Phase behavior of active and passive dumbbells

Area of Science:

• Physics
• Materials Science
• Statistical Mechanics

Background:

• Phase separation is crucial in various physical systems.
• Understanding active matter behavior is key to designing novel materials.

Purpose of the Study:

• Investigate phase separation in a mixture of active and passive 3D dumbbells.
• Analyze the impact of dumbbell asymmetry and composition on phase separation.
• Explore the structural ordering of components in the phase-separated state.

Main Methods:

• Constant density simulations of 3D dumbbells interacting via Lennard-Jones potentials.
• Varying activity ratio (χ), dumbbell asymmetry, and hot/cold dumbbell ratios.
• Two-phase thermodynamic method for entropy calculation.
• Analysis of bond orientation order parameters.

Main Results:

• Phase separation observed at χ>5.80 for symmetric dumbbells, higher than monomers (χ>3.44).
• Hot dumbbells exhibit high effective volume and entropy, driving cold dumbbell clustering.
• Cold dumbbells form solid-like structures (FCC, HCP) with random orientations.
• Critical activity decreases with increasing hot dumbbell fraction.
• Asymmetry of dumbbells does not affect critical activity for equal mixtures, but influences cold cluster ordering.

Conclusions:

• Active dumbbells can induce phase separation and ordering in passive components.
• The interplay between kinetic pressure, entropy, and virial pressure governs phase behavior.
• Dumbbell geometry and composition offer tunable control over self-assembly and material properties.