184 



SURFACES AND MEMBRANES 



where F is a correction factor and equal to 



and m is the mass of 



... B 



27r/(r/T 1/3 ) 



the drop falling from a tube of radius r. A table of correction factors 

 compiled by Harkins and Brown can be found in the International Criti- 

 cal Tables, Vol. 4, p. 435. With the aid of these tables the error in deter- 

 mining the surface energy is reducible to 0.1 per cent. 



A convenient form of dropping tube, or stalagmom- 

 eter, is shown in Fig. V-4. It is essentially a capil- 

 lary tube the end of which is flattened by compression, 

 carefully ground flat, and polished. The tube is filled, 

 with the liquid under examination, to the major en- 

 graving A. The number of drops which break away 

 from the lower plane surface C are counted while the 

 liquid level falls from the upper mark A to the lower 

 mark B. 



Since the measured surface energy decreases in mag- 

 nitude with rise in temperature, all observations must 

 be made at constant temperature. The experimental 

 results that can be obtained by this method are least 

 subject to error when the lower polished flat release- 

 surface possesses a sharp edge and the drops are 

 allowed to form slowly. Adam [1930] recommends a 

 total time of about ten minutes for the formation of 

 each drop. The surface-energy values will be found 

 too large if the drop forms too rapidly. The time 

 of formation can be shortened if the initial stages of 

 drop formation are rapid and if the final necking is allowed to develop 

 very slowly, for example during one minute, so that the drop eases 

 off very gently. 



In the use of a stalagmometer great care must be exercised to avoid the 

 contamination of the polished release-surface by grease. The slightest 

 trace of grease greatly lowers the surface energy of the water which is 

 used to calibrate the stalagmometer. In order that determinations may 

 be carried out at constant temperature and in the presence of saturated 

 vapor, drops must be allowed to form in a closed, temperature-controlled 

 vessel. 



Fig. V-4 

 agmometer 

 Traube. 



Zjla 



c 



, Stal- 

 after 



The Suspended-Ring Method 



If a biological fluid possesses a low content of surface-active solute, a 

 tensiometer method, which measures the force required to detach a cir- 

 cular ring from the surface, is highly recommended. A metal ring 



