New mechanism of cluster rotation in complex (dusty) plasmas

Abstract

Summary form only given. Complex (dusty) plasmas consist of fine solid particles suspended in a weakly ionized gas. It is often necessary to manipulate particles in a complex plasma of a gas discharge. Popular manipulation methods include modulating the discharge parameters (e.g., discharge voltage or current), using the radiation pressure of a focused laser beam, and using an electrically biased wire or external magnetic field. In the present study, we applied a rotating electric field ("rotating wall") to complex plasma. An additional "four- electrode box" was placed on the lower electrode of a capacitively coupled rf discharge in argon. The box consists of four vertical plates made of ITO-coated glass that are mounted on four insulating poles. The top and bottom of the box are open to allow the free flow of gas and plasma species. Polymer microspheres were suspended in the plasma sheath inside the box and formed a monolayer cluster of 20 to 160 particles. The ITO coating on the internal surfaces of the glass plates is conducting yet transparent; this allowed us to apply a certain electric potential to each plate independently to manipulate the particles and simultaneously illuminate and image them through the glass plates. A rotating electric field was created when the phase shift between sinusoidal signals on adjacent plates was set to I??/2. In this configuration, the absolute value of the horizontal component of the electric field at the location of particles is practically constant, but the field direction rotates uniformly in the horizontal plane. Rotating electric field caused the particle cluster to rotate. Clusters of different sizes rotated roughly as rigid objects. The maximum rotation speed of ~ 0.5 s"1 was achieved for the applied frequency of f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">appl</sub> >~ 10 kHz. At higher frequencies, f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">appl</sub> > 20 - 30 kHz, the cluster rotation reversed its direction and finally stopped at f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">appl</sub> > 100 kHz. For all f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">appl</sub> , the cluster rotation direction reversed with the reversal of electric field rotation. As the applied frequency was higher than the dust plasma frequency but lower than the ion plasma frequency, electrons and ions responded to the applied (screened) ac field and the particles responded to the time-averaged electric field. We discuss the mechanism of particle cloud rotation and possible applications of rotating electric fields in complex plasmas.