Past exam of the soft condensed matter course of the University of Cambridge / 2021 / Section B / 5 / c / Solution

For an ideal smooth homogeneous substrate in thermodynamic equilibrium, Young's equation iswhere $A$ denotes air, $S$ the solid, and $L$ the chosen liquid. It follows either by balancing tangential interfacial forces at the contact line or by varying the wetted area at fixed drop volume.

The dry Teflon–air surface energy is the same in the two measurements, so subtracting the two equations eliminates it:ThereforeThe water–Teflon interface has the higher interfacial free energy per unit area. Neither individual solid–liquid surface energy is determined without the solid–air value; the difference is determined.