Alignment and propulsion of squirmer pusher-puller dumbbells

Area of Science:

• Soft Matter Physics
• Fluid Dynamics
• Active Matter

Background:

• Microswimmers are artificial or biological entities capable of self-propulsion.
• Dumbbell structures of microswimmers allow for the study of collective behaviors and emergent properties.

Purpose of the Study:

• Investigate the hydrodynamic interactions and collective dynamics of microswimmer dumbbells composed of pusher-puller pairs.
• Analyze how varying active stress combinations influence the orientation, fluctuations, and swimming behavior of these dumbbells.

Main Methods:

• Mesoscale hydrodynamic simulations using the multiparticle collision dynamics (MPCD) approach.
• Modeling individual microswimmers as squirmers connected by a bond, allowing free rotational motion.

Main Results:

• Squirmers' flow fields significantly impact propulsion direction orientation, fluctuations, and overall dumbbell swimming.
• For unequal active stresses, strong orientational correlations emerge between propulsion directions and the bond vector.
• Alignment is orthogonal for strong pusher-weak puller pairs and antiparallel for strong puller-weak pusher pairs.

Conclusions:

• Active stress magnitude differences are crucial for orientational coupling in microswimmer dumbbells.
• The balance between pusher and puller activities dictates both the alignment of propulsion directions and the net swimming speed of the dumbbell.