Owens - Wendt model is used for calculating surface energy on liquid - solid interface and it is given by following equation: $$ \gamma_{sl} = \gamma_s + \gamma_l -2(\sqrt {\gamma_l^d \gamma_s^d} + \sqrt {\gamma_l^p \gamma_s^p}) $$

So, if we use liquid and solid of known surface energy as well as their components (dispersive and polar contributions) we can calculate surface energy of the interface.

It is stated that model has 2 assumptions:

  1. Total surface energy of any individual component (solid and liquid) is a sum of polar and dispersion contributions

  2. Dispersion and polar interactions between solid and liquid on the interface contribute to decrease of surface energy of the interface as geometric mean of individual contributions

Given these assumptions:

  1. How is the equation of this model derived (equation written in the question)?
  2. Why is there number 2 multiplying geometric mean contributions for decreasing surface energy of the interface since formula for geometric mean doesn't include that number?


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