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Electrospray

Electrohydrodynamic atomization (EHDA), or simply electrospray, is a physical process that concerns the disruption of a liquid into a spray of charged droplets when it is subjected to an intense electric field (EF). The history of electrospray research dates back to the late 16th century when William Gilbert discovered that, in the presence of a charged piece of amber, a drop of water deformed into a cone. Almost 300 years later Lord Rayleigh estimated the maximum amount of charge a liquid droplet could carry based upon equilibrium between electrostatic repulsion and surface tension, the so-called Rayleigh limit. In the early 20th century,
Zeleny published two important works about electrified liquid surfaces and in the 1960s, Taylor paved the way
for modern electrohydrodynamics (EHD). Examples of EHD-based technologies include electrospray-ionizationmass spectroscopy (ESI-MS) and the production of particles for medical and agricultural purposes. The different kinds of electrosprays generated under different conditions are classified as “spray modes.” The classification and characteristics of the different modes in EHDA are topics extensively explored by many authors. A complete list of references for the statements quoted here is given in the paper quoted at the end of this page.

Reverse movement and coalescence of water microdroplets in electrohydrodynamic atomization

In the case of water, droplets may reverse their paths flying back toward the liquid meniscus, sometimes making contact with it. Such reverse movement is caused by polarization of the water inside the strong electric field. To understand this phenomenon we developed a way to calculate the droplet charge using its trajectory obtained by high-speed imaging. The values found showed that these droplets are charged between 2.5% and 19% of their Rayleigh limit.

The bouncing effect

Some droplets return (bounce) to the meniscus and are completely reintegrated to it. This category is called the returning followed by “complete coalescence” (category C1, see Fig. 2).
In the second case, droplets reverse their path after being ejected from the nozzle, touch the meniscus, and are not completely reintegrated but just transferred part of their mass after the contact and then return with a smaller radius. This interaction is called bouncing with “partial coalescence” (category C2, see Fig. 3).
In the third case, after the breakup, droplets decelerate as they move downward and stop. After remaining in equilibrium for some time they restart their downward movement. Because the droplet and meniscus are not in direct contact in this category, it is called “noncoalescent” bouncing (category C3, see Fig. 4).

 
  Fig. 1: Electrospray: A liquid (water) is pumped through a charged nozzle.
  Fig. 2: Category C1 (complete coalescence): a droplet returns to the cone after being formed from a liquid ligament. The diameter of the
returning droplet is ∼80 μm, the time frame between images is ∼45 μs, the applied potential is −5.67 kV on the counter electrode, and the
flow rate used is 1 mL h−1.
 

 

 

 
 

Fig. 3: Bouncing with “partial coalescence,” or C2: a droplet collides with the meniscus with mass exchange. The droplet diameter before
collision with the meniscus is ∼70 μm, and after it is ∼40 μm; the time frame between images is ∼50 μs, the applied potential is 5.0 kV on
the nozzle, and the flow rate is 1 mL h−1.

 

 
 

Fig. 4: Noncoalescent bouncing (C3): nozzle to plate configuration with 6 kV applied on the nozzle. The plate was grounded and placed 4 cm
below the nozzle. Dashed lines are arbitrarily placed to guide the eye. The droplet diameter is ∼30 μm, and the time frame between images is
40 μs.

 

 

The presented experiments resemble classical EHDA: A dc electric field interacts with deionized water and creates an electrospray in different modes, dependent on the field strength. What was hitherto unknown, however, is that in the pulsating jet mode, polarization forces can create oppositely
charged droplets, thus changing repulsion into attraction and making them return to the cone from which they were ejected. Once there, there are different possibilities: coalescence, partial coalescence, or noncoalescence. We found that the different categories depend on the retention time of the droplet in relation to the meniscus oscillation period, i.e., the changing of the electric field strength.We believe that these results can be used to better understand phenomena like the Lenard’s effect and the buildup of the electric field inside thunderstorm clouds.

The interested reader will find a thorough description of the experiments including charge and pathway calculatios as well as a complete reference list in the corresponding papers: