where y/ is the critical surface tension, below which 0is zero and the surface is perfectly

wetted. This is considered to be a measure of the surface free energy of the solid. The

rationale behind this stems from Young's equation, represented as


 Ys = s/+ 1 Cos 0

where represents the surface tension (or free energy) and the subscripts s, sl, and 1 refer

to the solid-vapor, solid-liquid, and liquid-vapor interfaces respectively.[123] Essentially

this is a force balance between interfaces of a drop on a surface, with the solid-vapor and

solid-liquid opposing each other in the plane of the solid. The basis of this theory is that

with decreasing y/ towards ,, then the solid-liquid surface tension is minimized and the

solid-vapor surface tension will equal the liquid-vapor surface tension or surface free

energy.

 Contact angles were taken using the sessile drop method with 5 different liquids

of known surface tension on the surface. Ten readings were taken for each sample and

liquid by measuring the angle on both sides of the drop. The liquids used and their

corresponding surface tensions can be found in Table 3.4.




Table 3.4 Surface tension of liquids used for surface energy determination by contact
angle analysis[124]
 Liquid Surface Tension (mN/m)
 Water 73.05
 Methylene Iodide (Mel) 50.76
 N,N-Dimethylformamide (DMF) 37.1
 Acetonitrile (ACN) 29.30
 1-Propanol 23.78