AMAZING BEHAVIOR OF LIQUIDS --- SURFACE TENSION.
Have you tried observing as water drops from a tap that wasn't properly closed? I bet you have, but for the sake of those who haven't I will try and describe what happens.
If you watch the water drop from the tap, you will observe that the drop of water doesn't fall immediately, but takes a bit of time as if its trying to accumulate before it falls. You will also observe that it seems like the outer surface of the water is acting as a bag where the rest of the water molecules accumulate. This continues till it seems as if the bag can't bear the weight anymore, it begins to sag before it finally falls.
I know you must have wondered what caused that to happen. An event that must have amazed you the most is how a coin floats on water even though it is dense. At a point in my life I also wondered what caused these events, this lead to a series of research. If you will allow me, I will introduce you to one of the most amazing behavior observed in liquids. This behavior is called Surface Tension.
This discussion is going to be very interesting, I promise you that.
WHAT IS SURFACE TENSION
As was observed from the dripping tap it appears as if the water "bag was being stretched".
Thus we define surface tension as the tendency of the surface of a fluid to behave as if it is a stretched elastic skin, thus making it to acquire the smallest surface area possible.
For the sake of physics students, let me just add a little mathematics.
Surface tension is defined as the ratio of the surface force (F) to the length d along which the action of the surface force is felt. It is represented as gamma and is measured in N/m
Mathematically
Surface Tension = F/d
Try fillng a glass cup to the brim with water you will notice that it seems as if the water surface is curved at the edges. If you drop a pin on in, it doesn't sink. Before we understand this phenomenon I will like to define two types of forces: cohesive force and adhesive force.
Cohesive forces are forces of attraction between like molecules, while adhesive force is the force of attraction between unlike molecules. These forces occurs the same time, but one has to be more significant than the other.
Imagine the water in the cup as if it's divided into separate molecules of H20, then there must be a force that joins these molecules of water together such that if you try pouring out the water it doesn't fall out individually. The force that holds the water molecule together is the cohesive forces. Other liquids are held together by this force too.
Water molecules attract each other in all directions
Observe also that after pouring out the water from the glass cup, there was still drips of water held to the glass. The same also happens if you pour out wine from a glass cup. But when you pour mercury out of a glass cup not even a drop remains in the cup. Thus it is wise to think that there must have been an attraction that held the water drop to the cup, which doesn't really hold mercury to the same glass cup.
That force is called adhesive force since its between two unlike molecules (liquid and glass). Remember that we said these two forces are always present, it's just that one has to be greater than the other. Thus adhesive force of attraction is stronger in water and glass than mercury and glass. Therefore we say water wets glass but mercury doesn't wet glass.
WHAT CAUSES SURFACE TENSION
Using water as a test case, these cohesive forces (which is mostly Vander Waals forces) causes each molecule of water to attract other molecules around it, in every direction. Thus those molecules that are not in the surface are equally attracted in all directions.
Surface tension causes the surface of a liquid to curve inward or outward
It is not so for the molecules on the surface, they do not have the same number of molecules pulling them since they are pulled mainly by the water molecules inside and none from the outside. These unbalanced inward forces causes the surface of the liquid to become under tension (i.e is probably where the name was gotten from). These pressure being asserted on the water surface causes the surface of the liquid behave as if it's a stretched membrane.
Don't forget that the surface of the water is also acted on by adhesive forces, but those forces are not as powerful as the cohesive forces acting in the fluid, thus there effect is not significant. This causes the surface of the water to curve inward and not outward. If the adhesive attraction in water was stronger like in the case of liquids like mercury where the adhesive force of attraction is greater than the cohesive force, the surface of the water would be expected to curve outward, as observed in mercury.
EFFECTS OF SURFACE TENSION
Surface tension causes liquids to behave in a certain manner. I will be point out and also explain few of them.
- Rain drop:-
Have you noticed how water settles on a leaf after rainfall, even as it falls it appears as if it's spherical. When rain falls from the sky, or even drops on the leaf, the force of attraction of the molecules inside causes the liquid molecules to come together.
Rain drops are spherical because of surface tension
Since those on the outside are experiencing an uneven force, it will cause the drop of water to appear as a sphere. It is worth noting that the only reason why the raindrop is not as spherical as when it drops on a leaf is because of the gravitational pull on it.
- Insects walking on water
Have you noticed insects walk on water? Insect like the water strider is able to walk on water because of surface tension. When their foot touches water, due to surface tension they don't "pierce" the water, instead they cause the water surface to depress. When the water surface gets depressed, it tries to return back to the way it was before and in the process it lifts the foot of the water strider away from the fluid. The foots of water striders are designed in such a way that it exerts a very minimum potential energy on the surface of the fluid as they walk on the surface of the water. The same as how snow shoes enables you to walk on snow without your feet sinking into the snow.
Needles float on water as a result of surface tension
- Coin or needle floating on water
Another effect is noticed when substances more densed than water (like coin and needles) floats on water. The principle is the same as that of water striders. Before I forget to mention it, the flotation of a material is dependent on the nonwettability of the material.
- Capillarity
Capillarity is defined as the rise or fall of a liquid in a narrow tube. This action is very important, to be precise water from the ground gets distributed to all part of the plant due to capillary action. Capillary action is primarily as a result of surface tension.
Remember that we said earlier that cohesive and adhesive forces are always present but one is always more significant when compared to the other. When a narrow tube is dipped in water, it is observe that the water rises up the tube and the surface of the liquid curves outside. But when the same tube is dipped in liquids like mercury, the liquid level falls below the level of the fluids outside of the narrow tube.
The reason is because of the greater force of attraction between the molecule of the mercury. The molecules of the liquid prefers to attach themselves to themselves than with other molecules, thus it falls down the tube as if it's trying to "shy" away.
Water is distributed across a plant by capillary action
But that's not so in water, the force of cohesion is greater. Infact, they prefer others to themselves and as a result they easily rise up the test tube. Anyway, it is nice they prefer other molecule, because if they behave like mercury, we will all die of hunger since they won't rise up from the ground to all parts of the plant.
DISADVANTAGE OF SURFACE TENSION
All these while, I have been making surface tension sound good, but as some people will say everything that has an advantage has a disadvantage, it has a disadvantage.
When you try soaking your clothe in cold water (do you know that water is a detergent?) the dirt on the clothe doesn't get removed, except you use a detergent. But when you soak it in hot water, the first get removed.
Surface tension prevents the water molecules from entering the fabric, and soaking in to the pores and soiled areas, but when the water is heated or a detergent added to the cold water, the surface tension gets reduced thus allowing cleansing to occur.
CONCLUSION
The importance of this phenomenon (surface tension) cannot be over emphasized, and for that reason I will conclude by pointing out a great use of surface tension.
Surface tension is really what help us to get to our work place when it rains. An umbrella protects you from the rain with the help of surface tension. If there was nothing like umbrella, we will get to our work place drenched, except some of us who has cars, but even at that, you still need an umbrella.
If you really think it's the material used in making the umbrella that protects you, try touching the inside of the umbrella and watch as the surface tension breaks and your hand gets soaked with water.
There are still so many phenomenons in nature that will definitely wow you, and leave you mouth open. Sometimes the best thing to do after studying these phenomenons is to sit down and allow ourselves to be marveled by them.
REFERENCES
Examples of Surface tension effects
Capillary action as a result of surface tension
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