Rheology of interfaces stabilized by a 2D suspension of anisotropic particles: a classical irreversible thermodynamics theory

Surface rheological properties have a significant impact on the stability of particle-stabilized emulsions and foams. Interfaces stabilized by anisotropic particles display a highly nonlinear surface rheology, even at relatively small deformation rates. The nonlinearity of the response is the result of changes in the microstructure of the interface, induced by the applied deformation. The particles are oriented in the direction of the imposed flow field, and this leads to a decrease in the surface shear viscosity (shear thinning). In this paper we derive nonlinear constitutive equations for the surface stress tensor of an interface stabilized by a mixture of anisotropic particles and low molecular weight surfactants, using the classical irreversible thermodynamics formalism. These equations are valid in the low shear regime, where departures from linear behavior are still small. The effect of the microstructure of the interface on the rheological response is incorporated through the particle orientation tensor Qs. The constitutive equations are able to predict the shear thinning behavior observed experimentally for this type of interface.