Electrohydrodynamic spraying means

Details Electrohydrodynamic spraying means Electrohydrodynamic spraying means

2000-09-27

2004-01-20

Nguyen, Dinh Q. (Department: 3752)

Fluid sprinkling, spraying, and diffusing

Electrostatic type

C239S692000, C239S707000

Reexamination Certificate

active

06679441

ABSTRACT:

The present invention relates to electrohydrodynamic spraying (hereafter called EHDS) means.
EHDS is a means of producing sprays of electrically charged liquid droplets of millimetric, micron or submicron size.
EHDS essentially consists in applying an electric field to a liquid so as to induce, on the surface of this liquid, electric charges of the same polarity as the voltage applied to it. These charges, accelerated by the electric field, cause the drop of liquid to be transformed into a cone. A jet of liquid is produced at the apex of this cone, which jet fragments into droplets (spray) of millimetric, micron or submicron size.
Various liquid fragmentation modes may be obtained and have been described in the prior art (cf. especially Cloupeau and Prunet-Foch, 1989, J. Electrostatics 22, pp. 135-159). Mention may especially be made of the “drop-by-drop” mode which produces millimetric drops and the stable “cone-jet” mode which produces a bimodal particle size distribution of the spray (micron drops and submicron satellites).
Various means have been described in the prior art for making it possible to obtain an EHDS in stable “cone-jet” mode (a mode guaranteeing bimodal dispersion) in the case of liquids whose surface tension at room temperature is less than or equal to 0.055 N/m, such as ethanol, acetone and ethylene glycol. However, EHDS in “cone-jet” mode poses a problem in the case of liquids having a high surface tension, such as water or else liquids to which reactants or active principles having a surfactant effect have been added.
This is because the high surface tension of these liquids means that high potentials have to be applied to the liquid in order to produce an EHDS from them, this in turn creating a large electric field in the gas surrounding the liquid and, consequently, creating ionization phenomena in the gas. In air, at atmospheric pressure, these electrical discharges are mostly of pulse duration (dart leaders) and prevent the establishment of a “cone-jet” fragmentation mode in favour of a “cone-jet-glow” mode.
Thus, EP 0,258,016 describes an electrostatic spray system intended to allow the application of very thin surface coatings. This system is capable of spraying, in air at atmospheric pressure, liquids whose surface tension is less than 0.065 N/m, and preferably less than 0.050 N/m, but this is so only if the corona-type phenomena are avoided (“cone-jet” mode of fragmentation of the liquid). If discharges were to appear, EP 0,258,016 indicates that its device must be placed in a gas other than air, or in an atmosphere different from at atmospheric pressure. The teaching of EP 0,258,016 therefore leads a person skilled in the art to avoid discharge phenomena, which are regarded as spray destabilizers.
Various approaches have been proposed in the prior art for stabilizing the EHDS of such liquids, by preventing the formation of pulsed discharges in the gas surrounding them. Two types of approach may be identified: a first type of approach uses an increase in the dielectric strength of the gas surrounding the liquid by increasing the pressure of the gas and/or by employing gases other than air, such as CO
2
or SF
6
; a second type of approach uses an additional electrode placed near the cone and near the jet of liquid so as to reduce the radial electric field in the gas near the liquid. However, neither of these types of approach is satisfactory from the industrial standpoint: the first type requires means of controlling the atmospheric environment and the second type requires an additional high-voltage source.
To the knowledge of the Applicant, none of the devices described in the prior art therefore allows, in the case of liquids having a high surface tension, such as water, EHDS in air and at atmospheric pressure, without generating a pulsed discharge regime and without requiring the use of an additional electrode.
The present application relates to novel means allowing this problem to be solved and is aimed at overcoming the drawbacks of the means of the prior art.
In fact, the inventors have for the first time confirmed that an EHDS without a pulsed discharge regime could be established directly in air and at atmospheric pressure for liquids whose surface tension, as measured at room temperature, is greater than 0.055 N/m and, notably, greater than 0.065 N/m. They have in particular confirmed that such an EHDS can be obtained using an EHDS device complying with certain operating parameters and, most essentially, using an EHDS device comprising at least one liquid delivery duct
1
whose external diameter and internal diameter values, at the point of emergence of the biased liquid, satisfy an appropriate relationship within a predefined range of external diameters (cf. examples and graph in
FIG. 2
below). Such a relationship may especially correspond to a ratio of the (external diameter value) to the (internal diameter value) of greater than or equal to a fixed limiting value.
The inventors have in fact observed that the discharge regime in the gas (a continuous discharge regime—stabilizing glow—or a pulsed discharge regime—destabilizing dart leaders) is directly related to the divergence of the field in the gas. They have thus confirmed that, for liquids whose surface tension is greater than 0.055 N/m and, notably, greater than 0.065 N/m, it is essential, in order to produce the desired EHDS in air at atmospheric pressure, to choose external and internal diameters which make it possible to control:
the shape of the liquid, that is to say the geometry of the cone and of the jet of liquid; and
the potential drop in the liquid, that is to say the potential at the surface of the liquid; so as to control the divergence of the field in the gas (that is to say the variation in the electric field in the gas).
Thus, the first subject of the present invention is an electrohydrodynamic spray device comprising at least one duct
1
at one outlet of which a biased liquid can be sprayed. The device according to the invention makes it possible to spray, in air, at atmospheric pressure, a liquid whose surface tension, as measured at room temperature, is greater than 0.055 N/m and, notably, greater than 0.065 N/m, without generating a pulsed discharge regime. A means for demonstrating the absence of such a pulsed discharge regime comprises the measurement of the time variation of the current using a high-speed oscilloscope. According to one advantageous aspect, the device according to the invention is capable of spraying, in air and at atmospheric pressure, a liquid whose surface tension is greater than 0.055 N/m and, notably, greater than 0.065 N/m, by generating a continuous discharge regime, such as a corona-type discharge regime (or glow regime or Hermstein regime).
The device according to the invention is thus characterized in that it comprises means, and especially means of external and internal diameters, of the duct
1
, at the very least at the said outlet of the duct
1
, which spray, in air and at atmospheric pressure, a liquid whose surface tension, as measured at room temperature, is greater than 0.065 N/m, by generating a continuous discharge regime, such as a corona-type regime (or glow regime or Hermstein regime). Various means are known to those skilled in the art for monitoring the continuous nature of a discharge regime. Mention may especially be made of measurement of the electric current using a high-speed oscilloscope, the visual checking of the stability of the liquid cone formed and/or the particle size distribution measurements used for confirming the bimodal nature of the droplet size distribution. Such a bimodal distribution may especially correspond to a first, major droplet population (corresponding for example to 90% of the liquid volume sprayed), of larger average droplet size and to a second, minor droplet population (corresponding for example to 10% of the liquid volume sprayed), of finer average droplet size.
By the term “electrohydrodynamic spray device” we mean, in the present invention, a device capable of generating a spra

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