Bubbles in Dusty Plasma Liquids

Hong-Yu Chu, Chen-Ting Liao and Lin I
Department of Physics, National Central University, Chung-Li, Taiwan, R.O.C.
Abstract. Dusty plasma liquid can be formed by suspending micro-meter sized particles in a low pressure glow discharge.
The ablation by focusing a nsec laser pulse on one of the suspended dust particle generates a dense plume which expels dust
particles. The plasma bubble with dust depletion does not collapse after the laser pulse. It maintains its shape and travels
downwards at a speed higher than the dust acoustic wave. The bubbles interact with each other vertically but not horizontally.
The bubble also strongly interacts with the dust acoustic wave. The dynamical behaviors of bubble formation and the above
interactions are reported and discussed.
Keywords: dusty plasma, plasma bubble
PACS: 52.27.Lw, 52.35.Sb
INTRODUCTION
The dusty plasma consists of many negatively charged (about 104 electron / particle) micro-meter sized particles
suspended in weakly ionized low pressure plasma background. The suspended particles can be self-organized into
crystal or liquid state through the strong Coulomb coupling between particles. Under the proper spatial and temporal
scales, it has been used to investigate many interesting microscopic behaviors of solid and liquids at the kinetic
through direct optical microscopy [1, 2, 3]. On the other hand, the massive dust particles are strongly coupled with
the background plasma. It not only changes the dielectric property of the background plasma, but also generates new
collective dynamical behaviors, involving the motion of the massive dust particles. Self-organized dust density waves,
and plasma cavities or bubbles with dust depletion are the few good examples [4, 5, 6].
In our daily life, the formations of bubbles in liquids are interesting phenomena. Gas bubbles can be generated
through heating, external acoustic field, electrical discharge, or focused laser beam vaporizing the liquid or colloidal
particle suspensions. For example, the acoustic field drives the water molecules back and forth to form wave while the
field is weak. However, when the high frequency and large amplitude external field is applied, the water molecules
are incapable to follow the oscillation. A gas bubble is formed. On the other hand, gas bubble interacting with each
other through the surrounding fluid is also an interesting phenomenon. Similar to the gas bubble generating in the
laser-liquid interaction, the plasma bubble can be induced in the dusty plasma liquid by the intense laser pulse.
FIGURE 2. (a) Typical plume emission image measured at 50 ns delay and 1 ms exposure interval from a 38 mJ pulsed laser
ablating on a 5 mm polystyrene particle in dusty plasma. (b) The temperal evolution of the spatial intensity distribution at various
delay from 20 ns to 20 ms. The inset shows the curve at 50 ns delay fitting with Gaussian distribution function.
our previous work, a focused laser pulse (6 ns duration) is used to ablate a suspended dust particle [6]. An expanding
plume is generated which expels the surrounding dust particles and forms an traveling plasma bubble. In this work,
we investigate the initial plume generation, and the interaction between plasma bubble and the interaction between a
plasma bubble and the dust density wave.
EXPERIMENTAL SETUP
The experiment is conducted in a cylindrical symmetric rf dusty plasma system described elsewhere. The weakly
ionized glow discharge (ne » 109 cm¡3) is generated in Ar gas at about 300 mTorr using a 14 MHz rf power system at
2.2Wpower. Fig. 1 shows the sketch of the side view of the system setup. A hollow coaxial cylindrical trap with 3-cm
inner diameter and 14 mm height is put on the bottom electrode to confine the dusty plasma. Polystyrene particles
(5 mm in diameter) are dropped from the top of the chamber and then levitated by the sheath. The suspended dust
particles are aligned to form vertical chains by the wake field effect from the downward ion flow shown in Fig. 1. The
inter-chain distance a is about 0.3 mm. The dust mass and the charge are about 1.9£10¡10 g per dust and 5000 electron
per dust. The Debye length lD is between 100 and 200 mm. Along the center vertical axis, a Nd-YAG laser beam (532
nm wavelength, and 10 nsec pulse width) is passed vertically (upward) through the bottom glass electrode coated with
transparent conducting indium-tin-oxide thin film. It is focused to 0.1 mm at the center of the dusty plasma liquid inFIGURE 1.

FIGURE 4. (a)-(d) Sequential images of two horizontally separating plasma bubbles. The horizontal separation of the tw0 bubbles
does not change while they travel. (e)-(i) Sequential images of two separated plasma bubbles with vertical separtion. The lower
plasma bubbles start traveling downward at 2/60 s. However, the upper one start collapsing from it bottom edge at 2/60 s and
disappears at 4/60 s. Each image is exposured for 16 ms.
order to ablate a suspended dust particle. The emission intensity from the expanding plume, and the positions of dust
particles (illuminated by another Ar+ laser sheet) are monitored using a gated ICCD (Stanford Computer Optics, Inc.
: 4 QuickE) through a telescope.
EXPERIMENTAL RESULTS
Figure 2 shows the typical 2D intensity distribution of a snap shot and the sequential radial intensity distribution of
the white light emission from the plume after ablating one of the suspended particle using the pulsed laser. It is found
that the diameter (measured from the variance of the radial intensity distribution), D, follows the power law scaling
relation with time, D µ t0:31. The diameter of the dust bubble also follows the same scaling relation, although the
expansion of the dust bubble occurs much slower than that of the plume, i.e., in the time scale greater than 100 microsec.
Unlike the expansion of the plume through the ablation of a solid surface into the neutral background similar to
our pressure, the shock front is not observed in our plume image [7, 8]. The small mass of the ablated particle and the
small energy absorbed by the small particle make the plume too weak to drive a shock wave.
Figure 3 shows that, after the formation of the bubble, the bubble maintains its shape and travels downward with
a constant velocity. Due to the driving of the expansion plume, a dust bubble is formed. Since dust particles sink
electrons, there are more electrons generated through the electron impact ionization process under the higher electron
density in the dust-depletion region of the bubble. It compensates the outward diffusion loss, and sustains the denser
plasma which exerts outward drag on the surrounding dust particles to prevent the void from collapsing. The bubble
therefore maintains its shape. Under the downward ion wind toward the bottom electrode, the leading (trailing) edge
of the traveling bubble forms a repeller (attractor) for dust particles. The bubble is permeable to dust particles. Inside
the bubble, particles travel upward as the leading edge is depleted. Outside the bubble, particles move upward around
the bubble, similarly to the flow field of a dipole.
Figure 4 shows the interaction between two bubbles. A pair of bubbles can be generated by two simultaneous laser
pulses focused at different spots, through a beam splitter. Basically, the two horizontal bubbles do not interact with
each other. However, a pair of vertical bubbles strongly coupled with each other. Figure (a)-(d) shows that the two
horizontal bubbles travel downward independently. Figure (e)-(i) shows that the upper bubble quickly diminishes as
they travel downward. If we move the upper bubble away horizontally from the wake region, the upper bubble is not
affected again. The different behaviors of between the horizontal and the vertical pairs are very interesting. As shown
in Fig. 3(c) the repeller and attractor in the leading and trailing edge plays very important roles in the propagation of the bubble. When the two vertical bubbles become too close, the region between them cannot provide the upward flux
to sustain the shape and motion of the trailing bubble.
ACKNOWLEDGMENTS
This work is supported by the National Science Council under contract number No. NSC-0212-M008-042.
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