  The properties of liquid dispersions, such as foams or emulsions, depend
strongly on the volume fraction $\phi$ of the continuous phase. Concentrating
on the example of foams, we show experimentally and theoretically that $\phi$
may be related to the fraction $\phi_s$ of the surface at a wall which is
wetted by the continuous phase - given an expression for the interfacial energy
or osmotic pressure of the bulk system. Since the surface fraction $\phi_s$ can
be readly determined from optical measurement and since there are good general
approximations available for interfacial energy and osmotic pressure we thus
arrive at an advantageous method of estimating $\phi$. The same relationship
between $\phi$ and $\phi_s$ is also expected to provide a good approximation of
the fraction of the bubble or drop surface which is wetted by the continuous
phase. This is a parameter of great importance for the rheology and ageing of
liquid dispersions.
