Abstract:
An atomizing device, comprising a fine liquid tube, a holder to permanently fix the tube proximate to its exit end and an optional cap to homogeneously and repeatably disintegrate small liquid amounts is disclosed. A manufacturing method for reproducibly machining the atomizer assembly of the present invention is provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application relates to and claims priority from commonly owned U.S. Provisional Application Ser. No. 60/674,005, filed on Apr. 22, 2005, which is incorporated herein by reference. 
     
    
     FEDERALLY SPONSORED RESEARCH 
       [0002]    Not Applicable SEQUENCE LISTING OR PROGRAM 
         [0003]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of Invention 
         [0005]    The present invention relates to an atomizing device comprising a fine tube as fluid path and its method of manufacture for providing a repeatable performance in terms of droplet size and spatial droplet distribution. The invention is particularly suitable for coating medical devices and for creating fine aerosols. 
         [0006]    2. Background of the Invention 
         [0007]    Atomizing devices comprising a fine liquid tube as fluid line are used in various applications, such as medical nebulizers, chemical analysis of liquid samples and coating devices to atomize small amounts of liquids. 
         [0008]      FIG. 1  depicts an enlarged view of the front section of an exemplary atomizer known by the prior art comprising an inner liquid delivery tube and a support member to secure the liquid delivery tube. A cap may be provided at the exit end of the liquid tube to form an annular gap  106  between the inner liquid delivery tube and the cap surrounding the liquid tube. The support member includes a central bore, having an internal diameter larger than the outside diameter of tube  104 , for aligning the liquid tube. Additional points for alignment of the liquid tube (not shown) may be provided by central bores disposed within the support member having slightly larger diameters than the outside diameter of the tube. 
         [0009]    The liquid tube may be additionally secured by various mechanisms, such as compression fittings as described in U.S. Pat. App. No. US2005/0029442, collet type connections as described in U.S. Pat. No. 6,337,480 or brace like support structure as provided by U.S. Pat. Nos. 5,868,322 and 6,032,876. 
         [0010]    Optimum atomization and particle transport efficiencies generally depends on the spatial characteristics of the spray plume and on the droplet size which, in turn, depends on the shape of the atomizer tip and/or on the roundness and concentricity of the annular gap. This is particularly true, when an atomizing gas is provided through a comparatively small annular gap. 
         [0011]    However, with current atomizers there is relatively poor control over the concentricity between the tube support member assembly and the cap for the atomizing gas, resulting in a misalignment of liquid tube relative to cap. A stable and secure support of the liquid tube may not be ensured because the tube is generally not sufficiently supported proximate to the liquid exit. Mechanisms used to support the liquid tube are often connections that don&#39;t ensure precise and repeatable positioning of the liquid tube in relation to the cap due to assembly tolerances. For example, tolerances between the outside diameter of tube  104  and inside diameter of bore  103 , and tolerances between the location shoulder and cap, as shown in  FIG. 1 , may lead to an increased error in concentricity between the liquid tube and cap. In U.S. Pat. Nos. 5,868,322 and 6,032,876 an atomizing device is provided having an outer tube comprising a brace-like support structure for mechanically securing the inner liquid tube. However, precise concentricity of the liquid tube and outer tube may not be ensured due to assembly tolerances and possible imperfections of the inner and/or outer tube. In addition, the brace-like support structure comprises gas channels, which may cause constrictions within the flow path. Thus, turbulence in the gas flow may be produced resulting in an unstable flow field and inconsistent spray performance. Furthermore, the gas channels, which are disposed along the micro tube, constitute limitations for generating a flow field with an angular momentum, which may be desirable to improve the atomization process. 
         [0012]    Another drawback of conventional atomizers, which usually comprise premanufacturered liquid tubes, are imperfections of the tube in terms of roundness and surface quality as well as manufacturing limitations. Due to the relatively small outside diameter and long length of such liquid tubes it may be difficult or even not possible to compensate such imperfections by machining the tip region. To overcome given quality limitations of prefabricated liquid tubes and maximize concentricity it is desirable to machine or even shape the tip of the liquid tube, which is particularly beneficial in electrostatic spraying applications. In addition, there is a risk of misalignment of the liquid tube in relation to the cap when disassembling and reassembling the tube support member assembly during cleaning and maintenance operations, which may result in poor spray performance. 
         [0013]    It has been furthermore found that when pneumatic assisted atomizers are used, comprising a very fine liquid tube that is not sufficiently stabilized towards the atomizing end, the spray performance may change during the same spray run. Gas stream  105 , as depicted in  FIG. 1 , which exits the annular gap and surrounds the liquid tube, may have an impact on position and alignment of the liquid tube in relation to the gas orifice. Depending on operating conditions and atomizer configuration, the gas flow may cause the liquid tube to oscillate and change its position during operation, resulting in an inconsistent spray performance of the atomizer over time. 
         [0014]    Imperfections of the tube tip and/or in the annular region directly translate into an inhomogeneous spray pattern, a relatively wide size range of droplets and increased droplet sizes. In addition, the shape and surface quality of the atomizing end at the liquid exit may influence the droplet break up and may result in poor efficiency of the atomization process, particularly in the case of electrostatic atomization. The spray performance of pneumatic atomizers, in terms of symmetric spatial particle distribution and tight droplet size distribution, is closely related to the roundness and concentricity of the annular gap. Any imperfection and eccentricity between the axes of the liquid delivery tube and the cap can cause the flow of the atomizing gas to be cylindrically asymmetric with respect to the axis of the liquid exiting from the liquid delivery tube. Hence, inhomogeneous gas velocities within the annular gap will lead to nebulization by the atomizing gas that is different on different sides of the spray plume. 
         [0015]    Poor spray stability and droplets that are too large and polydisperse in size may result in poor reproducibility and often poor stability during operation which, in turn, may lead to coating defects or reduced sample analysis efficiency. 
       OBJECT OF THE INVENTION 
       [0016]    Accordingly, there is a need for an atomizing device that overcomes the aforementioned problems with the prior art and provides improved stability and reproducibility of precision spraying processes. 
         [0017]    One object is to provide an atomizing device comprising a tube holder assembly, wherein the liquid tube is permanently fixed within the holder proximate to the exit end of the liquid tube, and the error of concentricity between the fine liquid tube and holder is compensated by a final machining operation. 
         [0018]    Yet another object is to modify the shape of the liquid tube holder assembly, particularly of the tip of the liquid tube, to compensate imperfections of the liquid tube for improved atomizer performance. 
         [0019]    Another object is to provide an pneumatic atomizing device, comprising a tube holder assembly and cap, that ensures the concentric alignment of the tube in relation to the cap to reproducibly generate a uniform spray pattern and small droplets with a tight droplet distribution. 
         [0020]    Still another object is to provide an atomizing device having a flow path with minimum perturbation of the atomizing gas flow to generate a stable flow field and to achieve a consistent atomization. 
         [0021]    Another object is to provide a atomizing device having a compact and robust design that can be manufactured reproducibly, resulting in a repeatable performance from one atomizer to the next. 
         [0022]    A further object is to allow easy assembling and disassembling without the risk of misalignment of the air cap relative to the liquid tube. 
         [0023]    Yet another object is to provide a manufacturing method for machining the atomizer assembly, which allows shaping of the tip of the liquid tube and results in improved concentricity, roundness and surface quality. 
         [0024]    These and additional features and advantages of the invention will be more readily apparent upon reading the following description of exemplary embodiment of the invention and upon reference to the accompanying drawings herein. 
       SUMMARY 
       [0025]    In one embodiment of the present invention, a device for disintegrating a liquid into fine droplets is provided, comprising at least one fine liquid tube having an outer wall, an entrance end and an exit end, a cap, surrounding and essentially coaxial with the liquid tube, having an exit opening proximal to the exit end of the liquid tube, and a holder with at least one holding section, through which the liquid tube extends. The outer wall of the liquid tube is fixed within the holding section to prevent displacement of the liquid tube in any direction orthogonal to the holding section axis and to allow machining of the liquid tube holder assembly. At least a portion of the liquid tube and at least a portion of the holder are machined in order to compensate the error of concentricity between the axis of the liquid tube and the axis of the holder. The cap is connected to the machined portion of the holder to provide an annular intermediate space between the liquid tube holder assembly and cap. The annular intermediate space has at least one gas inlet feeding directly into it and at least one exit opening and is free of intermediate structures. In one or more embodiments, the gas inlet may be positioned such that a gas flow field with an angular momentum can be generated. The exit opening of the cap may be manufactured by internal turning to improve roundness. The machining operation of the holder and liquid tube assembly may be performed in one setting. The machining operation of the holder and liquid tube assembly may be performed by turning, using the same finishing cut for the holder and tube. The liquid tube can be permanently fixed within the holding section. The holding section may be disposed proximal to the exit end of the liquid tube. The atomizing device may additionally comprise means for forming an electric field at the exit end. 
         [0026]    In a further embodiment, a device for disintegrating a liquid into fine droplets is provided, comprising at least one fine liquid tube having an outer wall, an entrance end and an exit end, a holder with at least one holding section through which the liquid tube extends, and means for forming an electric field at the exit end to disintegrate the liquid. The outer wall of the liquid tube is fixed within the holding section to prevent displacement of the liquid tube in any direction orthogonal to the holding section axis and to allow machining of the liquid tube holder assembly. At least a portion of the liquid tube and at least a portion of the holder are machined in order to compensate the error of concentricity between the axis of the liquid tube and the axis of the holder. In one or more embodiments, the machining operation of the liquid tube holder assembly may be performed in one setting. The tube may be permanently fixed within the holding section. The exit end of the liquid tube may be machined so that the tip diameter is reduced and shaped in order to improve the performance of the device. The machining operation of the liquid tube holder assembly may be performed by turning, using the same finishing cut for holder and liquid tube. The liquid tube may be additionally secured and coupled to the electrical means through a compression fitting. 
         [0027]    In certain embodiments, a method for machining a device for disintegrating a liquid into fine droplets is provided, including a fine liquid tube having an outer wall, an entrance end and an exit end and a holder having at least one holding section for the liquid tube. This method comprises the steps of connecting the liquid tube to the holder so that the outer wall of the liquid tube is fixed within the holding section to allow machining of the liquid tube holder assembly, and machining the holder tube assembly so that at least a portion of the liquid tube and at least a portion of the holder are machined to compensate the error of concentricity between the axis of the liquid tube and the axis of the holder. In one embodiment, the cutting operation of the liquid tube holder assembly may be performed by turning, using the same finishing cut for liquid tube and holder. 
         [0028]    In still another embodiment, a method for machining a device for disintegrating a liquid into fine droplets is provided, including a fine liquid tube having an outer wall, an entrance end and an exit end and a holder having at least one holding section for the liquid tube. This method comprises the steps of machining the holding section for the liquid tube by internal turning, wherein at least a portion of the final holder shape is machined in the same setup, and connecting the liquid tube to the holder so that the outer wall of the liquid tube is fixed within the holding section and the liquid tube is located at a predetermined position in relation to the holder. 
     
    
     
       DRAWINGS 
         [0029]    The accompanying drawings, which are incorporated in and constitute a part of this specification, serve to explain the principles of the invention. The drawings are in simplified form and not to precise scale. 
           [0030]      FIG. 1  (Prior Art) is a longitudinal cross-sectional view of the front portion of a conventional atomizer; 
           [0031]      FIG. 2  is a longitudinal cross-sectional view of the front portion of the atomizer of the present invention; 
           [0032]      FIG. 3  is a diagrammatic representation of a manufacturing procedure for machining a tube holder assembly comprising a final machining operation; 
           [0033]      FIG. 4A  is a longitudinal cross-sectional view of a holder having a holding section for the liquid tube; 
           [0034]      FIG. 4B  is a longitudinal cross-sectional view of a tube holder assembly before machining; 
           [0035]      FIG. 4C  is a longitudinal cross-sectional view of a tube holder assembly after machining; 
           [0036]      FIG. 5A  is a longitudinal cross-sectional view of a atomizer (tube holder assembly) without cap 
           [0037]      FIG. 5B  is an expanded view of the tip region of the atomizer shown in  FIG. 5A ; 
           [0038]      FIG. 6A  is a longitudinal cross-sectional view of an atomizer comprising a cap and a connection to a high voltage source; 
           [0039]      FIG. 6B  is a longitudinal expanded view of the gas passage defined by the intermediate space between tube holder assembly and cap; 
           [0040]      FIG. 6C  is a 3-D view of the gas passage defined by the intermediate space between tube holder assembly and cap; 
           [0041]      FIG. 7  is a diagrammatic representation of a manufacturing procedure for machining a tube holder assembly; 
           [0042]      FIG. 8A  is a longitudinal cross-sectional view of a holder comprising a holding section for the liquid tube; 
           [0043]      FIG. 8B  is a longitudinal cross-sectional view of a tube holder assembly; 
           [0044]      FIG. 8C  is a longitudinal cross-sectional view of a mounting fixture; 
           [0045]      FIG. 9  (Prior Art) is a spatial droplet distribution generated by a conventional atomizer; 
           [0046]      FIG. 10  is a spatial droplet distribution generated by the atomizer of the present invention; 
           [0047]      FIG. 11  is a comparison of the droplet size distributions between an atomizer of the Prior Art and of the present invention; and 
           [0048]      FIG. 12  is a comparison of the COV between an atomizer of the Prior Art and of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS/PREFERRED EMBODIMENTS 
       [0049]    The invention provides a compact atomizing device for reproducibly forming droplets from a liquid with improved operational stability, reliability and reproducibility compared to prior art atomizing devices. The atomizer is designed to allow precise and repeatable machining of the liquid tube holder assembly according to the manufacturing procedure described later herein. The liquid tube is embedded in a holding section between the liquid tube and the surrounding holder, and is disposed towards the exit end to provide support for the liquid tube resulting in minimum perturbation of the atomizing gas flow. Broadly, the invention provides an atomizer comprised of at least one liquid tube holder unit, the liquid tube permanently embedded in the holder proximate to the liquid exit end. The liquid tube and holder are positioned in a concentric arrangement about a common central axis and the tube is secured in a centered position. In one or more embodiments, the invention further provides a cap which, when coupled with the liquid tube holder unit, provides a conduit for the atomizing gas. The cap may be removably secured by a thread and aligned through a centering section between the holder and cap, so that a concentric alignment between the liquid tube and the cap as well as repeatable assembly and disassembly can be provided. 
       DETAILED DESCRIPTION 
       [0050]    While the invention will be described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. On the contrary, the invention includes all alternatives, modifications and equivalents as may be included within the spirit and scope of the present invention. Details in the Specification and Drawings are provided to understand the inventive principles and embodiments described herein, to the extent that would be needed by one skilled in the art to implement those principles and embodiments in particular applications that are covered by the scope of the claims. 
         [0051]      FIG. 2  illustrates the front section of the atomizer of the present invention comprising liquid tube  11 , cap  3  and holder  2 , which is used to align the cap in relation to the tube. The holder is preferably made from stainless steel or from a polymeric material, such as polyetheretherketone (PEEK). The tube can be constructed from any material which is impervious to chemical attack by the solution to be sprayed and which allows machining by cutting. The tube is preferably made from stainless steel or from a polymeric material. The outside diameter of the tube may range between approximately 0.3 and 1.615 mm depending on the particular application. All dimensions used herein are suggestive and not intended to be restrictive. In this embodiment, liquid tube  11  extends axially within holder  2  to a position slightly beyond air cap  3 . Alternatively, the liquid tube may be flush with the cap or may be placed at a recessed position with respect to the cap. Tube  11  is permanently connected to support holder  2  by joint  17 . Joint  17  is disposed next to the atomizing end of tube  11  and secures tube  11  along its entire perimeter. The axis of the tip of tube  19  is concentric with the axis of location shoulder  34  for cap  3 . Cap  3  is aligned with location shoulder  34 , which locates the axis of the tube holder assembly to be concentric with the axis of cap  3 . Thus, air cap  3  can be easily removed for maintenance and cleaning of the atomizer without the risk of misalignment. Cap  3  is secured to the liquid tube holder assembly by thread  36  and a small annular gap  16  to permit passage of gas is provided. The tip diameter of tube  19  and cap orifice diameter  20  define the width of the annular gap. The holder secures the tube in a centered position and allows finish machining of the assembly and shaping of the tip of the tiny tube. For precise concentricity between the tube and holder, the tip of liquid tube  19  and the outside shape of the holder are machined in one setting, preferably by a turning operation. The contour of the machined section is illustrated by line  42 . The final machining operation of the tube holder assembly compensates errors in concentricity. Hence, a proper and stable alignment of tube  11  in relation to location shoulder  34  of holder  2  can be ensured. Besides the concentricity, the roundness of orifice  20  of cap  3  and the tip of liquid tube  19  are crucial for the uniformity and repeatability of the atomization process. For optimized roundness, orifice  20  of air cap  3  and tip of liquid tube  19  are preferably machined using a precision turning operation. 
         [0052]    In operation, the liquid is fed at the liquid inlet, while the atomizing gas is fed in gas inlet  5 , located in the cap, and flows through gas passage  6  defined by the intermediate space between the tube holder assembly and cap to the exit end aperture and exits the atomizer at the annular gap formed between the liquid tube and the cap orifice. The atomizing gas disintegrates the liquid when it exits the liquid orifice. The liquid and gas are mixed outside the atomizer to obtain an aerosol. 
         [0053]    By providing a secure connection and optimized alignment between tube  11  and holder  2  and by machining the assembly, the concentricity between the axis of liquid tube  19  and orifice  20  of cap  3  can be substantially optimized compared to prior art atomizers. Consequently, the annular flow of the atomizing gas is very uniform about the spray axis, resulting in a symmetrical spray pattern. 
         [0054]    To compensate for alignment errors between the liquid tube and holder and to obtain improved roundness of the tube tip, a manufacturing procedure as diagrammatically shown in  FIG. 3  and illustrated in  FIGS. 4A-C  may be adopted. 
         [0055]    Referring to  FIG. 4A , holder  2  may be manufactured from a solid rod. For best alignment, centering or holding section  37  and holder outside diameter  12  are preferably machined in the same setting by turning using a precision lathe. Internal turning is preferred for centering section  37  because there is a minimized risk of out-of-roundness compared to other machining operations such as drilling. Depending on the size of liquid tube  11 , centering section  37  may have a diameter of approximately 0.3 to 2 mm and a length ranging from 2 to 6 mm. In a next step shown in  FIG. 4B , liquid tube  11  is mounted within holder  2 . To ensure a stable connection between holder  2  and liquid tube  11 , at least one permanent joint is provided between the holder and the tube. Additional joints may also be provided for improved stability of the tube and to facilitate machining of the assembly. For example, member  18  may secure the tube at the liquid inlet end resulting in permanent joint  23 , which may be obtained through press or shrink fit or by using an adhesive. Alternatively, a removable connection, such as a compression fitting, can be used to obtain a non-permanent joint. Depending on the size and material of liquid tube  11 , permanent joint  17  between holder  2  and liquid tube  11 , which secures liquid tube  11  along its entire perimeter, can be obtained as described below. If joint  17  is obtained by a press or shrink fit connection, the diameter of centering section  37  is machined slightly smaller than the outside diameter of liquid tube  11 . Holder  2  may be heated and liquid tube  11  cooled down to create a temperature difference between both parts. Then, tube  11  is pressed into holder  2  until face  13  of liquid tube  11  and face  14  of holder  2  flush. Alternatively, a permanent joint may be obtained by bonding liquid tube  11  to holder  2 . After machining the internal diameter of holder  2  slightly larger than the outside diameter of liquid tube  11 , a thin film of adhesive can be applied to the outside surface of liquid tube  11  and/or to centering section  37  to secure liquid tube  11  to holder  2 . Then liquid tube  11  can be disposed within the holder so that face  13  of liquid tube  11  and face  14  of holder  2  flush.  FIG. 4C  depicts machined section  42  of the liquid tube holder unit after a final machining operation. Machined section  42  may extend from the center of liquid tube tip  19  to the outside cylindrical surface of holder  2  and may comprise location shoulder  34  and threaded section  36  for alignment and connection of the cap. The final machining operation compensates possible imperfections of premanufactured liquid tubes and allows shaping of the tip to customize the atomizing device for the particular application. 
         [0056]    The final machining operation of the tube holder unit of the atomizer, described later herein in  FIG. 6 , is illustrated in more detail in  FIGS. 5A  and B.  FIG. 5B  is an enlarged view of the tube holder unit having machined section  42 , which extends to a tapered portion towards the exit end of tube  11 . To facilitate machining of centering section  37  and to improve stability of the tube holder unit, a stainless steel tube with a relatively large outside diameter resulting in an enlarged holding section may by used. For example, when a tube with an outside diameter of 1/16 inch, an inside diameter of approximately 0.2 mm and a length of 2 inches (commercially available from Upchurch Scientific, Oak Harbor, USA) is used, a holding section with a comparatively long length of approximately 6 mm can be provided. The holder can be made from a polymeric material such as polyetheretherketone (PEEK). The tube holder unit is preferably machined by turning. The resulting machined section  42 , extends from atomizer tip  19  to the cylindrical outer surface of the holder and comprises two centering sections  34  and a threaded section  36  to mount the cap. The tip of the tube has a decreasing outside diameter towards the liquid orifice. Liquid tube  11  and holder  2  are machined using the same finishing cut, resulting in a smooth transition between the tapered section of liquid tube  11  and the tapered section of holder  2 . The tapered section of the tube holder assembly is precision machined to obtain a smooth shape and an impingement angle for the atomizing gas resulting in an unobstructed gas flow for improved effectiveness of the atomization process. In addition, a possible error of concentricity between the axis of liquid tube tip  19  and the axis of the machined outer shape of the holder can be compensated. 
         [0057]    By providing an atomizer designed to allow machining of the liquid tube holder unit, a superior quality of the annular gap in terms of concentricity, roundness and smooth finish can be obtained. In addition, a facilitated, repeatable and cost-effective manufacturing method for the tube holder unit and especially for the tip of the tube is provided. 
         [0058]      FIG. 6A  illustrates an exemplary atomizer according to the present invention comprising holder  2 , tapered liquid tube  11 , and optional cap  10 . The liquid tube holder unit is designed to compensate the errors in concentricity between tip  19  of liquid tube  11  and holder  2  and to provide superior roundness of the shaped tube. The liquid tube holder unit is machined according to the manufacturing procedure described in  FIG. 4  and  FIG. 5 . Tube  11  is made from stainless steel and holder  2  and cap  10  are fabricated from a dielectric material, polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE, or Teflon), and the like. Tube  11  is permanently fixed into holder  2  to facilitate subsequent machining of the tube holder assembly. For improved alignment of cap  10  in relation to holder  2 , two centering sections  7  having different outside diameters are provided. Liquid tube  11  is connected to holder  2  via permanent joint  17  obtained by press fit and via a compression fitting including ferrule  24  and nut  26 . The compression fitting is used to secure the tube and to couple the tube to a high voltage source via cable  25 . Union  29 , which is connected to tube  11  by an additional compression fitting comprising nut  27  and ferrule  28 , provides the liquid inlet port. 
         [0059]      FIG. 6B  shows gas passage  6  defined by the intermediate space between the tube holder assembly and cap, which is free of intermediate structures and has a decreasing cross-section area towards annular gap  16 . In operation, the atomizing gas is introduced at tangential gas inlet  5  and flows towards annular gap  16  provided between the cap and tube holder assembly. A gas flow with an angular momentum is generated, resulting in a flow field with axial and radial velocity components and increased shear forces at the atomizer orifice. The liquid flows through the liquid tube to the atomizing end and is broken up by the atomizing air into very fine droplets having a tight particle size distribution. The break up length of the liquid can be reduced by generating an angular momentum resulting in an improved atomization. In addition, gas passage  6  is designed to minimize turbulence and to produce a stable gas flow, thereby ensuring a consistent atomization of the liquid to be sprayed. 
         [0060]    In the presence of an electrical field the atomization process can be improved by electrically charging the liquid to a very high voltage as described below. Alternatively, the atomizer may also be used without cap  10  to atomize the liquid using only electrostatic energy. In such situations, centering sections  34  may be used for alignment of the tube holder unit. In operation, a fine spray of charged droplets is produced when the liquid flows from the end of the liquid tube and emerges from orifice  15  of tube  11  in the presence of a high electric field. The electric field causes a disruption of the liquid surface and charged liquid droplets are generated. Depending on the polarity of the electric field, positively or negatively charged droplets are produced. The formation of an electrospray plume depends mainly on the electric field distribution in the space proximal to exit end  19  of tube  11 , which, in turn, depends on the shape of the electrically conductive surfaces bordering this space. To enhance the electric field gradient in the space proximal to the face of exit end  19  and to improve the atomization, the edge face of exit end  19  may be shaped as a cone by ‘sharpening’ the end. Depending on the particular operating conditions it may also be formed as a blunt face 
         [0061]    When using a liquid tube made from a non-machinable material, such as a ceramic material or fused silica, the manufacturing procedure diagrammatically shown in  FIG. 7  and illustrated in  FIGS. 8A-C  is preferably employed. Referring to  FIG. 8A , holder  2  comprises location shoulder  34  for the cap and holding section  37  for the liquid tube. The diameter of location shoulder  34  and the inner diameter of liquid tube holder centering section  37  are machined in the same setup to ensure proper alignment of cap  3  in relation to liquid tube  1 . Location shoulder  34  for cap  3  is preferably manufactured by external turning and centering section  37  for liquid tube  1  by internal turning. In order to achieve an optimized concentricity, an internal turning operation is preferred compared to drilling and reaming. As shown in  FIG. 8B , holder  2  and liquid tube  1  are precisely aligned and fixed by a permanent joint. Depending on the material and size of the liquid tube a permanent joint may be obtained by shrink fit, press fit or bonding. For a shrink or press fit connection, the internal centering diameter of centering section  37  is machined slightly smaller than the outside diameter of liquid tube  1 . Holder  2  may be heated until its internal centering diameter is larger than the outside diameter of liquid tube  1  to obtain a shrink fit connection. In a next step, liquid tube  1  is placed into holder  2  at a predefined distance from holder tip. Alternatively, a permanent joint may be obtained by bonding liquid tube  1  to holder  2 . The internal diameter of centering section  37  of holder  2  is machined slightly larger than the outside diameter of liquid tube  1  and a thin film of adhesive is applied to the outside surface of liquid tube  1  to secure it to holder  2 . To ensure a repeatable and precise positioning of the liquid tube in relation to the holder tip a mounting fixture, such as illustrated in  FIG. 8C , may be used to assemble liquid tube and holder. 
         [0062]    In order to demonstrate the performance of the atomizing device of the present invention various spray tests have been conducted. The spatial droplet distribution and the droplet size distribution have been measured and compared to an exemplary atomizer known by the prior art. The atomizers used were pneumatic atomizers having a fine liquid tube with an internal diameter of approximately 0.2 mm. The prior art atomizing device comprises a removable tube secured using a compression fitting. The front section of prior art atomizing device is shown in  FIG. 1 . The atomizing device of the present invention includes a permanently fixed micro tube, fabricated according to the manufacturing procedure of  FIG. 3 . The spray pattern was measured 20 mm downstream from the nozzle orifice using an Optical Patternator. The liquid to be atomized (DI Water) was supplied by a syringe pump (manufactured by Hamilton Company, Reno, Nev.) at a flow rate of 15 ml/h and the gas (air) was fed at a pressure of 0.7 bar. 
         [0063]      FIG. 9  depicts the spray pattern of the prior art atomizing device. The spray pattern has an asymmetric spray distribution comprising coarse particles in the right portion, depicted by line  81 . The asymmetric spray distribution may be caused by inhomogeneous gas velocities within the annular gap resulting from a misalignment of the liquid tube in relation to the cap and/or from poor roundness of the annular gap. In contrast, the spray pattern of the atomizer of the current invention, as shown in  FIG. 10 , has a homogeneous spatial droplet distribution. 
         [0064]    To compare the atomizer performance in terms of atomization consistency, a droplet size analysis has been performed. The droplet sizes of both atomizing devices have been measured using the Helos BF Laser Diffractometer (manufactured by Sympatec, Lawrenceville, USA), which was located 30 mm downstream from the atomizer orifice. The liquid to be sprayed was supplied by a syringe pump (manufactured by Hamilton Company, Reno, Nev.) at a flow rate of 3.5 ml/h and the atomizing gas was fed at a gas pressure of 1.0 bar. Eight measurement runs have been conducted during a spray time of approximately 5 minutes. 
         [0065]    It has been shown, as depicted in  FIG. 11 , that the prior art atomizing device generates an inconsistent droplet size distribution, in particular for droplet sizes ranging from 0.8 to 4 microns and for droplet sizes of 25 microns or more. Furthermore, comparatively large droplets of 40 microns or more have been detected in four measurement runs. In contrast, the atomizing device of the current invention provides a comparatively homogeneous droplet size distribution during the whole spray run. 
         [0066]      FIG. 12  illustrates the coefficient of variation (COV) of both atomizers, which can be used as a measure for the spray performance variation over time. The coefficient of variation has been calculated for the ×16, ×50, ×64, ×90 values, shown in the table below, obtained during eight measurement runs for each atomizer. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 ATOMIZER INVENTION 
                 ATOMIZER PRIOR ART 
               
             
          
           
               
                 RUN 
                 ×16 [μm] 
                 ×50 [μm] 
                 ×84 [μm] 
                 ×90 [μm] 
                 ×16 [μm] 
                 ×50 [μm] 
                 ×84 [μm] 
                 ×90 [μm] 
               
               
                   
               
             
          
           
               
                 1 
                 1.52 
                 5.45 
                 10.42 
                 11.98 
                 2.99 
                 11.18 
                 19 
                 21.25 
               
               
                 2 
                 1.53 
                 5.46 
                 10.45 
                 12.02 
                 2.93 
                 11.09 
                 18.75 
                 20.89 
               
               
                 3 
                 1.52 
                 5.45 
                 10.42 
                 11.98 
                 2.75 
                 10.97 
                 18.65 
                 20.83 
               
               
                 4 
                 1.52 
                 5.46 
                 10.44 
                 12 
                 2.63 
                 11.11 
                 19 
                 21.25 
               
               
                 5 
                 1.52 
                 5.46 
                 10.44 
                 12 
                 2.58 
                 10.93 
                 18.52 
                 20.63 
               
               
                 6 
                 1.52 
                 5.46 
                 10.46 
                 12.04 
                 2.6 
                 10.95 
                 18.71 
                 20.9 
               
               
                 7 
                 1.52 
                 5.44 
                 10.41 
                 11.97 
                 2.61 
                 11.09 
                 19.13 
                 21.58 
               
               
                 8 
                 1.52 
                 5.44 
                 10.44 
                 12.02 
                 2.56 
                 11.01 
                 18.84 
                 20.95 
               
               
                   
               
             
          
         
       
     
         [0067]    Referring to  FIG. 12 , the COV values of the atomizer of the present invention are significantly smaller than the values obtained for the prior art atomizing device. The repeatable spray performance indicates that the liquid atomization has been improved by optimizing the atomization region in terms of concentricity between the liquid tube and annular gap, surface quality and by providing a securing mechanism which prevents misalignment of the liquid tube during operation. The results outline the advantages of the design and manufacturing methodology adopted for the atomizer of the present invention in terms of spray pattern quality and atomization consistency.