Monitoring droplet ejection and landing in electrospray with electric current and acoustic emission

Droplet ejection and landing phenomenon for electrospray was investigated using a simple capillary inkjet head and a planar-needle inkjet head. The waveform of electric current passing through between the inkjet head and a counter electrode was measured to investigate how a fine droplet is generated. The measurement of elastic waves using an acoustic emission (AE) sensor placed on the back of a counter electrode was carried out to detect the condition of droplet landing directly. The droplet ejection synchronizing with both a measured pulse of electric current and a generated elastic wave could be ascertained. Forming a single-peak pulse as the electric current was found to be a principal element in obtaining a very fine droplet with a volume of, for example, a few 10 fL. Moreover, a continuous drawing of minute dots was demonstrated on a conventional photopaper, and a fine dot line having a regulated pitch determined by the frequency of applied voltage and the shift velocity of the paper could be obtained synchronizing with both sensing methods.