Fluid sprinkling – spraying – and diffusing – Processes – Vibratory or magneto-strictive projecting
Reexamination Certificate
2001-08-14
2003-12-09
Hwu, Davis (Department: 3752)
Fluid sprinkling, spraying, and diffusing
Processes
Vibratory or magneto-strictive projecting
C239S102100, C239S102200
Reexamination Certificate
active
06659364
ABSTRACT:
The present invention relates to droplet generation and, more particularly, to the generation of liquid droplets from a membrane having one or more apertures.
In our European Patent EP-B-0615470, we describe an apparatus of this general type and there are numerous other examples of droplet generators of this type, see in particular, our European Patent EP-B-0732975.
Other prior art of possible relevance includes: EP-A-0 432 992, GB-A-2 263 076, EP-A-0 516 565, U.S. Pat. No. 3,738,574, EP-A-0 480 615, U.S. Pat. No. 4,533,082 & U.S. Pat. No. 4,605,167.
In our EP-B-0732975 we describe and claim a liquid droplet spray device comprising a perforate membrane; an actuator, for vibrating the membrane; and means for supplying liquid to a surface of the membrane, characterised in that perforations in the membrane have a larger cross-sectional area at that face of the membrane away from which liquid droplets emerge than at the opposite face of the membrane.
According to EP-B-0732975, such a device is a perforate membrane capillary wave droplet generator believed to be excited and to pin capillary waves within the small orifice of the membrane perforations. It employs a perforate membrane designed to spatially match the excited capillary wave field (i.e. one or more capillary waves pinned within each orifice), so that the coupling efficiency of energy transferred to the oscillating capillary waves is greatly enhanced over free-surface capillary wave generators. Thus, significantly lower power and lower cost devices can be employed to generate droplets using the capillary wave approach. However, even with the enhanced energy coupling provided in such devices, devices of this type together with the drive electronics needed to excite their operation still consume up to 10 W of power to operate in the 250 kHz-8 MHz frequency region to enable the production of small water droplets between 1 and 10 &mgr;m in diameter. It is desirable to create such small droplets, for example, in order to practice electrophotography as described in our International patent application no. PCT/GB96/01671.
The present invention is aimed particularly, but not solely, at reducing the power consumption necessary to produce small droplets using a device of the vibratory perforate membrane type. Drive electronics operating at higher frequencies in the range 250 kHz to 8 MHz are also generally more expensive than drive electronics otherwise similar in nature but operating at lower frequencies. The present invention, in allowing lower frequency operation of vibrating perforate membrane droplet generators, therefore also has utility in reducing the overall cost of such droplet generators, particularly in the production of small droplets relative to those of capillary wave type described above.
According to the present invention, there is provided a method of generating liquid droplets from an aperture in a membrane, the aperture having a diameter &phgr; at one face at most as large as its diameter at the other face, the method comprising
providing a liquid to the one face of the membrane adjacent the aperture, the liquid having a surface tension &sgr; and density &rgr;;
applying a pressure bias such that the pressure of the liquid at the one face of the membrane is lower than the pressure immediately adjacent to the opposing face of the membrane; and
while the bias pressure is applied, vibrating the membrane at a frequency f which is determined by the relationship:
f
≤
(
5
π
σ
ρ
φ
3
)
1
/
2
in order to generate droplets emergent from the other face.
The invention also includes a droplet generator for generating droplets of a liquid having a surface tension &sgr; and density &rgr;, the droplet generator comprising
a membrane having a first face and a second face, an aperture extending through the membrane from the first face to the second face, the aperture being of a diameter &phgr; at the first face at most as large as its diameter at the second face;
means for applying a pressure bias such that the pressure of the liquid at the first face of the membrane is lower than the pressure immediately adjacent to the second face of the membrane; and
a vibration generating means for causing the membrane to vibrate at a frequency f which is determined by the relationship:
f
≤
(
5
π
σ
ρ
φ
3
)
1
/
2
in order to generate droplets emergent from the other face.
The invention further includes a liquid atomisation head for a droplet generator for generating droplets of a liquid having a surface tension &sgr; and density &rgr;, the liquid atomisation head comprising
a membrane having a first face and a second face, an aperture extending through the membrane from the first face to the second face, the aperture being of a diameter &phgr; at the first face at most as large as its diameter at the second face; and
a vibration generating means for causing the membrane to vibrate at a frequency f which is determined by the relationship:
f
≤
(
5
π
σ
ρ
φ
3
)
1
/
2
in order to generate droplets emergent from the other face.
By “liquid atomisation head” is meant a device which, in use, converts liquid in contact with it into liquid droplets emergent from it. This includes aerosol generation and ink jet devices. “Liquid” is to be understood to include pure liquids, mixtures of liquids, solutions, and suspensions of solid particles within a liquid carrier.
According to a second aspect of the invention, the relationship in the statements of invention defined above is replaced by the relationship:
f
≤
(
3.6
×
10
-
4
·
π
φ
3
)
1
/
2
The relationship immediately above applies in the case, particularly, but not exclusively, of water, and falls wholly within the definition of the frequency relationship given previously above.
Preferably, the vibration generating means is integrally or intimately formed with the membrane.
Prior art devices of the vibrating perforate membrane type are most commonly known with perforations that decrease in size from the side of the membrane to which bulk liquid is supplied to the opposite, droplet-emergent, side. These are known as forward-tapered perforations and in devices having such forward-tapered perforations, droplet ejection consistent with the condition f≦(5Π&sgr;/&rgr;&phgr;
3
)
½
is understood from, for example U.S. Pat. No. 5,164,740. However, droplet ejection under that condition with devices in which the perforations are tapered as in devices according to the present invention, (‘reverse-tapered’), has not been previously observed. Furthermore, droplet generation in these circumstances cannot be understood from the capillary wave mechanism known from previous reverse taper devices as described, for example, in EP-B-0732975.
Ultrasonic droplet generators, which are preferably used in the present invention, typically have a number of harmonic vibration frequencies at which they will resonate. At some of these harmonic frequencies, a capillary wave may be excited within each membrane perforation or aperture. As the harmonic vibration frequency increases, so the capillary wavelength of excited capillary waves decreases and the emergent droplet diameter reduces accordingly. These features are known from the following equations:
&phgr;
d
≈&lgr;/3
and
λ
3
=
8
π
σ
ρ
f
2
(
1
)
where &phgr;
d
is the droplet diameter and &lgr; is the wavelength of the capillary wave.
According to the model presented in EP-B-0732975, an apparatus is provided with membrane orifices such that one or more such capillary waves fits within and is “pinned” by the relatively large opening of a perforation at the rear face of a membrane.
In this way, an otherwise large opening at the rear face of a membrane can produce a small droplet emerging from the front face of the membrane. For a given liquid (and hence &sgr; a
Blakey David Mark
Humberstone Victor Carey
Gossett PLLC Dykema
Hwu Davis
The Technology Partnership PLC
LandOfFree
Droplet generation method and device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Droplet generation method and device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Droplet generation method and device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3116369