Walking on Water

Water striders use surface tension to walk on water.

Have you noticed how some insects are able to walk on water? For example, you can often find water striders on the surface of ponds. How can they do this?

Water striders use a handy trick for this: surface tension. This tension arises because water molecules strongly attract each other, especially at the interface with air. The strength of the attraction at the water surface creates a thin layer, which can be used as a surface by water striders. Water striders have an additional adaptation that prevents them from sinking: the tiny hairs on their legs ensure that they retain air bubbles and further repel water.

The cleanliness of the water can affect the effectiveness of the jumping ability of water striders. Dirty water which contains pollutants (oils, detergents, etc.) or debris, can weaken the attraction between water molecules, which in turn reduces surface tension.

Pushpin: will it float, or will it sink? Discover for yourself how surface tension works.

• Give all students a pushpin/thumbtack (without plastic) and a glass.
• Clean the pushpin and glass with water only.
• Dry the pushpin.
• Place the pushpin, with the pin up, carefully in the glass of water.

Question 1: What happens?

• Add a drop of dishwashing soap in the glass of water.

Question 2: What happens now?

Question 3: Why do you think this is?

Alternative activity: students build an aluminum foil water strider around a paperclip and make it float, as can be seen here.

Simple explanation

Have you ever gone off the (high) diving board and landed badly? Painful huh? That is because the top layer of water is actually stronger than you think due to surface tension. Surface tension is a force which causes a layer of liquid to behave like a stretched elastic sheet or skin. The pushpin can stay on the water because of this top layer (surface tension). The detergent breaks the surface tension and the pushpin is too heavy to stay on the water without this strong layer.

More detailed explanation

Water molecules love each other. All molecules in a glass of water are attracted to each other (or pulled together) in the same amount, in all directions. The molecules at the surface are attracted to each other even more, because they don’t have water molecules on top of them. They get so close to each other that they form a strong layer, also known as ‘skin’. This skin prevents the pushpin (and other light enough items) from sinking. When adding soap, the soap particles come between the water particles, which makes the connection between the water particles weaker and the water loses its resilient layer, therefore, the pushpin sinks.

The surface tension of water arises because water molecules (H2O) strongly attract each other. This attraction forms hydrogen bonds (H bonds). The electrical charge of water particles is not evenly distributed, causing the positively charged front (H's) of one molecule to pull strongly on the negatively charged back (O) of the other molecule. The surface tension is so strong that it can carry insects and small objects that are heavier than water and would therefore sink. When soap is added to the water the surface tension decreases sharply, because many of the hydrogen bonds between water molecules are broken. The tail of a soap particle is repelled by water (is hydrophobic), leaving the particle on the surface. The bonds that soap particles form with water molecules are much weaker than the hydrogen bonds between the water molecules. In soap-contaminated water, the surface tension is too weak to support a pushpin or water strider, which will sink as a result.

Taking cues from the water strider, researchers have developed a robotic insect capable of walking on, and launching itself from, the water’s surface. The robotic version uses the same forces to jump as the water strider, pushing off without breaking the surface. It takes off with a downward force that never exceeds the surface tension of water.

Although the researchers wanted to explore a new possibility for a robot’s aquatic mobility, they can envisage an environmental application for their robotic water strider - monitoring pollution in waterways. In the future, environmental monitoring applications on dams, lakes, sea, etc. would become possible using a network of these robots with miniature sensors, an on-board power source and electronics.

Can you think of another application?

Some STEAM opportunities include:

• Identify the difference between the effects of air resistance, water resistnnce and friction.
• Observing and raising questions about how different animals adapt.
• Analyse advantage and disadvantage of different adaptations/behaviours.
• Carrying out simple tests.
• Making predictions.
• Using scientific evidence to answer a question.
• Apply learning to real world problems.

Can robots jump on water? (find out more, and here and here).

Learn more about the amazing Water Strider (find out more).