Capillarity and Surface Forces (Liquids) - Surface and Colloid Chemistry: Principles and Applications - page 24

Liquid

Drop

FIGure 2.10 Drop formation of a liquid in a tube

A simple method is to count the number of drops (e.g., 10 or more) and measure
the weight.
A more convenient method may be used, in which a fluid is pumped and the
drops are collected and weighed. Since in some systems (solutions) there may be
kinetic effects, care must be taken to keep the flow as slow as possible. This system
is very useful in studying what one finds in daily life phenomena: oil flow, blood cells
flowing through arteries, etc. In those cases where the volume of fluid available is
limited, this method may be used with advantage. By decreasing the diameter of the
tubing, one can work with fluids less than 1 mL. This may be the case for systems
such as eye fluids, etc.
The magnitude of γ can be determined from either the maximum weight or the
shape of the drop.
Maximum weight method: The “detachment” method is based upon the following:
to detach a body from the surface of a liquid that wets the body, it is necessary to over-
come the same surface tension forces that operate when a drop is broken away. The
liquid attached to the solid surface on detachment creates the following surfaces:

Initial stage: Liquid attached to solid
Final stage: Liquid separated from solid

In the process from the initial to the inal stage, the liquid molecules that were near
the solid surface have been moved away and are now near their own molecules. This
requires energy, and the force required to make this happen is proportional to the
surface area of contact and the surface tension of the liquid.
Methods of determining surface tension by measuring the force required to detach a
body from a liquid are therefore similar to the stalagmometer method described earlier.