In-flow measurement of molecular diffusion coefficients using differential dynamic microscopy.

Area of Science:

• Fluid dynamics
• Microfluidics
• Physical chemistry

Background:

• Diffusive mass transport is crucial in fields from medicine to oceanography.
• Experimentally measuring diffusion in flowing fluids is difficult due to shear-induced sample deformation.

Purpose of the Study:

• To develop a robust method for quantitatively measuring molecular diffusion coefficients in laminar flow profiles.
• To address the challenges of local sample deformation and varying shear rates in experimental diffusion studies.

Main Methods:

• Differential dynamic microscopy (DDM) combined with image velocimetry.
• Utilizing subresolution tracer particles within a rectangular microfluidic channel.
• Scanning through different focal planes to capture variations across the flow profile.

Main Results:

• Successfully measured local diffusion coefficients across a laminar flow profile.
• Quantified diffusion in the presence of varying shear rates.
• Demonstrated the method's robustness in a microfluidic setting.

Conclusions:

• Differential dynamic microscopy with image velocimetry offers a powerful approach to study diffusion in dynamic fluid environments.
• The method is adaptable to complex fluids with shear-dependent viscosity.
• Provides a foundation for broader applications in studying transport phenomena.