2012-06-13
10:15 at HCI J 574

Dynamics of complex fluid-fluid interfaces

Leonard Sagis

Wageningen University, NL & ETH Zürich, CH

Surface rheological properties often play an important role in the stability and stress-deformation behavior of emulsions, foam, biological fluids, or immiscible blends [1]. This is particularly true when the interfaces in these systems have a complex microstructure, for example, when the surface active components stabilizing the interface form a 2d gel phase, a 2d glass phase, or 2d (liquid) crystalline phase. Applied deformations induce changes in the microstructure of the interface, and the resulting changes in the surface rheological properties (such as surface shear thinning and thixotropic behavior) affect the behavior of the multiphase system on a macroscopic scale. Most currently available constitutive models for the surface extra stress tensor either do not account for the strain (rate) dependence of surface rheological properties, or are appropriate only for infinitesimally small rates, where departures from linear behavior are very small [1]. In this paper we will discuss recent advances in the development of nonlinear constitutive equations for the stress-deformation behavior of fluid-fluid interfaces in the framework of nonequilibrium thermodynamics (NET). Using the classical irreversible thermodynamics (CIT) framework, and the general-equation-for-the-nonequilibrium-reversible-irreversible-coupling (GENERIC) framework, we construct models that describe the effect of microstructural changes on the nonlinear response of an interface to a deformation through a dependence of the surface stress tensor on a set of scalar and a tensorial structural variables. We present the time evolution equations for these structural variables, and evaluate the ability of these types of models to describe shear thinning behavior of an interface stabilized by anisotropic colloidal particles in both simple and oscillatory shear. We find that both frameworks allow us to construct nonlinear expressions for the surface extra stress tensor capable of describing the shear thinning behavior observed experimentally for this type of interface, but the CIT model gives realistic predictions only for small departures from equilibrium, whereas the GENERIC framework allows us to create models valid also far from equilibrium. These results show that microstructural models developed using NET provide a valuable tool for the analysis of the highly nonlinear dynamics of multiphase systems with complex liquid-liquid interfaces.

[1] L.M.C. Sagis, Rev. Mod. Phys. 83 (2011) 1367.