Simulation of centrifugal separation of particles with equal sedimentation velocities
Conventional centrifugal sedimentation cannot readily separate particles with the same sedimentation velocities. Density gradient centrifugation, although highly selective, exposes delicate particles such as drug delivery system (DDS) nanoparticles and exosomes to high osmotic pressure that can cause shrinkage, rupture, or expansion. To separate particles with equal or similar sedimentation velocities while preserving their properties, we propose two methods that exploit thermal fluctuations (Brownian motion): the centrifugal Brownian ratchet method, which uses asymmetric structures, and the centrifugal wavy-channel method, which uses geometric barriers. Two-dimensional Brownian dynamics simulations combining Stokes-Einstein diffusion with centrifugal sedimentation were performed, and particle transport was analyzed using the Péclet number. In the ratchet method, particles with identical sedimentation velocities but different Péclet numbers showed statistically distinct mean arrival positions and times. In the wavy-channel method, periodic entropic barriers interacting with differences in transport properties produced arrival-time differences, achieving temporal separation even for particles with identical sedimentation velocities or Péclet numbers. Both methods separate particles without relying on sedimentation-velocity differences and are expected to preserve particle properties. Because they neither depend on electrical properties nor require a change of medium, they may also be applicable to particles in liquid metals, oils, organic solvents, and ionic liquids.