This means that although the difference between the liquid-solid and solid-air surface tension , formula_41 , is difficult to measure directly , it can be inferred from the liquid-air surface tension , formula_38 , and the equilibrium contact angle , formula_26 , which is a function of the easily measurable advancing and receding contact angles ( see main article contact angle ) .
This means that although the difference between the liquid-solid and solid-air surface tension , formula_20 , is difficult to measure directly , it can be inferred from the liquid-air surface tension , formula_17 , and the equilibrium contact angle , formula_5 , which is a function of the easily measurable advancing and receding contact angles ( see main article contact angle ) .