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Using the usual surface-energy symbols (S_sl for solid-liquid, S_sa for solid-air, S_la for liquid-air interfacial tensions), for which condition does the liquid spread out and completely wet the solid surface?
- S_sl >= S_sa + S_la
- S_sa >= S_sl + S_la
- S_la >= S_sa + S_sl
- None of these
Correct answer: S_sa >= S_sl + S_la
Solution
Spreading converts a solid-air interface into a solid-liquid plus a liquid-air interface. It happens spontaneously (complete wetting) when the original solid-air tension is at least as large as the sum of the two new ones.
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