Abstract:
An inkjet printing device includes: a flow path plate, a piezoelectric actuator and an electrostatic force applicator. The flow path plate includes an ink inlet, a pressure chamber and a nozzle. The piezoelectric actuator is configured to provide a first driving force, and the electrostatic force applicator is configured to provide a second driving force. The disclosed inkjet printing devices and methods combine piezoelectric and electrostatic techniques.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to Korean Patent Application No. 10-2009-0008848, filed on Feb. 4, 2009, in the Korean Intellectual Property Office, the entire contents of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     One or more example embodiments relate to inkjet printing devices using a combination of a piezoelectric technique and an electrostatic technique, and methods of driving the same. 
     2. Description of the Related Art 
     Conventional inkjet printing devices eject fine droplets of ink onto desired positions of printing media by using inkjet heads to print given, desired or predetermined images on printing sheets. The inkjet printing devices have been applied to a larger variety of fields, for example, flat panel displays (FPDs) such as liquid crystal displays (LCDs) and organic light emitting displays (OLEDs), flexible displays such as electronic paper (e-paper), printed electronics such as metal interconnection lines, and organic thin film transistors (OTFTs). Among process techniques for applying the inkjet printing devices to display devices or printed electronics, relatively high-resolution ultrafine printing techniques may be needed. 
     Related art inkjet printing devices may be classified as piezoelectric inkjet printing devices and electrostatic inkjet printing devices depending on how the ink is ejected. Specifically, related art piezoelectric inkjet printing devices eject ink by deforming a piezoelectric material, while related art electrostatic inkjet printing devices eject ink using an electrostatic force. In more detail, related art electrostatic inkjet printing devices operate based on the following two methods. In a first method, ink droplets are ejected using electrostatic induction. In a second method, charged pigments are accumulated using an electrostatic force and then ink droplets are ejected. 
     In the case of a piezoelectric inkjet printing device, because ink is ejected by using a drop on demand (DOD) technique, it is relatively easy to control a printing operation and drive the inkjet printing device. Also, the piezoelectric inkjet printing device generates ejection energy by mechanically deforming a piezoelectric material, and thus, any kind of ink may be used. However, the piezoelectric inkjet printing device does not produce ultrafine droplets having a size of several picoliters or less nor does it allow ink droplets to reach a desired position as compared with an electrostatic inkjet printing device. 
     The electrostatic inkjet printing device may produce ultrafine droplets, is relatively easy to drive, and allows ink to be ejected in a desired direction. As a result, the electrostatic inkjet printing device is more appropriate for relatively precise printing processes. However, because it is difficult to form separate ink flow paths in an electrostatic inkjet printing device by using an electrostatic induction technique, ink is relatively difficult to eject via a plurality of nozzles by using the DOD technique. Also, when charged pigments accumulate due to an electrostatic force, the ejection rate of ink droplets and the kind of ink is limited because it is necessary to accumulate relatively highly dense pigments. 
     Moreover, in the related art the amount of ejected ink droplets is proportional to the diameters of nozzles of inkjet printing devices. Thus, it is necessary to reduce the sizes of nozzles to eject fine ink droplets. However, a reduction in the sizes of the nozzles makes it difficult to manufacture precise nozzles and causes the nozzles to clog more frequently, thereby reducing reliability. 
     SUMMARY 
     One or more example embodiments provide an inkjet printing device using a technique that is a combination of a piezoelectric technique and an electrostatic technique, and a method of driving the inkjet printing device for ejecting fine ink droplets. 
     At least one example embodiment provides an inkjet printing device. According to at least this example embodiment, the inkjet printing device includes a flow path plate, a plurality of pressure chambers and a plurality of nozzles. The flow path plate includes an ink inlet through which ink is supplied. The plurality of pressure chambers are filled with the supplied ink, and the ink filled in the plurality of pressure chambers is ejected through the plurality of nozzles. The inkjet printing device further includes a piezoelectric actuator and an electrostatic force applicator. The piezoelectric actuator is configured to provide a pressure change in the ink filled in the plurality of pressure chambers as a first driving force used to eject ink droplets from the plurality of nozzles. The electrostatic force applicator is configured to apply an electrostatic force to the ink filled in the plurality of nozzles as a second driving force used to eject the ink droplets from the plurality of nozzles. 
     At least one other example embodiment provides an inkjet printing device. According to at least this example embodiment, the inkjet printing device includes a flow path plate, at least one pressure chamber and at least one nozzle. The flow path plate includes an ink inlet through which ink is supplied. The at least one pressure chamber is filled with the supplied ink, and the ink filled in the at least one pressure chamber is ejected through the at least one nozzle. The inkjet printing device further includes a piezoelectric actuator and an electrostatic force applicator. The piezoelectric actuator is configured to provide a pressure change in the ink filled in the at least one pressure chamber as a first driving force used to eject an ink droplet from the at least one nozzle. The electrostatic force applicator is configured to apply an electrostatic force to the ink filled in the at least one nozzle as a second driving force used to eject the ink droplet from the at least one nozzle. 
     Yet at least one other example embodiment provides an inkjet printing device. According to at least this example embodiment, the device includes a flow path plate, a piezoelectric actuator, and an electrostatic force applicator. The flow path plate includes an ink inlet, at least one pressure chamber configured to be at least partially filled with ink supplied via the ink inlet, and at least one nozzle configured to eject the ink at least partially filling the at least one pressure chamber. The piezoelectric actuator is configured to provide a pressure change in the ink at least partially filling the at least one pressure chamber as a first driving force to eject an ink droplet from the at least one nozzle. The electrostatic force applicator is configured to apply an electrostatic force to the ink at least partially filling the at least one nozzle as a second driving force to eject the ink droplet from the at least one nozzle. 
     Yet at least one other example embodiment provides an inkjet printing device. According to at least this example embodiment, the device includes a flow path plate, a piezoelectric actuator, and an electrostatic force applicator. The flow path plate includes an ink inlet, at least one pressure chamber configured to be at least partially filled with ink supplied via the ink inlet, and at least one nozzle configured to eject the ink at least partially filling the at least one pressure chamber. The piezoelectric actuator is configured to generate a first driving force for ejecting an ink droplet from the at least one nozzle by reducing a volume of the at least one pressure chamber. And, the electrostatic force applicator is configured to generate a second driving force for ejecting the ink droplet from the at least one nozzle by increasing the volume of the at least one pressure chamber. 
     According to at least some example embodiments, the ink inlet may be formed on a top surface of the flow path plate, the at least one pressure chamber may be formed in the flow path plate, and/or the at least one nozzle may be formed on a lower surface of the flow path plate. The flow path plate may further include manifolds and a restrictor connecting the ink inlet and the at least one pressure chamber. The flow path plate may further include a damper connecting the at least one pressure chamber and the at least one nozzle. The flow path plate may be formed of a plurality of substrates. 
     According to at least some example embodiments, the piezoelectric actuator may include a lower electrode, a piezoelectric layer, and an upper electrode that are sequentially stacked on a top surface of the flow path plate. A first power source is connected between and configured to apply a voltage between the lower electrode and the upper electrode. 
     The electrostatic force applicator may include a first electrostatic electrode and a second electrostatic electrode disposed to face each other. A second power source is connected between and configured to apply a voltage between the first electrostatic electrode and the second electrostatic electrode. The first electrostatic electrode may be disposed on a top surface of the flow path plate, and the second electrostatic electrode may be spaced apart from a lower surface of the flow path plate. 
     According to at least some example embodiments, a guide rod may be formed in the at least one nozzle. The guide rod may extend along the center axis of the at least one nozzle. The guide rod may be supported by a bridge fixed to an inner wall surface of the at least one nozzle. The guide rod may protrude from a lower surface of the flow path plate to have a given, desired or predetermined length. 
     At least one other example embodiment provides a method of driving the inkjet printing device. According to at least this example embodiment, the piezoelectric actuator is deformed to reduce a volume of the at least one pressure chamber by applying a first voltage to the piezoelectric actuator. The piezoelectric actuator is deformed to increase the volume of the at least one pressure chamber by applying a second voltage to the piezoelectric actuator, and the second voltage applied to the piezoelectric actuator is removed. 
     According to at least some example embodiments, an electrostatic force may be applied to the ink filled in the at least one nozzle by applying an electrostatic voltage to the electrostatic force applicator. The electrostatic voltage may be maintained at least while applying the first voltage and the second voltage to the piezoelectric actuator. When applying of the first voltage to the piezoelectric actuator, a meniscus of the ink filled in the at least one nozzle may be deformed to a convex shape. When applying of the second voltage to the piezoelectric actuator, the convex meniscus having a radius of curvature smaller than an inside diameter of the at least one nozzle may be formed at the center portion of the at least one nozzle, and the ink of a protruding convex portion may be ejected in the form of a droplet due to the electrostatic force. When applying of the second voltage to the piezoelectric actuator, an ink droplet having smaller sizes than the at least one nozzle may be ejected. 
     When removing the applied second voltage applied to the piezoelectric actuator, the piezoelectric actuator, the pressure of the plurality of pressure chambers, and the meniscus of the ink filled in the at least one nozzle may return to their original states. 
     At least one other example embodiment provides a method of driving the inkjet printing device. According to at least this example embodiment, the piezoelectric actuator may be deformed to increase a volume of the at least one pressure chamber by applying a second voltage to the piezoelectric actuator. The second voltage applied to the piezoelectric actuator may be removed. 
     According to at least some example embodiments, an electrostatic force may be applied to the ink filled in the at least one nozzle by applying an electrostatic voltage to the electrostatic force applicator. Before applying the second voltage to the piezoelectric actuator, the piezoelectric actuator may be deformed to reduce a volume of the at least one pressure chamber by applying a first voltage to the piezoelectric actuator. In the applying of the first voltage to the piezoelectric actuator, a meniscus of the ink filled in the at least one nozzle may be deformed to a convex shape. The electrostatic voltage may be maintained at least while applying the first voltage and the second voltage to the piezoelectric actuator. 
     Before applying the second voltage to the piezoelectric actuator, a meniscus of a front portion of the guide rod may be deformed to the convex shape due to a surface tension caused by the guide rod. When applying of the second voltage to the piezoelectric actuator, the convex meniscus having a radius of curvature smaller than an inside diameter of the at least one nozzle may be formed in the front portion of the guide rod, and the ink of a protruding convex portion may be ejected in the form of a droplet due to the electrostatic force. When applying the second voltage to the piezoelectric actuator, an ink droplet having smaller sizes than the at least one nozzle may be ejected. 
     When removing of the applied second voltage applied to the piezoelectric actuator, the piezoelectric actuator, the pressure of the at least one pressure chamber, and the meniscus of the ink filled in the plurality of nozzles may return to their original states. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The general inventive concept will become apparent and more readily appreciated from the following description of example embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a cross-sectional view of an inkjet printing device according to an example embodiment; 
         FIG. 2  is a diagram for explaining a method of driving the inkjet printing device shown in  FIG. 1  according to an example embodiment; 
         FIG. 3  shows a driving waveform applied in the method shown in  FIG. 2  according to an example embodiment; 
         FIG. 4  shows a driving waveform applied in the method shown in  FIG. 2  according to another example embodiment; 
         FIG. 5  is a cross-sectional view of an inkjet printing device according to another example embodiment; 
         FIG. 6  is a plan view of nozzles, a guide rod, and a bridge shown in  FIG. 5 ; 
         FIG. 7  is a diagram for explaining a method of driving the inkjet printing device shown in  FIG. 5  according to an example embodiment; 
         FIG. 8  is a diagram for explaining a method of driving the inkjet printing device shown in  FIG. 5  according to another example embodiment; 
         FIG. 9  shows a driving waveform applied in the method shown in  FIG. 8  according to an example embodiment; and 
         FIG. 10  shows a driving waveform applied in the method shown in  FIG. 8  according to another example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. In this regard, the example embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the example embodiments are merely described below by referring to the figures to explain aspects of the general inventive concept. 
     Various example embodiments will now be described more fully with reference to the accompanying drawings in which some example embodiments are shown. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. 
     Detailed illustrative example embodiments are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. This invention may, however, may be embodied in many alternate forms and should not be construed as limited to only the example embodiments set forth herein. 
     Accordingly, while example embodiments are capable of various modifications and alternative forms, embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments to the particular forms disclosed, but on the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of the invention. Like numbers refer to like elements throughout the description of the figures. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, method steps or actions, these elements, steps or actions should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or,” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element or layer is referred to as being “formed on,” another element or layer, it can be directly or indirectly formed on the other element or layer. That is, for example, intervening elements or layers may be present. In contrast, when an element or layer is referred to as being “directly formed on,” to another element, there are no intervening elements or layers present. Other words used to describe the relationship between elements or layers should be interpreted in a like fashion (e.g., “between,” versus “directly between,” “adjacent,” versus “directly adjacent,” etc.). 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a,” “an,” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Further still, it should also be noted that in some alternative implementations, the steps/functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the steps/functionality/acts involved. In addition, the order of the steps/actions/operations/interactions may be re-arranged. 
       FIG. 1  is a cross-sectional view of an inkjet printing device according to an example embodiment. 
     Referring to  FIG. 1 , the inkjet printing device according to the example embodiment includes a flow path plate  110 , a piezoelectric actuator  130 , and an electrostatic force applicator  140 . The electrostatic force applicator  140  is configured to provide a driving force for ejecting ink. 
     The flow path plate further includes an ink flow path. The ink flow path further includes an ink inlet  121  through which ink is supplied, at least one (e.g., a plurality of) pressure chambers  125  containing the supplied ink, and at least one (e.g., a plurality of) nozzles  128  for ejecting ink droplets. Example embodiments will be discussed herein, for the sake of clarity, as including a plurality of pressure chambers and a plurality of nozzles. 
     The ink inlet  121  may be formed on the top surface of the flow path plate  110  and is connected to an ink tank that is not shown. Ink is supplied from the ink tank to the flow path plate  110  via the ink inlet  121 . The pressure chambers  125  are formed in the flow path plate  110 , and store the ink supplied via the ink inlet  121 . 
     Still referring to  FIG. 1 , the flow path plate  110  further includes manifolds  122  and  123  and a restrictor  124 , which connect the ink inlet  121  and the pressure chambers  125 . The nozzles  128  eject the ink filled in the pressure chambers  125  in the form of droplets and are connected to the pressure chambers  125 , respectively. The nozzles  128  may be formed on the bottom surface of the flow path plate  110 , and may be arranged in one or more lines (e.g., in one line or two lines). The flow path plate  110  may include a plurality of dampers  126  that connect the pressure chambers  125  and the nozzles  128 . 
     The flow path plate  110  may be formed of a material having a highly fine workability, for example, a silicone substrate. The flow path plate  110  may have a stacked structure including a plurality of substrates stacked sequentially. In one example, the flow path plate  110  may be formed by bonding first through third substrates  111  through  113 , which are sequentially stacked, using a silicone direct bonding (SDB) process. In this example, the ink inlet  121  may pass perpendicularly through a substrate disposed on the uppermost portion of the flow path plate  110  (e.g., the third substrate  113 ). The pressure chambers  125  may be formed on or within the bottom portion of the third substrate  113  to have a given, desired or predetermined depth. The nozzles  128  may pass perpendicularly through a substrate disposed on the lowermost portion of the flow path plate  110  (e.g., the first substrate  111 ). The manifolds  122  and  123  may be formed on or within the second substrate  112  disposed between the first and third substrates  111  and  113 . The dampers  126  may pass perpendicularly through the second substrate  112 . 
     Although the flow path plate  110  is described above as including three substrates  111  through  113 , example embodiments are not limited thereto. Rather, the flow path plate  110  may include one substrate, two substrates, or four or more substrates. Furthermore, an ink flow path formed in the flow path plate  110  may be shaped in various ways. 
     The piezoelectric actuator  130  provides a pressure change as a first driving force for ejecting the ink to the pressure chambers  125 . In the example embodiment shown in  FIG. 1 , the piezoelectric actuator  130  is disposed on the top surface of the flow path plate  110  so as to correspond to the pressure chambers  125 . The piezoelectric actuator  130  includes a lower electrode  131 , a piezoelectric layer  132 , and an upper electrode  133 , which are stacked sequentially on the top surface of the flow path plate  110 . The lower electrode  131  functions as a common electrode, while the upper electrode  133  functions as a driving electrode for applying a voltage to the piezoelectric layer  132 . A first power source  135  is connected between the lower electrode  131  and the upper electrode  133 . The piezoelectric layer  132  is deformed by a voltage applied from the first power source  135  such that the portion of the third substrate  113  corresponding to the upper wall of the pressure chambers  125  is deformed. The piezoelectric layer  132  may be formed of a given, desired or predetermined piezoelectric material, for example, a lead zirconate titanate (PZT) ceramic or similar material. 
     The electrostatic force applicator  140  applies an electrostatic force as a second driving force for ejecting ink to the nozzles  128 . The electrostatic force applicator  140  includes first and second electrostatic electrodes  141  and  142 , which are disposed to face each other. The electrostatic force applicator  140  further includes a second power source  145  connected between and configured to apply a voltage between the first and second electrostatic electrodes  141  and  142 . 
     Still referring to the example embodiment shown in  FIG. 1 , the first electrostatic electrode  141  is disposed on the flow path plate  110 . As shown, the first electrostatic electrode  141  may be disposed on the top surface of the flow path plate  110  (e.g., on the top surface of the third substrate  113 ). The first electrostatic electrode  141  may be disposed on a region where the ink inlet  121  is formed so as to be spaced apart from the lower electrode  131  of the piezoelectric actuator  130 . The second electrostatic electrode  142  may be disposed a given, desired or predetermined distance apart from the bottom surface of the flow path plate  121 . Recording media P on which ink droplets ejected via the nozzles  128  of the flow path plate  110  are printed may be loaded on the second electrostatic electrode  142 . 
     The inkjet printing device having the above-described structure uses an ink ejecting technique that is a combination of a piezoelectric technique and an electrostatic technique, thereby obtaining merits of the piezoelectric technique and the electrostatic technique. For example, the inkjet printing device according to at least this example embodiment ejects ink using a drop on demand (DOD) technique, thereby controlling a printing operation and producing ultrafine droplets more easily, as well as allowing ink to be ejected in a desired direction, thereby appropriately performing a more precise printing process. 
       FIG. 2  is a diagram for explaining an example embodiment of a method of driving the inkjet printing device shown in  FIG. 1 .  FIG. 3  shows a driving waveform applied in the method shown in  FIG. 2  according to an example embodiment. 
     Referring to  FIGS. 2 and 3 , at S 202 , a voltage is not applied to the piezoelectric actuator  130 , and the second power source  145  applies a given, desired or predetermined electrostatic voltage VE between the first and second electrostatic electrodes  141  and  142 . In this regard, because a relatively small amount of electrostatic force is applied to ink  129  of the nozzles  128 , a meniscus M of the ink  129  is in a static state. 
     At S 204 , a first voltage VP 1  is applied to the piezoelectric actuator  130  to deform the piezoelectric actuator  130  thereby reducing volumes of the pressure chambers  125 . The electrostatic voltage VE applied between the first and second electrostatic electrodes  141  and  142  is maintained. Thus, the pressure of the pressure chambers  125  increases so that the meniscus M of the ink  129  of the nozzles  128  is deformed to a convex shape. In this case, an electric field is collimated at the convex meniscus M so that positive charges in the ink  129  move toward the second electrostatic electrode  142  and are collected at the end portion of the nozzles  128 . 
     At S 206 , a second voltage VP 2  is applied to the piezoelectric actuator  130  to deform the piezoelectric actuator  130  thereby increasing volumes of the pressure chambers  125 . The electrostatic voltage VE applied between the first and second electrostatic electrodes  141  and  142  is maintained. Thus, the pressure of the pressure chambers  125  is reduced so that the meniscus M of the ink  129  of the nozzles  128  sinks, whereas the center portion of the meniscus M is deformed to the convex shape due to an electrostatic force applied between accumulated charges and the second electrostatic electrode  142 . As a result, the convex meniscus M having a smaller radius of curvature than an inside diameter of the nozzles  128  is formed at center portions of the nozzles  128 . 
     In general, an electrostatic force F E  is proportional to an amount of charges q and an intensity E of the electric field as shown in equation 1 below. The amount of charges q is proportional to the intensity E of the electric field as shown in equation 2 below. The electrostatic force F E  is proportional to a square of the intensity E of the electric field as shown in equation 3 below. As shown below in equation 4, the intensity E of the electric field is proportional to the electrostatic voltage V E , but inversely proportional to the radius of curvature r m  of the meniscus M. Thus, the electrostatic force F E  applied to the ink  129  of a portion that protrudes relatively sharply from the end portion of the nozzles  128  is inversely proportional to a square of the radius of curvature r m  of the meniscus M as shown in equation 5. 
     
       
         
           
             
               
                 
                   
                     F 
                     E 
                   
                   ∝ 
                   
                     q 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     E 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   q 
                   ∝ 
                   E 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   
                     F 
                     E 
                   
                   ∝ 
                   
                     E 
                     2 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   E 
                   ∝ 
                   
                     
                       V 
                       E 
                     
                     
                       r 
                       m 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   
                     F 
                     E 
                   
                   ∝ 
                   
                     
                       ( 
                       
                         
                           V 
                           E 
                         
                         
                           r 
                           m 
                         
                       
                       ) 
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     As shown above, the electrostatic force F E  applied to the ink  129  of the relatively sharply protruding portion increases so that the radius of curvature r m  of the meniscus M at the center portion of the nozzles  128  is further reduced, which further increases the electrostatic force F E . The ink  129  of the relatively sharply protruding portion is ejected in the form of droplets  129   a  from the nozzles  128 . In this regard, because the ink  129  sharply protrudes from the center portion of the nozzles  128 , relatively small (e.g., very small) sizes of ink droplets  129 ′ are ejected as compared to sizes of the nozzles  128 . The ink droplets  129   a  move to the second electrostatic electrode  142  due to the electrostatic force F E  and are printed on the recording media P. 
     Referring back to  FIG. 2 , at S 208 , if the second voltage V P2  applied to the piezoelectric actuator  130  is removed, the piezoelectric actuator  130  returns to an original state and the pressure of the pressure chambers  125  returns to an original state, so that the sunken meniscus M also returns to an original state. In this regard, the electrostatic voltage V E  applied between the first and second electrostatic electrodes  141  and  142  is maintained. 
     Although the electrostatic voltage V E  applied between the first and second electrostatic electrodes  141  and  142  is maintained during the actions S 202  through S 208 , the electrostatic voltage V E  may be maintained only during some of actions S 202  through S 208  as described below. 
       FIG. 4  shows a driving waveform applied in the method shown in  FIG. 2  according to another example embodiment. 
     Referring to  FIG. 4 , in this example embodiment the electrostatic voltage V E  applied between the first and second electrostatic electrodes  141  and  142  is maintained during actions S 204  and S 206 , but not during actions S 202  and S 208  in which the meniscus M is maintained in a static state. 
     As described above, the method of driving the inkjet printing device according to at least this example embodiment ejects the ink droplets  129   a  that are smaller (e.g., much smaller) than the nozzles  128 . In more detail, ultrafine droplets having a size of several picoliters or less are ejected via the nozzles  128  having relatively large diameters (e.g., several μm through several tens of μm), without the need to reduce the sizes of the nozzles  128 . The nozzles  128  have relatively large diameters while ejecting ultrafine droplets, which reduces a possibility of the nozzles  128  getting clogged, thereby increasing reliability. Furthermore, the electric field is focused on a part of the ink meniscus M, thereby maintaining a relatively low electrostatic voltage when generating a given, desired or predetermined amount of electrostatic force. 
       FIG. 5  is a cross-sectional view of an inkjet printing device according to another example embodiment.  FIG. 6  is a plan view of the nozzles  128 , a guide rod  128   a , and a bridge  128   b  shown in  FIG. 5 . Because the inkjet printing device shown in  FIGS. 5 and 6  is the same as the inkjet printing device shown in  FIG. 1  except for the construction of the nozzles  128 , only the nozzles  128  will be described below with reference to  FIGS. 5 and 6 . 
     Referring to  FIGS. 5 and 6 , the guide rod  128 a may be disposed in the nozzles  128  along a center axis of the nozzles  128 . In this example embodiment, the guide rod  128   a  protrudes from the lower surface of the flow path plate  110  to have a given, desired or predetermined length. The guide rod  128   a  is supported by the bridge  128   b . The bridge  128 b is fixed to an inner wall surface of the nozzles  128 . 
       FIG. 7  is a diagram for explaining an example embodiment of a method of driving the inkjet printing device shown in  FIG. 5 . The driving waveform shown in  FIG. 3  is applied to the method of driving the inkjet printing device shown in  FIG. 7 . 
     Referring to  FIGS. 3 and 7 , at S 702 , no voltage is applied to the piezoelectric actuator  130 , and the second power source  145  applies the given, desired or predetermined electrostatic voltage V E  between the first and second electrostatic electrodes  141  and  142 . Because a relatively small amount of electrostatic force is applied to the ink  129  of the nozzles  128 , the meniscus M of the ink  129  is in a static state. However, the meniscus M of a front portion of the guide rod  128   a  slightly protrudes due to a surface tension caused by the guide rod  128   a  disposed at the center portion of the nozzles  128 . 
     At S 704 , the first voltage V P1  is applied to the piezoelectric actuator  130  to deform the piezoelectric actuator  130  thereby reducing volumes of the pressure chambers  125 . In this regard, the electrostatic voltage V E  applied between the first and second electrostatic electrodes  141  and  142  is maintained. Thus, the pressure of the pressure chambers  125  increases such that the meniscus M of the ink  129  of the nozzles  128  is deformed to a convex shape. An electric field is collimated at the convex meniscus M so that positive charges in the ink  129  move toward the second electrostatic electrode  142  and collect at the end portion of the nozzles  128 . 
     At S 706 , the second voltage V P2  is applied to the piezoelectric actuator  130  to deform the piezoelectric actuator  130  thereby increasing volumes of the pressure chambers  125 . The electrostatic voltage V E  applied between the first and second electrostatic electrodes  141  and  142  is maintained. Thus, the pressure of the pressure chambers  125  is reduced such that the meniscus M of the ink  129  of the nozzles  128  sinks, whereas the center portion of the meniscus M maintains the convex shape due to an electrostatic force applied between accumulated charges and the second electrostatic electrode  142 . In this regard, the convex meniscus M is more easily formed in the front of the guide rod  128   a  due to a surface tension caused by the guide rod  128   a . Thus, the convex meniscus M having a smaller radius of curvature than an inside diameter of the nozzles  128  is formed at center portions of the nozzles  128 . 
     As described above, the electrostatic force F E  applied to the ink  129  of the relatively sharply protruding portion increases, so that the radius of curvature r m  of the meniscus M of the center portion of the nozzles  128  is further reduced, which further increases the electrostatic force F E . The ink  129  of the relatively sharply protruding portion is ejected in the form of droplets  129   a  from the nozzles  128 . In this regard, because the ink  129  sharply protrudes from the center portion of the nozzles  128 , relatively small (e.g., very small) size ink droplets  129 ′ are ejected as compared to the sizes of the nozzles  128 . The ink droplets  129   a  move toward the second electrostatic electrode  142  due to the electrostatic force F E  and are printed on the recording media P. 
     Still referring to  FIG. 7 , at S 708 , if the second voltage V P2  applied to the piezoelectric actuator  130  is removed, the piezoelectric actuator  130  returns to an original state and the pressure of the pressure chambers  125  returns to an original state, so that the sunken meniscus M also returns to an original state. In this regard, the electrostatic voltage V E  applied between the first and second electrostatic electrodes  141  and  142  is maintained. 
     Although the example embodiment shown in  FIG. 7  is described above with regard to the electrostatic voltage V E  applied between the first and second electrostatic electrodes  141  and  142  being maintained during actions S 702  through S 708 , the electrostatic voltage V E  may be maintained only during actions S 704  and S 706  as shown in  FIG. 4 . 
     The method of driving the inkjet printing device shown in  FIG. 7  more easily forms the meniscus M having a pronounced bulge at the center portion of the nozzles  128  by applying the surface tension caused by the guide rod  128   a  disposed at the center portions of the nozzles  128  and the electrostatic force as well. 
       FIG. 8  is a diagram for explaining a method of driving the inkjet printing device shown in  FIG. 5  according to another example embodiment.  FIG. 9  shows a driving waveform applied in the method shown in  FIG. 8  according to an example embodiment. 
     Referring to  FIGS. 8 and 9 , at S 802 , no voltage is applied to the piezoelectric actuator  130 , and the second power source  145  applies the given, desired or predetermined electrostatic voltage V E  between the first and second electrostatic electrodes  141  and  142 . Because a relatively small amount of electrostatic force is applied to the ink  129  of the nozzles  128 , the meniscus M of the ink  129  is in a static state. However, the meniscus M of a front portion of the guide rod  128   a  slightly protrudes due to a surface tension caused by the guide rod  128   a  disposed at the center portion of the nozzles  128 . Positive charges accumulate in the slightly bulging portion of the front portion of the guide rod  128   a  due to the electrostatic force. 
     At S 804 , the second voltage V P2  is applied to the piezoelectric actuator  130  to deform the piezoelectric actuator  130  thereby increasing volumes of the pressure chambers  125 . In this regard, the electrostatic voltage V E  applied between the first and second electrostatic electrodes  141  and  142  is maintained. Thus, the pressure of the pressure chambers  125  is reduced so that the meniscus M of the ink  129  of the nozzles  128  sinks, whereas the center portion of the meniscus M (e.g., the front portion of the guide rod  128   a ) maintains the convex shape due to an electrostatic force applied between accumulated charges and the second electrostatic electrode  142  and due to a surface tension caused by the guide rod  128   a . 
     Because the method shown in  FIG. 8  does not perform, for example, action S 704  shown in  FIG. 7 , a relatively small (e.g., very small) amount of the ink  129  remains in the front portion of the guide rod  128 a, and thus, the meniscus M has a relatively small (e.g., very small) radius of curvature. Therefore, the electrostatic force F E  applied to the ink  129  remaining in the front portion of the guide rod  128   a  increases, so that the ink  129  is ejected in the form of the droplets  129   a . The ink droplets  129   a  move toward the second electrostatic electrode  142  due to the electrostatic force F E  and are printed on the recording media P. 
     Referring still to  FIG. 8 , at S 806 , if the second voltage V P2  applied to the piezoelectric actuator  130  is removed, the piezoelectric actuator  130  returns to an original state and the pressure of the pressure chambers  125  returns to an original state, so that the sunken meniscus M also returns to an original state. In this regard, the electrostatic voltage V E  applied between the first and second electrostatic electrodes  141  and  142  is maintained. 
     As described above, the method of driving the inkjet printing device shown in  FIGS. 8 and 9  ejects the ink droplets  129   a  having ultrafine (e.g., very ultrafine) sizes compared to those described with reference to  FIG. 7  because the relatively small (e.g., very small) amount of the ink  129  remains in the front portion of the guide rod  128   a  disposed at the center portions of the nozzles  128 . 
       FIG. 10  shows a driving waveform applied in the method shown in  FIG. 8  according to another example embodiment. 
     Referring to  FIG. 10 , the electrostatic voltage V E  applied between the first and second electrostatic electrodes  141  and  142  is maintained during action S 804 , but not during actions S 802  and S 806  in which no voltage is applied to the piezoelectric actuator  130  and the meniscus M is maintained in a static state. 
     It should be understood that the example embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each example embodiment should typically be considered as available for other similar features or aspects in other embodiments.