For heterogeneous surfaces (consisting of two or more different types of material), the contact angle of a drop of liquid at each point along the three phase contact line with a solid surface is a result of the surface tension of the surface at that point. For example, if the heterogeneous regions of the surface form very large domains, and the drop exists entirely within a homogeneous domain, then it will have a contact angle corresponding to the surface tension of that homogeneous region. Likewise, a drop that straddles two domains of differing surface tensions will have different contact angles along the three phase contact line corresponding to the different surface tensions at each point.  However, with sufficiently small domains (such as in those of a block copolymer), the observed surface energy of the surface approaches the weighted average of the surface energies of each of the constituents of the surface:
where:
: is the overall surface energy of the polymer
: is the fraction of the  component of the polymer's surface
: is the surface energy of the  component
This occurs because as the size of the homogeneous domains become very small compared to the size of the drop, the differences in contact angles along different homogeneous regions becomes indistinguishable from the average of the contact angles.
The observed contact angle is given by the following formula:
where:
:is the fraction of  component
: is the contact angle  component
If the polymer is made out of only two different monomers, it is possible use the above equation to determine the composition of the polymer simply by measuring the contact angle of a drop of liquid placed on it:
where:
: is the observed contact angle
: is the area fraction of one component, and  the area fraction of the other.
: and  are the contact angles of the first and second components of the polymer.