August 7, 2020
Race water droplets on a wax paper track
On your mark, get set, go!
Liquid water droplets are held together by a concept called cohesion, meaning the molecules that make up water are drawn together, creating the shape of the droplet.
In this activity, you will set up and run an exciting race, pitting water droplets against each other on a track of your own design. It is a simple activity, but it has infinite possibilities as you consider all the different shapes your track can take. Race to beat your best time or put your droplet up against someone else’s droplet for bragging rights.
As the water droplet races down your racetrack, it is exhibiting another concept called adhesion, which is how much (or in this case how little) the water droplet sticks to another object. As you conduct this activity, consider how it would work differently on work on different racetrack surfaces.
You’ll need about 5 minutes of prep time for this activity and can expect about 15 minutes of learning time. This is best suited for students in elementary school grades.
- Pipette or eyedropper (Note: Placing a piece of tape at the end of a straw and poking a pinhole will work as a pipette.)
- Copy paper
- Sheet of wax paper
- Food coloring (optional)
- Paper towel
Part I – Wax Paper vs. Copy Paper
- Cut out a piece of wax paper to be the same size as the copy paper.
- Mark a starting line and a finish line on each.
- Decorate each as a racetrack (optional).
- Fill pipette/eyedropper with water.
- Place a droplet of water at the starting line of the wax paper and the copy paper.
- With a partner, begin blowing each droplet with a straw to see which surface allows the water to move faster. You can also do this by yourself, timing with a stopwatch if you want.
- Reflect on what happened. On which paper did the water have less adhesion (less stickiness)? How do you know? Why do you think that is?
- Feel free to test other types of paper to see how they behave differently.
Part 2 – Wax Paper vs. Wax Paper
- Using wax paper, design a racetrack for your next round of races, with at least two lanes.
- Using your dropper or pipette, place a droplet of water at the starting line of each lane. Use food coloring in the water to make it look even more interesting.
- On “Go,” begin blowing the droplets, using a straw, down the racetrack to see which droplet gets to the finish line first. If you are doing this by yourself, feel free to use a stopwatch and race against time to see what your best time could be to complete the course.
- If you really want to challenge yourself, add more curves to your racetrack!
How to adjust for younger and older learners
For younger learners, have them practice using a straw to blow a drop of water on a sheet of wax paper. Ask them to share what they see and notice about the water droplet’s behavior. Does blowing harder or softer change how the water moves? Try drawing a circle or target zone on the wax paper. Challenge your child to try to get the water drop into the target zone. Practice using the new vocabulary adhesion and cohesion in a sentence.
With older learners, have them test different types of papers to see which papers have the most adhesion and the least. Additionally, have students record their times for each race, measure the distance traveled and calculate the speed of their droplets using d=rt formula, where “d “stands for distance, “r” stands for the (average) rate of speed and “t” stands for time.