Structure of AOT reverse micelles under shear.

Area of Science:

• Colloid and Surface Science
• Soft Matter Physics
• Rheology

Background:

• Reverse micelles are self-assembled aggregates crucial in various chemical and biological processes.
• Understanding their structural transitions under different conditions is key to controlling their properties.

Purpose of the Study:

• To investigate the structural evolution of AOT/water/isooctane reverse micelles with varying water content.
• To correlate these structural changes with rheological behavior under static and shear conditions.

Main Methods:

• Rheometry was employed to measure viscosity and flow properties.
• Small-angle X-ray scattering (SAXS) was used to determine micelle shape and structure.

Main Results:

• SAXS confirmed spherical micelles at low water content, transitioning to elongated (cylindrical) micelles at higher content.
• A coexistence of spherical and cylindrical micelles was observed even above the percolation threshold.
• Rheometry showed a viscosity increase correlated with micelle shape transition.
• Under shear, low water content micelles behaved as Newtonian fluids, while high water content micelles exhibited shear-thinning behavior.
• Shear forces aligned cylindrical micelles with flow, increasing the anisotropy parameter.

Conclusions:

• Water content significantly dictates the shape of AOT reverse micelles, transitioning from spherical to cylindrical.
• This shape transition directly influences the bulk rheological properties, particularly viscosity and shear response.
• Shear flow induces alignment of elongated micelles, enhancing structural order without altering micelle dimensions.