Date:02/07/2009 URL: http://www.thehindu.com/thehindu/seta/2009/07/02/stories/2009070250241300.htm

Bubble formation

Why do bubbles form in a vessel containing water when water falls into it from a height?

SIDARTH. S

Thrissur, Kerala

Water is a liquid with high fluidity, or in other words, the viscosity of water is very low. That is why it is very easy to deform water locally with very little force.

Many other thick liquids, like honey or oil, with high viscosity do not locally deform so easily. Such liquids maintain a compact and coherent stream when poured from a height.

On the other hand, when water falls from a height, the stream gets fragmented enclosing air filled gaps inside the stream. The water layer surrounding the air gaps close during the downward travel and air pockets form. This happens when water falls rather rapidly.

When we open a tap slightly so that a slow stream of water comes out, the stream is practically transparent and does not have air pockets in it, but if the tap is opened widely, a large amount of water gushes out with higher speed and the stream breaks into fragmented parallel streams and encloses many air pockets rendering it a white look. When this mixture of air and water reaches the collection “tub” the air pockets float up and appear as bubbles.

Further, the falling water stream carries some amount of air along with it due to drag and this air also penetrates into the water in the tub down to some depth as a bubble, which pops up due to considerable difference of density of air and water.

In addition to this, bubbles form in the tub also due to another effect. When water falls onto the surface of water in the tub, locally the water is pushed aside forming a temporary cavity, The water in its vicinity, on all sides rush towards it and close in resulting in the formation of bubbles.

This effect is clearly noticeable on making water fall from a height drop by drop on an existing mass of water in a tub. The drops, on reaching the bottom, would displace the local mass of water scooping out a cavity which soon fills in enclosing one or two bubbles.

H. K. SAHU

Scientific Officer

Indira Gandhi Centre for Atomic Research

Kalpakkam, Tamil Nadu