CHAPTER IX 



-, SURFACE TENSION 



Few physical forces have been resorted to more frequently 

 in explanation of vital processes than has surface tension. 

 While speculation has gone too far in most instances, there are 

 undoubtedly many protoplasmic phenomena which involve 

 surface-tension changes, some few of which may in part be 

 determined by such changes. 



Surface tension is an expression of surface energy and is present 

 at all interfaces. The "surface" of water is an interface between 

 water and air; that it is under tension may be demonstrated by 

 pouring more water into a glass already full to the brim until 

 the surface is curved above the edge of the glass. The water 

 that stands above the brim is held there by a surface-tension 

 membrane. If a needle is carefully lowered on to the surface of 

 water, it will remain in suspension (if slightly oily). The surface 

 film of the water holds it there. Once below the surface, it 

 rapidly falls. When droplets of water are being formed, as when 

 dripping from a faucet, they grow larger and larger until they 

 suddenly fall. They break loose at the moment when the surface 

 film can no longer hold the water within them. When free in 

 the air, liquid droplets assume a spherical shape. Their suspen- 

 sion at the mouth of the faucet, their size before falling, and their 

 spherical shape when free are all determined by the tension of the 

 water at their surface. What is true of the surface of liquids 

 when in contact with air is also true of liquids in contact with 

 other liquids with which they do not mix and of liquids in contact 

 with solids. In all cases, a surface or interfacial film is formed. 



Unbalanced intermolecular attraction at the surface is the 

 physical basis of surface tension. It results in a crowding of 

 the molecules there and the formation of a film (Fig. 93). A 

 molecule within a liquid is attracted by, and itself attracts, all 

 the molecules surrounding it at not too great a distance. Such a 

 molecule would, if uniform distribution existed, have in its imme- 

 diate neighborhood 12 or 14 others, 6 of which would be visible 



157 



