Rheology of gas/liquid and liquid/liquid interfaces with aqueous and biopolymer subphases

Abstract

The stability, performance and quality of multiphase fluid products such as emulsions or foams are strongly influenced by the structure and rheology of the interfacial film between two immiscible liquids or between air and a liquid. In this study, an interfacial shear rheometer for both stress- and strain-controlled measurements at liquid/liquid and gas/liquid interfaces is used to study the interfacial rheology of the water-insoluble surfactant sorbitan tristearate. The subphase is either pure water or an aqueous viscous solution of κ-carrageenan. Air/water and air/solution as well as oil/solution interfaces with different bulk viscosity ratios are investigated. Experiments were performed in steady and oscillatory shear, as well as in stress relaxation (strain step) and creep recovery (stress pulse) mode. A setup combining interfacial rheometry with Brewster-angle microscopy (BAM) is presented and used for real-time imaging of the gas/liquid interface during rheological experiments. The problem of fracture in interfacial films during shear flow even at small deformations is addressed using both interfacial rheometry and BAM images.

Acknowledgements: Parts of this work were supported by the Swiss National Science Foundation (SNF Project No. 2100–063976). The authors are thankful to the staff at the mechanical workshop of the Laboratory of Food Process Engineering (ETH Zürich) for their assistance in building and setting up the Brewster-angle microscope. We are also grateful to Fabien Rubli (ETH) for assistance with the measurements presented here. Heiko Stettin and Matthias Prenzel at Physica Messtechnik GmbH are acknowledged for their work on the analysis software.