Side-view cross section of the PK-3 Plus vacuum chamber. Neon plasma is generated by applying rf voltages to a pair of parallel-plate electrodes. Microparticles are introduced by a dispenser and viewed from the side by video cameras, not shown here. The microparticles are charged and confined stably. A single isolated microparticle would rest anywhere on an equilibrium surface where the net force is zero.
(a) Sketch of the force balance for microparticle confinement. At equilibrium, under microgravity conditions, microparticles experience electric and ion drag forces that are opposite and are almost equal, as well as a small thermophoretic force due to a temperature gradient. The electric field drives an outward ion flow. (b) A cross section of the entire microparticle suspension viewed from the side by the overview camera. The two electrodes are visible at the top and bottom. The suspension has a central void and it is displaced upward, so that it had only a single layer at the lower void boundary. (c) Single layer imaged by the high-resolution camera. The field of view is indicated in (b) by the yellow rectangle. (enhanced online). (d) Number density of microparticles. This graph indicates that the sample is spatially nonuniform over the entire field of view; consequently, we perform most of our spectral analysis for microparticle data in . [URL: http://dx.doi.org/10.1063/1.3204638.1]10.1063/1.3204638.1
(a) Power spectrum for microparticle velocity fluctuations in the direction. Microparticle velocities and positions computed by tracking particle motion yield this spectrum. Fits are shown for a theoretical expression for a damped harmonic oscillator driven by white noise (dashed curve) and a Lorentzian (dashed-dotted curve). The central peak indicates the presence of oscillations in the random motion of microparticles and its width indicates damping. The peak is a resonance frequency that we attribute to the confining forces. (b) Wave spectrum with color representing energy (in arbitrary units) as a function of wave number and frequency. The wave frequency does not noticeably vary with wave number, i.e., the wave spectrum lacks the signature of wave dispersion. The power spectrum (a) was computed from the wave spectrum (b) by averaging over .
Article metrics loading...
Full text loading...