Theoretical models successfully describe the spreading in inert systems.

Inert systems follow definite flow pattern and in most cases a single function is sufficient to describe the whole kinetics. Similar is the case of spreading kinetics. Surface energetics has to be considered while determining the driving force for spreading. Thermodynamic treatment of wetting in inert systems is simple and based on free energy minimization where as that in reactive systems is quite complex. The effect of these factors on wettability is discussed.

Wetting is affected by large number of factors including liquid properties, substrate properties and system conditions. Among these, sessile drop and wetting balance techniques are versatile, popular and provide reliable data. Various methods have been developed over the years to evaluate wettability of a solid by a liquid. Reliable and reproducible measurements of contact angle from the experiments are important in order to analyze the wetting behaviour. Nevertheless Young's equation is the most fundamental starting point for understanding of the complex field of wetting.

The derivation of Young's equation is made under the assumptions of spreading of non-reactive liquid on an ideal (physically and chemically inert, smooth, homogeneous and rigid) solid, a condition that is rarely met in practical situations. Young's equation gives the equilibrium contact angle in terms of interfacial tensions existing at the three-phase interface. A liquid spreading on a substrate with no reaction/absorption of the liquid by substrate material is known as non-reactive or inert wetting whereas the wetting process influenced by reaction between the spreading liquid and substrate material is known as reactive wetting. Wettability is a tendency for a liquid to spread on a solid substrate and is generally measured in terms of the angle (contact angle) between the tangent drawn at the triple point between the three phases (solid, liquid and vapour) and the substrate surface.

Non reactivity balance