Some kinds of guard plates have been adopted in the electrospray generation because the axial electric field induced by them can increase the stability of the electrospray (A. Jaworek and A. Krupa, 1995). However, there are some lacks in the research for the effects of the guard plate on the electrospray system. The purpose of the paper is to evaluate the electrospray characteristics for the cases of with or without the guard plate and a distance from a capillary’s tip to the guard plate. In addition to that, the stable cone-jet voltage range, droplet size, spray angle and spray current are selected as the main characteristics of electrospray.
Four types of nozzles are used in this experiment. One is an ordinary electrospray nozzle without the guard plate (nozzle I) and the others are electrospray nozzles with the guard plate (nozzle II, nozzle III, nozzle IV). The distance from the capillary’s tip to the guard plate surface is 5 mm for the nozzle II, and 10 mm for the nozzle III and IV respectively. In case of the nozzle II and nozzle III, the input voltage of the guard plate and capillary are same. But in case of the nozzle IV, the input voltage of the guard plate is controlled independently from the capillary (0, 5, 10 kV). At the standard condition, the cone-jet mode is generated by supplying ethanol at a various liquid flow rate (5~30㎕/min) with the syringe pump applying a high positive voltage to a stainless-steel capillary’s tip (0.5 mm i.d., 0.9 mm o.d.) located at the height of 30 mm above the ground electrode.
The range of the stable cone-jet mode is measured for each nozzle. In the stable cone-jet mode, a cone shape is fixed, the liquid ligament is turn towards axial direction, and the spray current is constant at fixed flow rate. From these results, it is thought that the guard plate can induce the wider range of the cone-jet voltage and the higher cone-jet voltage. On the other hand, as the tip-plate distance decreases, the input voltage for the stable cone-jet is higher, and the range of the voltage becomes wider.
The spray current is measured by nano electrometer for each nozzle. In the stable cone-jet mode and fixed flow rate, the spray current is increased little by little, but almost same as the input voltage increases. On the other hand, the spray current increases as the flow rate increases and the input voltage highly increase.
The size distributions of electrospray droplets from the Taylor cone in cone-jet mode are measured by a laser diffraction system. At fixed flow rate (Q), the size distributions consist of on fairly monodisperse classes of droplets with diameter. As the flow rate increases, the droplet size increases. In case of a same flow rate, droplet sizes are almost same for each nozzle. When the flow rates are 5, 10, 20 ㎕/min, the droplet size (SMD) is similar about 2.5, 6.9, 10.2㎛ , respectively for each nozzle. Thus the effect of the guard plate on the droplet size can be ignored. However, when the flow rate is 30㎕/min, the droplet size is rapidly increases for each nozzle. The cause of these results is the unstable broken-up of the liquid ligament at high flow range condition.
For each nozzle, the spray at the cone-jet mode is visualized using a CCD camera, a telephoto lens and argon ion laser. The spray angle including satellites is determined by the image processing. The spray angle of nozzle I is about 68℃, that of nozzle II is about $17^\circ$, and that of nozzle III is about 24℃. For the nozzle IV, when the input voltage of guard plate is 0, 5, 10 kV, the spray angle is about 119℃, 33℃, 18℃, respectively. In addition to that, the spray angle isn’t affected by the flow rate, however the spray angle becomes smaller with the guard plate. It can be understood as the axial electric field strengthened by the guard plate accelerated a positive charged droplet toward the axial direction.
In case of using the guard plate in the electrospray, the stable cone-jet mode can be generated at the higher voltage and in the wider range of the input voltage. The guard plate merely affected the droplet size, however it can make the spray angle smaller. Moreover the spray current can be increased because the input voltage can be applied as the higher voltage.