178 SURFACES AND MEMBRANES 



dynes per centimeter. If these spheres are brought in contact they will 

 coalesce to form one sphere whose diameter is 0.6 cm and whose potential 

 surface energy is E = O.SQtT ergs per square centimeter. The differ- 

 ence, or 0.14wT, is the loss in potential surface energy that has been 

 transformed into kinetic energy, which manifests itself as a rise in 

 temperature of the newly formed sphere. For water, where T = 72.75 

 ergs/cm 2 at 20° C, this rise amounts to 7.5 X 10 -7 calorie. The con- 

 verse case possesses physiological interest. How much energy would be 

 used to break the above drop into ten thousand equal spherical 

 fragments? 



Surface Energy of Spreading Cells 



In the response of living cells to contact with solid bodies, the surface 

 energy and contact angles developed at the interfaces play an important 

 part, if not a major one, in determining the physical response of cells to 

 changes in environment. 



The behavior of a cell towards a horizontal flat surface with which it 

 may come in contact and attain an equilibrated shape or, if surface con- 

 ditions are favorable, spread to molecular film thickness has many 

 biological implications. The familiar adhesiveness exhibited by blood 

 cells and the spreading of phagocytes in the process of ingesting small 

 particles are typical examples. 



CP interface 



P PG interface Plasma 



G Glass ~~ ) 



Fig. V-l. Cell immersed in plasma before contact with glass surface. 



The complex phenomena of the response of a living cell to a flat 

 surface or spherical particle with which it makes contact is best under- 

 stood if an idealized simple case is first analyzed and examined for the 

 physical implications involved. The existence of a perfect fluid is 

 assumed which is immersed in plasma of the same density as the cell. 



