Importance of solubility on droplet ejection from liquid gallium interacting with inductively coupled helium plasma

Understanding the interaction between plasma and liquid is an important issue for various applications of plasmas. We observed the ejection of droplets from liquid gallium interacting with an inductively coupled helium plasma. In this work, we separately controlled the ion flux (1-6 mA/cm2 ), the ion energy (0-300 eV), and the temperature of the liquid gallium (50-300 ℃) to investigate the mechanism of the droplet ejection by the plasma-liquid interaction. The ion flux and the liquid gallium temperature affected the threshold condition at which droplets started to be ejected. On the other hand, the ion energy was not involved in the initiation condition of the droplet ejection at all. We observed the formation of bubbles on the liquid surface. The ejection of droplets was observed at the timing of the collapse of the bubbles, indicating that the mechanism of the droplet ejection is closely related to the dissolvation of helium in the liquid gallium. The bombardment of helium ions, which are accelerated in the sheath electric field, causes the supersaturation in the amount of helium atoms in the liquid gallium. The supersaturation leads the formation of bubbles in the liquid, and the collapse of the bubbles is responsible to the droplet ejection.