Patent Application
20230053695
This disclosure relates to creation of aerosol droplets, more particularly to creation of aerosol droplets using an array of electrified wicks.
Creating aerosol droplets with submicron diameters presents a considerable engineering challenge. Conventional spray nozzles, in which water is forced through a narrow orifice, produce mists with droplet diameters in the tens of microns to several millimeters. To decrease droplet size by a factor of ten, the pressure for a given nozzle must increase by more than 2,000 times. The pressures needed to produce submicron droplets require large amounts of energy, and can quickly lead to nozzle failure.
Other atomizers, like the ultrasonic nebulizers found in home humidifiers, can produce droplets with diameters of less than ten microns. However, these cannot produce smaller droplets without extremely high frequencies and high power requirements.
Electrospray atomization can produce submicron droplets. A large electrical field deforms the liquid surface at the end of a capillary, deforming it into a so-called Taylor cone. After formation of the cone, a narrow jet emits from the liquid surface quickly forming into small droplets. Electrospray is high tunable, produces droplets within a narrow size distribution, and produces charged droplets which are unlikely to coalesce into bigger drops.
The minimum field required to form the Taylor cone is:
where γ is surface tension, rc is capillary radius, ε0 is permittivity, and θ is the Taylor cone angle, 49.3°. For water, which has a high surface tension of 73 mN/m, the turn-on field will exceed the breakdown strength of air, approximately 3 kV/mm, if the capillary radius is smaller than 1.2 mm.
In practice, arcs and air ionization occur at much lower fields, requiring much larger diameter capillaries. As capillary diameter increases, capillary pressure decreases, and steady feeding becomes difficult. Workarounds for water electrospray exist, such as operating in a vacuum, using high strength breakdown gas, or adding chemical surfactants to lower the surface tension. However, these increase system cost for particle production, or are undesirable for applications such as outdoor coatings and sprays.
According to aspects illustrated here, there is provided an electrospray generator including a first electrode, a reservoir of liquid adjacent the first electrode, at least one wick having one end in the reservoir in contact with the liquid, a second electrode spaced a distance away from the wick, and a power source connected to one or more of the first and second electrodes.
According to aspects illustrated here, there is provided a method of generating a spray including inserting first ends of one or more wicks into a reservoir of liquid, the reservoir having a base electrode adjacent the liquid, positioning one or more offset electrodes at distance from second ends of the one or more wicks, and applying a voltage to at least one of the base electrode and the one or more offset electrodes to create an electric field, the electric field causing the liquid to move through the one or more wicks and form droplets in a spray.
The embodiments here enable electrospraying of water and other high surface tension liquids in air at atmospheric pressure and without the need for surfactants. Embodiments generally include an array of cylindrical wicks. Each wick has one end submerged in a liquid bath with an electrode at the bottom adjacent the liquid. Water travels up the wick via capillary action. In another embodiment, the wick end sits in a pressurized bath and has a liquid-tight seal to enable transport of liquid through the wick. A second electrode resides a distance at least two times the wick diameter away from and parallel to the other ends of the wick or wicks. An electrical field applied between the two electrodes causes the Taylor cone to form allowing extraction of droplets. Air flow, around or across the wicks, may direct the generated particles away from the electrode and towards the collector.
A second electrode, referred to as on offset electrode, 18 lies a predetermined distance away from the reservoir. In one embodiment this distance is twice the diameter of the one or more wicks. In one embodiment the electrode is a flat plate. In another embodiment the electrode contains holes such as 19 aligned with the wicks so that ejected droplets pass through the electrode. A voltage source 20 provides a voltage to the second electrode 18, with the first electrode 16 being grounded at 24, or the opposite. Alternatively, one electrode would be at a first potential and the other at a different potential. As long as a voltage differential exists, the result is an electric field. The voltage source has the capability to generate a voltage in the range of 1 kV to 70 kV, in one embodiment, the voltage source provides 20 kV, and in another it provides 50 kV. The current from the voltage source remains relatively low, In yet another embodiment there is a third electrode, an additional offset electrode, 17 positioned a further distance from the first electrode, for the purpose of accelerating droplets which pass through the second electrode. This third electrode is held at a higher positive or lower negative potential than the second electrode, in the case where the first electrode is grounded.
When the electrodes are activated, the liquid moves up through the wick and forms a Taylor cone between the wick 12 and the electrode 18. As the cone breaks up to form droplets, an air flow source 29, which may comprise a fan, allows the system to direct the droplets in a desired direction. The system may include other air direction components, such as baffles, not shown.
The wicks may have several different variations. For example, in
Regardless of the configuration of the wicks, the array 30 of wicks such as 12 have one end in the reservoir with an electrode 24 at the bottom of the common reservoir and an electrode 18 spaced above the reservoir. The fan previously shown in
In
In this manner, an array of wicks that can each form a Taylor cone to form submicron droplets without requiring excessive pressure or frequency requirements. Formation of submicron droplets allows for many different applications, including marine cloud brightening, formation of thin coatings and nanoparticle formation.
All features disclosed in the specification, including the claims, abstract, and drawings, and all the steps in any method or process disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in the specification, including the claims, abstract, and drawings, can be replaced by alternative features serving the same, equivalent, or similar purpose, unless expressly stated otherwise.
It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the embodiments.
