Limiting coalescence by interfacial rheology: over-compressed polyglycerol ester layers.

The stabilisation of foam bubbles is of both scientific interest and technological importance. In this respect, the study of polyglycerol ester-based nonionic surfactants is highly relevant as they lead to remarkable foam stability. The present work investigates the thermodynamic and rheological properties leading to this stability with a particular focus on out-of-equilibrium conditions of the surface. These may occur after bubble coalescence events, which lead to a compression of the interface. To separate the effects of such a compression on surface tension and the extra stresses which arise due to deformation of the interface, a double wall ring geometry mounted in a ribbon trough was used. Surface tension and the linear viscoelastic properties are shown to be affected differently by compression of the surface. The surface tension relaxes to an equilibrium value while the surface moduli will continue to rise which each compression step. The presence of multilayered structures, which have been reported for polyglycerol esters, explains how these differences arise. The polyglycerol ester-based nonionic surfactants are an example of materials where a true surface rheological response, related to the extra and deviatoric stresses, can be used to limit coalescence. [ABSTRACT FROM AUTHOR]