viscous.

• If you have played with green slime, you may have noticed that it behaves differently from liquids and solids.

Introduction

One of the characteristics that allows us to recognise the three states of aggregation is viscosity, i.e., the resistance to flow of a gas or liquid. In the following experiment we will learn how to identify it.

Materials

• Starch powder • White glue • Food colouring • Water • A spoon • A glass • A bowl • A self-sealing bag.

Build

We are going to make homemade green slime.

• In half a glass of warm water, dilute a teaspoon of starch.

• In a bowl, add one tablespoon of this solution and two tablespoons of white glue. A few drops of food colouring can be added.

• Stir the mixture, first with a spoon and then with your fingers. Place the resulting paste in the self-sealing bag and put it in the refrigerator.

Experiment

• Does the paste flow more or less easily than honey? And compared to oil? Is it a liquid or is it a solid? Is viscosity the same as density?

• Pour the paste carefully into a glass and make it slide. Measure the time it takes to reach the edge. Do the same with the oil and the water; in this way, we can compare the viscosity of these materials.

• Knead the paste with your hands and roll it into a ball. We observe that the more force that is applied, the less it behaves like a liquid, i.e., its viscosity increases.

Conclusions

The paste we have prepared behaves like a very viscous liquid. Moreover, its viscosity increases when we subject it to some kind of stress, such as tensions, torsion, or if we hit it. This is why we say that some fluids, such as the one we have made, can behave like solids. These fluids are called non-Newtonian fluids, and they are very common. Look up information on everyday non-Newtonian fluids, and relate it to the behaviour of ketchup and chocolate.