Dynamic properties of interfaces in soft matter: Experiments and theory

The dynamic properties of interfaces often play a crucial role in the macroscopic dynamics of multiphase soft condensed matter systems. These properties affect the dynamics of emulsions, of dispersions of vesicles, of biological fluids, of coatings, of free surface flows, of immiscible polymer blends, and of many other complex systems. The study of interfacial dynamic properties, surface rheology, is therefore a relevant discipline for many branches of physics, chemistry, engineering, and life sciences. In the past three to four decades a vast amount of literature has been produced dealing with the rheological properties of interfaces stabilized by low molecular weight surfactants, proteins, (bio)polymers, lipids, colloidal particles, and various mixtures of these surface active components. In this paper recent experiments are reviewed in the field of surface rheology, with particular emphasis on the models used to analyze surface rheological data. Most of the models currently used are straightforward generalizations of models developed for the analysis of rheological data of bulk phases. In general the limits on the validity of these generalizations are not discussed. Not much use is being made of recent advances in nonequilibrium thermodynamic formalisms for multiphase systems, to construct admissible models for the stress-deformation behavior of interfaces. These formalisms are ideally suited to construct thermodynamically admissible constitutive equations for rheological behavior that include the often relevant couplings to other fluxes in the interface (heat and mass), and couplings to the transfer of mass from the bulk phase to the interface. In this review recent advances in the application of classical irreversible thermodynamics, extended irreversible thermodynamics, rational thermodynamics, extended rational thermodynamics, and the general equation for the nonequilibrium reversible-irreversible coupling formalism to multiphase systems are also discussed, and shown how these formalisms can be used to generate a wide range of thermodynamically admissible constitutive models for the surface stress tensor. Some of the generalizations currently in use are shown to have only limited validity. The aim of this review is to stimulate new developments in the fields of experimental surface rheology and constitutive modeling of multiphase systems using nonequilibrium thermodynamic formalisms and to promote a closer integration of these disciplines.

Figure 1

Surface rheology plays a central role in understanding macroscopic dynamic behavior of multiphase soft condensed matter. When combined with experimental structure evaluation on the molecular scale, theoretical modeling (statistical physics, nonequilibrium thermodynamics), and computer simulations (for example, nonequilibrium molecular dynamics), surface rheology can be used to link information on the molecular scale of a system to its dynamics on the macroscopic scale.

Figure 2

$Q_{xy}^s$, $Q_{xx}^s$, and ${\epsilon }_{\mathrm{eff}}^s$ as a function of time, for three different shear rates $\dot{\gamma }$: 0.1, 0.08, and $0.05{\mathrm{s}}^{-1}$. Parameters: $K_1=1\mathrm{Ns}/\mathrm{m}$, $K_3=-4.0\mathrm{Ns}/\mathrm{m}$, $\alpha =8.0\mathrm{J}/\mathrm{kg}$, $L_1^s/{\rho }^s=10.0$, $L_2^s/{\rho }^s=10.0$, $L_3^s/{\rho }^s=-5.0$, ${\nu }_3=2({\nu }_1+{\nu }_2)/{\rho }^s=-1.5{\mathrm{s}}^{-1}$, $D_r^s=1.0{\mathrm{s}}^{-1}$, and ${\rho }^s=2.0\times {10}^{-6}\mathrm{kg}/{\mathrm{m}}^2$.

Figure 3

A multiscale multidisciplinary approach to model dynamics of multiphase soft condensed matter systems, combining surface rheology with experimental structural evaluation on molecular and microscales, theoretical modeling, and numerical simulations.