Wednesday, December 17, 2014

Surface Tension Explanation basing on Molecular Theory

To explain the surface tension of a liquid we can depend on molecular theory. As per the molecular theory each molecule is having a certain molecular range up to where it can influence the surrounding molecules. Taking that molecule at the center and the molecular Range as  radius we can draw a sphere and that sphere is called as sphere of influence.

Let us consider a liquid in a vessel and three  different molecules at three different locations. One molecule is at the middle of the liquid surface, one molecule is on the surface of the liquid, another molecule is near to the surface of the liquid. For each of them we can draw the sphere of influence as shown.

The molecule who is there completely inside the water it is completely filled with the water molecules.The molecule at the centre is equally pulled by the surrounding molecules in all directions and it is in a kind of equilibrium state. In this case there is no resultant force acting on the molecule and there is no force due to the surface tension on that.

Let us consider the molecule that is close to the surface of the liquid. Its lower sphere of influence is full of liquid molecules whereas the upper sphere of influence only partially filled. The molecule is equally pulled by both the left and the right side molecules. But the force on the molecule in the downward direction is more than that of the upward  direction because of the more number of the molecules in the lower sphere of influence. Thus this molecules experience a little bit of the downward force due to the surface tension.

Let us consider the molecule on the surface of the liquid itself. Its lower sphere of influence is full of liquid molecules where as the upper spirit of influence completely empty. Thus the molecules on the surface of the liquid experience maximum download force. It is the same for all the molecules on the surface and this generates a certain force on the surface of the liquid  called a surface tension.

For the molecules who are close to the surface of the container ,there is a adhesive force which is dominating the cohersive force in general.Therefore the molecules there stick to the surface of the container itself whereas the remaining molecules comes in the downward direction because of the force of attraction in the downward direction. Hence the surface of the liquid will change from a plain shape to a curved shape. This is what do you mean by stretching elastic membrane.

Because of the surface tension the liquid surface molecules always tries to acquire  a minimum surface area. This is simply to reduce their potential energy and to get the better stability. This can be easily observed in the following example.

Let us take a thin wire and metal frame and immerse it in a soap solution. We can arrange a lightweight thread which is connected between both the ends gently on this soap solution and by default it acquires jig jag  shape. If you punch it a little bit at the Centre there will be the soap only between the thread that the frame but no soap solution within the thread. The soap solution that is there between the thread and frame tires to acquire minimum surface area by pulling the thread from all the direction so that the thread will finally acquire a circular shape as shown. 

The shape is simply circular because among all the shapes,area of circle is having a largest possible area. As the thread is occupying the maximum area the soap solution is obviously going to occupy the minimum area. This is due to the surface tension.

Even the water drops thrown into the air always acquires a spherical shape due to the surface tension only. Among all the shapes the sphere is having the least possible area and hence  water drops are acquire spherical shape. If the size of the drop is big,because of dominating gravitational force it is not going to acquire the exact spherical area and it will be something like a dumbbell and other different shapes.

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