Water Repellency

Have you ever looked at a lotus leaf on a rainy day? You may have noticed nothing special besides a large green leaf with some water drops on its surface. But have to ever wonder why these water drops never soak through the leaf? They are always floating on the surface. Likewise, ducks do not get wet but have water rolling down on their plumage.

How does it work? Water repellency can be found in few man-made products like Teflon, Zipel, or 3M Scotch Tape. The phenomenon, how ever, is common in nature. It is amazing that ducks and lotus leaves have such properties without an artificial water repellent coating.

Water repellency is a matter’s property to repel water. Its meaning is quite different from having water resistance or being water proof because it works by having surface tension.

The surface tension is force that liquid sustains the minimum surface area. This is caused by the attracting force in the molecules of liquid. Each molecule is pulled equally in all directions by neighboring liquid molecules, which results zero net force. On the surface of the liquid, the molecules are pulled inwards by other molecules under them. Therefore, all of the molecules at the surface are subject to an inward force of molecular attraction which can be balanced only by the resistance of the liquid to compression. Thus the liquid holds itself together to achieve the lowest surface area possible. This property is surface tension. Due to this property, water drops take the shape of spheres. The stronger surface tension is, the more round the sphere it forms. It becomes flattened if surface tension is weak.

What happens when the two materials with different surface tension meet? The material with stronger surface tension retains its shape and is repelled on the material with weaker one. In the opposite case, the material with weaker surface tension on the stronger one becomes absorbed or flattened. This is an amazing physical property that can be found in many substances of the universe.

Our ancestors understood this property well to invent many useful devices. One of these is papillote, oiled paper. Underlying this invention is the fact that oiled paper pushes out water instead of absorbing it. The reason is that surface tension of water is stronger than that of oil.
The more the discrepancy of surface tension is, the stronger the effect of water repellency. People became to find objects with weaker surface tension such as silicon and paraffin oil.

Although silicon and paraffin oil repel water effectively, these cannot repel similar oil components with similar surface tension. Oil repellency is quite more complicated than water repellency.

Teflon came out in 1938. Teflon is a compound of carbon and fluorine rounded with fluorine molecules with intensive chemical bond, which have less surface tension than oil.

Teflon forms fluorine layer on fabric with strong surface tension such as cotton and prevents it from getting stained with water or oil. The repellency becomes more effective as the fluorine compound molecules get smaller in size and numerous in their number.

Nanotex, which produces Nanocare, asserts that their fluorine molecules are in nano size. Accordingly, the company’s products with stand laundry washing and repel water effectively. These products are mentioned in the chapter,“Nanocare vs. Teflon”in more detail. The sizes of their molecules are around 100 nanometers. It is interesting that the actual size is quite bigger than that of the molecules in Teflon produced by Invista. So one may say that Nanocare is cheating by calling their products nano.

Mercury is the substance with the strongest surface tension on Earth. So it is repelled on the surface of all other matters and forms spheres.

The surface tension of water is 72 dyne/cm. Cotton has exactly the same surface tension of 72 dyne/cm. So cotton cannot repel water and pants made in cotton gets wet in a matter of seconds. Olive oil coating on cotton would result surface tension of 32 dyne/cm. Water on this cotton fabric would be repelled and form beads.

Synthetic fabrics such as nylon (46 dyne/cm) and polyester (43 dyne/cm) have weaker surface tension than water. So they naturally repel water. This is the reason why most bathing suits are made of synthetic fiber. If you want your cotton pants to repel olive oil, cover them with paraffin oil (26 dyne/cm) or silicon (24 dyne/cm). These are actual fabric coating methods. They are called“wax coating”and“silicon coating”.

The substance with the least surface tension is fluorine compound. So far there is no substance with lesser surface tension than this. Teflon made of this fluorine compound has 15 dyne/cm surface tension. So Teflon is invincible against water or oil!

Let’s get back to the lotus leaf. The leaf not only repels water but even prohibits it from making any contact. The water beads on the lotus leaf appear white, not transparent. The reason for this is that water reflects itself since it is not touching the lotus leaf. Total reflection is happening here.

Total reflection is an optical phenomenon that occurs when the light travels from a denser medium (water) to a lesser dense medium (air) at a greater angle than the critical angle. The light stops crossing the boundary altogether and is completely reflected.

The reason that the lotus leaf repels water is its oily surface with weaker surface tension than water. The lotus leaf’s surface tension is similar to that of olive oil, around 32 dyne/cm. So it naturally repels water with surface tension of 72 dyne/cm.

I believe enough has been said about water repellency. There is another interesting phenomenon with these water beads on the lotus leaf. How come they are floating on the leaf?

This is possible because there are micro-mini buds on the leaf’s surface. This bud is only about 1 nanometer. Water forms into beads over these buds and floats on them because they are much smaller in size. Empty space is formed between the buds and water drops and the total reflection occurs. So water is not even touch ing the leaf.

To be exact, the contact dimension of the lotus leaf and a water drop is less then 2~3%. So the water beads cannot stick but roll down. Unlike inert water drops on a plain surface, the water beads on the lotus leaf catch dirt and stains on the leaf when rolling down. The beads actually clean the leaf. This is called the lotus effect.

The core properties in the lotus effect are surface tension and minimization of the contact area. These two have to come together to make the cleansing effect work properly. Some say that the leaf’s buds are entirely responsible for creating the lotus effect but this is wrong. What if we pour alcohol on the leaf?

The surface tension of alcohol is 23 dyne/cm. Since the leaf’s surface tension is about 30 dyne/cm, alcohol will just wet the leaf as shown in the picture. Unlike water, alcohol does not become beads. If water is poured on alcohol, it will be repelled since alcohol has much less surface tension than water.

The surface tension is affected by the temperature. The lower the temperature is, the stronger the surface tension gets. Therefore, hot water has less surface tension than cold water. So cotton pants would get wet easier in hot water.