Dilational and shear rheology of adsorption layers at liquid interfaces

Abstract

Dynamic properties of interfaces are of increasing interest in science and in practice as they give insight into interactions and processes at interfaces rather than equilibrium properties. The general ideas on mechanical interfacial properties as an important part of dynamic properties were established long ago by Gibbs and Boussinesq. Now on the basis of new techniques, better experiments can be performed which allow a more and more quantitative understanding. The mechanical behaviour of interfaces, modified by soluble adsorption layers or insoluble monolayers of surfactants or polymers, is the subject of many actual studies. Computer-driven instruments using new sensors and very sophisticated methodologies enable us to perform very complex and sensitive measurements which were impossible until recently. Numerous studies of interfacial shear and dilational rheology have been reported and use a large variety of techniques. Shear experiments are most useful for polymer and mixed polymer-surfactant adsorption layers and insoluble monolayers and give access to interaction forces in two-dimensional layers. Dilational interfacial properties however are most frequently studied for soluble adsorption layers of surfactants and mixtures of polymers and surfactants. This overview gives an introduction to the interfacial rheology and discusses some specific theoretical aspects necessary to interpret experiments. Experimental techniques to perform shear and dilational experiments at liquid interfaces are summarised and only the most recent developments are described in more detail. Examples are given to demonstrate how the experiments work and what output can be expected.