Phase separation and coexistence of hydrodynamically interacting microswimmers

Area of Science:

• Physics
• Soft Matter Physics
• Statistical Mechanics

Background:

• Spherical microswimmers exhibit collective behavior, including phase separation into dense clusters and disordered surrounding at high self-propulsion.
• Hydrodynamic interactions play a crucial role in the collective dynamics of microswimmers.

Purpose of the Study:

• To investigate the influence of hydrodynamics on the phase behavior of model microswimmers (squirmer) in a quasi-two-dimensional geometry.
• To explore the dynamics of phase separation and determine phase diagram binodal lines.

Main Methods:

• Utilizing multi-particle collision dynamics (MPCD) for particle-based simulations.
• Employing the squirmer model to represent microswimmers with defined propulsion characteristics.
• Analyzing coarsening dynamics and phase boundaries in a quasi-two-dimensional system.

Main Results:

• Observed diffusive coarsening dynamics followed by ballistic cluster compactification.
• Determined phase diagram binodal lines of Péclet number versus density.
• Found that gas binodals shift to lower densities with increasing mean density or cluster size, explained by extended pressure balance.
• Pushers and pullers show shifted binodal lines to higher Péclet numbers compared to neutral squirmers.
• Identified a transition from a hexagonal solid to a disordered fluid dense phase upon lowering the Péclet number.

Conclusions:

• Hydrodynamics significantly impact microswimmer phase separation, influencing cluster dynamics and phase boundaries.
• The Péclet number and swimmer type (pusher/puller) critically determine phase behavior and transitions.
• A pressure balance model, extended with hydrodynamic contributions, effectively explains observed density shifts in phase separation.