GB2106039A - Thermal ink jet printer - Google Patents

Abstract

An ink-on-demand ink jet printing device is disclosed typically having an ink-containing capillary with an orifice for ejecting ink, and an ink heating mechanism 16 in close proximity to the orifice. In operation, the ink heating mechanism is quickly heated, transferring a significant amount of energy to the ink, thereby vaporizing a small portion of the ink and producing a bubble 12 in the capillary. This in turn creates a pressure wave which propels an ink droplet from the orifice onto a closeby writing surface. By controlling the energy transfer to the ink, the bubble quickly collapses before any ink vapor can escape from the orifice. <IMAGE>

Description

SPECIFICATION Thermal ink jet printer This invention is concerned with improvements in or relating to thermal ink jet printers.

Recent advances in data processing technology have spurred the development of a number of high speed devices for rendering permanent records of information. Alphanumeric non-impact printing mechanisms now include thermal, electrostatic, magnetic, electrophotographic, and ionic systems.

Of particular import in these developing systems has been ink jet printing technology, because it offers a simple and direct method of electronically controlling the printed output and has the special advantage of being non-contact, high speed, and particularly well adapted to plain paper printing.

Generally, ink jet systems can be categorized into three basic types: continuous droplet ink jets in which droplets are generated continuously at a constant rate under constant ink pressure, electrostatically generated ink jets, and ink-on-demand jets (or impulse jets). This invention is concerned primarill with this latter system.

The primary approach in commercially available ink-on-demand systems has been to use piezoelectric crystals to propel ink from the orifice of a tube of narrow cross-section. Atypical example of this approach is described in U.S. Patent Specification No. 3,832,579. Here a small cylindrical piezoelectric transducer is tightly bound to the outer surface of a cylindrical nozzle. Ink is brought to the nozzle by an ink hose connected between the broad end of the nozzle and an ink reservoir. As the transducer receives an electrical impulse, it generates a pressure wave which accelerates ink toward both ends of the nozzle. An ink droplet is formed when the ink pressure wave exceeds the surface tension of the meniscus at the orifice on the small end of the nozzle.

In these piezoelectric ink jet systems, a principal problem is associated with the relative disparity in size between the piezoelectric transducer and the ink jet orifice. The transducer is generally substantially larger than the orifice, thereby limiting either the minimum separation of the jets or the number of jets which can be used on a given print head. Furthermore, piezoelectric transducers are relatively expensive to produce and are not amenable to amny of the modern semiconductorfabrication techniques.

Another type of ink-on-demand system is described in U.S. Patent Specification No. 3,174,042.

This system utilizes a plurality of ink containing tubes. Electrodes in the tubes contact the ink and upon a trigger signal an electric current is passed through the ink itself. This current flow heats the ink by virtue of a high 12R loss (where I is the current and R is the resistance of the ink), vaporizes a portion of the ink in the tubes, and causes ink and ink vapor to be expelled from the tubes.

The principal drawbacks of this steam-type system are the serious difficulties in controlling the ink spray, and the constraints on ink conductivity, since a highly conducting ink requires a large current flow to achieve the required vaporization, and therefore unduly restricts the types of ink which might be used.

Despite the fact that both of these systems have been known for many years, the technology of ink-on-demand ink-jet printing has yet to resolve the fundamentai problems associated with each of these devices.

The present invention provides an ink jet printing device, comprising capillary means for holding ink, said capillary means having an orifice for ejecting ink; and heating means in proximity to said orifice for transferring a quantity of energy to said ink sufficient to vaporize a portion of said ink in said capillary means, and to eject a droplet of said ink from said orifice, said quantity of energy being insufficient to also cause ink vapor to escape from said orifice.

In a device as set forth in the last preceding paragraph, it is preferred that said heating means is capable of successively transferring said quantity of energy to said ink in a desired sequence to cause a corresponding sequence of ink droplets to be ejected from said orifice.

In a device as set forth in the last preceding paragraph, it is preferred that said capillary means comprises a substrate and a top, said substrate and said top being configured with an interstitial space for holding ink, and said interstitial space being connected to said orifice.

In a device as set forth in the last preceding paragraph, it is preferred that said substrate and said top meet to form an interface, said orifice being located at said interface.

In a device as set forth in the last preceding paragraph but one, the orifice may be located in the top.

In a device as set forth in either one of the last two immediately preceding paragraphs, it is preferred that said heating means comprises a resistor. Preferably said resistor is located within said interstitial space.

Alternative to the arrangement set forth in the last preceding paragraph but four, said capillary means may comprise a membrane and a substrate, said membrane and said substrate being configured with a space therebetween for holding ink, said space being connected to said orifice.

In a device as set forth in the last preceding paragraph, it is preferred that said heating means is located on said membrane opposite said ink.

In a device as set forth in the last preceding paragraph, it is preferred that said membrane is flexible.

In a device as set forth in the last preceding paragraph, it is preferred that said heating means comprises a resistor.

In a further alternative device as set forth in the last preceding paragraph but eight, it is preferred that said capillary means comprises a top and a membrane, said top and said membrane being configured with a space therebetween for holding ink, said space being connected to said orifice, said orifice being located in said top.

In a device as set forth in the last preceding paragraph, it is preferred that said resistor is located on said membrane opposite said ink.

In a device as set forth in any one of the last six immediately preceding paragraphs, it is preferred that said membrane comprises at least one of silicon carbide, silicon dioxide, silicon nitride, and boron nitride.

In a device as set forth in any one of the last thirteen immediately preceding paragraphs, it is preferred that said capillary means comprises a plurality of said orifices, said capillary means holding said orifices in a fixed relationship to each other; and a plurality of said heating means is provided, each heating means corresponding to a particular orifice.

In a device as set forth in the last preceding paragraph, it is preferred that said orifices are arranged in a straight line.

The present invention further provides a method of propelling a droplet of liquid from an orifice in a liquid-contaning capillary, comprising the step of transferring a quantity of energy to said liquid sufficient to vaporize a portion of said liquid causing a bubble in said liquid containing capillary, the bubble causing a droplet of liquid to be ejected from said orifice, said quantity of energy being insufficient also to cause vaporized liquid to escape from said orifice.

In carrying out a method as set forth in the last preceding paragraph, it is preferred that the step of transferring a quantity of energy to said liquid comprises the steps of heating said portion to a temperature below the point at which a bubble will nucleate in said liquid; and quickly heating said portion to a temperature near the superheat limit of said liquid to cause bubble nucleation. Alternatively, the step of transferring a quantity of energy to said liquid comprises the steps of thermally contacting said portion with a resistor; passing a current through said resistor to heat said portion to a temperature below the point at which a bubble will nucleate; and passing a larger current through said resistor to heat said portion quickly to a temperature near the superheat limit to cause bubble nucleation.

In carrying out a method as set forth in the last preceding paragraph, it is preferred that said current varies substantially as the square root of the inverse of time in said step of passing a current through said resistor to heat said portion to a temperature below the point at which a bubble will nucleate.

Said liquid preferably comprises ink.

In accordance with the illustrated preferred embodiments, the present invention provides an inkcontaining capillary having an orifice for ejecting ink, and an ink heating mechanism located in close proximity to the orifice (e.g., a resistor located in or adjacent to the capillary).

In operation, the ink heating mechanism is quickly heated, transferring a significant amount of energy to the ink, thereby vaporizing a small portion of the ink and producing a bubble in the capillary. This in turn creates a pressure wave which propels an ink droplet from the orifice into a closeby writing surface. With proper selection of the location of the ink heating mechanism relative to the orifice and careful control of the energy transfer, the bubble will quickly collapse on or near the ink heating mechanism before any ink vapor can escape from the orifice.

Hence, many of the existing hardware and software schemes used for implementing other dot matrix printing systems are readily adapted to the present invention.

In the preferred embodiments, there are primarily two geometric configurations. In the first configuration, ink is ejected from an orifice located at the end of a capillary tube, while in the second configuration, the ink capillary essentially defines a plane, the ink being ejected from an orifice in a direction orthogonal to that plane. Each of these configurations has the advantage over the prior art of being suitable for mass production using standard electronicfabrication techniques. Furthermore, the dimensions of print heads which can be produced using the principle of the invention are virtually unlimited, allowing very large arrays and very high resolution, since no space-consuming piezoelectric crystals are required. Furthermore, the electrical conductivity of the inks which can be used is not a constraint on the technique.

There now follows a detailed description which is to be read with reference to the accompanying drawings of embodiments of the present invention; it is to be clearly understood that these embodiments have been selected for description to illustrate the invention by way of example and not by way of limitation.

Figure 1 is a disassembled view of a device according to the invention; Figure 2 is a view of the device of Figure 1 in its assembled form; Figure 3 is a cross-sectional view of the device shown in Figures 1 and 2; Figure 4 depicts the time sequence of events involved in the production of an ink droplet; Figure 5 shows a typical voltage profile which is involved with bubble formation; Figure 6 shows a variation of the voltage profile involved in bubble formation; Figure 7a is a disassembled view of a multiple-jet, edge-shooter print head; Figure 7b shows a device of Figure 8a in its assembled form; Figure 8 is a cross-sectional view of another embodiment of an edge shooter print head; Figure 9a is a disassembled view of a side-shooter print head; Figure 9b is a view of the print head of Figure 1 Oa in its assembled form; ; Figure lOa is an oblique view of a multiple-jet, side-shooter print head; Figure 10b is an oblique view of the top of the substrate of the device shown in Figure 10a; and Figure ii is an oblique view of another multiplejet, side-shooter print head.

Shown in Figure 1 is a construction diagram of a thermal ink jet printer. Figure 2 depicts the related product after assembly. The basic construction is that of a substrate 11 typically sapphire, glass, or some inert composite material, such as coated metal or coated silicon, part of one surface of substrate 11 being covered with a thin film metallization layer 13.

The thin film metallization has been configured to provide a narrow nonconducting strip 14 of width D1 (-0.003") and a conducting strip of width D2 (--0.003") to create a resistor 16 in metallization layer 13. A resistance of approximately 3 ohms is appropriate. In a typical configuration, resistor 16 is located at a distance D3 (nominaily 0.006" but generally in the range 0.002" < D3 < 0.01") from the edge of substrate 11. Bonded to the top of thin film metallization 13 is a capillary block 15, typically glass, having a capillary channel 17 with an orifice on each end.

Channel 17 is approximately 0.003" wide by 0.003" deep, corresponding in width to nonconducting strip 14 in metallization layer 13.

Behind capillary block 15 and on top of substrate 11 is a reservoir wall 19 for holding ink in a reservoir 24 in juxtaposition with capillary block 15. Channel 17 draws ink by capillary action from reservoir 24 to the vicinity of the orifice opposite the reservoir. As seen in Figure 2, in its compieted configuration the printer has two electrodes 23 and 25 which are attached to thin film metallization layer 13 for applying an electrical potentiai difference across resistor 16. Figure 3, a cross-section of the thermal ink jet printer of Figures 1 and 2, shows the relative configurations of ink 21, capillary block 15, resistor 16 and a printing surface 27. In operation, the distance D5 between the printer orifice and the printing surface 27 is of the order of 0.03".

Figure 4 shows, in cross-section, a time sequence of events during one cycle of operation of the printer. As a voltage is applied to electrodes 23 and 25, the current through resistor 16 causes joule heating and superheats the ink, which, with proper control nucleates at a prescribed time, creating a bubble 12 over resistor 16 as shown in Figure 4a. The bubble continues to expand very rapidly toward the orifice as shown in Figure 4b, but its expansion is limited by the energy transferred to the ink. By maintaining careful control of the total energy, and the time distribution of energy fed into resistor 16, the bubble can be made to grow to a wide range of sizes. Care is taken, however, to ensure that the total energy absorbed by the ink is not so great as to expel vapor from the orifice.Instead, the bubble begins to collapse back onto resistor 16 as shown in Figure 4c, while the forward momentum imparted to the ink from the bubble expansion acts to propel a droplet of ink from the orifice (it should be noted, however, that the droplet can be accompanied by one or more satellites depending on the ink used, the orifice geometry, and the applied voltage). Afterthe drop has left the orifice, the bubble completely collapses back on or near its starting location as shown in Figure 4d. The ink then begins to refill by capillary action (Figure 4e), and the ink droplet subsequently lands on the printing surface.Figure 4f shows the channel filled to its original position, ready for another cycle. Printingis then accomplished by successively applying a voltage to resistor 16 in an appropriate sequence while the orifice and the printing surface are moved transversely relative to each other to create a desired pattern.

Clearly, with the above device, the particular dimensions, including those of the substrate, capillary block, and capillary channel, can vary over a wide range depending on the desired mass, construction material and techniques, droplet size, capillary filling rate, ink viscosity, and surface tension.

Also, in contradistinction to prior art devices, it is neither necessary that the conductivity of the ink be commensurate with a high 12R heat loss nor that the ink be electrically conductive at all.

An essential feature of the invention is that the impulse required to eject a droplet of ink from the orifice is caused by the expansion of a bubble, rather than by a pressure wave imparted by a piezoelectric crystal or other device. Careful control over the energy transfer from resistor 16 to the ink ensures that ink vapor does not escape from the orifice along with the droplet. Instead, the bubble collapses back onto itself eliminating any ink vapor spray. Furthermore, careful control of the time sequence of the energy transfer is exceedingly important.

Although a single square current pulse of about 1 amp with a duration of about 5 lisec through resistor 16 will accomplish the above result, such a straightforward approach is not generally applicable to various jet configurations. In addition, problems arise when it is desired to produce a larger bubble, for example, to accomodate a larger orifice or to obtain a higher ejection velocity for the droplet. If the pulse is made longer to provide more energy to the ink, the statistical nature of bubble formation can cause substantial time jitter. On the other hand, if the pulse height is increased to ameliorate the problem of time jitter, the substantially higher current densities required can result in early burnout of the resistor due to electromigration.

Each of the these problems can be substantially eliminated with the approach shown in Figure 5.

Here, no DC level is required, but a precursor pulse IP is used to preheat the ink in the vicinity of resistor 16 at a rate low enough to avoid bubble nucleation.

Precursor pulse IP is followed by a nucleation pulse IN which very quickly heats resistor 16 to near the superheat limit of the ink, i.e., the point at which a bubble spontaneously nucleates in the ink. The bubble nucleus so formed grows very rapidly, its mature size being determined by the volume of the ink heated by precursor pulse IP. During the growth phase of the bubble, the voltage across resistor 16 is generally reduced to zero, since the heat transfer to the ink is very ineffective during this time and sustaining the current can result in overheating of the resistor.

In a typical configuration, resistor 16 is about 3 ohms, the pulse height of precursor pulse IP is of the order of 0.3 amps with a pulse width TP of approximately 40 usec, and the pulse height of nucleation pulse IN is of the order of 1 amp with a pulse width TN of approximately 5 sec. Since these parameters can vary quite widely, however, it is more appropriate to view them in terms of the typical ranges which are encountered in operation: O < R < 100 & O < IP < 3 amps with 10 < TP < 100 usec; and 0.01 < IN < 5 amps with O < TN < 10 usec.

Many other schemes for control of bubble formation are also available, e.g., pulse spacing modula tion or pulse height modulation. Still another scheme is shown in Figure 6. In this approach, the precursor pulse decreases in magnitude from its initial value of approximately 0.5 amps to a value of approximately 0.2 amps just before nucleation pulse beings. The shape of the precursor pulse as a function of time varies as 1/, which keeps the resistor at approximately a constant temperature, thereby optimizing the energy distribution in the ink before nucleation and decreasing the required nucleation pulse width while concurrently enhancing nucleation reproducability.

Shown in Figures 7a and 7b is an inkjet print head having more than one orifice, demonstrating the principles of the invention in a form more nearly commensurate with its commercial appliction. This so called "edge-shooter" device is made up of a substrate 71 and capillary block 75 having several ink capillary channels 77, located at the interface of the substrate and the capillary block. Typical materials used for substrate 71 are electrical insulators such as glass, ceramics, or coated metal or silicon, while the materials used for capillary block 75 are generally chosen for their ease of manufacture in regard to ink capillary channels 77. For example, capillary block 75 is typically made of molded glass, etched silicon, or etched glass.In its construction, substrate 71 and capillary block 75 can be sealed together in a variety of ways, for example, by epoxy, anodic bonding or with sealing glass. The distances D6 and D7 corresponding to the channel spacing and channel widths, respectively, are determined by the desired separation and size of the ink jets. Channel 79 is a reservoir channel to supply ink to the ink capillary channels 77 from a remote ink reservoir (not shown).

A plurality of resistors 73 is provided on substrate 71 with a resistor on the bottom of each capillary channel 77. Also provided is a corresponding number of electrical connections 72 for supplying electrical power to the various resistors 73. Both resistors 73 and electrical connections 72 can be formed using standard electronic fabrication techniques, such as physical or chemical vapor deposition.

Typical materials for electrical connections 72 are chrome/gold (i.e., a thin underlayer of chromium for adhesion, with an over-layer of gold for conductivity), or aluminum. Suitable materials for the resistors 73 are typically platinum, titanium-tungsten tantalum-aluminum, diffused silicon, or some amorphous alloys. Other materials would also clearly be appropriate for these various functions; however, some care must be taken to avoid materials which will be corroded or electroplated out with the various inks which might be used. For example, with wter base inks, both aluminum and tantalum-aluminum exhibit these problems at the currents and resistivities typically used (i.e., with resistors in the range of 3 to 5 ohms and currents of the order of 1 amp).

However, even thsese two materials can be used if a proper passivation layer is provided to insulate the electrical conductors and resistors from the ink.

Shown in Figure 8 in another configuration for an edge-shooter ink jet print head shown in crosssection. In this configuration, the thermal energy for creating a bubble in the ink is provided by resistor 83. As in the previous embodiment, the resistor 83 is located at a small distance (-0.003") from the orifice of an ink channel 82 (note: the cross-section of Figure 8 has been taken through resistor 83, so that the ink channel orifice is not shown). In this embodiment, there is provided a substrate 81, typically of glass, which is bonded to an etched silicon capillary block 89, which defines ink channel 82. Overlying capillary block 89 and ink channel 82 is a membrane 87, usually made of a heattolerant, electrically non-conductive, thermally conductive, flexible material, such as silicon carbide, silicon dioxide, silicon nitride, or boron nitride.Resistor 83 is deposited on membrane 87 by standard techniques, and electrical power is provided to resistor 83 by a metallization layer 85 on each side of the resistor.

The advantage of this configuration relative to a non-flexible structure is that it improves device lifetime. Also, construction tehcniques are simplified since the structure consisting of substrate 81, capillary block 89, and membrane 87 can be essentially complete before the resistor and metallization layer are applied. Further, as in the previous embodiment, this particular structure is easily adapted to multiple channel devices and mass production techniques.

Other variations of this concept of a resistor on a flexible membrane will occur to those skilled in the art. For example, by appropriate choice of materials, the flexible membrane as a separate structure could be eliminated entirely by providing a resistor which is itself flexible and self-supporting.

Shown in Figure 9a and 9b is yet another configur ationforathermal ink jet print head, a so called "side-shooter" device. In this configuration a substrate 91 is provided, typically constructed of glass or other inert, rigid, thermally insulating material. Electrical connections to a resistor 93 are provided by two conductors 92 in much the same manner as the construction shown in Figures 7a and 7b. Two plastic spacers 94 are used for maintaining the separation of substrate 91 from a top 95, thereby providing a capillary channel 96 for ink to flow to the resistor.

Clearly, however, many other techniques are available for providing an appropriate spacing. For example, instead of plastic, the glass substrate itself could be etched to provide such a channel.

The top 95 in this embodiment is typically composed of silicon in order to provide a convenient crystalline structure for etching a tapered hole which acts as an orifice 97 for the ink jet. Orifice 97 is located directly opposite resistor 93, and can be fabricated according to the method described in U.S.

Patent No.4,007,464 issued February 8,1977, entitled "Ink Jet Nozzel", by Bassous, et al. Orifice 97 is typically of the order of 0.004". It is important to note that many other materials could also be used for top 95 of the side-shooter ink jet; for example, a metal layer could be used with holes immediately opposite their respective resistors, or even a plastic top could be used.

Shown in Figure 10a is a typical configuration which might be used in a commercial realization of a side-shooter system having multiple jets. In this embodiment substrate 101 is typically glass on which two glass spacers 104 are placed for holding ink 102. A silicon top 105 is provided having a series of etched tapered holes as represented by hold 107.

Each hole is recessed in a trough 108 so that a thicker top can be used to provide better structural stability to the device in order to support a larger print head system for multiple jets. Element 109 is a fill tube which is connected to a remote reservoir (not shown) in order that a continuous supply of ink can be provided to the resistor/orifice system.

Figure lOb is a view of a portion of substrate 101 from the top. Here, a second resistor 106 is shown which also lies along trough 108 of Figure 10a.

Electrical power is supplid to resistors 103 and 106 by two independent electrical connections 110 and 111 respectively, and by a common ground 112. In order to prevent ink from being ejected from orifice 107 when resistor 106 fires, a barrier 113 is provided between resistors 106 and 103. In the above configuration, barrier 113 is typically constructed of glass, silicon, photopolymer, glass bead-filled epoxy, or electroless metal deposited onto the substrate or the inside surface of the top. Additional methods for providing barriers become available if a metal top is used. For example, barriers could be metal placed directly onto the inside surface of the metal top.

Another embodiment of the side-shooter print head is shown in Figure 11,which incorporates the membrane and external resistor of Figure 8. The details of this embodiment are identical to those of Figure 10, except that the substrate has been replaced buy a membrane 120, again typically of silicon carbide, silicon dioxide, silicon nitride, or boron nitride, and a substrate 121. Located on membrane 120 and external to the ink is a resistor 123. As in the previous examples, electrical connection to resistor 123 is provided by two conductors 122. Substrate 121 is provided for structural stability and is usually etched glass, or etched silicon, and has a recess near resistor 123 to permit flexing of membrane 120.

Clearly, there are many other embodiments which could be configured with various kinds of materials and with many different geometries depending on the particular nature and needs of the application.

For example, within certain limits and depending on the inks which are used, larger orifices lead to larger drop size and smaller orifices lead to smaller drop size. Similarly, the maximum frequency for the ejection of ink drops depends on the thermal relaxation time of the substrate and the refill time.

Electrical characteristics of the ink can also result in different geometric configurations. For instance, should current flow through the ink become a problem because of highly conductive inks, passivations layers can be placed over the resistors themselves and other the conductors in order to avoid conduction.

Claims (23)

1. An inkjet printing device, comprising: capillary means for holding ink, said capillary means having an orifice for ejecting ink; and heating means in proximity to said orifice for transferring a quantity of energy to said ink sufficient to vaporize a portion of said ink in said capillary means, and to eject a droplet of said ink from said orifice, said quantity of energy being insufficient to also cause ink vapor to escape from said orifice.

2. A device according to claim 1, wherein said heating means is capable of successively transferring said quantity of energy to said ink in a desired sequence to cause a corresponding sequence of ink droplets to be ejected from said orifice.

3. A device according to claim 2, wherein said capillary means comprises a substrate and a top, said substrate and said top being configured with an interstitial space for holding ink, and said interstitial space being connected to said orifice.

4. A device according to claim 3, wherein said substrate and said top meet to form an interface, said orifice being located at said interface.

5. A device according to claim 3, wherein said orifice is located in said top.

6. A device according to either one of claims 4 and 5, wherein said heating means comprises a resistor.

7. A device according to claim 6, wherein said resistor is located within said interstitial space.

8. A device according to claim 2, wherein said capillary means comprises a membrane and a substrate, said membrane and said substrate being configured with a space therebetween for holding ink, said space being connected to said orifice.

9. A device according to claim 8, wherein said heating means is located on said membrane opposite said ink.

10. A device according to claim 9, wherein said membrane is flexible.

11. A device according to claim 10, wherein said heating means comprises a resistor.

12. A device according to claim 2, wherein said capillary means comprises a top and a membrane, said top and said membrane being configured with a space therebetween for holding ink, said space being connected to said orifice, said orifice being located in said top.

13. A device according to claim 12, wherein said resistor is located on said membrane opposite said ink.

14. A device according to any one of claims 8 to 13 wherein said membrane comprises at least one of silicon carbide, silicon dioxide, silicon nitride, and boron nitride.

15. A device according to any one of the preceding claims wherein said capillary means comprises a plurality of said orifices, said capillary means holding said orifices in a fixed relationship to each other; and a plurality of said heating means is provided, each heating means corresponding to a particular orifice.

16. A device according to claim 15 wherein said orifices are arranged in a straight line.

17. An ink jet printing device substantially as hereinbefore described with reference to the accompanying drawings.

18. A method of propeiling a droplet of liquid from an orifice in a liquid-containing capillary, comprising the step of transferring a quantity of energy to said liquid sufficient to vaporize a portion of said liquid causing a bubble in said liquid containing capillary, the bubble causing a droplet of liquid to be ejected from said orifice, said quantity of energy being insufficient also to cause vaporized liquid to escape from said orifice.

19. A method according to claim 18, wherein the step of transferring a quantity of energy to said liquid comprises the steps of: heating said portion to a temperature below the point at which a bubble will nucleate in said liquid; and quickly heating said portion to a temperature near the superheat limit of said liquid to cause bubble nucleation.

20. A method as in claim 18, wherein the step of transferring a quantity of energy to said liquid comprises the steps of: thermally contacting said portion with a resistor; passing a current through said resistorto heat said portion to a temperature below the point at which a bubble wil nucleate; and passing a larger current through said resistorto heat said portion quickly to a temperature near the superheat limit to cause bubble nucleation.

21. A method according to claim 20, wherein said current varies substantially as the square root of the inverse of time, in said step of passing a current through said resistor to heat said portion to a temperature below the point at which a bubble will nucleate.

22. A method according to any one of claims 18 to 21 wherein said liquid comprises ink.

23. A method of propelling a droplet of liquid from an orifice in a liquid-containing capillary substantially as hereinbefore described with reference to the accompanying drawings.