Quasi-equilibrium phase coexistence in single component supercritical fluids.

Area of Science:

• Thermodynamics
• Fluid Dynamics
• Materials Science

Background:

• Supercritical fluids are typically considered homogeneous, but exhibit anomalous properties.
• Research on supercritical fluids has focused on equilibrium states, neglecting out-of-equilibrium dynamics.
• Industrial handling of substances like CO2 and Argon involves non-equilibrium processes.

Purpose of the Study:

• Investigate the out-of-equilibrium behavior of supercritical fluids under dynamic conditions.
• Characterize the formation and persistence of structures during compression-expansion cycles.
• Explore the implications of these non-equilibrium phenomena for industrial applications.

Main Methods:

• Subjecting Argon to successive compression-expansion cycles, crossing the critical point.
• Utilizing light scattering experiments to observe fluid behavior.
• Developing a kinetic rate model to explain observed phenomena.

Main Results:

• Observed formation of sub-micron liquid droplets and nanometer-scale clusters during expansion cooling.
• These structures are distinct from supercritical density fluctuations and persist.
• A kinetic rate model successfully explains the exchange between droplets and clusters.

Conclusions:

• Supercritical fluids exhibit significant non-equilibrium behavior, including the formation of distinct phases.
• Observed droplet and cluster formation persists longer than expected.
• Understanding these non-equilibrium aspects is crucial for optimizing industrial processes involving supercritical fluids.