IWNET12
IWNET12
Dynamics of complex uid-uid interfaces
Leonard M. C. Sagis1
1 Wageningen University, the Netherlands
Abstract
Surface rheological properties often play an important role in the stability and stress-deformation behavior of
emulsions, foam, biological uids, 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) aect 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 innitesimally 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 uid-uid 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 eect 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 nd 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.
References
[1] L.M.C. Sagis, Rev. Mod. Phys. 83 (2011) 1367.
E-mail: Leonard.Sagis@wur.nl

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