Several theological studies indicate that in ancient Egypt the gods could walk on water, as could Buddha and Jesus. Humanity associates the ability of walking on water with the divinity of gods.
Besides a few insects, for example, water striders, in the animal kingdom, it makes one wonder whether it is the divinity which (for whatever reason) human beings do not possess to walk on water, or whether walking on water has something more fundamental to the phenomenon itself.
Many of us are familiar with the floating needle or this floating paper clip trick. In fact, the scientific explanation for these phenomena is commonly referred to “Surface Tension.”
Generally, surface tension is a property of a liquid that causing it to act, as if it has an elastic skin encasing its surface. This is a property with intermolecular cohesive forces between the molecules.
A molecule in the bulk of a liquid experiences interactions with other molecules from all sides, whereas molecules at the surface experience interactions with adjacent molecules and adjacent layers underneath. Therefore, molecules at the surface will experience a greater net force of attraction than molecules in the bulk.
In terms of force, surface tension (γ) is defined as the force (F) acting across the surface per unit length (L) directed perpendicular to the surface, with the units newton per meter.
γ = 1/2 x (F/L) Nm-1
In terms of energy, surface tension is defined as the isothermal energy it takes to increase the surface area by one square meter, with the units joules per meter square.
γ = W/ΔA Jm-2
In addition, surface tension is an intensive property of a liquid. In general, if we replaced the water underneath the water striders with ethanol, they would not float because the surface tension of ethanol is not greater than that of water.
Besides that, floating is not totally reliant on surface tension. Object floating also relies heavily on the force of gravity.
The object’s weight must be of such mass (the force downward (Fw) due to gravity) must be countered by the intermolecular force (Fs) of the surface molecules, that is, surface tension.
It is important to note that the definition of floating in this case is reserved only for objects that are completely above the surface of the water and not some, or partially submerged. In addition, only the force of gravity is repelling against the intermolecular forces of the liquid and not the surface tension of the liquid.
Factors affecting Surface Tension
Many aspects can affect the surface tension of a liquid including its nature, its environment, and its purity. Let’s examine each factor in more detail:
1. Temperature.
The surface tension of liquid decreases with increasing temperature. As liquid molecules become more energetic, they will move freely in random directions and will thus weaken their intermolecular bonds.
Whenever we mention the value for surface tension of a liquid, we must also include the temperature at which it exists.
Furthermore, the value of surface tension will continue to degrease with increasing temperature until a critical temperate, where the intermolecular forces of the liquid and gaseous phase are equal, and the liquid loses any limits to how much it can expand.
For small changes in temperature, the value of surface tension can be approximated linearly in temperature from the following relation:
γt = γ0 (1-αt)
Where γt and γ0 are the values of surface tension at temperature t°C and 0°C, and α is the temperature coefficient of surface tension. A practical example of this can be observed in a bowl of soup.
Cold soup does not taste as good as hot soup because the surface tension of hot soup is lower than the surface tension of cold soup; therefore, hot soup can cover more area on your tongue.
In terms of, covering more taste receptors makes the soup somehow taste better to your brain.
2. Impurities.
Pure substances are usually never found in nature; there are probably other substances either at the surface of liquid or dissolved inside, and almost always will for dilutes contain various impurities.
These impurities change the amount of surface tension compared to pure liquid depending on the concentration of the impurities.
Sparingly soluble impurities can and often do decrease the amount of surface tension of a liquid in the same way as impurities.
The reason for this is that the adhesive force between the liquid molecule and impurity molecule, is less than, and because of weaker intermolecular forces of attraction between impurities against the solvent, surface tension is decreased.
A perfect example is phenol added to water; which decreases surface tension and provide better cleaning than pure water.
Impurities that are highly soluble can actually increase a liquid’s surface tension very minimally.
Generally speaking, the adhesive force created between the liquid and that impurity increased instead due to the increase intermolecular forces of attraction created between a solute that is soluble and and uniformly compatible with the solvent, and surface tension this increased overall.
Do not confuse surface tension with increased density of a solution.
3. Surfactants.
A surfactant is an agent that is surface active. Surfactants are amphiphilic molecules. All amphiphilic molecules have both a hydrophilic and hydrophobic part.
In an aqueous system, the hydrophilic part is water loving and the hydrophobic part of the surfactant is water hating and relatively non-polar. Surfactants can decrease the surface tension of water when they adsorb themselves to the interface.
In general, if water is mixed with a very small amount of surfactants (for example, hand soap), the surfactant will reduce the water’s surface tension to the point that the water droplet is weak and easily breaks apart. Soap increases the cleaning effectiveness of water by reducing the cohesive forces among the water molecules.
Examples of Surface Tension
Surface and interfacial tensions can be very important for many common processes. Consequently, many chemicals and methods have been developed for changing surface and interfacial tensions.
1. A Drop of Liquid.
Whether it is walking through rain, spilling a morning coffee, or putting in eye drops, we encounter quite a few instances of liquids splashing from a solid surface.
Often we do not notice these events, but, if we began to pay closer attention, we would see that the basic physics governing the language of liquid droplets is incredibly complex.
The hydrodynamics of a liquid drop that falls freely is intriguing. What we see from the drop, is that it appears as though the drop has a “skin” that is holding it together as a sort of sphere.
The character of the drop is the lowest energy state when the maximum amount of water molecules are surrounded by other water molecules, just barely holding together, and therefore the drop will have the least surface area and be a sphere.
Gravity gives a force that deforms this ideal shape into a nearly-oval shape that we see. Without that force acting on the drop, as well as other forces, drops of all liquids would take on roughly a spherical shape.
2. Soaps and Detergents.
Soaps and detergents are common items found in the bathroom, laundry, and cleaning supplies; after all, isn’t washing your face just splashing water, applying soap, rinsing again, and voila! all the dirt is gone? Maybe it’s not that straightforward after all though.
Soaps and detergents are rather complex chemicals and work in a special way. Water molecules like to stick together because of intermolecular forces.
Soaps and detergents assist in cleaning because they lower the surface tension of water, thus enabling more liquid water to penetrate into the pores and dirty areas.
Soap consists of long chains of carbon and hydrogen atoms bonded with ionic molecules.
The soap molecule has a hydrophilic (or water-loving) end and a hydrophobic (or water-hating) end that attaches easily to grease.
When you wash, the grease-loving end of the soap molecule attaches to whatever oil or fat is in the stain, allowing the water to then seep in underneath.
The oil particles in the stain are now loose and surrounded by soap molecules and are carried off by a water flow of washing!
3. Washing with Hot Water.
In the previous example, we discussed that increasing the surface tension of water can when cleaning effect of water. Even though soaps and detergents will significantly reduce the surface tension of water, simply warming the water can have the same effect.
The basic mechanics behind the cleaning remains the same, to reduce water’s surface tension so that the water may disperse over an area.
When water is heated, the increase in energy causes the water molecules to start moving randomly. The increase in energy reduces the intermolecular forces that bind water together in the ionic state when it is cold.
While washing clothing in hot water is not always a good option because it can damage the clothes themselves, always observe the clothing warnings on the label for the temperatures for the proper wash temperature of the water.
4. Clinical Test for Jaundice.
Surface tension is important in analytical chemistry. It helps save millions from disease and death from jaundice! Surface tension is critical with Hay’s Test.
Hay’s test (also known as Hay’s sulfur flower test) is a chemical test to test for the presence of bile salts in urine. Bile salts are salts derived from four different bile acids: cholic acid, deoxycholic acid, chenodeoxycholic acid, and lithocholic acid.
Bile acids bind glycine or taurine to make decorative salts. Bile salts are secreted with bile into the small intestine to act as surfactants to emulsify fat.
Once emulsion is created with surface tension reduction on the fat droplets, the fat can be broken down by enzymes that digest them.
Bile salts are absorbed in the terminal ileum and enter the blood supply and then transported back to the liver where they are excreted in bile that will help digest more fat.
Bile salts, in combination with bilirubin, the yellow (chole) pigment will be present in your urine if you have obstructive jaundice. Hay’s test is a simple test where a fresh urine sample at room temperature is collected and then powdered sulfur is sprinkled over it.
If bile salts are present, when sulfur is sprinkled, the particles sink to the bottom of the urine sample, because bile salts lower the surface tension.
If the sulfur powder remains on the surface of urine, there are no bile salts present and thus the test is negative for jaundice.
5. Water Striders.
Water striders are insects that walk on water. They are referred to as water skeeters, water scooters, water bugs, pond skaters, water skimmers, Jesus bugs or water skimmers. They benefit from surface tension.
Much of their advantage relies on their long, thin, hydrophobic legs allowing the body weight of the water strider to be dispensed over a large area.
The surface tension of water provides the necessary barrier against gravity causing the water strider to sink.
Nonetheless, if for any reason a water strider gets submerged by natural causes like waves, its legs have lots of small hairline on its body trapping a sufficient quantity of air, to provide buoyancy to bring the water strider back to the surface while also providing air bubbles to breathe underwater.
6. Capillary Action.
Capillary action is the ability of a liquid to flow in narrow spaces without the help of gravity. As you think about capillarity, think about molecules being like sticky balls.
The attraction between unlike substances such as glass and water is referred to as adhesion and the attraction between like substances is referred to as cohesion.
When a glass tube is dipped into water, the adhesion between the glass tube and water causes a thin film of water to be drawn up over the inner surface of the tube and over the outer surface of the tube.
Surface tension causes the thin film of water to contract. The film on the outer surface contracts to make a round edge whereas the film on the inner surface contracts more which raises water in 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:
h=2T/ρrg
Where T= surface tension, r = radius of the tube, g = 9.8 ms−2 and ρ = density of the liquid.
This effect should be seen with the liquid molecules that are found in 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 been to a chemistry lab or if you closely look at a mercury thermometer, you may notice that the liquid present inside the tube or tumbler has a surface that is most often concave or convex.
The curvature of the surface at the top of a fluid column in a narrow tube is due to the relative strength of the forces that result in the surface tension of the fluid (cohesive forces), and the adhesive forces to the walls of the container.
Generally, if the particles of the liquid are more attracted to the container (adhesion) than to each other (cohesion), a concave meniscus is formed – this results in the liquid being able to rise the walls of the container between water and glass.
In contrast, if the particles in the liquid are attracted to each other more than the container material: example, mercury and glass, a convex meniscus is formed.
Meniscus formation is widely used in contact angle and surface tension computations in surface science. The meniscus outline is established with a balance or optically with a digital camera while computing the touch angle.
For surface tension measurements, the measuring probe has a contact angle of zero, and the surface tension is extracted from menisci with a mass measurement.
8. Bubbles.
Have you ever tried to make bubbles with pure water? It never works. People often misconceive that water does not have the requisite surface tension to sustain a bubble, and that soap increases it, but soap actually decreases the pull of surface tension – usually to around one-third of what plain water provides.
When bubbles are going to last any length of time, plain water will be a problem because the surface tension provided by plain water is too high, which is where soap comes in to play.
Another issue with pure water bubbles is evaporation – they become thin quickly and pop. The internal pressure of a bubble is determined by the strain of the surface and the radius of the bubble – the pressure differential between inside and out.
If we think about it as two hemispheres, we can see that the internal pressure that forces the two hemispheres apart is opposed by the surface tension going around the circle.
9. Tears of wine.
Anyone who has ever enjoyed a glass of wine may have noticed a phenomenon called tears of wine. The tears of wine appear as a ring of clear wine around the top of the level of wine in the glass from which the wine ascends, drops and falls back into the wine.
They are also known as wine legs, wine fingers, wine curtains, or church windows. Wine is mainly a mix of alcohol and water with dissolved sugars, acids, colorants, and flavorings.
When poured into a glass, the capillary action allows the wine to cling to the side of the glass. The climbing film has evaporated alcohol and water, but since alcohol has a higher vapour pressure than water, it evaporates first.
The decline in alcohol concentration enables the fluid to increase its surface tension allowing for additional liquid to be pulled up from the rest of the wine as the rest of the wine has a lower surface tension due to its higher alcohol content.
The wine ascends the side of the glass and can only hang there until the droplets fall back of their own weight.
10. Reptilian Envenomation.
The idea of dying from a snakebite is a ghastly thought. However, there is a clever mechanism behind snakebites which most of us believe snakes kill by injecting the venom they shoot out of their fangs.
Researchers have recently calculated the amount of venom snakes produce when they strike and claim that many snakes do not kill using a tube-like syringe to inject the venom under pressure, but instead from the force of surface tension along an open groove.
Very few snakes inject their venom but rattlesnakes are a well known example. Their fangs resemble hypodermic needles in that they fire venom from a poison gland in the snake’s head into prey with high intensity.
In most snakes however, because of the surface tension difference between the venom inside and outside of the groove, the lethal fluid from the venom gland leaves the gland and travels to the groove in the snake’s fang through capillary action.
In general, researchers report that surface tension leaves the venom held still within the grooves while the fangs are in the air.
But as the fangs penetrate the scar tissue, the grooves and tissue take a tubular shape that enhances the surface area and minimizes the surface energy, thus pulling the venom into the wound.