I want to consider all reasons for decreasing dynamic viscosity after addition of cosurfactant.
In principle, yes, there's a direct relation - if the surface is highly elastic (high adsorption), the droplet surface is tangentially immobile, while at low adsorption/elasticity the surface is mobile, which decreases the effective viscosity. In practice, I doubt that it is important since a minute quantity of adsorbed surfactant is enough to bring about strong Marangoni effect and immobilize the surface entirely. Marangoni effect in mixed films has not been investigated, as far as I know, but I guess the mobility is again close to zero at any condition met in practice. I also guess that the cosurfactant most important effects will be through droplet deformability (i.e., the interfacial tension), the depletion effect and the interaction between droplets.
It depends on the microemulsion structure. If it is in the form of spherical drops, then the interfacial elasticity will have no effect. Viscosity will be close to the calculation for a hard sphere fluid (from Einstein to Bachelor to Quemada, as disperse phase volume fraction increases). The elasticity will have a big effect if the structure is bicontinuous, as then deforming and breaking interfacial films is an essential part of the flow process. Softer interface = lower viscosity
Dear Jacub,
First of all I should say that the rigidity of your droplets crust is directly depends on water/surfactant (in the case of water/oil microemulsions) or oil/surfactant (in the case of oil/water microemulsions) and these ratios are inversly related to the rigidity, but I think the viscosity of your microemulsion depends on the concentration of surfactant in the bulk fluid beside the rigidity, I mean If you want to have more viscose microemulsion, you should decrease water/surfactant ratio and rise surfactant concentration in the microemulsion.
Dear Alan,
I found Your reasoning very useful and i clarified me a lot. But I need literature confirmation of this information (softer interface = lower viscosity) and I cannot find. Do You have something at hand?
Regards
JP
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