Page 10 - AN INTRODUCTION TO SURFACE CHEMISTRY By ERIC KEIGHTLEY RIDEAL

SURFACE ENERGY OF LIQUIDS 5
(6) Langmuir's differential method for thin insoluble films on
liquid surfaces.
Whilst the dynamic methods include
(T) The ripple method.
(8) Vibrating jets of fluid.

(9) Vibrating drops.
(10) Rate of flow of liquids in capillary tubes.
These methods are capable both of endless variations to suit any
particular set of circumstances and of numerous modifications
limited only by the ingenuity of the experimenter.

3. The angle of contact.
Whilst certain of these methods involve measurements only at
the liquid-vapour or liquid-liquid interfaces involved; in the static
methods we must usually consider besides the interfacial energy of
the two fluids, that between each of them and a solid
There is at present no means of measuring satisfactorily the energy
at any solid surface but the two energies of this kind may be
eliminated from the calculation by taking into consideration the
"angle of contact," in the following manner (Poynting and Thom-
son, Properties of Matter, p. 139).

Let A represent the solid, B, C the two fluids, EG, GD, GF the
two solid-fluid and the fluid-fluid interfaces respectively, the line
GF forming an angle a with ED. This angle is called the angle of
contact of the system. Then since FG represents an equilibrium
configuration the potential energy of the system in this position
must be a minimum, so that an infinitesimal displacement of GF
to G' «will not cause an alteration in the energy of the system.