Let S_la be the surface tension of the liquid-air interface, S_sa the surface tension of the solid-air interface, and S_sl the surface tension of the solid-liquid interface. The angle of contact theta satisfies which relation?

cos theta = (S_sa - S_sl)/S_la
cos theta = (S_sa - S_la)/S_sl
cos theta = (S_sl - S_sa)/S_la
None of these

Correct answer: cos theta = (S_sa - S_sl)/S_la

Solution

At the three-phase contact line, the horizontal equilibrium of the surface-tension forces gives Young's relation. Solving for cos theta yields the contact-angle expression.

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