Measurement of net electric charge and dipole moment of dust aggregates in a complex plasma

Understanding the agglomeration of dust particles in complex plasmas requires knowledge of basic properties such as the net electrostatic charge and dipole moment of the dust. In this study, dust aggregates are formed from gold-coated mono-disperse spherical melamine-formaldehyde monomers in a radiofrequency (rf) argon discharge plasma. The behavior of observed dust aggregates is analyzed both by studying the particle trajectories and by employing computer models examining three-dimensional structures of aggregates and their interactions and rotations as induced by torques arising from their dipole moments. These allow the basic characteristics of the dust aggregates, such as the electrostatic charge and dipole moment, as well as the external electric field, to be determined. It is shown that the experimental results support the predicted values from computer models for aggregates in these environments.

Figure 1

A representative single frame of rotating aggregates showing four particles. Rotation of P1–P3 about the vertical axis was observed, while P4 was moving from right to left without any rotation, probably due to its small horizontal electric dipole moment.

Figure 2

(a)–(g) Sequence of images showing the complete rotation of cluster P1 from Fig. 1 consisting of eleven 8.93 $\mu \mathrm{m}$ gold-coated mf particles $(\Delta t=0.0167\text{s})$, and (h) its reconstructed 3D model. (i)–(o) P2 from Fig. 1 consisting of eight particles $(\Delta t=0.0117\text{s})$, and (p) its reconstructed 3D model. Spatial scaling is the same for all the subimages.

Figure 3

A sequence of images showing a sample interaction between two charged dust particles (P5) and (P6). P5 begins in the bottom right and P6 begins in the top left ($\Delta t=0.001$ s).

Figure 4

Proposed structure of aggregate P5 consisting of different numbers of monomers. (a,b) Eight monomers, with the location of the displaced monomer indicated by an arrow, (c) seven monomers, and (d) six monomers, with the locations of the removed monomers indicated by arrows.

Figure 5

Illustration of image processing. (a) A raw image loaded into ImageJ. (b) A binary image after setting a pixel value threshold in the program for the image shown in (a). (c) Image analysis by fitting ellipses snugly around the detected aggregates in (b) through the “analyze particle” function.

Figure 6

Time evolution of the $X$ and $Y$ components of center-of-mass position for two interacting particles, P5 and P6, superposed on four frames from Fig. 3, with color indicating the time.

Figure 7

Orientation angle vs time for P5 and P6, with the solid line showing the time of closest approach.

Figure 8

A superposition of four frames showing the interactions between two aggregates from (a) experimental data with a similar color of aggregates in each frame, and (b) simulation using ${\mathrm{aggregate}}_{-}\mathrm{builder}$ code with a similar color of aggregates at each time step. Not all monomers are visible.

Figure 9

A superposition of four frames from ${\mathrm{aggregate}}_{-}\mathrm{builder}$ code with a similar color of aggregates at each time, showing the interaction of P6 with (a)–(c) the corresponding aggregate in Figs. 4, 4(c), and 4(d), respectively.