Dynamics of multiphase systems with complex microstructure. II. Particle-stabilized interfaces
Sagis, L.M.C. - \ 2013
Physical Review. E, Statistical nonlinear, and soft matter physics 88 (2013)2. - ISSN 1539-3755 - 9 p.
fourier-transform rheology - in-water emulsions - superficial viscosity - bending rigidity - surface - liquid - viscoelasticity - liquid/liquid - fluid - gas/liquid
In this paper we use the GENERIC (general equation for nonequilibrium reversible-irreversible coupling) nonequilibrium thermodynamics framework to derive constitutive equations for the surface extra stress tensor of an interface stabilized by a two-dimensional suspension of anisotropic colloidal particles. The dependence of the surface stress tensor on the microstructure of the interface is incorporated through a dependence on a single tensorial structural variable, characterizing the average orientation of the particles. The constitutive equation for the stress tensor is combined with a time-evolution equation describing the changes in the orientation tensor as a result of the applied deformation field. We examine the predictions of the model in in-plane steady shear flow, in-plane oscillatory shear flow, and oscillatory dilatational flow. The model is able to predict the experimentally observed shear thinning behavior in surface shear flow, and also the experimentally observed emergence of even harmonics in the frequency spectrum of the surface stress in oscillatory dilatational flow. Our results show that the highly nonlinear stress-deformation behavior of interfaces with a complex microstructure can be modeled well using simple structural models like the one presented here.
Normal stresses in surface shear experiments
Sagis, L.M.C. - \ 2013
The European Physical Journal. Special Topics 222 (2013)1. - ISSN 1951-6355 - p. 99 - 103.
in-water emulsions - interfacial permeability - general formalism - bending rigidity - complex fluids - dynamics - viscoelasticity - thermodynamics - liquid/liquid - gas/liquid
When viscoelastic bulk phases are sheared, the deformation of the sample induces not only shear stresses, but also normal stresses. This is a well known and well understood effect, that leads to phenomena such as rod climbing, when such phases are stirred with an overhead stirrer, or to die swell in extrusion. Viscoelastic interfaces share many commonalities with viscoelastic bulk phases, with respect to their response to deformations. There is however little experimental evidence that shear deformations of interfaces can induce in-plane normal stresses (not to be confused with stresses normal to the interface). Theoretical models for the stress-deformation behavior of complex fluid-fluid interfaces subjected to shear, predict the existence of in-plane normal stresses. In this paper we suggest methods to confirm the existence of such stresses experimentally.
Rheology of interfaces stabilized by a 2D suspension of anisotropic particles: a classical irreversible thermodynamics theory
Sagis, L.M.C. - \ 2011
Soft Matter 7 (2011)17. - ISSN 1744-683X - p. 7727 - 7736.
nonequilibrium thermodynamics - superficial viscosity - general formalism - latex-particles - complex fluids - liquid - liquid/liquid - dynamics - viscoelasticity - gas/liquid
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.