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Advances in Colloid and Interface Science (v.133, #2)
Review of non-reactive and reactive wetting of liquids on surfaces by Girish Kumar; K. Narayan Prabhu (pp. 61-89).
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. 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. Young's equation gives the equilibrium contact angle in terms of interfacial tensions existing at the three-phase interface. 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. Nevertheless Young's equation is the most fundamental starting point for understanding of the complex field of wetting.Reliable and reproducible measurements of contact angle from the experiments are important in order to analyze the wetting behaviour. Various methods have been developed over the years to evaluate wettability of a solid by a liquid. Among these, sessile drop and wetting balance techniques are versatile, popular and provide reliable data.Wetting is affected by large number of factors including liquid properties, substrate properties and system conditions. The effect of these factors on wettability is discussed. Thermodynamic treatment of wetting in inert systems is simple and based on free energy minimization where as that in reactive systems is quite complex. Surface energetics has to be considered while determining the driving force for spreading. Similar is the case of spreading kinetics. Inert systems follow definite flow pattern and in most cases a single function is sufficient to describe the whole kinetics. Theoretical models successfully describe the spreading in inert systems. However, it is difficult to determine the exact mechanism that controls the kinetics since reactive wetting is affected by a number of factors like interfacial reactions, diffusion of constituents, dissolution of the substrate, etc. The quantification of the effect of these interrelated factors on wettability would be useful to build a predictive model of wetting kinetics for reactive systems.
Keywords: Wetting; Contact angle; Thermodynamics; Kinetics; Modeling
Impact of atomic force microscopy on interface and colloid science by H.-J. Butt; R. Berger; E. Bonaccurso; Y. Chen; J. Wang (pp. 91-104).
Since its invention twenty years ago the atomic force microscope (AFM) has become one of the most important tools in colloid and interface science. The reason for this impact is that the AFM allows doing experiments on length, time, force, and energy scales, which are not accessible by any other technique. These experiments can be carried out under natural conditions, for example in liquid environments. In this paper we specify the length and time scales involved, give examples where by using the AFM relevant questions in colloid and interface science have been solved, and we discuss future perspectives.