Do hydrodynamically assisted binary collisions lead to orientational ordering of microswimmers?

Area of Science:

• Physics of complex systems
• Fluid dynamics
• Statistical mechanics

Background:

• Understanding collective motion in microswimmer systems is crucial for fields ranging from biology to materials science.
• Hydrodynamic interactions play a significant role in the emergent behavior of self-propelled particles.
• Previous models often simplify or neglect detailed collision dynamics.

Purpose of the Study:

• To investigate the fundamental mechanisms driving the onset of collective motion in model microswimmer systems.
• To analyze the impact of binary collision dynamics on emergent collective behaviors.
• To develop and validate a simplified model for microswimmer interactions.

Main Methods:

• Conducted three-dimensional direct numerical simulations (DNS) to capture detailed hydrodynamic interactions.
• Analyzed the dynamics of binary collisions between model microswimmers.
• Developed a simplified Binary Collision Model (BCM) based on DNS data.

Main Results:

• The simplified Binary Collision Model (BCM) accurately reproduces the collective behavior observed in DNS for most conditions.
• Global alignment in microswimmer systems can largely be attributed to pairwise binary collisions.
• Discrepancies between DNS and BCM emerge during strong density fluctuations, associated with pseudo-sound phenomena.

Conclusions:

• Binary collisions are a dominant factor in the emergence of collective motion in these microswimmer systems.
• The developed BCM offers a computationally efficient alternative to full DNS for studying collective behaviors.
• Further investigation is needed for systems exhibiting strong density fluctuations and pseudo-sound phenomena.