052 – Spread of a substance on the water surface

Propulsion by recoil  

Washing-up liquid or alcohol on water  

Introduction to:  

• Recoil principle  
• Surface tension  

Material:  

• Styrofoam board (thin, thickness approx. 2 mm, e.g. from a fast food box) / light wood / cardboard  
• Scissors / sharp knife  
• Washing-up liquid or liquid detergent / disinfectant / alcohol (e.g. ethanol, methanol, propanol)  
• Large container with water  

perhaps:  

• a piece of paper  
• Match  
• Glue  

Instructions:  

• Cut out a rectangular plate in the shape of a boat from the polystyrene.  
• Cut a thin channel with a hole at the beginning in the boat.  
• Decorate the boat as you like with a small sail.  
• Check if the boat floats.  
• Add a drop of dish soap to the hole.  

Observation:  

• The boat starts moving forward with a jerk.  

Explication: 

• Both a drop of dish soap and a drop of alcohol spread evenly on a water surface.  
• The reasons are differences in the density and surface tension as well as the specific surface energy of the substances.  
• The fuel can only spread backwards through the slot in the boat, resulting in a driving force to the front (recoil force). 
• After some time, the alcohol dissolves in the water and the experiment can be carried out again. Not so with washing-up liquid, where the experiment can only be carried out once.