Bow Shock Formation in a Complex Plasma

A bow shock is observed in a two-dimensional supersonic flow of charged microparticles in a complex plasma. A thin conducting needle is used to make a potential barrier as an obstacle for the particle flow in the complex plasma. The flow is generated and the flow velocity is controlled by changing a tilt angle of the device under the gravitational force. A void, microparticle-free region, is formed around the potential barrier surrounding the obstacle. The flow is bent around the leading edge of the void and forms an arcuate structure when the flow is supersonic. The structure is characterized by the bow shock as confirmed by a polytropic hydrodynamic theory as well as numerical simulation.

Figure 1

Schematic drawing of the experimental glass chamber YCOPEX device. The side view (above) shows the tilted chamber and the top view (below) shows the reservoir region of microparticles in the left of the gate and the experimental region in the right of the gate.

Figure 2

Wave velocity $C_d$ as a function of screening parameter $\kappa$. Measured values (solid circles) are compared with three-dimensional and two-dimensional dust acoustic velocities ($C_{\mathrm{DA}}^{3\mathrm{D}}$, solid line and $C_{\mathrm{DA}}^{2\mathrm{D}}$, dotted line) and dust lattice velocity ($C_{\mathrm{DL}}$, dashed line).

Figure 3

Typical examples of microparticle flow around a conducting needle [top views: (a)–(c), and side view: (d)]. The needle is shown as a point in the void [(a)–(c)]. The Mach number and velocities are (a) 0.8 ($54\pm 8\mathrm{mm}/\mathrm{s}$), (b) 1.6 ($116\pm 24\mathrm{mm}/\mathrm{s}$), (c) 2.0 ($142\pm 29\mathrm{mm}/\mathrm{s}$), and (d) 1.3 ($91\pm 20\mathrm{mm}/\mathrm{s}$), respectively. The exposure times are $1/10\mathrm{s}$ for (a)–(c) and $1/20\mathrm{s}$ for (d). The void structures are characterized by the $y$-directional spread of (a) 249, (b) 228, and (c) 218 in the unit of the Debye length, respectively.

Figure 4

The density ratio, $n_{dp}/n_{d0}$, versus Mach number, $M$. The open circles are the experimental results and the solid circles are the simulation results. The lines are based on Eq. (3) with $\gamma =5/3$, 2.2, and 3.

Figure 5

Simulation results of the density contour plots for Mach numbers (a) $M=1.3$ and (b) $M=2.0$. The mark $X$ represents the position of the obstacle charge.