Hydrodynamic interactions in dense active suspensions: From polar order to dynamical clusters.

Area of Science:

• Physics
• Soft Matter Physics
• Fluid Dynamics

Background:

• Microscopic active particles in fluid exhibit complex collective behaviors.
• Hydrodynamic interactions are crucial for understanding their dynamics across various length scales.

Purpose of the Study:

• To investigate the distinct roles of short-range and long-range hydrodynamic interactions.
• To analyze their impact on collective dynamics, phase separation, and emergent ordering.

Main Methods:

• Development of a calculational framework to decouple hydrodynamic contributions.
• Systematic analysis of near-field (lubrication) and long-range forces.
• Simulation and theoretical modeling of active particle systems.

Main Results:

• Lubrication forces are as critical as long-range interactions in many-body behavior.
• Near-field interactions suppress motility-induced phase separation, favoring open, gel-like clusters.
• A globally polar ordered phase emerges in neutral swimmers, driven by near-field forces.

Conclusions:

• Hydrodynamic interactions, particularly near-field lubrication, fundamentally shape active matter collective behavior.
• The interplay between short-range and long-range forces dictates emergent structures and ordering.
• Near-field forces can dominate alignment mechanisms, leading to novel ordered phases.