Active billiards: Engineering boundaries for the spatial control of confined active particles

Area of Science:

• Physics
• Biophysics
• Soft Matter Physics

Background:

• The spatial organization of self-propelled particles is sensitive to container boundaries, unlike equilibrium gas molecules.
• Understanding boundary effects is key to controlling confined active particles geometrically.

Purpose of the Study:

• To propose and demonstrate a novel boundary method for controlling confined active particles.
• To enable geometric control and concentration amplification of active matter.

Main Methods:

• A boundary method based on the flux transfer formalism, treating particles as rays with infinite persistence length.
• Application to swimming microalgae (Euglena gracilis) in light-defined billiard geometries.

Main Results:

• Euglena exhibits nearly Lambertian scattering, leading to uniform distributions in simple cavities.
• A stacked multistage billiard geometry achieved exponential cell concentration amplification.
• The boundary method successfully designed a system for spatial control and sorting.

Conclusions:

• The flux transfer boundary method offers powerful geometric control over confined active matter.
• This approach enables precise manipulation, concentration, and potential sorting of microorganisms.
• The method has broad applications in active matter research and robotic navigation.