= Young's equation
{c}
{title2=$\gamma_{SV}=\gamma_{SL}+\gamma_{LV}\cos\theta$}
For a smooth homogeneous solid in <thermodynamic equilibrium> with a liquid and vapor, <Young's equation> relates their three interfacial tensions to the <contact angle> measured through the liquid. A virtual increase $dA$ of wetted area changes the interfacial <free energy> by $(\gamma_{SL}-\gamma_{SV}+\gamma_{LV}\cos\theta)dA$. Setting this to zero proves the equation. Roughness, pinning and chemical heterogeneity can make an observed apparent angle differ from this ideal equilibrium value. The equation does not determine each solid–liquid <surface energy> from a contact angle alone, but two liquids on the same unchanged dry substrate allow the solid–vapor term to be eliminated by subtraction.
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