Wetting index is basically a chemical feature of certain fluids. Solvents stay engaged with the product much longer. Therefore, you have a higher wetting index, and it stays in contact with the surface. Water is a poor wetting index because it sheets off residues quicky, if you have ever waxed a car then throw water on it, you’ll know what I’m referring to. However, if you put soap on the waxed car, the soap will stay longer because it attacks the soils. The wetting index is better because it keeps the solvent in contact with the contaminants for the removal and it also makes it easier to fit different configurations or custom components.