Multiple temperatures and melting of a colloidal active crystal

Area of Science:

• Condensed Matter Physics
• Soft Matter Physics
• Statistical Mechanics

Background:

• Thermal fluctuations drive defect formation and melting in equilibrium crystals.
• Active solids possess non-equilibrium fluctuations from self-propelled units, impacting their properties.
• The melting behavior of active crystals remains largely unexplored.

Purpose of the Study:

• To investigate the melting dynamics of a 2D colloidal crystal activated by swimming bacteria.
• To understand the interplay between solvent temperature and active temperature on crystal properties.
• To explore how active fluctuations affect melting transitions and energy equipartition.

Main Methods:

• Utilized a system of repulsive paramagnetic colloidal particles in a 2D confinement.
• Activated the crystal with a bath of light-driven E. coli (bacteria).
• Varied magnetic field strength and light intensity to control fluctuations and activity.
• Performed numerical simulations and developed a minimal model of an active particle in a periodic potential.

Main Results:

• A single effective temperature governs relaxation modes and melting for short active fluctuation persistence.
• For persistent active noise, energy equipartition is violated, leading to multiple effective temperatures.
• Melting occurs at a lower critical Lindemann parameter in active crystals compared to equilibrium crystals.
• Phenomenology confirmed by numerical simulations and a minimal active particle model.

Conclusions:

• Solvent and active temperatures cooperate to define dynamic and thermodynamic properties of active colloidal crystals.
• Active fluctuations introduce non-equilibrium effects that alter the melting scenario compared to equilibrium systems.
• The persistence of active noise is crucial in determining whether a single or multiple temperatures describe the system's behavior.