Intermembrane spacing and velocity profiling of a lamellar lyotropic complex fluid under flow using x-ray diffraction

Area of Science:

• Rheology
• Fluid Dynamics
• Materials Science

Background:

• Understanding fluid behavior in confined geometries is crucial for various industrial processes.
• Non-Newtonian complex fluids exhibit unique flow properties that differ from simple liquids.
• Characterizing velocity profiles in such fluids is challenging but essential for accurate modeling.

Purpose of the Study:

• To employ X-ray diffraction for determining velocity profiles in a non-Newtonian complex fluid.
• To investigate the flow behavior of a concentrated undulating membrane system in a capillary tube.
• To correlate intermembrane spacing with shear rate and shear stress.

Main Methods:

• Utilized X-ray diffraction to measure intermembrane separation (d) under laminar flow conditions.
• Employed a Couette flow cell to apply simple shear and measure d as a function of shear rate and stress.
• Mapped intermembrane spacing across the capillary tube's cross-section to derive shear-rate profiles.
• Performed numerical integration to calculate the fluid's velocity profile.

Main Results:

• Observed a logarithmic dependence of intermembrane separation on shear rate.
• Found a linear relationship between fractional intermembrane spacing and shear stress.
• Successfully calculated shear-rate profiles within the capillary.
• Derived accurate velocity profiles, demonstrating both shear thickening and plug flow shear thinning behaviors.

Conclusions:

• X-ray diffraction is an effective method for extracting velocity profiles from complex fluids.
• The undulating membrane system exhibits distinct rheological behaviors (shear thickening/thinning) under laminar flow.
• The study provides valuable insights into the microstructural dynamics of complex fluids in confined flow.