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Sketch of the experimental setup. The side view of the quasi-two-dimensional plasma crystal. Spherical microparticles (black particles) are confined in the weak parabolic confinement potential above the rf electrode and are illuminated with a horizontal laser sheet. The binary agglomerates (striped particles) levitate at a smaller height than the spherical particles without forming vertical pairs. The defocused top view image of the binary agglomerates can be identified with distinct interference fringe pattern (black stripe). Under typical experimental conditions, the vertical separation of the binary agglomerates from the monomers, d is substantially smaller than the horizontal inter-particle separation, Δ.
Illustration of (a) focused and (c) defocused top-view images of two dimensional plasma crystal with monodisperse spherical particles. The quasi-two-dimensional plasma crystal with (b) focused (with brighter particles) and (d) defocused (with interference fringe patterns) top-view images are shown. The “Delaunay triangulation” for (e) two dimensional and (f) quasi-two-dimensional plasma crystal has been performed. It is clear that the presence of many brighter particles destroys the crystalline order and the system may emerge as disordered solid. In the quasi-two-dimensional plasma crystal they levitate just below the monodisperse spherical particles, but do not form vertical pairs—focus-defocus.mov (enhanced online). [URL: http://dx.doi.org/10.1063/1.4729755.1]10.1063/1.4729755.1
(a) and (b) Top-view defocused images of the brighter particles which contain distinct interference fringe patterns. The interference fringe pattern can be stationary or can rotate in both clockwise/anti-clockwise directions (yellow arrow) with additional spinning motion (red arrow). (c) Side-view defocused images of brighter particles with vertical interference fringe patterns. They do not rotate but move in left or right directions.
Orientation angle of the interference fringe on the defocused images versus time. The rotational velocities are different for different particles as shown in the figure: ∼0.08 Hz (red circles) and ∼0.21 Hz (blue circles).
(a) Cluster of 36 particles was formed within an additional confinement using a copper ring and a silicon wafer. The 4 striped particles were identified (inset). They dropped on the silicon wafer after the plasma was switched off. (b) The optical microscopic imaging of four binary agglomerates. The scale length (5 μm) is shown with black line. Similar results were obtained with different small clusters of particles. These observations indicate that the top-view defocused images of the particles with distinct interference fringe patterns can be binary agglomerates.
Observation of binary agglomerates by the long-distance microscope (a) in presence of diffuse background light but in absence of laser and (b) in presence of laser. The simultaneous observations from top- and side-view were performed—top-side-view.mov (enhanced online). [URL: http://dx.doi.org/10.1063/1.4729755.2] [URL: http://dx.doi.org/10.1063/1.4729755.3] [URL: http://dx.doi.org/10.1063/1.4729755.4]10.1063/1.4729755.210.1063/1.4729755.310.1063/1.4729755.4
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