A Method to Measure the Diffusion Coefficient in Liquids.

Area of Science:

• Physical Chemistry
• Fluid Dynamics
• Materials Science

Background:

• Molecular diffusion homogenizes solute concentration, influencing reaction rates in biological and chemical systems.
• Current methods like Dynamic Light Scattering and Fluorescent Correlation Spectroscopy require prior knowledge of fluid or tracer properties.
• Direct measurement of diffusion coefficients (D) is crucial for understanding mixing and dilution processes.

Purpose of the Study:

• To introduce a novel optical method for directly measuring the diffusion coefficient (D) of solutes and suspensions in liquids.
• To validate the method using spherical colloids and subsequently apply it to non-spherical tracers.
• To quantify tracer mixing within a confined diffusion chamber and assess the impact of chamber size.

Main Methods:

• Directly measuring the tracer's concentration spatial profile using optical techniques within a diffusion chamber.
• Utilizing a diffusion chamber to confine the tracer and fluid.
• Employing Einstein-Stokes relation for initial validation with spherical colloids.

Main Results:

• The novel method successfully measured the diffusion coefficient (D) for both spherical and non-spherical tracers.
• Diffusion-limited mixing in confined spaces was observed to be faster than in unconfined domains.
• The finite size of the diffusion chamber significantly impacts the estimation of D and tracer mixing degree.

Conclusions:

• The proposed optical method offers a direct and versatile approach to measure diffusion coefficients (D) without prior tracer or fluid property knowledge.
• Accurate estimation of D and tracer mixing requires accounting for the finite dimensions of the diffusion chamber.
• This technique advances the understanding of diffusion and mixing dynamics in confined environments.