Microscopic Model of Surface Tension

Let us consider a water-air interface. If we assume cubic geometry of water molecules, six neighboring molecules are present inside bulk of the fluid for each molecule. A surface molecule has five nearest liquid molecules with one missing above in the gas. The area covered by a single molecule is roughly A (0.3 nm). The typical intermolecular bond energy, A , in a liquid is of the order of couple of thermal energies, A 50 meV. 

Note that for water-air interface at 20 °C, a = 72.9 mJ/m, which is close the microscopic estimation of equation (5.4). 

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