186 



SURFACES AND MEMBRANES 



breaks through the liquid surface. The liquid surface is adjusted so 

 that, for its final reading at rupture, the beam lies in a horizontal position. 

 If the ring has a mean circumference of 4.00 cm and is used on pure 



water at room temperature, the 

 value of the correction factor F will 

 be approximately 0.990. Since F 

 can be determined for any given 

 case, the apparatus is sensitive to a 

 force of about 0.1 dyne and has the 

 advantage of being able to use small 

 quantities of liquid at high operat- 

 ing speeds. 



Certain experimental precautions 

 are essential if a moderately high 

 degree of precision is desired in the 

 use of this instrument: 



1. The liquid should be covered 

 by an inverted glass funnel to pre- 

 vent cooling by evaporation. 



2. The liquid and the vapor 

 above it must be kept at a con- 

 stant temperature. 



3. The vessel used to contain the 

 sample should be of such a shape and size that the surface need not be 

 corrected for curvature of meniscus of the liquid. 



4. The design of the vessel should be such that the surface of the 

 liquid may be renewed or swept by a clean glass bar. Tables of correc- 

 tion factors are supplied by the manufacturers of this instrument for the 

 particular ring adopted. 



Since the surface energy of water and other liquids increases appreci- 

 ably with a decrease in temperature, it is necessary to know the tem- 

 perature and the temperature coefficient of the liquid under test. The 

 temperature coefficient of water between 15 and 25° C is, according to 

 Harkins, 0.154 erg/cm 2 /°C; between 10 and 60° C its surface energy 

 may be expressed by 



T = 75.796 



Fig. V-5. Cenco-duNouy precision 

 tensiometer. (Courtesy Central Scien- 

 tific Company.) 



0.145* - 0.00024* 2 



The Effect of Surface Impurities on Surface-Energy Measure- 

 ments 



In determining the speed with which a substance spreads over the 

 surface of a liquid, the ring method of measuring surface energy is used 



