Nondiffusive decay of gradient-driven fluctuations in a free-diffusion process.

Area of Science:

• Physical Chemistry
• Fluid Dynamics
• Soft Matter Physics

Background:

• Understanding concentration fluctuations is crucial for diffusion processes in mixtures.
• Previous studies often focused on shorter wavelengths, inaccessible to techniques like dynamic light scattering.
• The role of buoyancy in modifying diffusion dynamics at large scales remained less explored.

Purpose of the Study:

• To experimentally investigate the static and dynamic properties of long wavelength concentration fluctuations.
• To measure fluctuation amplitudes and temporal correlations for wave vectors below the limits of dynamic light scattering.
• To analyze the influence of buoyant forces on diffusion in a glycerol-water mixture with a stabilizing concentration gradient.

Main Methods:

• Employed the shadowgraph method for precise measurements.
• Measured mean-squared amplitude and temporal correlation functions of concentration fluctuations.
• Focused on wave vectors (q) significantly smaller than those typically studied.

Main Results:

• Observed normal diffusive decay (Dq^2) for wave vectors above a cutoff wave vector (qC).
• Found non-diffusive behavior below qC, where longer wavelengths decayed more rapidly than shorter ones.
• Confirmed that buoyant forces significantly alter fluctuation decay rates at long wavelengths.

Conclusions:

• Buoyancy-driven effects dominate the dynamics of long wavelength concentration fluctuations in stratified fluids.
• The study validates theoretical predictions of non-diffusive decay for q < qC.
• Provides new insights into diffusion mechanisms in complex fluid systems.