Separation of velocity distribution and diffusion using PFG NMR

Area of Science:

• Physical Chemistry
• Materials Science
• Rheology

Background:

• Pulsed field gradient (PFG) Nuclear Magnetic Resonance (NMR) is a powerful technique for studying molecular motion and fluid dynamics.
• Standard PFG-NMR experiments face challenges in differentiating between diffusion and directed flow, particularly at low velocities.
• Signal attenuation in PFG-NMR arises from both diffusion and the distribution of velocities, complicating flow analysis.

Purpose of the Study:

• To present a modified PFG-NMR approach for accurately investigating flow processes, especially at very low velocities.
• To differentiate the contributions of diffusion and directed motion to the NMR signal attenuation.
• To demonstrate the capability of the method for analyzing slow flow in a rheometer cell.

Main Methods:

• Implementing a PFG-NMR experiment where velocity is incremented at a constant gradient strength.
• Analyzing NMR signal phase modulation and attenuation patterns to decouple diffusion and directed motion effects.
• Utilizing the Fourier conjugation relationship between velocity and gradient strength to enhance flow sensitivity.

Main Results:

• The modified PFG-NMR method successfully separates the velocity distribution into diffusion and directed motion components.
• Incrementing velocity at constant gradient strength isolates the signal attenuation caused by the distribution of directed motion.
• The technique preserves phase modulation, a key indicator of flow, even for very low velocities masked by diffusion.

Conclusions:

• This PFG-NMR strategy enables the measurement of extremely low velocities, overcoming limitations of conventional methods.
• The ability to distinguish directed motion from diffusion significantly enhances the understanding of complex flow phenomena.
• The method is validated for studying slow flow dynamics within a rheometer, offering new insights into material behavior.