Complex-plasma manipulation by radiofrequency biasing

This paper presents an experimental study on the nature, the dimensions and the timescale of the perturbation introduced by radiofrequency (rf) biasing of areas adjacent to the plasma. The analysis of the rf sheath, and of the charging of particles in it, has disclosed a levitation force on particles, which is substantially different from the dc one often used in complex plasmas. Experimentally, the rf heavily loaded sheath presents characteristics completely different from the normal case V-rf less than or equal to V-dc. Regions of extra ionization and complex electrostatic structures arise. These have been visualized by nanoparticles grown in the plasma. A variety of equilibrium positions for a controlled number of microparticles (injected) can be achieved by fine balancing of dc and rf on a pixel with the neighbouring sheath kept under control. In certain situations gravity is completely compensated, allowing the study of three-dimensional clusters. The motion of clusters from 4 to about 100 particles is simultaneously monitored by a three-dimensional visualization based on two laser lights modulated in intensity. This method enables the study of time-varying effects, such as transitions and vibrations, as well as the study of static structures and lattice defects. At pressures below 40 Pa in large clusters a poloidal motion appears.