Formation of Bubbles, Blobs, and Surface Cusps in Complex Plasmas

Investigations of the dynamical evolution of a complex plasma, in which a vertical temperature gradient compensates gravity, were carried out. At low power the formation of microparticle bubbles, blobs, and spraying cusps was observed. This activity can be turned on and off by changing control parameters, such as the rf power and the gas pressure. Several observational effects indicate the presence of surface tension, even at small “nanoscales” of a few 100’s of particles. By tracing the individual microparticle motion the detailed (atomistic) dynamics can be studied as well as the pressure dependence of the forces. A possible mechanism that could drive the observed phenomena is analogous to the Rayleigh-Taylor instability.

Figure 1

Observed fluid activity in a complex plasma. Top: Two microparticle bubbles ($6.81\mu \mathrm{m}$ particles, $\Delta T=65\mathrm{K}$, argon plasma). The dashed line indicates the midplane of the plasma chamber. Bottom: (a) Microparticle blob [field of view (FoV) $8\times 7{\mathrm{mm}}^2$, $6.8\mu \mathrm{m}$ particles]. (b) Microparticle spray (FoV $11\times 11{\mathrm{mm}}^2$, $4.8\mu \mathrm{m}$ particles). (c) Microparticle bubble (FoV $9\times 9{\mathrm{mm}}^2$, $6.8\mu \mathrm{m}$ particles). A movie showing bubbles, blobs, and cusps (FoV $29\times 19{\mathrm{mm}}^2$, $6.8\mu \mathrm{m}$ particles, $\Delta T=64.5\mathrm{K}$, Ar) is available online [21].

Figure 2

Color-coded superposition of pictures demonstrating the movement of microparticles (a) inside a blob (FoV $4\times 4{\mathrm{mm}}^2$, $7.2\mu \mathrm{m}$ particles, $\Delta T=63\mathrm{K}$, argon plasma) and (b) while a bubble is forming (FoV $11\times 10{\mathrm{mm}}^2$, $4.8\mu \mathrm{m}$ particles, $\Delta T=49.5\mathrm{K}$, argon plasma).

Figure 3

Three-dimensional slice showing a particle blob (colors inverted, FoV $11\times 2\times 12{\mathrm{mm}}^3$, $6.8\mu \mathrm{m}$ particles, $\Delta T=63.5\mathrm{K}$, argon plasma) recorded by horizontal scanning at a speed of $8\mathrm{mm}/\mathrm{s}$. The particles on the bottom correspond to the region below the void. Note the irregular structure of the void edge with indentations as mentioned in the text.

Figure 4

Mean velocity of particles moving upwards versus gas pressure. The results correspond to different particle diameters $2a$ and temperature differences between the electrodes $\Delta T$ (diamond, triangle, star, and bold line are for $2a=6.8\mu \mathrm{m}$, $\Delta T=63–64.5\mathrm{K}$; cross and square are for $2a=4.8\mu \mathrm{m}$, $\Delta T=45–48\mathrm{K}$). The red square indicates measurements taken while the plasma was turned off. The bold line shows an experiment with a gradual pressure increase, in other experiments the pressure was kept constant. The error bars indicate the FWHM of the velocity distributions. Inset: Open symbols show the excess of the critical temperature needed for the appearance of bubbles (above the equilibrium temperature needed to compensate gravity [17]). Filled symbols show the total temperature jump between the electrodes and the gas predicted by 18 in the creep flow regime.

Figure 5

Surface tension effects ($6.8\mu \mathrm{m}$ particles, $\Delta T=64.5\mathrm{K}$, $p=18\mathrm{Pa}$, argon plasma). (a) Breaking up of a bubble lid (FoV $5\times 3{\mathrm{mm}}^2$). The arrows indicate the size of the opening as well as the direction of particle motion. The vertical column shows three moments in time. We estimate the volume of the lid and the mean particle distance and calculate the surface tension from the total mass of the particles moved during the time needed for opening the bubble. Figures (b) and (c) show contours of the particle cloud at successive moments in time (gray scale and upwards displacement, marked by horizontal lines, indicate time) during (b) spray formation (FoV $9\times 4{\mathrm{mm}}^2$, time step 0.48 s) and (c) evolution of a conic cusp (FoV $8\times 6{\mathrm{mm}}^2$, time step 0.4 s). The red dashed lines show a Taylor cone with limiting angle of 98.6° [10].