# Effects of contact angles

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The spreading coefficient is: $S=\sigma _{s}-\left( \sigma _{sl}+\sigma _{lv} \right)$

This is often called the “initial” spreading coefficient.

If the spreading coefficient is positive, the liquid “spreads” on the solid (b); if the spreading coefficient is negative, the liquid “partially spreads” on the solid (a). If the contact angle is less than 90o, the liquid is said to “wet” the solid; if the contact angle is greater than 90o, the liquid is said to “not wet” the solid.