Measurements of interfacial shear viscoelasticity with an oscillatory torsional viscometer

Abstract

The stress-deformation behavior of a non-Newtonian surface subject to an oscillatory shearing flow was theoretically as well as experimentally investigated. The rheological properties of such an interface were interpreted using the complex viscosity surface fluid model. The measurements of surface shear viscoelasticity of an air-liquid surface were performed with the in-house built oscillatory torsional interfacial viscometer. The behavior of the liquid-liquid interface was described based on the exact solution obtained from the analysis of the interfacial rheometer, assuming a non-Newtonian surface and Newtonian bulk phases. The interfacial angular velocity distributions are illustrated as functions of surface viscous and elastic components described by the complex viscosity surface fluid model. An extensive experimental investigation of the surfactant concentration and temperature dependence of the surface viscoelasticity of aqueous gelatin solutions was carried out. The surface rheological behavior encompasses both shear thickening and shear thinning, below and above a characteristic frequency, respectively. The characteristic frequency is independent of temperature, except at 35°C. The surface shear elasticity attains maximum values at a characteristic frequency which is independent of surfactant concentration and temperature, except at 40°C. The temperature dependence of the surface shear viscoelasticity and the origin of the characteristic frequency are explained in terms of the two conformations of gelatin molecules, namely, the α-conformation at 40 and 45°C, and the β-conformation at 35°C. In general, it was found that the temperature and surfactant concentration dependence of the surface shear elasticity was more complex than the corresponding dependence of the surface shear viscosity. Moreover, the experimental results proved the appropriateness of the complex viscosity surface fluid model and the viability of the oscillatory torsional interfacial rheometer.