Hydrodynamic suppression of phase separation in active suspensions.

Area of Science:

• Active Matter Physics
• Fluid Dynamics
• Statistical Mechanics

Background:

• Active matter systems, composed of self-propelled units, exhibit complex behaviors.
• Motility-induced phase separation (MIPS) is a proposed emergent phenomenon in active matter.
• Previous studies on active Brownian disks suggested MIPS is a generic behavior.

Purpose of the Study:

• To investigate the role of hydrodynamic interactions in active disk suspensions.
• To explore the full range of area fractions, from dilute to close-packed.
• To determine if MIPS occurs in hydrodynamically interacting active disk systems.

Main Methods:

• Numerical simulations incorporating full hydrodynamic interactions.
• Exploration of various squirmer area fractions.
• Comparison with simulations of active Brownian disks under similar conditions.

Main Results:

• Hydrodynamic interactions strongly suppress motility-induced phase separation.
• MIPS is significantly less prevalent in hydrodynamically simulated active disks compared to Brownian disk models.
• An analytical argument explains the suppression mechanism due to fluid flow coupling.

Conclusions:

• Hydrodynamic interactions are critical in determining the phase behavior of active matter.
• MIPS is not a universally generic phenomenon in all active matter systems.
• The findings necessitate re-evaluation of models for active suspensions, emphasizing fluid dynamics.