Many theological studies suggest that the gods in ancient Egypt could walk on water, and so could Buddha and Jesus. Humans associate this unique behavior with the divinity of gods. But few insects such as water striders in the animal kingdom make us wonder if it is the divinity that humans lack to walk on water or there is something more fundamental to the phenomenon itself. Many of us are also familiar with the trick of a floating needle or a paper clip on water. The scientific explanation for these phenomena is associated with the term “Surface Tension.” In general terms, surface tension is a property of a liquid that makes it behave as if its surface is enclosed in an elastic skin. The intermolecular cohesive forces among the molecules are responsible for this property. A molecule in the interior of a liquid experiences interactions with other molecules from all sides, but the molecules present on the surface are only affected by the molecules present alongside and the molecules present in the layer below them. Therefore, they experience a greater force of attraction than the molecules present in the interior.
In terms of force, surface tension (γ) is defined as the force (F) acting over the surface per unit length (L) directed perpendicular to the surface, with the units newton per meter.
γ = 1/2 × (F/L) {Nm}^{-1}
Whereas, in terms of energy, it is defined as the energy required to increase the surface area isothermally by one square meter, with units being joules per meter square.
γ = W/ΔA {Jm}^{-2}
Moreover, surface tension is an intensive property of a liquid. In general, if we replace the water under the water striders with ethanol, they will not be able to float because the surface tension of ethanol is less than that of water. Nevertheless, floating does not only depend on the surface tension, but the force of gravity also presents an equally important factor. For an object to float on a liquid surface, it must have a weight such that the force ({F}_{w}) acting downward due to gravity must be counterbalanced by the intermolecular force ({F}_{s}) of the surface molecules, i.e., surface tension. It is important to notice that only objects that are completely above the surface of the water, not partially submerged, are considered to be as floating in this case. Also, it is not the surface tension of the liquid that is being affected due to the force of gravity.
Factors affecting Surface Tension
The value of surface tension for a particular liquid can be affected by the nature of the liquid, its surroundings, and the purity of the liquid. Let’s discuss these factors in a detailed manner:
1. Temperature
The surface tension of liquid decreases with an increase in temperature. As the temperature increases, the liquid molecules become more energetic and start moving in random directions, thereby decreasing the strength of the intermolecular bonds. Therefore, when we assign a value for the surface tension of a liquid, we explicitly state the value of the temperature at which it exists. Moreover, the value of surface tension keeps on decreasing with the increase in temperature, and it reaches zero at the critical temperature as the intermolecular forces in both liquid and gaseous state become equal, and the liquid can expand without any restriction. For small temperature differences, the value of surface tension varies linearly with temperature given by the relation:
{γ}_{t} = {γ}_{0} (1-αt)
where {γ}_{t} and {γ}_{0} are the values of surface tension at the temperature t °C and 0 °C, respectively, and α is the temperature coefficient of surface tension. A practical example of this effect can be observed while enjoying the soup. A hot soup tastes much more delicious than a cold one because the surface tension of a hot soup is lower than that of the cold soup; and therefore, it spreads over a larger area of the tongue. This in terms means that covering more taste receptors somehow makes the brain interpret the soup as tastier.
2. Impurities
The presence of impurities either on the liquid surface or dissolved in it considerably affects the force of surface tension depending upon the degree of contamination. The addition of sparingly soluble impurities to a liquid can decrease its surface tension. This happens because adhesive force between the liquid molecule and the impurity molecule is less than cohesive force among liquid molecules, and because of a weaker intermolecular force of attraction among the molecules of solvent and solute, surface tension decreases. For instance, adding phenol to the water can lower down its surface tension, and provides better cleaning than pure water.
On the other hand, highly soluble impurities can slightly increase the surface tension of a liquid. In general, the adhesive force between the liquid molecule and that impurity molecule increases because of the stronger intermolecular force of attraction between a soluble compound and solute, hence surface tension increases. However, one should not confuse surface tension with the increase in density of the solution.
3. Surfactants
The term surfactant is a shorthand notation for the surface-active agents. They are amphiphilic molecules that consist of hydrophilic and hydrophobic portions of the chemical structure. In an aqueous environment, the hydrophilic portion is water-loving and polar, whereas the hydrophobic part of the surfactant is water-hating and relatively non-polar. Surfactants reduce the surface tension of water by adsorbing at the interface. In general, if the water is mixed with a tiny amount of surfactants (for example, hand soap), it lowers the water’s surface tension so the water drop becomes weaker and breaks apart easily. Soaps increase the cleaning effect of water by decreasing the cohesive forces among water molecules.
Examples of Surface Tension
Surface and interfacial tensions can have a strong influence on several essential phenomena in everyday life. As a result, many chemicals and techniques have been developed for modifying the surface and interfacial tensions.
1. A Drop of Liquid
Whether walking through the rain, spilling the morning coffee, or putting an eye drop, we come across several liquids splashing off the solid surfaces. Most of the time, such events go unnoticed, but if one pays a significant amount of attention, they would notice that the basic physics that governs the dynamics of liquid droplets is extremely rich of complexities. The hydrodynamics of a liquid drop under free fall is fascinating. It is easy to see that the drop seems to have a “skin” holding it into a sort of sphere. The lowest energy state for this drop occurs when the maximum number of water molecules are surrounded by other water molecules without falling apart, meaning that the drop should have the minimum possible surface area, which is a sphere. The effect of gravity distorts this ideal sphere into a nearly-oval shape that we see. In the absence of other forces, including gravity, drops of virtually all liquids would be approximately spherical.
2. Soaps and Detergents
Soaps and detergents seem like simple things that we can find in our bathrooms, laundry area, or cleaning supplies. Just splash some water on your face, apply soap, rinse again, and all the dirt is gone, right? Still, the chemistry behind this phenomenon is not that simple. Soaps and detergents are quite tricky chemicals, and they work in a really interesting way. Water molecules prefer to stick together through intermolecular forces. Soaps and detergents help the cleaning by lowering the surface tension of the water so that it can more readily soak into pores and soiled areas. The soap molecules are composed of long chains of carbon and hydrogen atoms along with other ionic molecules. At one end of the chain is a configuration of atoms that likes to be in water (hydrophilic). The other end, however, shuns water (hydrophobic) but attaches easily to grease. In washing, the “grease-loving” end of the soap molecule attaches itself to the oil or fat present on the stain, letting water flow underneath. The particles of stain become loose and surrounded by soap molecules, to be carried off by a flood of water.
3. Washing with Hot Water
In the previous example, we noted that by lowering its surface tension, the efficiency of water in cleaning can be improved. Although surface tension of water can significantly be reduced by soaps and detergents, warming up the water can also do the trick. The basic mechanics behind the cleaning stays the same, reducing water’s surface tension so that it can disperse over a wider surface area. Due to the increase in energy, the molecules of water start moving spontaneously when heated. It reduces the strength of intermolecular forces that keep the molecules together when the water is cold. Washing clothes with hot water, however, is not always advisable as it can damage the clothing itself. On the cloth, one can look for the warning label that shows the water’s temperature lable.
4. Clinical Test for Jaundice
Surface tension also plays an essential role in analytical chemistry. This property of liquids helps saving millions of lives around the world from jaundice via Hay’s Test. Hay’s test, also known as Hay’s sulfur flower test, is a chemical test used for detecting the presence of bile salts in the urine. Bile salts are salts of four different types of bile acids: cholic, deoxycholic, chenodeoxycholic, and lithocholic. These bile acids interact with glycine or taurine in order to form complex salts. Bile salts pass through the bile into the small intestine and serve as surfactants to emulsify fat. They reduce the surface tension on fat droplets so that the fat can be broken down by enzymes. Bile salts are processed in the terminal ileum and enter the bloodstream from where the liver takes them and re-excretes them in the bile. Bile salts, along with bilirubin, can be detected in urine in cases of obstructive jaundice. Hay’s test is performed by taking a fresh urine sample at room temperature and sprinkling sulfur powder on it. If bile salts are present, sulfur particles sink to the bottom because of the lowering of surface tension by bile salts. If sulfur particles remain on the surface of urine, bile salts are absent, and the test gives a negative result for jaundice.
5. Water Striders
Water striders are a family of insects that are capable of walking on water. They are also known by other common names such as water skeeters, water scooters, water bugs, pond skaters, water skippers, Jesus bugs, or water skimmers. They use the surface tension of water to their advantage. Combined with their long, thin, and hydrophobic legs, which allow the weight of the water strider body to be distributed over a large surface area, the surface tension of water provides them with necessary shielding from the force of gravity that can cause them to sink. Nevertheless, if by any natural cause such as waves, a water strider submerges, the tiny hairline present on its body traps enough air to provide buoyancy to bring the water strider back to the surface again, while also providing air bubbles to breathe underwater.
6. Capillary Action
Capillary action is the ability of a liquid to flow through narrow spaces without the assistance of, and in opposition to, external forces such as gravity. When thinking of capillarity, think of molecules as sticky balls. The attraction between unlike substances such as glass and water is called adhesion, whereas the attraction between like substances is called cohesion. When a glass tube is dipped into water, the adhesion between glass and water causes a thin film of water to be drawn up over the inner and outer surface of the tube. Surface tension causes this film to contract. The film on the outer surface contracts enough to make a round edge, whereas the film on the inner surface contracts more, and it raises water within it until the adhesive force is balanced by the weight of the water lifted. The narrower the tube, the higher is the water level in order to balance adhesion force. The mathematical expression for the height (h) of a liquid column is given by:
h = \frac {2T} {ρrg}Where T= surface tension, r = radius of the tube, g = 9.8 {ms}^{-2}, and ρ = density of the liquid.
This effect can be seen in the liquid molecules present between the hairs of a paintbrush, in a thin tube, in porous materials such as paper, in some non-porous materials (such as liquified carbon fibre), or in a biological cell.
7. Formation of a Meniscus
If you have ever been to a chemistry lab or if you have closely looked at a mercury thermometer, you might have noticed that the top of the liquid present inside the tube or tumbler is mostly either concave or convex. This curvature of the surface at the top of a column of fluid in a narrow tube is caused by the relative strength of the forces responsible for the surface tension of the fluid (cohesive forces) and the adhesive forces to the walls of the container. In general, when the particles of the liquid are more strongly attracted to the container (adhesion) than to each other (cohesion), a concave meniscus develops, allowing the liquid to ascend the walls of the container between water and glass. Conversely, when the particles in the liquid have a greater attraction to each other than to the container material, for example, between mercury and glass, a convex meniscus forms. Menisci formation is widely used for the calculation of contact angles and surface tension in surface science. The outline of the meniscus is determined with a balance or optically with a digital camera while calculating the touch angle. In a surface tension measurement, the measurement probe has a contact angle of zero, and the surface tension can be obtained by measuring the mass of the menisci.
8. Bubbles
Have you ever tried to make bubbles with pure water,? It never works. There is a common misconception that to sustain a bubble, water does not have the requisite surface tension and that soap enhances it, but soap simply reduces the pull of surface tension – usually to around a third of that of plain water. For bubbles to last for any amount of time, the surface tension in plain water is just too high and therefore soap comes into play. One other problem with pure water bubbles is evaporation: the surface quickly becomes thin, causing them to pop. The pressure differential between a bubble’s inside and outside depends on the strain of the surface and the bubble’s radius. By visualizing the bubble as two hemispheres, the relationship can be accomplished by noticing that the internal pressure that tends to force the hemispheres apart is counteracted by the surface tension that works around the circumference of the sphere.
9. Tears of wine
Anyone who has ever enjoyed a glass of wine might have noticed the phenomenon called tears of wine manifested as a ring of clear liquid near the top of the level of wine in the glass from which droplets continuously form and drop back into the wine. It is also referred to as wine legs, fingers, curtains, or church windows. Wine is mostly a mixture of alcohol and water with dissolved sugars, acids, colourants, and flavourings. When it is poured into a glass, the capillary motion allows the wine to climb up the side of the glass. As it does so, the rising film evaporates both alcohol and water, but due to its higher vapour pressure, the alcohol evaporates faster. The resulting drop in alcohol concentration allows the liquid to increase the surface tension, allowing more liquid to be drawn from the remainder of the wine, which has a lower surface tension due to its higher alcohol content. The wine moves up the sides of the glass and forms droplets that fall back under their own weight.
10. Reptilian Envenomation
The prospect of death from a snakebite is an awful one. Nonetheless, a snakebit has a very ingenious mechanism involved. Most of us believe that the snakes kill by injecting the venom fired through their fangs. Surprisingly, it is only now that scientists have carefully calculated the snake venom flow, they claim that many snakes kill not by a syringe-like tube injecting poison under pressure, but rather by the force of surface tension along an open groove. However, few snakes do inject their venom, the rattlesnake being a well-known example. The fangs of a rattlesnake are like hypodermic needles, firing venom into prey from a poison gland in the snake’s head at high intensity. But in other snakes, because of the variations in surface tension between the venom inside and outside of the groove, the lethal fluid from the venom gland travels through the open groove in the snake’s fang by capillary action. In general, the researchers have found that surface tension holds the venom in the groove while the fangs are in the air. However, the grooves and tissue form a tubular shape as the fangs enter the skin, which increases surface area and minimizes surface energy, thus drawing the venom in.
In the above image of snake fangs, A is for a banded snake while B) is for a groove snake. In B) the fang is embedded in prey tissue, increasing the surface area to let the venom flow