Explanation of Formation of Drops and Bubbles

For a drop on a solid surface: cosθ = \[\frac {T_2-T_1}{​T_3}\]​​,

where T₁ = solid-liquid,

T₂ = solid-air, 

T₃ = liquid-air surface tension

• When T₂ > T₁, cos θ is +ve, angle is acute — liquid partially wets the solid. (e.g., Kerosene on glass)
• When T₂ < T₁, cos θ is −ve, angle is obtuse — liquid does not wet the solid. (e.g., Mercury on glass)
• When T₂ − T₁ = T₃, cos θ = 1, θ = 0° — liquid completely wets and spreads over the solid. (e.g., Pure water on clean glass)
• When T₂ − T₁ > T₃, cos θ > 1, which is impossible — no drop forms, liquid simply spreads; equilibrium is not possible.