Velocity measurement by coherent x-ray heterodyning

Area of Science:

• Soft Matter Physics
• Materials Science
• Fluid Dynamics

Background:

• X-ray Photon Correlation Spectroscopy (XPCS) is a powerful tool for studying dynamics in materials.
• Traditional XPCS (homodyne) faces limitations in measuring slow flow velocities due to signal decay.
• A need exists for enhanced techniques to accurately quantify slow fluid motion.

Purpose of the Study:

• To introduce and validate a novel small-angle coherent x-ray scattering technique for measuring slow flow velocities.
• To extend XPCS capabilities by incorporating heterodyning for improved sensitivity to flow dynamics.
• To provide a robust method for characterizing fluid flow profiles in microfluidic systems.

Main Methods:

• Developed a heterodyne XPCS technique by mixing scattering from moving tracers with a static reference.
• Implemented small-angle coherent x-ray scattering experiments.
• Analyzed temporal intensity fluctuations to extract flow velocity information.

Main Results:

• Successfully measured flow velocities in a viscous fluid ranging from 0.1 to 10 μm/s.
• Demonstrated the ability to resolve Poiseuille flow profiles in a rectangular channel.
• Validated the theoretical framework for heterodyne XPCS in various flow regimes.

Conclusions:

• Heterodyne XPCS significantly enhances the measurement of slow flow velocities compared to traditional methods.
• The technique is versatile and applicable to different flow types, including uniform and Couette flows.
• This advancement offers a valuable tool for rheological studies and microfluidic characterization.