Surface Tension of liquids (For beginners only)

1. SURFACE TENSION

2. Before we start, let us ask ourselves….
What makes water molecules stick together to
form a drop of water?

3. What allows those cool water bugs to walk on water?

4. WHAT IS SURFACE TENSION?
Surface tension is a force which causes a layer of liquid to behave like an
elastic sheet or skin.
The “ANSWER” is : SURFACE TENSION
Causes of Surface Tension
Various intermolecular forces, such as Van der Waals forces, draw the
liquid particles together. Along the surface, the particles are pulled
toward the rest of the liquid.

5. Examples of Surface Tension
Drops of water:- When using a water dropper, the water does not
flow in a continuous stream, but rather in a series of drops. The shape
of the drops is caused by the surface tension of the water.
The only reason the drop of water isn't completely spherical is that
the force of gravity pulling down on it. In the absence of gravity, the
drop would minimize the surface area in order to minimize tension,
which would result in a perfectly spherical shape.

6. Insects walking on water
Several insects are able to walk on water, such as the water strider.
Their legs are formed to distribute their weight, causing the
surface of the liquid to become depressed, minimizing the
potential energy to create a balance of forces so that the strider can
move across the surface of the water without breaking through the
surface. This is similar in concept to wearing snowshoes to walk
across deep snowdrifts without your feet sinking.

7. SURFACE TENSION EXAMPLE – FLOATING
An object like a safety pin or
needle can carefully be placed
on the surface of the water
and float even though it is
quite heavy in relation to the
water. Once the surface is
agitated and the surface
tension is broken, the object
will quickly sink.

8. From these examples we can define Surface Tension as….
Surface tension is the ability of fluid surfaces to contract into the
smallest possible surface area. Surface tension is described as
the phenomenon that occurs when the surface of a liquid comes
into contact with another phase (it can be a liquid as well).
Liquids appear to have the smallest possible surface area. The
liquid’s surface looks like an elastic sheet.

9. EFFECTS OF TEMPERATURE ON SURFACE TENSION
As the temperature decreases, surface tension of a liquid increases and start
minimizing the surface area. Because when temperature decreases, molecular thermal
activities also decreases and as a result of that intermolecular cohesive force increases
and finally surface tension increases.
For example, when we see
the surface of the COLD milk
(after boiling), due to the
increasing surface tension
the layer of fats on the milk
surface appear in folds.

10. On the other hand, as the temperature increases, surface tension of a
liquid decreases. Because when temperature increases, molecular
thermal activities also increase and as a result of that intermolecular
cohesive force decreases and finally surface tension decreases.
For example, when a
perfume is sprayed, the
liquid perfume gets exposed
to external temperature and
the surface tension of the
liquid perfume decreases.
Thus it quickly gets
converted to gas.

11. Formula for Surface Tension
Mathematically, the surface tension is defined as the force (F) acting
on the length (l) of the surface. Thus, mathematically it is given as:
T = F / l
SI Unit : N/m
On the other hand, W = F x S
So, F = W/S
Therefore, T = W/S x l
= W/A { Since S x l = A}
SI Unit : Joule/metre square
Thus, the ratio of the work done (W) and the change in the area of the surface (A)
is termed surface tension.

12. Howto Calculate Surface Tension?
The surface Tension of any substance can easily be calculated using the surface tension
formula. Let us take an example to understand the calculation of Surface Tension in
detail.
Example: Find the surface tension of the liquid with a dragging force of 12 N when the
length at which the force acts is 4 m.
Solution:
Given, F = 12 N & L = 4 m
According to the Surface Tension formula, T = F/L
T = 12/4
T = 3 N/m

13. Thank You
Dr. Pranabjyoti Das
Arunodoi Junior College