A ternary mixture at the border of Soret separation stability

Area of Science:

• Fluid Dynamics
• Thermodynamics
• Chemical Physics

Background:

• Ternary mixtures exhibiting the Soret effect can undergo triple-diffusive convection in thermal fields.
• The stability of such mixtures is governed by the net separation ratio (Ψ), which is near zero for the studied toluene-methanol-cyclohexane mixture.

Purpose of the Study:

• To experimentally and numerically examine the behavior of a ternary mixture near the stability boundary.
• To classify the evolution of concentration patterns under varying conditions of Ψ and initial temperature.

Main Methods:

• Microgravity measurements of Soret coefficients for a toluene-methanol-cyclohexane mixture.
• Experimental and numerical simulations of the mixture in an elongated cell with differential heating.
• Classification of observed patterns based on Ψ and initial temperature (mean, linear, or cold).

Main Results:

• Four distinct regimes of pattern evolution were identified: motionless, metastable, and convective.
• Cross-diffusion of toluene leads to a positive Soret coefficient (S_T1) and a negative thermodiffusion coefficient (D_T1).
• A temporarily unstable density pattern emerges when Ψ > 0 due to cross-diffusion fluxes.
• An initially cold mixture exhibits dissimilar Soret separation, forming a long-lived inverse density gradient, ultimately resulting in motionless (Ψ > 0) or convective (Ψ < 0) patterns.

Conclusions:

• The interplay of Soret effect, cross-diffusion, and thermal gradients dictates the hydrodynamic stability and pattern formation in ternary mixtures.
• The observed inverse density gradient is a key phenomenon influencing the final state of the system.
• Understanding these complex behaviors is crucial for applications involving multi-component fluid systems.