Fluorescent microspheres as tracer particles in dusty plasmas

A technique has been developed to derive the dynamic properties of an ensemble of particles in dusty plasmas by using fluorescent tracer particles. The tracers form an integral part of the entire dust system. The dynamic properties of the dust system is reconstructed from the motion of the tracer. This technique allows long-duration experiments and is applied to a two-dimensional Coulomb cluster where the flow field, intershell rotation, and stagnation points of the flow have been identified.

Figure 1

Scheme of the experimental setup. One of the trapped dust particles is the fluorescent tracer particle. See text for details.

Figure 2

Snapshot of the dust cloud. (a) Image of camera No. 1 with all particles and (b) image of camera No. 2 that only sees the fluorescent tracer particle. In (a) the position of the tracer is marked by the arrow.

Figure 3

Position of the dust particles (gray dots) and trajectory of the tracer particle (solid line) during (a) the first $40\mathrm{s}$ and (b) the entire $3600\mathrm{s}$. The illumination laser is coming from the left in this representation.

Figure 4

Histogram of the step width of the tracer particle from one frame to the next. The arrows indicate the nearest-neighbor distance $b$ in the cluster and $b∕2$.

Figure 5

(Color) Flow field of the tracer particle. The arrows indicate the instant velocity of the tracer particle at the particle position. The length and the color of the arrows correspond to the particle speed.

Figure 6

Fraction of the time-resolved tracer particle trajectory in the time frame between 1050 and $1200\mathrm{s}$. The gray shading denotes the temporal evolution to more easily follow the trajectory.

Figure 7

Displacement of the tracer from its origin versus time. Only the first $500\mathrm{s}$ are shown here for clarity.

Figure 8

Flow field of the entire cluster using particle image velocimetry. The arrows indicate the mean velocity of all particles at the grid positions. The length and the color of the arrows correspond to the particle speed.