CROSS-REFERENCES TO RELATED APPLICATIONS 
     This is a Continuation-In-Part of U.S. Ser. No. 09/575,152, now Abandoned, filed on May 23, 2000. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     FIELD OF THE INVENTION 
     This invention relates to a fluidic sealing structure. More particularly, this invention relates to a liquid displacement assembly that incorporates a fluidic seal. 
     REFERENCED PATENT APPLICATIONS 
     This application is a continuation-in-part application of U.S. application Ser. No. 09/575,152. The following applications and patents are hereby incorporated by reference: 
     
       
         
               
               
               
               
               
             
           
               
                   
               
             
             
               
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     Some applications are temporarily identified by docket numbers. The docket numbers will be replaced by the corresponding U.S. Ser. No. when available. 
     BACKGROUND OF THE INVENTION 
     As set out in the above referenced applications/patents, the Applicant has spent a substantial amount of time and effort in developing printheads that incorporate micro electro-mechanical system (MEMS)—based components to achieve the ejection of ink necessary for printing. 
     As a result of the Applicant&#39;s research and development, the Applicant has been able to develop printheads having one or more printhead chips that together incorporate up to 84 000 nozzle arrangements. The Applicant has also developed suitable processor technology that is capable of controlling operation of such printheads. In particular, the processor technology and the printheads are capable of cooperating to generate resolutions of 1600 dpi and higher in some cases. Examples of suitable processor technology are provided in the above referenced patent applications/patents. 
     The Applicant has overcome substantial difficulties in achieving the necessary ink flow and ink drop separation within the ink jet printheads. 
     Each of the nozzle arrangements of the printhead chip incorporates one or more moving components in order to achieve drop ejection. The moving components are provided in a number of various configurations. 
     Generally, each nozzle arrangement has a structure that at least partially defines a nozzle chamber. This structure can be active or static. 
     When the structure is active, the structure moves relative to a chip substrate to eject ink from an ink ejection port defined by the structure. In this configuration, the structure can define just a roof for the nozzle chamber or can define both the roof and sidewalls of the nozzle chamber. Further, in this configuration, a static ink ejection formation is provided. The active structure moves relative to this formation to reduce a volume of the nozzle chamber in order to achieve the necessary build up of ink pressure. The static formation can simply be walls defined by the substrate. In this case, the active structure is usually in the form of a roof that is displaceable into and out of the nozzle chamber to achieve the ejection of ink from the ink ejection port. 
     Instead, the static formation can extend into the nozzle chamber to define an ink ejection area that faces a direction of ink drop ejection. The active structure then includes sidewalls that move relative to the static formation when the active structure is displaced to eject ink. 
     It will be appreciated that some form of seal is required between the active structure and the static formation to inhibit ink from escaping from the nozzle chamber when the active structure is displaced towards the substrate and ink pressure is developed in the nozzle chamber. 
     When the structure defining the nozzle chamber is static, an ink ejection member is usually positioned in the nozzle chamber. The structure also has a roof with an ink ejection port defined in the roof. The ink ejection member is often connected to an actuator that extends through a wall of the structure. The ink ejection member is actuated by the actuator to be displaceable towards and away from the roof to eject ink from the ink ejection port. 
     It will be appreciated that a seal is required at a juncture between the actuator or ink ejection member and the wall. 
     Applicant has found that it is convenient to use a surface tension of the ink to set up a fluidic seal between the active and static components of the nozzle arrangements. The fluidic seal uses surface tension of the ink to set up a meniscus between the active and static components so that the meniscus can act as a suitable seal to inhibit the leakage of ink. 
     Cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface “film” which makes it more difficult to move an object through the surface than to move it when it is completely submersed. 
     Surface tension is typically measured in dynes/cm, the force in dynes required to break a film of length 1 cm. Equivalently, it can be stated as surface energy in ergs per square centimeter. Water at 20° C. has a surface tension of 72.8 dynes/cm compared to 22.3 for ethyl alcohol and 465 for mercury. 
     As is also known, a liquid can also experience adhesive forces when the molecules adhere to a material other than the liquid. This causes such phenomena as capillary action. 
     Applicant has found that an effective fluidic seal can be achieved by utilizing the phenomena of surface tension and adhesion. 
     A particular difficulty that the Applicant has discovered and addressed in achieving such a fluidic seal is the problem associated with excessive adhesion or “wetting” when a meniscus is stretched to accommodate relative movement of the active and static components. In particular, wetting occurs when the relative movement overcomes surface tension and an edge of the meniscus moves across a surface, to which the meniscus is adhered. This results in a weakening of the meniscus due to the larger area of the meniscus and increases the likelihood of failure of the meniscus and subsequent leaking of ink. 
     The Applicant has conceived this invention in order to address these difficulties. Furthermore, the Applicant has obtained surprisingly effective fluidic seals when addressing these difficulties by developing sealing structures that support such fluidic seals. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, there is provided a liquid displacement assembly which comprises 
     a first displacement member; 
     a second displacement member, the first and second displacement members together defining a volume in which liquid is received, at least one of the first and second displacement members being movable with respect to the other to displace liquid from the volume; and 
     a sealing formation positioned on each displacement member, the sealing formations being shaped so that a fluidic seal is interposed between the sealing formations when the liquid is received in the volume, each sealing formation having a liquid adhesion surface positioned between side surfaces of the sealing formation and directed towards a plane of reference oriented substantially orthogonally with respect to a direction of relative displacement of the sealing formations, the sealing formations being spaced from each other to define a region in which a meniscus can be formed so that opposed edges of the meniscus adhere to respective adhesion surfaces of the sealing formations and the sealing formations being shaped so that each liquid adhesion surface is interposed between the side surfaces of each respective sealing formation and the plane of reference. 
     According to a second aspect of the invention, there is provided a printhead chip for an ink jet printhead, the printhead chip including 
     a substrate; and 
     a plurality of nozzle arrangements positioned on the substrate, each nozzle arrangement comprising
         a static ink ejection member positioned on the substrate;   an active ink ejection member, the static and active ink ejection members together defining a nozzle chamber in which ink is received, and the active ink ejection member including a roof that defines an ink ejection port, the active ink ejection member being displaceable towards and away from the substrate to reduce and subsequently enlarge the nozzle chamber so that ink in ejected from the ink ejection port; and   a sealing formation positioned on each ink ejection member, the sealing formations being shaped so that a fluidic seal is interposed between the sealing formations when ink is received in the nozzle chamber, each sealing formation having a liquid adhesion surface positioned between side surfaces of the sealing formation and directed towards a plane of reference oriented substantially orthogonally with respect to a direction of relative displacement of the sealing formations, the sealing formations being spaced from each other to define a region in which a meniscus can be formed so that opposed edges of the meniscus adhere to respective adhesion surfaces of the sealing formations, the sealing formations being shaped so that the liquid adhesion surfaces are interposed between the side surfaces and the plane of reference.       

     According to a third aspect of the invention, there is provided a printhead chip for an ink jet printhead, the printhead chip comprising 
     a substrate; and 
     a plurality of nozzle arrangements positioned on the substrate, each nozzle arrangement comprising
         sidewalls and a roof that are arranged on the substrate, the sidewalls and the roof defining a nozzle chamber with an ink ejection port defined in the roof;   an ink ejection member that is positioned in the nozzle chamber, the ink ejection member being displaceable towards and away from the roof to eject ink from the ink ejection port and the ink ejection member extending through the sidewalls to be connected to an actuator positioned outside of the nozzle chamber; and   at least one sealing formation positioned on the ink ejection member and at least one complementary sealing formation positioned on at least one of the sidewalls and the roof, the sealing formations being shaped so that a fluidic seal is interposed between the, or each sealing formation and the, or each, complementary sealing formation when ink is received in the nozzle chamber, each sealing formation having a liquid adhesion surface positioned between side surfaces of the respective sealing formation and directed towards a plane of reference oriented substantially orthogonally with respect to a direction of relative displacement of the sealing formations, the sealing formations being spaced from each other to define a region in which a meniscus can be formed so that opposed edges of the meniscus adhere to respective adhesion surfaces of the sealing formations, the sealing formations being shaped so that the liquid adhesion surfaces are interposed between the respective side surfaces and the plane of reference.       

     The invention is now described, by way of example, with reference to the accompanying drawings. The following description is not intended to limit the broad scope of the above summary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, 
         FIG. 1  shows a schematic side view of a pair of sealing formations to indicate a disadvantage associated with such a configuration; 
         FIG. 2  shows a schematic side view of a pair of sealing formations of a first embodiment of a liquid displacement assembly, in accordance with the invention; 
         FIG. 3  shows a schematic side view of a pair of sealing formations of a second embodiment of a liquid displacement assembly, in accordance with the invention; 
         FIG. 4  shows a schematic side view of a pair of sealing formations of a third embodiment of a liquid displacement assembly, in accordance with the invention; 
         FIG. 5  shows a schematic side view of a pair of sealing formations of a fourth embodiment of a liquid displacement assembly, in accordance with the invention; 
         FIG. 6  shows a schematic side view of a pair of sealing formations of a fifth embodiment of a liquid displacement assembly, in accordance with the invention; 
         FIG. 7  shows a schematic sectioned side view of a nozzle arrangement of a first embodiment of a printhead chip, in accordance with the invention, in a quiescent condition; 
         FIG. 8  shows a schematic sectioned side view of the nozzle arrangement of  FIG. 7  in an operative condition; 
         FIG. 9  shows a plan sectioned view of the nozzle arrangement of  FIG. 7 , taken through IX—IX in  FIG. 7 ; and 
         FIG. 10  shows a schematic sectioned side view of a nozzle arrangement of a second embodiment of a printhead chip, in accordance with the invention, in an operative condition. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention is directed towards the use of surface tension in order to provide a fluidic seal. Cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. Liquid molecules at a surface of a body of liquid do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface “film” which makes it more difficult to move an object through the surface than to move it when it is completely submersed. Surface tension is typically measured in dynes/cm, the force in dynes required to break a film of length 1 cm. Equivalently, it can be stated as surface energy in ergs per square centimeter. Water at 20° C. has a surface tension of 72.8 dynes/cm compared to 22.3 for ethyl alcohol and 465 for mercury. 
     Applicant has found that it is this surface tension is high enough in certain liquids to serve as a fluidic seal, provided that there are suitable formations to support a meniscus carrying the surface tension. 
     Surface tension plays a role in what is known as capillarity. This manifests itself when the liquid of the meniscus “wets” a surface supporting the meniscus. Wetting occurs when a contact angle defined between an edge of the meniscus and the surface reaches zero degrees. This wetting results in adhesive forces being set up between the liquid molecules and the molecules of the material defining the surface. When the adhesive forces are greater than the cohesive forces defining the surface tension, the edge of the meniscus is drawn along the surface, resulting in an increase in size of the meniscus. In water, for example, the adhesive forces between water molecules and the walls of a glass tube are stronger than the cohesive forces. Thus, the water can be drawn through such a tube against gravity, provided the tube is thin enough. 
     A fluidic seal is used when it is necessary to prevent liquid from escaping between components that move relative to each other. A particular advantage of a fluidic seal is that it uses the properties of the liquid to achieve sealing. It follows that the need for specialized sealing materials is obviated. However, it is important that displacement of edges of a meniscus defining the fluidic seal be constrained. This displacement can result in an increase in meniscus area. This increase also increases forces counteracting the surface tension, resulting in a breakdown of the meniscus and subsequent leaking. The Applicant has noted that movement of an edge of a meniscus can be substantially curtailed if the surface to which the edge is adhered is directed away from a direction of force exerted on the meniscus by such factors as gravity and liquid pressure. 
     In this description, a plane of reference, indicated by a reference line  11  is shown in the drawings. This is merely for ease of description. Furthermore, for the sake of convenience, the plane of reference is assumed to be horizontal, regardless of the fact that, as a whole, the various embodiments shown can be in any number of different orientations with respect to a true horizon. Still further, a direction towards the plane of reference  11  is assumed to be downward and a direction away from the plane of reference is assumed to be upward. 
     An example of an unsuitable sealing structure is indicated by reference numeral  10  in FIG.  1 . The solid lines indicate the sealing structure  10  in a quiescent condition, while the dotted lines indicate the sealing structure  10  in an operative condition. In this example, a sidewall  12  of an active liquid displacement member moves vertically relative to a complementary sidewall  14  of a static liquid displacement member. The purpose for this displacement can be multifold. However, in this example, the purpose is for increasing and subsequently decreasing pressure of a liquid  16  positioned in a chamber, such as a nozzle chamber  18 . The sidewall  12  is displaced towards and away from a substrate  20  as indicated by an arrow  22 . 
     As can be seen, the complementary sidewall  14  has a vertically extending external surface  26 . When the structure  10  is in a quiescent condition, a meniscus  24  is formed between a free edge  28  of the sidewall  12  and the external surface  26 . When the structure  10  moves into the operative condition, a contact angle defined between the meniscus  24  and the external surface  26  reaches zero degrees, and the liquid  16  wets the external surface  26 . As a result, the liquid  16  simply follows the external surface  26  towards the substrate  20  as shown by the dotted lines  30 . The meniscus  24  then expands to an extent to which the cohesive forces are broken and the liquid  16  leaks from between the sidewalls  12 ,  14 . 
     In  FIGS. 2  to  6 , there are shown various sealing structures that are suitable, to a greater or lesser extent, for inhibiting leakage of the liquid. All these structures form part of respective liquid displacement assemblies that fall within the scope of this invention. It is to be understood that the principles elucidated by these examples are applicable to a wide range of dimensions. The Applicant is presently involved in MEMS—based structures, and these examples are well suited to such structures. In the background to the invention it is set out that the Applicant has developed printhead technology in which up to 84 000 nozzle arrangements are incorporated into a single printhead. The printhead can include one or more printhead chips that span a print medium. 
     In accordance with this invention, each of the nozzle arrangements can include any of the sealing structures as shown in  FIGS. 2  to  6 . It follows that in this application, the sealing structures are on a microscopic scale, with sidewalls having a thickness of only a few microns. Further, a gap between the sidewalls is also only a few microns wide. It will be appreciated that such dimensions enhance the effects of surface tension. However, such small dimensions also enhance such phenomena as capillarity. It follows that the sealing structures should be dimensioned to inhibit excessive capillarity. 
     It is to be appreciated that, while the scale of the nozzle arrangements developed by the Applicant are microscopic, this invention finds application on the macroscopic scale as well. For example, with liquids and materials having certain characteristics, it is possible that the sidewalls and a gap between the sidewall could-be visible by the naked eye. In other words, the sidewalls and the gap could have transverse dimensions that are measured in millimeters and large fractions of a millimeter. 
     It is to be noted that the orientation of the structures in  FIGS. 1  to  6  is not intended to indicate their practical orientation in use. It follows that the effect of gravity should not be taken into account in these examples. 
     As set out in the background, the MEMS—based printhead is the product of an integrated circuit fabrication technique. Silicon dioxide is widely used in such techniques. As is known, silicon dioxide is simply an extremely pure glass. It follows that in this application, the sidewalls  12 ,  14  can be in the form of glass or a glass-like material. Furthermore, most inks are substantially water-based. It follows that interaction between the sidewalls  12 ,  14  and the liquid  16  can be similar to an interaction between glass and water. 
     Thus, in the structure  10 , since the liquid  16  is water-like and the sidewalls  12 ,  14  are of a glass-like material, capillarity will manifest itself between the sidewalls  12 ,  14  and could draw the liquid  16  out between the sidewalls  12 ,  14  so that leakage occurs between the sidewalls  12 ,  14 . This is especially so when the sidewall  12  is displaced relative to the sidewall  14 . 
     In  FIG. 2 , reference numeral  32  generally indicates a sealing structure, of a liquid displacement assembly, in accordance with the invention, that is suitable, under predetermined conditions, for setting up an effective fluidic seal to inhibit such leaking. With reference to  FIG. 1 , like reference numerals refer to like parts, unless otherwise specified. 
     The structure  32  has a complementary sidewall  34 . A sealing formation  36  is positioned on the complementary sidewall  34 . A first horizontal section  38 , a second vertically downward section  40  and a third horizontal section  42  that extends towards the complementary sidewall  34  define the sealing formation  36 . Thus, the sealing formation  36  has a re-entrant transverse profile. 
     In this example, the third horizontal section  42  defines a liquid adhesion surface  44 . When the sealing structure  36  is in a quiescent condition, a meniscus  46  is formed between the free edge  28  of the sidewall  12  and an outer edge  48  of the liquid adhesion surface  44 . As indicated by the dotted lines  50 , when the sealing structure  36  moves into an operative condition, the meniscus  46  is positioned between the free edge  28  and an inner edge  52  of the liquid adhesion surface  44 . Furthermore, since the surface  44  effectively turns upwardly and away from the plane of reference  11 , the meniscus  46  is unable to extend past the inner edge  52 . This serves to inhibit excessive enlarging of the meniscus  46  and subsequent leaking in the manner described above. 
     In  FIG. 3 , reference numeral  54  generally indicates a sealing structure, of a liquid displacement assembly, in accordance with the invention, that is also suitable, under certain conditions, for setting up a fluidic seal that inhibits such leaking. With reference to  FIGS. 1 and 2 , like reference numerals refer to like parts, unless otherwise specified. 
     The sealing structure  54  has a complementary sidewall  56 . A sealing formation  58  is positioned on the complementary sidewall  56 . The sealing formation  58  is in the form of an outwardly extending horizontal ledge  60 . The ledge  60  defines a horizontal liquid adhesion surface  62 . 
     When the structure  54  is in a quiescent condition, a meniscus  64  is defined between the free edge  28  of the sidewall  12  and an outer edge  66  of the liquid adhesion surface  62 . When the structure  54  is in an operative condition, the meniscus  64  moves into the condition shown by dotted lines  68 . 
     It will be appreciated that it is undesirable that the meniscus  64  reaches the complementary sidewall  56 , since this will result in wetting of the complementary sidewall  56  and subsequent leakage. A simple force analysis reveals that whether the meniscus  64  does reach the complementary sidewall  56  depends on a contact angle that is defined between the meniscus  64  and the complementary sidewall  56 . This contact angle increases as the sidewall  12  moves downwardly and is dependent on the extent of downward movement. It follows that the structure  54  is functional between certain ranges of movement of the sidewall  12 . 
     In  FIG. 4 , reference numeral  70  generally indicates a sealing structure, of a liquid displacement assembly, in accordance with the invention, that is suitable, under certain conditions, for setting up a fluidic seal that inhibits leaking. With reference to  FIGS. 1  to  3 , like reference numerals refer to like parts, unless otherwise specified. 
     The sealing structure  70  includes a complementary sidewall  72 . A sealing formation  74  is positioned on the sidewall  72 . The sealing formation  74  includes an outwardly and horizontally extending first section  76  and a downwardly extending vertical second section  78 . The second section terminates facing the plane of reference  11 . It follows that a free end of the sealing formation  74  defines a liquid adhesion surface  80 . It also follows that the sealing formation  74  has a re-entrant profile. 
     In this example, a meniscus  82  extends from the free edge  28  of the sidewall  12  to an outer edge  84  of the liquid adhesion surface  80 , when the structure is in a quiescent condition. In the operative condition, the meniscus  82  extends from the free edge  28  to an inner edge  86  of the surface  80  as indicated by dotted lines  88 . In view of the preceding material, it will be appreciated that an extent of movement of the meniscus  82  is dependent on a thickness of the second section  78 . 
     As set out above, in MEMS—based devices, such as the nozzle arrangement developed by the Applicant, the thickness of such a wall member is only a few microns. It is therefore extremely difficult to use such techniques to achieve a liquid adhesion surface that is much narrower than a few microns, using conventional integrated circuit fabrication techniques. Furthermore, the constraints on the extent of expansion of the meniscus  82  provided by the sealing structure  70  are sufficient to provide a workable fluidic seal. 
     In  FIG. 5 , reference numeral  90  generally indicates an optimum sealing structure, of a liquid displacement assembly, in accordance with the invention. With reference to  FIGS. 1  to  4 , like reference numerals refer to like parts, unless otherwise specified. 
     The sealing structure  90  is substantially the same as the sealing structure  70 , with the exception that a free end  92  of the sidewall  12  is tapered to define a vertex. A free end  94  of the second section  78  is also tapered to define a vertex. 
     In this optimum example, a meniscus  96  extends between the vertices  92 ,  94 . It will thus be appreciated that a surface area of the meniscus  96  remains substantially unchanged as the structure  90  is displaced into its operative condition, as indicated by dotted lines  98 . The reason for this is that the liquid adhesion surface defines by the vertices  92 ,  94  is dimensioned on a molecular scale, thereby providing practically no scope for movement of an edge of the meniscus  96 . 
     While the structure  90  is optimum, it is extremely difficult to achieve the structure  90  with conventional integrated circuit fabrication techniques, as set out above. As is known, integrated circuit fabrication techniques involve deposition and subsequent etching of various layers of material. As such, tapered forms, such as those of the structure  90  are not practical and are extremely difficult and expensive to achieve. 
     In  FIG. 6 , reference numeral  100  generally indicates a sealing structure, of a liquid displacement assembly, in accordance with the invention, that is suitable, under certain conditions, for setting up a fluidic seal. With reference to  FIGS. 1  to  5 , like reference numerals refer to like parts, unless otherwise specified. 
     The structure  100  is substantially the same as the structure  70 . However, a lip  102  is positioned on the second section  78  so that the lip  102  and the free end of the second section  78  define a liquid adhesion surface  104 . The lip  102  is a structural requirement that is determined by required alignment accuracy in a stepper process used in the fabrication of the sealing structure  100 . 
     In this example, a meniscus  106  is set up between the free edge  28  of the sidewall  12  and an outer edge  108  of the lip  102  and the surface  104  when the structure is in a quiescent condition. The meniscus  106  extends from the free edge  28  of the sidewall  12  and an inner edge  110  of the surface  104 . 
     The lip  102  does serve to increase the area of the surface  104  over the area of the surface  80 . As set out above, this could be undesirable. However, the lip  102  is required for the stepper alignment process mentioned above and its exclusion could lead to fabrication errors that would outweigh any advantages that may be achieved by excluding the lip  102 . 
     In  FIGS. 7 and 8 , reference numeral  120  generally indicates a nozzle arrangement of a first embodiment of a printhead chip, in accordance with the invention, for an ink jet printhead. With reference to  FIGS. 1  to  6 , like reference numerals refer to like parts, unless otherwise specified. 
     The nozzle arrangement  120  is one of a plurality of such nozzle arrangements positioned on a substrate  122  to define the printhead chip of the invention. As set out in the background, an ink jet printhead developed by the Applicant can include up to 84 000 such nozzle arrangements. It follows that it is for the purposes of convenience and ease of description that only one nozzle arrangement is shown. In integrated circuit fabrication techniques, it is usual practice to replicate a large number of identical components on a single substrate that is subsequently diced into separate components. It follows that the replication of the nozzle arrangement  120  to define the printhead chip should be readily understood by a reader of ordinary skill in the art. 
     In the description that follows the substrate  122  is to be understood to define the plane of reference  11  used in the preceding description. It follows that the same orientation naming conventions apply in the following description. 
     In  FIG. 7 , the nozzle arrangement  120  is shown in a quiescent condition and in  FIG. 8 , the nozzle arrangement  120  is shown in an operative condition. 
     An ink inlet channel  128  is defined through the substrate  122  to be in fluid communication with an ink inlet opening  130 . 
     The nozzle arrangement  120  includes a static ink ejecting member  124  and an active ink ejecting member  126 . The static ink ejecting member  124  has a wall portion  136  that is positioned on the substrate  122  to bound the ink inlet opening  130 . The active ink ejecting member  126  includes a roof  132  and a sidewall  134  that depends from the roof  132  towards the substrate  122 . The sidewall  134  is positioned outside of the wall portion  136 , so that the sidewall  134  and the wall portion  136  define a nozzle chamber  138 . 
     An ink ejection port  140  is defined in the roof  132  and is aligned with the ink inlet opening  130 . 
     The wall portion  136  includes a sidewall  142  that extends from the substrate  122  towards the roof  132 . A ledge  144  is positioned on the sidewall  142  and extends horizontally towards a position above the ink inlet opening  130 . A sealing formation  146  is also positioned on the sidewall  142  and extends outwardly from the sidewall  142 . 
     The sidewall  134  has a free end  148  that has a rectangular transverse profile. The sealing formation  146  has a horizontal first section  150  that extends from an upper end of the sidewall  142 . A vertical second section  152  extends downwardly from an end of the first section  150 . A lip  154  extends horizontally and outwardly from the second section  152 . It follows that the sealing formation  146  is the same as the sealing formation  74  of the sealing structure  100  shown in FIG.  6 . Further, the sidewall  134  is positioned relative to the sealing formation  146  so that the sidewall  134  and the sealing formation  146  define a sealing structure  156  that is substantially the same as the sealing structure  100 . It follows that the lip  154  and the vertical second section  152  define an ink adhesion surface  158 . 
     As can be seen in  FIGS. 7 and 8 , a meniscus  160  is formed between the free end  148  of the sidewall  134  and the ink adhesion surface  158  when the nozzle chamber  138  is filled with ink  162 . Thus, a fluidic seal is set up between the sealing structure  156  and the sidewall  134 . The operation and purpose of this fluidic seal has been fully described earlier in this description. As can be seen in the drawings, the roof  132  and sidewall  134  are displaced vertically downwardly towards the substrate so that an ink drop  164  is formed outside of the ink ejection port  140 . During this displacement, an edge of the meniscus  160  moves from one side of the ink adhesion surface  153  to an opposed side to accommodate this movement. When the roof  132  and the sidewall  134  move back into the position shown in  FIG. 7 , the ink drop  164  separates from the remainder of the ink  162  in the nozzle chamber  138 . 
     The sealing structure  156  and the ledge  144  have a vertically facing surface area that is sufficient to facilitate the ejection of ink, as described above, when the roof  143  is displaced towards the substrate  122 . 
     The nozzle arrangement  120  includes a pair of symmetrically opposed thermal actuators  166  that act on the roof  132  to eject the ink drop  164 . Each thermal actuator  166  is connected to suitable drive circuitry (not shown) arranged on the substrate  122 . Details of the thermal actuators are set out in the above referenced applications and are therefore not set out in this description. 
     Each thermal actuator  166  is in the form of a bend actuator. It follows that a suitable connecting structure  168  is positioned intermediate each thermal actuator  166  and the roof  132 . The connecting structures are configured to accommodate the different forms of movement of the roof  132  and the actuators  166 . Further details of these connecting structures  168  are provided in the above referenced applications and are therefore not set out here. 
     In  FIG. 10 , reference numeral  170  generally indicates a nozzle arrangement of a second embodiment of a printhead chip, in accordance with the invention. With reference to  FIGS. 1  to  9 , like reference numerals refer to like parts, unless otherwise specified. 
     As with the nozzle arrangement  120 , the nozzle arrangement  170  is one of a plurality of such nozzle arrangements set out on a substrate  172  to define the printhead chip of the invention. The reasoning behind this as been set out above and applies here as well. As with the previous embodiment, the substrate  172  is assumed, for the purposes of convenience, to define the plane of reference  11  referred to earlier in this description. Thus, the orientation terminology referred to earlier is used in the following description. 
     A sidewall  174  and a roof  176  are positioned on the substrate  172  to define a nozzle chamber  178 . An ink ejection port  180  is defined in the roof  176 . 
     The substrate  172  includes silicon wafer substrate  184 , a CMOS layer  186  that defines drive circuitry for the nozzle arrangement  170  and an ink passivation layer  188  positioned on the CMOS layer  186 . 
     An ink ejection member in the form of a paddle  182  is positioned in the nozzle chamber  178 . The paddle  182  is connected to a thermal bend actuator  190  with a connecting member  192  interposed between the paddle  182  and the thermal bend actuator  190 . 
     The thermal bend actuator  190  is connected to the CMOS layer  186  with suitable vias  194  so that the thermal bend actuator  190  can be driven by the drive circuitry. The thermal bend actuator  190  and its operation are fully described in the above referenced applications and these details are therefore not set out here. The thermal bend actuator  190  serves to displace the paddle  182  through an arc towards and away from the ink ejection port  180 . In  FIG. 10 , the nozzle arrangement  170  is shown in an operative position with the paddle  182  displaced towards the ink ejection port  180  so that ink  196  within the nozzle chamber  178  is ejected from the ink ejection port  180  to form a drop  198 . The drop  198  separates from the ink  196  when the paddle  182  returns to a quiescent condition and ink pressure in the nozzle chamber  178  drops. The nozzle chamber  178  is in fluid communication with an ink inlet channel  200  defined in the substrate  172 , so that the nozzle chamber  178  can be refilled with ink once the drop  198  has been ejected. This occurs when the pressure drop mentioned above is equalized. 
     The connecting member  192  and roof  176  define an upper sealing structure  202 . The connecting member  192  and the sidewall  174  define a lower sealing structure  204 . 
     The upper sealing structure  202  includes a sealing formation in the form of an outer, elongate plate  206  positioned on an inner side  208  of the connecting member  192  adjacent an upper surface  210  of the connecting member  192 . When the nozzle arrangement  170  is in a quiescent condition, the plate  206  is positioned in a vertical plane. 
     The upper sealing structure  202  includes a further sealing formation in the form of an inner, elongate plate  212  that is positioned on the roof  176 . The inner elongate plate  212  is horizontally aligned with the outer plate  206 , when the nozzle arrangement  170  is in a quiescent condition. Further, a gap  214  defined between the plates  206 ,  212  is such that a meniscus  216  is formed between the plates  206 ,  212 , the meniscus  216  extending between upper edges  218 ,  220  of the plates  206 ,  212 , respectively. 
     The edges  218 ,  220  are proud of the surface  210  and the roof  176 , respectively. Thus, an extent of movement of edges of the meniscus  216  is determined by a thickness of the plates  206 ,  212 . It follows that when the paddle  182  is displaced towards and away from the ink ejection port  180 , as described above, the meniscus  216  defines a fluidic seal to inhibit leaking of the ink  196 . As set out above, the reason behind this is that a contact angle of the meniscus  216  with the plates  206 ,  212  does not reach zero degrees during movement of the connecting member  192  relative to the roof  176 . 
     The lower sealing structure  204  includes a lower sealing formation in the form of a downward projection  222  defined by the connecting member  192 . The sidewall  174  defines a sealing formation in the form of a re-entrant wall portion  224  positioned on the substrate  172 . The re-entrant wall portion  224  includes an outer rim  226  that is horizontally aligned with the downward projection  222  when the nozzle arrangement  170  is in a quiescent condition. A meniscus  228  extends between the downward projection  222  and the outer rim  226  when the nozzle chamber  178  is filled with the ink  196 . 
     As is clear from the drawings, the sealing structure  204  is similar in form to the sealing structures  70  and  90  shown in  FIGS. 4 and 5  respectively. The operation and advantages of the sealing structure  204  are therefore clear and need not be described at this stage. It follows that the meniscus  228  defines a suitable fluidic seal that inhibits the leaking of ink during operation of the nozzle arrangement  170 .
A liquid displacement assembly includes a first displacement member and a second displacement member. The first and second displacement members together define a volume in which liquid is received. At least one of the first and second displacement members is movable with respect to the other to displace liquid from the volume. A sealing formation is positioned on each displacement member. The sealing formations are shaped so that a fluidic seal is interposed between the sealing formations when the liquid is received in the volume. Each sealing formation has a liquid adhesion surface positioned between side surfaces of the sealing formation and directed towards a plane of reference oriented substantially orthogonally with respect to a direction of relative displacement of the sealing formations. The sealing formations are spaced from each other to define a region in which a meniscus can be formed so that opposed edges of the meniscus adhere to respective adhesion surfaces of the sealing formations. The sealing formations are shaped so that each liquid adhesion surface is interposed between the side surfaces of each respective sealing formation and the plane of reference.