Further illustrations of surface tension
We have already seen that soap solution, detergents, and alcohol will lower surface tension of water. Camphor has a similar effect. When a piece of champion thrown on to the surface of water it slowly dissolves and causes a reduction surface tension in its immediate neighbourhood. The stronger pull exerted by surrounding uncontaminated water sets up a movement of the surface and camphor is carried along with it. After a time the concentration of the sur: solution becomes uniform and all movement ceases. The above experiment illustrates the principle of a toy duck, made of light p material, which swims about on the surface of water when a small piece of earn; is attached to it. A similar toy in the shape of a small metal boat has a reserve mentholated spirit. A short wick dips in the spirit and trails over the stern. Red of surface tension occurs where the spirit contaminates the water and he boat forward. Quantitatively, the surface tension is defined as the tangential force in the acting normally per unit length across any line in the surface. (The word “no here means “perpendicularly”.)
The tension in the surface of a liquid is well illustrated by a soap film. A wire frame with a piece of cotton tied across it is dipped into soap or detergent solution so that a film is formed. When the film on one side of the cotton is removed by touching it with a filter-paper the tension in the film on the opposite side pulls the cotton into an arc of a circle. A similar frame has a length of cotton with a loop in it. The loop is pulled into the form of a circle when the film is removed from its centre (Fig. 13.8).