Dynamics of monodisperse micrometre-sized metal droplets at low non-dimensional wavenumbers

This work studies the break-up process of a jet into droplets at low non-dimensional wavenumbers. The work is based on a novel droplet generator suitable for high temperature fluids including molten metals employing a piston resonant structure capable of generating high amplitude pressure excitations in the fluid medium. The relation between the excitation strength and the resulting jet break-up characteristics such as break-up length and in particular the temporal stability of the resulting droplet stream is studied. It is found that a given minimum excitation voltage is required for uniform droplet generation and that the temporal stability ameliorates further with increasing excitation levels. This analysis is complemented with in situ measurements of the generated pressure excitation signal in a molten metal. It is shown that the novel droplet generator can produce a continuous monodisperse stream of micrometre-sized molten tin droplets at low non-dimensional wavenumber of ≃0.12. This work is of particular importance for Laser Plasma produced (LPP) Extreme Ultraviolet (EUV) light sources, which require a highly stable stream of monodisperse micrometre-sized molten tin droplets as well as for 3D metal printing.