On-demand Electrohydrodynamic Jetting of an Ethylene Glycol and Water Mixture—System of Controlled Picoliter Fluid Deposition

Abstract

On-Demand Electro Spraying or Single Event Electrospraying (SEE) is a method to jet small amounts of fluid out of a nozzle with a relatively large diameter by switching on and off an electrical field between nozzle and substrate. The jet decomposes in many fl droplets, the total amount of volume deposited is up to 5 pl. A pipette is considered in which an electrode is mounted close to the exit of the nozzle. The electrode is connected to a high voltage power amplifier. By switching on the electrical field the apparent surface tension drops, the meniscus deforms into a cone and fluid starts to flow towards the nozzles. The moment the cone has reached the Taylor cone dimensions, from the tip of the cone a jet emerges and a number of charged fl droplets fly towards the substrate. This process stops when the pulse is switched off. After switching off the meniscus returns slowly to its equilibrium position. The process is controlled by different time constants, such as the slew rate of the power amplifier and the RC time of the electrical circuit composed of the electrical resistance in the fluid contained in the nozzle between electrode and meniscus, and the capacitance of the gap between the meniscus and the flat substrate. Another time constant deals with the fluid flow during the growth of the meniscus, directly after switching on the pulse. This fluid flow is driven by hydrostatic pressure and opposed by viscous drag in the nozzle. The final fluid flow during droplet formation is governed by the balance between by the drag of the charge carriers inside the fluid, caused by the current associated with the charged droplets leaving the meniscus and the viscous drag. These different phenomena will be discussed theoretically and compared to experimental results.