Technology

Electrohydrodynamic atomization is a method for breaking up and dispersing a conducting fluid into a beam of charged nanodroplets. To initiate an EHD beam, the electrostatic stress applied to a fluid meniscus must exceed the surface tension forces which hold the meniscus intact.

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 electrostatic field at the spraying end of the capillary.

EHD uses extremely low volumes of fluid. Typical usage rates can be in the range of .6 to 2 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 after use.

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 electrostatic field overcomes the meniscus surface tension, resulting in the breakup of the meniscus into an EHD nanodroplet beam.

Since the nanodroplets created through the EHD process are multiply charged, their acceleration can be controlled through the electrical field between the exit of the capillary and the item to be cleaned.

While EHD nanodroplets can be quite small, they are of massive size relative to ions in an ion beam. EHD nanodroplets expend their energy over an extended area of the target, causing simultaneous liftoff and removal of micron and submicron particulates, organic film, and metallic contaminants. The energy of an EHD nanodroplet is shared by the large number of nucleons in the nanodroplet. This results in energies well below material sputtering thresholds, preventing direct etching or damage to surfaces during the contaminant removal process.

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 the contaminants and substrate. Nanodroplets can be created for best coupling of momentum transfer to particles.

Spray

The “Taylor cone” mode of EHD spray can be instructive.

EHD Taylor Cone

In this photo of an active EHD spray, a conductive fluid is delivered to the exit of a capillary tube. The inside diameter of this capillary is 50 microns, while the outer diameter is approximately 800 microns. This end of the capillary is actually square-cut.

The fluid at the exit of the capillary configures itself into a cone due to the electrostatic forces in the vicinity. The sharp tip of the cone geometry results in the highest localized electric field strength. A high-velocity jet of fluid about 1 micron in diameter sprays from the tip of the cone. This results in a “virtual nozzle”, as if a capillary tube with an inside diameter of 1 micron were able to be fabricated.

The high velocity jet then breaks up into discrete nanodroplets which diverge because they are the same charge polarity. This EHD beam is in equilibrium, as the volumetric flow rate of fluid delivered to the spray site through the capillary tube equals the ability of the electric field to extract the same volumetric rate of fluid.

History

EHD electrostatic atomization owes its origins to early investigations of high voltage liquid discharges from capillary tubes. In 1914, J. Zeleny, then at the University of Minnesota, first studied the behavior of droplet emission from glass capillary tips held at high positive and negative potentials. Zeleny went on to identify various modes of droplet emission associated with meniscus changes at capillary tips as a function of the applied voltage.

In the 1950’s, G. Taylor further analyzed EHD emission, and studied the characteristics of the cone of liquid that can form at the capillary tip. These cones have since been referred to as “Taylor cones”.

In the 1960’s, electrostatic spraying underwent intensive research for microthruster applications in spacecraft. These charged particle propulsion systems relied on EHD atomization in a vacuum to provide micro-Newton thrust levels. EHD-based microthrusters have been deployed in space as “colloid propulsion”.

Ultrafine, rapidly solidified, amorphous powders can be created using EHD. This technique has successfully created submicron spherical particles from metals, alloys, and ceramics.

EHD is a vital element of electrospray mass spectroscopy (ESMS), which is used to analyze high-molecular weight molecules for biochemical applications.

In the 1990’s, Dr. Julius Perel and John F. Mahoney, both now with EHD Technology Group, recognized the ability of EHD to remove contaminants from surfaces. This initiated the current work with EHD in nanotechnology applications.

Patents

EHD Technology Group holds issued and pending patents for EHD cleaning. In addition, years of EHD-related research and characterization have created a large pool of Intellectual Property (IP) and specific knowledge related to all aspects of EHD.

This growing portfolio of IP is available for licensing, and EHD Technology Group is available for specific projects to address additional nanotechnical applications.