LIQUID-METAL ION SOURCES AND ELECTROSPRAYS OPERATING IN CONE-JET MODE: SOME THEORETICAL COMPARISONS AND COMMENTS

Abstract

The electrospray and the liquid-metal ion source (LMIS) are similar, in that both are electrohydrodynamically driven devices operating in cone-jet mode. There are some interesting resemblances and differences between their theories. This paper offers some comments on these, with particular emphasis on the factors that determine the details of cone-jet shape. A survey is presented of the LMIS physics relevant to such a comparison. New LMIS theory is presented about the nature of the driving forces on the LMIS jet, the theoretical prediction of voltage drop along the jet, the taper of a LMIS jet, and the relative magnitudes of the surface tension and inertial terms. A comparison of the physics of the electrospray and the LMIS suggests that an important difference between them is the size of the voltage drop along the jet. This voltage drop has its origin in differences in the nature and mobility of the charge carriers. These differences cause differences in the charge-transport processes in the jet and in the nature of the driving forces. The LMIS is primarily driven by Maxwell normal stresses but the electrospray by tangential stresses on the jet sides. The basic differences in charge-transport mechanism and in driving forces cause other differences in behaviour. It is concluded that scaling laws derived for electrosprays are not expected to apply to the LMIS. More generally, it is concluded that the LMIS and the standard (“reasonable conductivity”) electrospray have related but qualitatively different theories, that may be understood as different cases of a general theory of electrohydrodynamically driven jets. The theory of the shape of the liquid cone and the vena contracta region is probably common, but the phenomena and theories associated with the jet and the emission region are different. There seems enough common ground to make linked research studies valuable.