Basic principle of Electro Hydrodynamic Atomization (EHDA)

EHDA or electrospray refers to a process where a liquid jet breaks up into droplets under influence of electrical forces.

The formation of droplets depends on:

• The strength of the electric stresses.
• The liquid surface relative to the surface tension stress.
• The kinetic energy of the liquid leaving the nozzle.

The Technology is based on an aerosol reactor set-up, which is a patented method to produce nanoparticles by EHDA. A cone-jet is used to pump liquid through a nozzle at a low flow rate (µL/hour to mL/hour). An electric field is applied between the nozzle and a counter electrode. This electric field induces a surface charge in the growing droplet at the nozzle. Due to this surface charge, and due to the electric field, an electric stress is created in the liquid surface. If the electric field and the liquid flow rate are in the appropriate range, then this electric stress will overcome the surface tension stress and transform the droplet at the nozzle into a conical shape, the Taylor cone. The tangential component of the electric field accelerates the charge carriers (mainly ions) at the liquid surface toward the cone apex. These ions collide with liquid molecules, so accelerating the surrounding liquid. As a result, a thin liquid jet emerges at the cone apex. Depending on the ratio of the normal electric stress over the surface tension stress in the jet surface, the jet will break up due to axis symmetric instabilities (also called varicose instabilities) or due to varicose instabilities and lateral instabilities (called kink instabilities). At a low normal electric stress ratio in the varicose break-up mode, the desired monodisperse droplets are produced.

The droplets produced by EHDA carry a high electric charge close to the Rayleigh charge limit. To avoid Rayleigh disintegration of the droplets, the droplets are completely or partially neutralized. Rayleigh disintegration happens when the mutual repulsion of electric charges exceeds the confining force of surface tension, as a result, the droplets evaporate. Neutralization is with ions of opposite charge created by corona discharge.
The resulting mist as generated by an EHDA device is often referred to as a “soft mist”. The picture illustrates the difference in appearance between such a soft mist as generated by an EHDA device and the typical plume pattern as generated by a Metered Dose Inhaler.

 Typical MDI plume pattern (left) versus EHDA soft plume pattern (right).
Source: Battelle Memorial Institute