Acoustic trapping of active matter

Area of Science:

• Physics
• Soft Matter Physics
• Active Matter Physics

Background:

• Confining microorganisms and self-propelled particles aids in analyzing active systems.
• Standard optical traps face challenges with large trapping radii needed for studying swimmers over their run length.

Purpose of the Study:

• To introduce and utilize a novel acoustic tweezer for confining self-propelled particles in 2D.
• To investigate the behavior of confined active matter over distances larger than their run length.

Main Methods:

• Development of a near-harmonic acoustic trap.
• Confining self-propelled particles in two dimensions.
• Analyzing particle motion and density distributions under varying confinement strengths.

Main Results:

• Demonstrated confinement of self-propelled particles over distances larger than their run length.
• Observed a crossover from Boltzmann-like to perimeter-peaked density with increasing confinement.
• Identified crystallization into close-packed structures at high concentrations, followed by a traveling wave 'explosion' upon trap removal.
• Measured 'swim pressure,' a mechanical pressure exerted by self-propelled bodies.

Conclusions:

• Acoustic tweezers offer a novel solution for trapping active matter over large scales.
• Confined active matter exhibits distinct phase transitions and collective behaviors.
• The study provides a method for quantifying the unique mechanical pressure generated by self-propelled particles.