Lotus Effect

The tiny bumps on the surface of lotus leaves help them repel water and stay clean.

Imagine painting your house with a paint that repels dirt and water. While your neighbours have to repaint regularly, your house stays clean. What is the 'miracle' of the coating on your wall? Material engineers and scientists took inspiration from the leaves of the lotus flower, which remain clean even when living in a muddy environment.

1. Ask pupils to split into groups.
2. Each group receives a collection of hydrophilic vs. hydrophobic surfaces (e. g. a piece of glass or a salad leaf vs a piece of plastic, kale, broccoli, brussel sprout, cabbage, tulip, water lily or nasturtium leaves).
3. Ask pupils to touch each leaf and discuss their different surfaces. Is there any difference?
4. Invite pupils to add a drop of water into their leaves and observe the result. What happens? After the water drops have rolled off, what do pupils notice about the different leaves? Are they dry or wet? Do the leaves interact with water in the same way? (No, salad leaves stay wet, while kale leaves are covered by water droplets).

Variation: Put water on two parts of a kale leaf, one untouched and the other rubbed. What happens? (the untouched surface will have balled up water drops; the rubbed surface will have flattened drops that stick to the leaf). Why?

Explain to pupils that in relation to water, leaves (surfaces) can be divided into two groups: hydrophilic (water loving = capture water, like salad leaves) and hydrophobic leaves (water hating = repel water, like lotus or kale leaves).

Introduce pupils to the lotus effect, and that it inspired scientists to develop innovative surface coatings. See video here.

Ask the pupils to come up with ideas how the lotus effect could be used in other fields and if this can help improve the sustainability of different products. See below for suggestions.

The surface of the lotus leaf contains microscopic bumps that prevent water molecules from adhering to the surface. Instead, the water rolls right off, and picks up any dirt or oil on the surface of the lotus leaf along the way (source: Ask Nature).

Simple explanation

Leaves in nature have different shapes, forms and surfaces. These are all adaptations to survive. Just imagine the leaves of a lotus plant ... they live in a muddy environment, what special strategy have they evolved to survive? Self-cleaning ability. The surface of the lotus leaf has lots of tiny bumps covering their surface. When it rains, water droplets find it hard to ‘grip’ onto the leaf, and instead join together into larger droplets and just run off, carrying any dirt on the leaf with them. This is called the lotus effect.

More detailed explanation

Scientific analysis of hydrophobic plants found that their surface is made up of waxy nanocrystals. The crystals form a water-repellent layer, which is increased by the roughness of the surface. This makes the surface superhydrophobic, and the contact angle between the leaf and the water drop is about 150 degrees (compared to an angle made by water droplets on glass that is almost zero).

Material scientists experiment with mimicking the lotus effect, creating surfaces that are waterproof and have self-cleaning properties. As the materials self-clean, the need for chemical cleaners is reduced which is better for the environment.

Examples of innovative paints and coatings using nanoscale structures on surfaces inspired by water-repellent and self-cleaning properties of natural organisms include: 

• StoColor® Lotusan® is a lotus-inspired paint that keeps buildings clean. Any dirt on the surface is simply carried away when it rains. (read more here) 
• GreenShield™ is an oil-, water-, and stain-repellant finish for upholstery fabric that also uses lotus-inspired nano particles. (read more here)   
• Nanotex company uses nanotechnology to improve performance of fabrics to repel rain, sleet and snow. (read more here)
• Lotus coating from NASA repels dirt and dust to protect space equipment. (read more here)

Some STEAM opportunities include:

• Consider how the properties and shape of different materials makes them suitable for different tasks.
• Asking questions and making observations.
• Carrying out simple tests.
• Making predictions.
• Apply learning to real world problems.

Watch this experiment on the lotus effect (watch here).

More details on hydrophobicity (read more here).

Read about hydrophobicity and biomimetic materials (read more here).