Hands-on Activity Exploring the Lotus Effect

Learning Objectives

After this activity, students should be able to:

• Describe the behavior of water on superhydrophobic surfaces.
• Define "lotus effect" and describe its water-repellent and self-cleaning properties.
• Explore how damage to Nano-Tex^TM cloth affects its "lotus effect" properties.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

International Technology and Engineering Educators Association - Technology

State Standards

Materials List

Each group needs:

• 2 lotus leaf pieces, or other plant with a superhydrophobic surface (see note below); keep fresh leaves in a sealed plastic bag and refrigerated until ready to use
• 4 pieces of Nano-Tex™ cloth, 1 small (3 x 3 cm) and 3 larger (10 x 10 cm); see note below
• cup of water
• pipette
• dirt
• lab gloves
• tweezers
• ice cube
• Petri dish
• The Lotus Effect Worksheet, one per person

Note about leaves: The leaves of the following plants also demonstrate superhydrophobicity: broccoli, Brussels sprout, cabbage, collard greens, kale, taro (elephant's ear), tulip, turnip greens, and water lily. It is best if leaves are fresh, or have been kept refrigerated since being cut. Alternatively, purchase dried lotus leaves from an Asian grocery, although the dried leaves do not work well for the third section of the lab (Motion and the Lotus Leaf). Keep in mind that the oils from your hands will eventually destroy the effect; wear gloves if you plan to use the same leaf often.

Note about Nano-Tex™ cloth: Purchase garments made of Nano-Tex™ cloth at the stores listed at https://www.nanotex.com/. We used a $15 Ashworth Nano-Tex shirt purchased from Amazon. Other companies make products that demonstrate similar water- and stain-repellent effects.

Worksheets and Attachments

Introduction/Motivation

(Begin with the associated lesson, and its Introduction/Motivation talk, to set the stage for conducting this activity with students. The lesson provides photos, short videos and background information on superhydrophobic surfaces, including condensation and the Wenzel and Cassie Baxter wetting states, and real-life engineering applications.)

How many of you have ever spilled something on yourself at lunch? If you do that at school, you just have to make do with a stain on your shirt until you get home. (Students probably have a few good stories to share about this.) What if you could make your clothes out of something that could not stain?...So whatever you spilled on yourself would just run right off and leave you clean and dry? Well today we are going to do just that.

Lotus plants (as well as some other plants) have a special surface on their leaves that allows water to flow right off of it without sticking, and take any dirt on the leaf off with it. This is an example of biomimicry — which is when we study a process or object in nature and figure out how to "mimic" its effect or properties with a synthetic product. Right now, scientists and engineers are learning how to copy the superhydrophobicity of lotus leaves and apply it to everything from clothing to outdoor paints.

(At this point, conduct the pre-lab questions activity as described in the Assessment section.)

In this lab, you will observe the superhydrophobicity phenomenon on a natural item and a synthetic material designed to mimic nature.

Procedure

Background

These are suggested procedures, which you may need to alter, depending on student level, time constraints, and availability of materials.

Before the Activity

• Cut pieces of lotus leaf (or other plant with a superhydrophobic surface). Seal fresh leaves in a plastic bag and refrigerate until ready to use.
• Cut pieces of Nano-Tex™ cloth.
• Gather materials and organize the lab stations.
• Make copies of The Lotus Effect Worksheet.

With the Students

Have students perform this lab at their desks, or group students into pairs to work at lab benches.

To guide students' lab investigations, hand out the worksheets, which include instructions and procedures, a table to record data and observations, and questions to answer as they go.

A. Pre-Lab

Place an ice cube in a small dish and lay a piece of a lotus leaf on top of it (see Figure 1). Set aside for Part C. While students are working on Part B, condensation will form on the leaf (see Figure 6).

B. Water and the Lotus Leaf

Have students perform the following experiments on the lotus leaf, recording their observations on the worksheets. Then, do the same using the piece of Nano-Tex™ cloth.

• Place a drop of water on it and observe its behavior (see Figure 2).

  

• Submerge it into a glass of water and observe its appearance (see Figure 3).

  

• Observe the item after it is removed from the water (see Figure 4).

  

• Sprinkle dirt (or pepper or glitter) on the item. Then place a few drops of water on it and tilt it so that water runs through the dirt (see Figure 5).

  

C. Motion and the Lotus Leaf

• Have students retrieve the lotus leaves on ice cubes that were set aside earlier.
• For this part of the experiment to work, at least a few water droplets about 1 mm in diameter must be on the leaf. If not, have students breathe gently on the lotus leaf and ice, as if trying to fog up a window. It may take a few breaths.
• Tilt the leaf to the side slowly. Expect the water droplets to stay in place (see Figure 6).
• Return the leaf to the horizontal orientation and introduce mechanical energy by using your other hand to gently tap the leaf or flick it with the tweezers. Watch the drops make the transition from the "sticky" Wenzel state to the superhydrophobic Cassie-Baxter state and bounce off the leaf.

  

D. Damage and the Lotus Effect

If you scratch the Nano-Tex™ cloth deep enough to make visible marks, the scratched area is no longer superhydrophobic and water sticks to the fabric at that spot (see Figure 7).

• Have student teams determine ways in which they can damage the larger pieces of cloth safely, and define "slightly," "moderately" and "highly" damaged.
• Have students label the three larger pieces of cloth "slightly," "moderately" and "highly" damaged and proceed to damage them accordingly.
• Have students pipette water onto each cloth and record how the damage affected the superhydrophobic ability of its ability to shed water.
• Have students describe situations in which the damaged cloth may be too fragile to be used.

  

Conclustion

• After the lab is finished, lead a class discussion comparing results and answers.
• Assign students to write answers on separate sheets of paper, to the two concluding questions of on the worksheet.

Assessment

Pre-Activity Assessment

Pre-Lab Questions: After the introductory talk, but before conducting the lab, have students write on note cards their answers to the following questions.

1. How are hydrophobic and hydrophilic different?
2. What is the lotus effect?
3. How is the lotus flower's importance in Asian religions related to this effect?

Activity Embedded Assessment

Worksheet Observations: Circulate throughout the classroom, checking each student's observations, as recorded on the The Lotus Effect Worksheet table. Ask questions about student answers to gauge their comprehension, and help them gain clearer pictures of the concepts in the lab and their extensions to real-world engineering problems and products (such as self-cleaning, stain-repellent garments).

Post-Activity Assessment

Concluding Questions: Assign students to write answers (on separate sheets of paper), to the concluding questions of the lab.

• The Nano-Tex™ cloth is an example of biomimicry. The self-cleaning properties of the lotus plant were studied and a synthetic version of its surface was developed. When this treatment is added to fabric, the garments repel dirt and stains just like the lotus plant. What other products could benefit from a self-cleaning surface? Include at least three examples and explain how the self-cleaning ability would be especially useful for each application.
• Water droplets formed from dew or other condensation may act differently on supherhydrophobic surfaces than water poured or dropped on these surfaces. Does this limit the applications for self-cleaning surfaces? How could some products be modified to help self-cleaning products work even for water condensation? Write at least one paragraph to answer each question.

Share: Ask students to share what they learned about the lotus effect with their families.

Troubleshooting Tips

• Direct students to avoid handling the lotus leaf with their hands, as the oil from their skin destroys the superhydrophobic effect. Use lab gloves and/or tweezers.
• If using fresh cuttings, store them in a sealed plastic bag in a refrigerator until ready to use. The superhydrophobic effect lasts about three hours after the leaves are removed from the refrigerator.
• Instead of dirt, consider using pepper or glitter for the self-cleaning demonstration.
• For Part B, dried lotus leaves can be successfully used when students test the superhydrophobic properties of the leaf and Nano-Tex™ cloth. However, dried lotus leaves work poorly for the condensation demonstration in Part C.
• For Part C to work, the water droplets from condensation must be at least 1 mm or larger. If you have time and space, place the leaves on the ice cubes 1-2 hrs before class to give water droplets more time to form. As mentioned elsewhere, the size of the water droplets can also be increased by students breathing on the lotus leaves and ice cubes, as if trying to fog up a window or mirror.

Activity Extensions

Use plants and fabrics that are not superhydrophobic to show how water behaves differently on them.

References

Copyright

Contributors

Supporting Program

Acknowledgements

This digital library content was developed under an NSF CAREER Award (CBET- 08-46705) and an RET supplement (CBET-10-09869). However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.