Introduction to EHD
Configuration
Spraying
Modeling
Electrohydrodynamic (EHD) atomization is a method for breaking up and dispersing a conducting fluid into a beam of charged nanodroplets. EHD is a "green" technicnology because it uses extremely low volumes of fluid. Usage rates can be less than 10 microliters per minute per nozzle. This aspect of EHD can allow a significant reduction in the costs associated with high volumes of process chemistries and their disposal.
Configuration
In a typical EHD system, a small fluid reservoir holds the conductive process chemistry to be sprayed, and an electrical contact in the reservoir applies a potential to the fluid. A pneumatic controller applies a controlled pressure to the fluid in the reservoir, resulting in a flow of fluid from the reservoir, through a capillary tube, and into the electric field at the spraying end of the capillary.
A schematic is shown below.
Spraying
As the EHD spray starts, current flows through the conductive fluid in the capillary. In addition, charged ions in the fluid migrate in opposite directions in the capillary, resulting in an uneven distribution of charge. Ions of the same polarity as the voltage applied to the reservoir migrate to the far, or spraying, end of the capillary. There, the electric field overcomes the meniscus surface tension, resulting in the breakup of the meniscus into an EHD nanodroplet beam.
Control of the EHD nanodroplets is implemented electrically, through reservoir charging levels and electric field manipulation at the capillary exit. Speed and size can be varied, resulting in a wide range of process settings to match the nanodroplets to a specific application.
The mean radius and velocity of these sprays can be estimated from experimental measurements and vary with the conductivity of the sprayed solution. EHDTG models show that in one run of experiments, we generated mean radii and velocities shown here:
Modeling
In particle removal applications, nanodroplets may impact surfaces with impulse forces exceeding the Van de Waals and electrostatic adhesion forces that bond the contaminants to the surface. Some studies have shown that similar localized, high pressure impacts of 0.1-100 GPa may desorb organic films. In other cases, a chemical reaction may assist in breaking the bond to the surface through various electrochemical or steric effects; and may be enhanced through an appropriate design of fluid composition.
EHD provides a unique set of parameter “dials” which can be varied over a wide process range in order to generate an EHD spray to solve a specific problem.