= Interfacial-energy difference from two contact angles
{title2=$\gamma_{SL_1}-\gamma_{SL_2}=\gamma_{L_2V}\cos\theta_2-\gamma_{L_1V}\cos\theta_1$}
Subtracting <Young's equations> for two liquids on the same solid eliminates the unknown solid–vapour <interfacial energy>. The result assumes equilibrium angles and an unchanged solid surface. Swelling, contamination or <contact-angle hysteresis> can invalidate this cancellation as a comparison of equilibrium material properties.
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