Surface Tension and its Examples

Surface Tension

Surface tension is the property of a liquid by virtue of which the free surface of a liquid behaves like a stretched-membrane tending to decrease the surface area.

Consider a free surface of a liquid. Draw an imaginary line AB on it. The molecules lying on one side of the line tend to pull away from the molecules lying on the other side. Due to this action of the molecules the free surface of the liquid tends to decrease in area, producing a tension in it. Now we define surface tension as the force per unit length acting on either side of the imaginary line. The direction of this force is tangential to the surface and perpendicular to the imaginary line. If ’F’ is the force acting on a length ‘L’, then the surface tension “T”.

Examples Showing Surface Tension

(a) In general a steel needle, if dropped in water, will sink because the density of steel is greater than that of water. However if the needle is made slightly oily and then placed on the surface of water in a horizontal position, it will float leaving a depression of water under it.

This means that the needle is not floating in ordinary sense. It is the surface tension of water which supports it and stops from sinking.

(b) A metallic ring is dipped into a soap solution and a light moist loop of cotton thread of any shape is gently placed over the soap film. When the soap film within the loop is touched with a hot needle so as to break it, the loop takes a circular shape. Every point on the thread experiences a radically outward force of equal magnitude due to surface tension in the soap film out side the loop. These forces being due to surface tension lie in the plane of the soap film.

(c) When a drop of olive oil is left gently inside a mixture of spirit and water (the density of the mixture being equal to that of olive oil) with the help of a pipette, the olive oil drop assumes a perfectly spherical

Shape. Due to the surface tension, a liquid tends to keep its surface area minimum. For a given volume, the surface area of a sphere is minimum. Due to this reason the rain drops are spherical in shape.

(d) You may have observed water droplets falling from a tap, the drops are spherical in shape and is due

to surface tension that minimizes the surface of the drop similarly. Molten lead when allowed to fall through the end of a narrow tube, the lead drops assume spherical shape due to surface tension. Lead shots are manufactured in this way in factories.