A Guider’s Primer to Fluid Dynamics

We often take water for granted. It and other liquids are plentiful on Earth, but actually quite rare in the universe. This is because liquids, such as water, can only exist within a very narrow range of temperature and pressures.

Liquids are a type of fluid, meaning they can flow to take the shape of any container you pour them into. They exist somewhere between a solid and gaseous states and can contain multiple types of molecules. The atoms in liquids stick together due to special intermolecular glue knowns as cohesive forces. The interplay between forces acting on liquids are responsible for many interesting properties of liquids, as we will explore here.

Tie-Dye Milk

Materials

Pick Up Kit

• Some liquid food colouring in several colours
• Cotton Swabs

At Home Materials

• Shallow dish or plate
• Small cup or bowl
• Milk
• Dishwashing liquid or liquid hand soap
• A tray or newspapers to work on (optional)
• Wear old clothes, as this activity may stain them

What to Do

1. Add enough milk to cover the bottom of the dish. Works best with a thin layer.
2. In the small cup or bowl, mix together 1 tbsp of water and 1 tbsp of the dish liquid (some detergents may work better than others).
3. Put several drops of food colouring into the milk. Space them out in the milk so you can see what happens when you break the surface tension.
4. Dip a Q tip into the dish soap mixture and then touch the wet swab to the milk. Do not stir! The detergent will break the surface tension of the milk and the food colouring will swirl around as if by magic!
5. You can keep re-wetting your Q tip with the soapy water and touching it to the milk. Sometimes it works to touch the swab to the bottom of the plate and hold it there for a few seconds.

What to Talk About

• Imagine that the surface of liquids is a stretched elastic skin, like the surface of a balloon full of air. The scientific name for the way the “skin” of a liquid holds together is surface tension.
• When the skin of the liquid is broken by the detergent, food colouring and milk move and swirl around in interesting patterns on the milk’s surface.
• Let’s wonder how the fat content of milk affects the surface tension. If you are delivering this activity virtually, find out if the Brownies used different varieties of milk (2%, Skim, Homo, etc.)
• Let’s wonder what would happen if we put more milk in our bowls, the next time.
• We can also wonder, does the concentration of the soap matter? What if we put a drop of undiluted dish detergent into our milk?

Ice Cube on a String

Materials

Pick Up Kit

• Piece of kitchen twine or yarn approx 15 cm long

At home Materials

• Ice Cube(s)
• Glass of room temperature water
• Scissors
• Salt

What To Do

1. Drop a few ice cubes into your water.
2. Try to pick up the ice cube using just the string. (It won’t work)
3. Dip the string into the water to wet it, lay it across the ice cube and sprinkle a generous amount of salt over the string/ice.
4. Wait a minute or two and now try to lift it. (it should work now)

What to Talk About

• Does this seem familiar to anyone? Does anyone put down salt outside after shovelling the walk?
  • Normally, ice melts and water freezes at 0C. Adding salt, however lowers the temperature at which ice can melt and water can freeze!
• Observe the ice and the string, what do you notice?
  • The salt makes the ice surrounding the string begin to melt, stealing heat from the surrounding water. The cold water then refreezes around the string, which allows you to lift it from the glass.
• Do you think this will work at ANY freezing temperature?
  • No. Different chemicals change the freezing point of water.
  • Salt can thaw ice at up until -9C.
  • Other deicing chemicals are used on our roads because they can work at much lower temperatures (up to -29C!)
• Let’s wonder if there are any other kinds of things we might try this activity with. Sugar? Epsom salts? Baking soda?