How to Moisturize and Seal Afro Haircuts

Fabrics Water, kohlrabi and lettuce leaves, wax paper, coal powder or soot, flour, finely divided garden soil, textile piece made of wool, textile piece with hydrophobic nano-coating (from the NanoboX of the VCI), water-based adhesive, self-adhesive fabric, paper 80 g, Plastic film, glass cleaner
equipment Slide, candle, pipette, glass plate approx. 20 × 30 cm made of picture frame, set square with angle measurement, scissors, bucket

 
Experiment series 1: lotus effect
 
If you can't get a lotus leaf, go to the weekly market. There you will find numerous vegetables whose leaves show the same effect. Kohlrabi or nasturtium leaves, for example, work extremely well. If such leaves are “soiled” with flour or soot, they can be easily cleaned with drops of water. In those places where the water droplets run, clean paths are formed on the leaf.
 

 


So that the lotus effect is better understood, the students are given observation and research tasks, for example in this order:
  • Put a drop of water on a piece of woolen textile and describe the shape of the drop.
  • Create droplets that best match the spherical shape!
  • Explain the shape of the water droplet based on the surface tension.
  • Why doesn't the drop go into the tissue?
  • Explain the disadvantage of this effect when washing.
  • How does the detergent solve the problem?
  • Soil a kohlrabi leaf with potato starch and let water droplets run over it.
  • Submerge the kohlrabi leaf in a bucket of water. Can it be wetted?
  • Carry out the same experiments with other leaves and materials (lettuce leaf, nano-coated piece of textile, wax paper, normal paper).
  • Experiment with different types of pollution (coal powder, flour, soot as a combustion product of paper, etc.).
  • Does a water-based glue adhere to the leaf?
  • How does the kohlrabi leaf solve the problem? Look at an electron microscope picture of the leaf surface of a lotus plant!
  • Do the dirt particles adhere better to the water or better to the leaf surface?
  • Explain the lotus effect in your own words!
  • Make a drawing how you explain the effect of a self-cleaning car paint in the model.
  • Additional task: Get the nasturtium seeds and grow the plant. Experiment with the leaves!
It is important that students vary the experiments and find the explanations themselves. This website, the associated CD-ROM or other media sources serve as help.

 
Experiment series 2: Adhesive effect on two solid substances

One can first introduce the gecko and its achievements. It would also be interesting to ask whether a gecko can walk on Teflon or on a glass plate coated with sand or flour. The answer to this is not given at first.
 
A 20x30cm glass plate made of real glass with beveled edges is placed at an angle (e.g. 30 ° inclination) so that a 2x2cm square of paper just sticks. You have to make sure that the glass plate is absolutely free of grease and that the pieces of paper cut out (80 gram paper) are completely flat. The pieces of paper are pressed down. The test series can also be carried out with plastic foils. Now ask the students to find out what affects adhesion:
  • Check if the piece of paper sticks or slips off!
  • How does the angle of inclination affect the result?
  • Does the adhesion change if the pieces of paper were previously "soiled" with potato starch?
  • Look for further ideas how the adhesion can be improved or worsened!
Adhesive films are available in office supplies. They stick to a shop window without glue and can be easily removed again. With these foils you can carry out similar experiments directly on a window.
 
 
Experiment series 3: Adhesion effect on a solid and water
 
A slide becomes sooty with a candle flame. Then you hold it a little at an angle and put drops of water from a pipette on it. The water droplets do not stick to the glass, but glide over the surface at lightning speed. In the film you can still see how the water droplets carry away soot particles that do not adhere firmly enough to the burnt-in soot layer.
 
 
 

The interesting question now is what happens when you let a drop of water run over a smooth glass surface. Which material exerts more "attraction" (more adhesion) on the water? The glass plate coated with soot or the glass alone? There are still many possible variations:
  • How do water droplets behave on other materials, for example on plastics, on textiles or other water-attracting substances, on different types of paper?
  • Can two glass plates be pulled apart like microscope slides if you put a layer of water between them? How do you get them apart?
 
Explanations on the phenomena level

A drop of water sticks differently depending on the material. If it does not adhere to it, its shape approximates a sphere and it glides over it more easily. This is the case with soiling or a layer of soot. If it adheres well, the spherical shape will flatten or be destroyed. This is the case on a clean glass plate. Filter papers are water-attracting, they soak up with water. If you put a layer of water between two glass plates, the adhesion is increased enormously.
 
The lotus leaf has knobs on its surface, which are covered with wax balls. These prevent contact with the water droplet. Even dirt particles do not find a good hold, they are washed off with the water droplets from the rain. This is how the self-cleaning principle works in nature. The lotus plant must absolutely prevent soiling on the leaves, as it needs a lot of sunlight to develop its large flowers.
 
The gecko has many hairs with split ends on its feet. The unbelievable sum of one billion such ends per gecko result in excellent adhesion to the ground. A damp surface improves this adhesive effect even more. Even the gecko slips on non-stick coatings such as Teflon. On the other hand, it adheres excellently to glass plates.
 


Enlarge image

Red wine on a hydrophilic cotton tea towel (left),
Red wine drops on a hydrophobic, nano-coated fabric (right)
 

Explanations using the technical terms
 
The adhesion is the reason why a material can adhere to a glass plate at all. There are forces that prevail between two materials. The fact that the water drop takes on a round shape is a result of the cohesion, i.e. those forces that occur within the drop. The spherical shape is energetically favorable for the water drop. The spherical shape forms the smallest possible surface, it is a result of the surface tension at the water-air interface. If the external adhesion becomes too strong, the spherical shape is abandoned because it is energetically less favorable. This is partly the case with a clean glass plate.
 
If you moisten two slides with water and place them next to each other, they can hardly be peeled off from each other. The adhesive effect is then very great. If you place a drop of water on a water-attracting (hydrophilic) tea towel, the cohesion in the drop is also overcome. It finds one Wetting instead, the drop of water spreads in the cloth.
 
With the lotus effect, the nub structure with the extremely water-repellent (hydrophobic) wax layer prevents any adhesion, so that no wetting can take place even in those places where the water droplets lie. A carbon black coating is also hydrophobic and prevents wetting because it still contains residues of unburned paraffin. In textiles coated with nanoparticles, 200nm small, hydrophobic nanoparticles sit close together and prevent dirt particles from coming into contact with the textile material. The adhesive effect on the dirt particles is so low that they can be washed off by normal rain.
 
The gecko uses the van der Waals forces, the intermolecular forces that act when the billion adhesive ends on the feet come into contact with a glass plate, for example. Since moisture increases the adhesion, geckos can hold on to slightly damp material even better. Non-stick coatings such as Teflon prevent adhesion, so that even the gecko slips.