Surface tension boats

by Fellowship Agency September 28, 2017

Explore surface tension with this fun and simple activity.


Learning outcomes

You will be able to:

  • Explore surface tension practically
  • Design experiments to determine what makes the fastest surface tension boat.
  • Make links to Newtons 3rd Law of Motion.

You will need:

  • 1 x Gratnells A3 (D3) or Art (A1) tray
  • Tap water
  • 10ml washing up liquid or liquid soap in a small beaker or Gratnells mini tray
  • 2 x 3ml plastic pipette
  • 1 x Recycled piece of thick card about A5 in size to make a boat
  • 1 x scissors
  • 1 x stopwatch (optional)
  • Camera or video recording equipment (optional)

What to do:

  • Place the A3 or Art tray on a flat surface and fill to ~3cm deep with tap water.
  • Cut out two boats from recycled thick card using the template provided.
  • Start your video recording device (optional).
  • Gently float both boats on the surface of the water at one edge of the tray, evenly space them to ensure each boat has space around it. Be careful not to disrupt the surface.
  • Using the plastic pipettes, simultaneously place one drop of liquid soap into the central reservoir space of each boat.
  • Observe the effect on the boats.
  • Stop your recording device.
  • Optional: Time how long it takes for your boat to reach the other side of the tray.

What is happening?

When the boat is placed gently on the surface of the water, it rests on the surface and is suspended by surface tension. Upon putting a drop of soap solution (detergent) into the reservoir space, the boat accelerates rapidly and stops only when it hits the tray wall. Soap molecules try to spread (disperse) evenly over the surface of the water. Since they are confined in the central reservoir of the boat, their only way out is through the narrow rear channel. Rapid movement of the soap molecules through the narrow channel creates a directional force which drives the boat forward. This is also linked to Newton’s 3rd Law of Motion; for every action, there is an equal and opposite reaction, i.e. soap molecules go one way, the boat goes the other.

Other things to try…

There are lots of potential variables you could investigate:

  • Size of boat
  • The material of boat – e.g. thick card, thin polystyrene, corriflute
  • Size and shape of the central reservoir and channel
  • Brand of soap
  • Concentration of soap
  • Amount of soap added
  • Weight of the boat – you could experiment by adding small weights like paperclips or using a thicker material to make it.
  • The shape of the boat – can you make your own template?
  • Measure the time taken for your boat to travel the length of the tray each time you change a variable.
  • How many repeats do you need? Do you need to use a fresh boat each time?
  • Tip: You will have to change the water regularly, once the surface tension has been disrupted with the detergent, subsequent tests will be less effective.
  • Share the videos and photographs of your experiments on social media using #WhatsInMyTray

Health & Safety

As with all Gratnells Learning Rooms What’s In My Tray Activities, you should carry out your own risk assessment prior to undertaking any of the activities or demonstrations.