THE PRINCIPLES OF FILM-FOR^IING SPRAYS^ 

 By William Moore 



When a spray is applied to plants having leaves with a waxy sur- 

 face or heavy cuticle, it usually collects in drops and rolls off the foli- 

 age instead of forming a film over the surface of the leaf. In this 

 paper an attempt is made to explain the forces which cause the 

 spray to collect into drops or to spread out into a thin film, and to 

 point out how the more desirable film-forming spravs mav be made. 

 No definite recommendation of any particular substance will be made, 

 further work is necessary before a decision can be reached as to 

 which material will give the best results when cost and other factors 

 are considered. 



When a drop of liquid falls through the air it assumes a spheri- 

 cal shape, as this form gives the smallest surface for a given volume. 

 The reduction of the surface to a minimum is explained by the un- 

 equal forces of attraction exerted on the molecules at the surface of 

 the liquid. In the interior of the drop any particular molecule is 

 surrounded on all sides by similar molecules and the force of attrac- 

 tion acting on the molecule is the same on all sides. If, however, a 

 molecule at the surface of the drop is considered, it will be seen that 

 on all sides but one it is surrounded by the molecules of the liquid, 

 while on the external side it is in contact with the dissimilar mole- 

 cules of the air. Since the attraction between the molecules of the 

 liquid and those of the air is less than the attraction between the sim- 

 ilar molecules of the liquid, there is a definite force tending to pull 

 the molecule at the surface into the interior of the liquid. This force 

 will reduce the number of molecules in the surface to the smallest 

 possible number, thus producing a minimum surface. The liquid will 

 exhibit at its surface a certain tension which is known as surface 

 tension. 



If the surface of the liquid is to be increased, more molecules 

 must be brought into the surface layer, and since the strongest pull 

 on the molecule is toward the interior, sufficient force must be applied 

 to overcome this attraction of the like molecules. The force neces- 

 sary to bring sufficient molecules from the interior of the liquid to 



1 Published with the approval of the Director as Paper No. 2.>5, of the .Toiinial 

 Series of the Minmesota Agricultural Experiment Station. 



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