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Surface Tension and Life
Jan 1, 2014

Do you know how a steel blade can float on the water? Or how can some insects stride on a pond? How do your contact lenses stay in position on your eyes? And how does water reach the higher parts of plants?

While wandering near a creek, have you ever seen bugs walking on the surface of the water? Have you felt any resistance when you hit the surface of the sea with your palm? Have you ever thought about what causes these to happen?

The two examples of events given above happen to be related to "surface tension." This situation is described as the force per distance unit that is generated in the opposite direction of the direction of expansion between two different surfaces. It can take place in between two different liquid layers, as well as among liquid-gas and liquid-solid layers. For example, the surface tension of a liquid forms in the transitional region where liquid and gas molecules make contact. The source of this force generated on the liquid's surface is the intermolecular attractions that hold the liquid molecules together. Each molecule in the liquid is pulled via opposite but equal forces by neighboring molecules, thus no single force is acting on the molecules. However, the molecules on the surface are only surrounded by one side, therefore they are pulled inwards with a net force (Figure 1), causing a tension similar to an inflated balloon on the surface of the liquid.

When we look carefully to a stagnant pool of water in a container, the surface of the water seems to be covered with a thin layer of film, resembling a stretched membrane. In order for a substance to enter or leave the body of water successfully, it must puncture this membrane. In other words it has to overcome this intermolecular force. If a steel blade is laid horizontally on the surface of the water slowly, it floats despite that it is made of denser steel because it cannot overcome this surface tension. Surface tension is the principle responsible for the trampoline-like behavior of liquid surfaces. Many insect species created for aqueous habitats can maintain their lives on the water via their adapted leg parts. The best example of this is the water strider. This insect lives on water by taking advantage of water surface tension. Though the surface tension principle is a requirement to be on the water, it is also necessary that the strider not to stick to the surface. Therefore, this insect is also equipped with a paddle made of waxy hairs at the end of their legs (Figure 2).

Forces of cohesion and adhesion

The intermolecular force of a liquid among the same kind of molecules is called the "cohesion force," and intermolecular attraction between different types of liquid molecules is called the "adhesion force." These forces of adhesion and cohesion determine the behavior of a liquid in a container. If some mercury is put in a glass tube, because the cohesive forces among the mercury atoms is greater than the adhesive forces in between the glass container and the mercury, the mercury assumes a convex shape. Here, mercury has a tendency to reduce its contact with the glass and does not wet it. In contrast to mercury, when water is put inside the tube, the surface layer between the water and air takes an inward concave shape. This is caused by the greater adhesion force between the water and glass compared to the intermolecular cohesion forces of water. Water wets the glass since it has a tendency to spread towards the greatest surface possible (Figure 3).

When there is a thin layer of water or tea left in between a tea glass and its plate, the adhesion force glues the glass and plate together. Since the adhesion force is greater than the weight of the plate, the glass cup can be lifted together with the plate. Contact lenses also stay in position on the eyes without falling through the help of adhesion forces. Tears strongly pull both cornea and the contact lens together, holding it in place.

The capillary effect

A liquid inside a thin vertical tube is pulled upwards by the inner surface of the tube until the adhesion force becomes balanced with the liquid weight. This event is called the capillary effect or capillarity. Liquids naturally rise in narrow channels if there is sufficient adhesion force. This effect is enhanced in narrow tubes due to the smaller volume of the liquid, but reduced in wider tubes because of gravity. Therefore, there is an inverse ratio between the channel diameter and liquid height in capillarity.

The reason a sponge absorbs water effectively is the easy rise of water in the capillary openings of the sponge. In a similar fashion, there are small openings found in paper napkins and towels. When a napkin makes contact with a wet surface, water is pulled inside the small openings with capillary action, thus removing the water from the surface. This is because the adhesion force in between the napkin tissue and water is greater than the cohesion force of the water molecules. This principle is also utilized while getting blood samples with capillary tubes. In addition, the removal of continuously excreted tears by the capillary ocular ducts that extend into the nasal cavity is another example of this wise law.

Capillary action is also important for the transportation of water molecules from humid parts towards drier areas in soil, providing for the spread of water. The same principle is also vital to nourishment of trees. Every part of a tree encompasses capillary channels, all the way from the tips of the roots to very ends of the branches. Water molecules are transported to the leaves against gravity when they enter the tips of these capillary channels at the roots. Even though the adhesion forces between the water molecules and the root's tissues win the war against gravity, at a certain height, this force becomes equal to the gravitational pull, thus not allowing water molecules to climb higher. This is the ultimate height a tree reaches. Capillarity also affects internal water pressure of a tree, leaf size, photosynthesis, and other factors. This is why the leaves of a tree are usually bigger on lower branches compared to higher ones (Figure 4).

The surface tension of water is the highest among the known values of other liquids and this has very significant biological effects. If the surface tension of water was to be lower, like other liquids, it would not be able reach the higher parts of plants through capillary action, thus preventing the survival of taller plants. The vegetation waits patiently as nourishment is delivered to its roots. Water has been assigned a vital role in this service.

The water-dependent survival of plants is made possible through the capillarity and surface tension. Could this amazing phenomenon, in which the capillarity is on duty to water the leaves on the highest branches of the tallest trees to ensure the maintenance of life, take place via blind atomic interactions or accidental occurrences?

How do liquid droplets get their shape?

Objects with a wider surface will have a greater surface tension. Since the force of surface tension, acting on per unit distance, is equal to the surface energy per surface area, a wider surface requires greater accumulation of energy on the surface. All the matter in the universe tends to stay at a lowered energy level. Therefore, it is ideal for objects to reduce their surface area. When the surface area to volume ratio of the known geometric shapes is investigated, the smallest ratio is found to belong to a sphere. A small value of this ratio means the most reduced surface area per volume. Among enclosed containers of equal volume, a sphere is also the one with the smallest surface area. When two equal volume watermelons of spherical and cubical shape are peeled, the spherical one will produce the least amount of rinds.

Because of the reasons mentioned above, liquids take a droplet shape immediately when they fall, reducing their surface area. That is why a water droplet dripping from a faucet, a falling rain drop, and a droplet on a leaf are all in the shape of a sphere (Figure 5). It is the same principle that makes planets and other heavenly bodies resemble a globular form. This indeed points to an Almighty Power who plans the motions, positions, and assignments of all the objects, from particles to giants, managing and dispatching them as The Self-Existent One holding everything together.

Factors affecting surface tension

Temperature increase is directly proportional to a decrease in the surface tension in most liquids. When the temperature of a liquid rises, so does the kinetic energy of the particles in it, making these particles move faster. This leads to a weakened intermolecular attraction that binds molecules together. Since this change affects the particles at the surface, it decreases the tension. Improved soaking of hands and laundry can be achieved with warm water during cleaning because heat reduces the surface tension. This helps with better cleaning results in a shorter amount of time.

In a similar fashion, soap and detergents also reduce the surface tension of water. If a small soap bubble is placed on a water droplet, the droplet spreads away instantly. This indeed tells us that the soap bubble reduces surface tension.

If a substance dissolves in a pure material, surface tension is found to change depending on the solute and the solvent structure. For example, salt decreases the surface tension of water. Salt weakens the intermolecular bonds of the water molecules, and therefore reduces the cohesion and surface tension. That's why sea waves foam when they hit shore.

Can surface tension be associated with the ability of unconscious and primitive atoms as the principle behind many functions and tasks in the lives of plants and animals? Do such wondrous events happen by chance? Isn't this principle such a blessing of the One who easily provides what is necessary to all living things, nourishing them in time according to their needs?