Vortex formation driven by the particle flow at the interface of a phase-separated binary complex plasma under microgravity condition

Abstract Microparticles of two sizes are confined in a dc discharge in a glass tube with polarity switch in the PK-4 laboratory on board the International Space Station. Small and big particles separate from each other presumably due to the unbalance of the force under microgravity condition, forming an ellipsoidal interface. Particles close to the symmetric axis of the cylindrical glass tube are driven by a manipulation laser and a particle flow is generated. The flow velocity depends not only on the laser current but also on the configuration and location of the particle cloud. Counterintuitively, it is observed that a vortex can be formed at the interface, only if the flow velocity is below a certain critical value. Our experiments provide a great opportunity to study the new facets of vortex formation at particle-resolved level.