Direct imaging of long-range concentration fluctuations in a ternary mixture

Area of Science:

• Physical Chemistry
• Thermodynamics
• Fluid Dynamics

Background:

• Understanding critical phenomena in multi-component mixtures is crucial for thermodynamics and fluid dynamics.
• Thermal fluctuations significantly influence concentration dynamics near the consolute critical point.
• Accurate measurement of the mutual mass diffusion coefficient (D) is essential for characterizing mixture behavior.

Purpose of the Study:

• To investigate concentration fluctuations enhanced by thermal fluctuations in a ternary mixture of methanol, cyclohexane, and partially deuterated cyclohexane.
• To determine the mutual mass diffusion coefficient (D) near the consolute critical point using direct imaging.
• To compare experimental diffusion coefficients with theoretical predictions.

Main Methods:

• Employed a direct imaging technique utilizing a low-coherence white-light source and red filter.
• Analyzed fluctuation images with a differential dynamic microscopy algorithm to determine correlation time (τ).
• Calculated the mutual mass diffusion coefficient (D) from the correlation time and compared with numerical estimations.

Main Results:

• Successfully visualized and analyzed concentration fluctuations in the ternary mixture.
• Determined the mutual mass diffusion coefficient (D) very near and above the consolute critical point.
• Experimental D values closely align with predictions from the Stokes-Einstein diffusion law incorporating Kawasaki's correction.

Conclusions:

• Direct imaging combined with differential dynamic microscopy is effective for studying critical phenomena.
• The study validates theoretical models for diffusion near critical points in ternary mixtures.
• Findings contribute to a deeper understanding of mass transport in complex fluid systems.