Method and apparatus for synchronizing droplet formation in a liquid stream
United States Patent [1 1 Eaton [451 Apr. 15, 1975 METHOD AND APPARATUS FOR SYNCHRONIZING DROPLET FORMATION IN A LIQUID STREAM James H. Eaton, Armonk, NY.  
 International Business Machines Corporation, Armonk, NY.  
 Jan. 31, 1974 Inventor:  
 Assignee:  
 Filed:  
 Appl. No.:  
 US. Cl. 346/1; 239/13; 239/133; 346/140 Int. Cl. G01d 15/18 Field of Search 346/75, 140, 1; 239/133, 239/134, 135, 13; 118/620, 641, 302  
 References Cited UNITED STATES PATENTS I Ii INK 3.731.876 5/1973 Showalter 239/13 Primary E.raminerJoscph W. Hartary Attorney, Agent, or Firm-Frank C. Leach, Jr.  
 [57] ABSTRACT A source of energy is selectively applied to a liquid stream to reduce the surface tension of the liquid and is applied before the stream would randomly break up into droplets. Both the quantity of energy applied and the time period that the energy is applied are controlled to control the time to droplet breakoff and the time between droplets by selectively reducing the surface tension of segments of the stream. The source of energy can be high intensity light, which is converted by the stream to heat energy, or a source of heat (resistive or inductive) with the resistive heat being applied to the stream by conduction and the inductive heat being converted by the stream to heat energy.  
 19 Claims, 6 Drawing Figures HIGH INTENSITY UGHT SOURCE SUPPLY INTENSITY CONTROL DEFL MEANS I DEFL. 22 J SIGNAL SOURCE T0 INK RETURN PATENTEDAPR I 5&amp;975  
 SHEEIlflF 2 man INTENSITY ucm SOURCE i6 1 FIG. 1  
  17 1a INK SUPPLY INTENSITY MOD. CONTROL 1 i5 ,0 DEFL. o o o MEANS DEFL. 22 21 f SIGNAL SOURCE T0 INK l RETURN W HIGH INTENSITY INK LIGHT SOURCE 32 SUPPLY 51 comm DEFL. SIGNAL SOURCE PS-JEKHEMFR] 5:975 3.878.518 saw 2 i 2 40 CONTROL FlG.4  
 50 r CONTROL METHOD AND APPARATUS FOR SYNCHRONIZING DROPLET FORMATION IN A LIQUID STREAM ln some ink jet printing embodiments. the droplets are selectively deflected in a direction perpendicular to the paper motion to place the droplets on the desired position of the paper. Where droplets are not wanted. they are selectively deflected into a gutter to catch them before they can impinge on the paper.  
  In other embodiments. an array of nozzles is used to form a droplet stream for each desired spot position on the paper. Thus. ifall droplets were to hit the paper. the paper would be uniformly covered with ink. Printing is accomplished by selectively removing unwanted droplets from the stream. usually by deflecting them into a gutter.  
  The most common means for selectively deflecting the droplets is to selectively place an electrical charge on the droplets and then to pass the droplets through a uniform electric field for deflection as shown and described in U.S. Pat. No. 3.596.275 to Sweet. The amount of deflection for a given droplet is then proportional to the previously selected electric charge and inversely proportional to its mass and velocity. The charge on a droplet is determined by the electric field on the droplet at its moment of breakoff from the stream.  
 The usual method of charging the droplet involves applying a voltage to a cylinder (charging tunnel) surrounding the point of breakoff of the droplet from the stream. Thus. it is important to control both the time and position of the breakoff so that a given timed voltage sequence will appropriately charge the droplets. If the droplets do not break off at the correct time and position. they can receive an incorrect charge and. thus. be deflected to an undesired position.  
  The correct time and position is determined by synchronizing the droplets so that they pass through the cylinder (charging tunnel) at uniform intervals of time and with the correct phase. in addition. the disturbance causing the breakoff is modified in amplitude to give the desired breakoff point within the charging tunnel.  
  Various means for obtaining synchronization of the droplets have been previously suggested. For example. mechanical forces have been applied to a nozzle by a piezoelectric device. for example. at a desired frequency.  
  When using a mechanical arrangement for creating the breakup ofthc stream into droplets. the mechanical force applied to one nozzle to produce a vibrating frequency of the nozzle can have an effect on an adjacent nozzle of the ink jet streams if the ink streams are disposed on five mil centers. for example. The transmission of the mechanical vibrations to an adjacent nozzle can alter the phase and breakoff point of the droplets of an adjacent nozzle.  
  The present invention satisfactorily solves the foregoing problem by producihg synchronous formation of the droplets without any mechanical force being applied. Thus. with the method and apparatus of the present invention. the nozzles for more than one ink stream. if such is required. can be placed very close to each other without the droplet forming means for one of the ink streams having any effect on any ofthe adjacent ink streams. Thus. droplet formation from each ink stream. if more than one is required. can be effectively controlled with the method and apparatus of the present invention.  
  The present invention accomplishes the foregoing through causing a thermal change or disturbance within the ink stream prior to the time that the stream would randomly break up into droplets. The random breakup of a stream into droplets depends upon its surface tension. its velocity. and its diameter with the breakup occurring after the stream leaves a confined passage unless the passage should be coated with a material that the liquid does not wet such as Teflon. for example.  
  By regulating the thermal change or disturbance in the ink stream. the breakup ofthe stream into droplets is controlled to cause synchronous formation of the droplets and break off at the desired point. The thermal change or disturbance in the stream is controlled through regulating a source ofenergy. which produces this thermal change or disturbance. as to the time it is applied. the length of the segment of the stream to which it is applied. and the quantity of energy applied.  
  By creating the thermal change or disturbance in spaced segments of the stream. the temperature of the spaced segments of the stream is increased so that the surface tension of the spaced segments of the stream is reduced to cause synchronous formation of the droplets. Since the surface tension of the stream decreases as the temperature of the stream increases. the source of energy. which creates the thermal change or disturbance in the spaced segments of the stream. results in a lowering of the surface tension of the spaced seg ments of the stream to cause breakup of the stream into droplets in the desired relation. The period between successive applications of a quantity of energy and the stream velocity control the spacing between the droplets. The breakoff point is determined primarily by the energy applied in each pulse.  
  The surface tension of a stream is directly proportional to the internal pressure within the stream so that the decrease in the surface tension in a segment of the stream causes a reduction in the internal pressure in that segment of the stream. This decrease in the internal pressure has the same effect as an increase in the diameter ofthe segment of the stream since the internal pressure of the stream is inversely proportional to the diameter of the stream. By reducing the internal pressure ofa first segment of the stream. a second and adjacent segment has an increase in its internal pressure relative to the first segment to force the liquid in the second segment to the first segment of the stream. This results in a further reduction in the pressure in the already lowered internal pressure of the first segment of the stream since it increases the diameter of the first segment of the stream to accommodate the larger volume so that a positive feedback is provided to cause the stream to break up into droplets.  
  By selecting the segments or sections in which the source of energy creates a thermal change or disturbance so that the segments or sections defined by the selected application of the source of energy are normally no greater than the average length of the droplets when they break up randomly. synchronization of the formation of the droplets from the stream can be obtained. Thus. the small changes in surface tension in the spaced segments of the stream trigger a positive feedback and lead to controlled droplet formation. The breakoff point is determined primarily by the amplitude of the thermal changes since an increase in amplitude causes the breakoff to occur closer to the nozzle.  
  The present invention contemplates using a modulated heat or high intensity light source as the source of energy. When a resistive heat source is employed. the heat is supplied to the ink stream by conduction to cause the thermal change or disturbance in the stream. When the heat source is inductive. heat conversion occurs within the stream to produce the thermal change or disturbance in the stream. When modulated light from a high intensity light source is the source of energy to cause the thermal change or disturbance of the stream. heat conversion occurs within the stream. Of course. the stream must not be transparent to the light for there to be the conversion ofthe light to heat within the stream.  
  An object of this invention is to synchronously form droplets from a liquid stream by the intermittent application of a source of energy directly to the stream.  
  Another object of this invention is to selectively alter the surface tension of segments of a liquid stream to synchronously form droplets from the stream.  
  A further object ofthis invention is to produce a thermal change in a liquid stream before random breakup of the liquid stream would occur.  
  The foregoing and other objects. features. and advan tages of the invention will be apparent from the following more particular description of preferred embodiments of the invention as illustrated in the accompanying drawings.  
 In the drawings.  
  FIG. I is a schematic view showing a source of energy being applied to a liquid stream to cause synchronous formation of droplets therefrom and the droplets producing printing on a recording surface.  
  FIG. 2 is a schematic view of another form of the source of energy being applied to the liquid stream to cause synchronous formation of droplets therefrom.  
  FIG. 3 is a fragmentary sectional view of a portion of a nozzle and showing a modification of the present invention in which an electric heater is the source of energy to a liquid stream and taken along line 33 of FIG. 4.  
  FIG. 4 is a schematic end elevational view of the nozzle of FIG. 3 with the electric heater and its control.  
  FIG. 5 is a fragmentary sectional view of another embodiment for disposing an electric heater within a nozzle and taken along line 55 of FIG. 6.  
  FIG. 6 is a schematic end elevational view of the noz zle of FIG. 5 with the electric heater and its control.  
  Referring to the drawings and particularly FIG. I, there is shown an ink supply 10. Ink. which can be magnetic or non-magnetic. is supplied under pressure from the ink supply to a nozzle II.  
  A pressurized ink stream I2 passes from the nozzle ll through an o ening [4 thereof. As the pressurized ink stream I2 exits from the opening 14 of the nozzle 1]. it is subjected to a modulated light beam 15, which is supplied from a high intensity light source I6. The high intensity light source 16 can be a gas laser such as a helium-neon gas laser. for example. The laser is continuous since it cannot be turned on and off at the desired frequency such as 100 kiloHertz. for example.  
  To intermittently supply the light beam I5 from the high intensity light source 16 to the stream 12. a modulator I7 is disposed between the high intensity light source 16 and the ink stream 12. The modulator 17 can be any suitable type in which a control 18 can effectively start and stop the application of the light beam 15 to the ink stream I2.  
  The modulator 17 could include. for example. a glass with a chrome mask having a slit through which light could pass. The modulator 17 could include an accoustic deflector. for example. which would shift the light in response to the control I8 so that a focusing lens. for example. within the modulator I7 would not focus the beam at the slit in the chrome mask. When the accoustic deflector is inactivated by the control 18. the beam is focused at the slit so that the beam passes through the slit and another lens front which it exits from the modulator 17.  
  Each time that the light beam 15 is applied to a segment ofthe ink stream I2, which must not be transparent to the light beam [5. the light is converted to heat within the segment of the ink stream I2. This conversion of the light to heat increases the temperature of the segment of the ink stream I2 to reduce its surface tension since the surface tension of the ink stream I2 is inversely proportional to the temperature of the ink stream 12.  
  Each time that the light beam I5 is applied to a segment of the ink stream 12 for a predetermined period of time. the surface tension of the stream I2 is reduced in the segment of the ink stream 12 subjected to the light beam 15. The velocity of the ink stream 12 determines the segment subjected to the light beam IS during the predetermined period oftime. As a result of the reduction of the surface tension of the spaced segments of the ink stream 12. the ink stream 12 breaks into droplets 19 of substantially uniform size with the droplets 19 having substantially uniform spacing therebetween.  
  Prior to the droplets 19 being formed. the stream I2 enters a deflection means 20, which is connected to a deflection signal source 21. in which the droplets 19 are charged at the moment of breakoff in the manner more particularly shown and described in the aforesaid Sweet patent. The deflection signal source 2] applies a signal to the deflection means 20 to determine whether each of the droplets [9 falls into a gutter 22 from which the droplet can be returned to the reservoir of the ink supply 10 or strikes a recording surface such as a moving paper 23. which would be moving horizon tally in a plane perpendicular to the drawing. The position on the paper 23 at which each of the droplets I9 strikes the paper 23 also can be determined by the strength of the signal to the deflection means 20 from the deflection signal source 21. Of course. if a plurality of the ink streams 12 were used. then the deflection means 20 would either cause the droplet 19 to fall into the gutter 22 or to strike the same position on the paper 23 each time. In this case. the paper 23 would be mov ing vertically with the plurality of nozzles disposed in a horizontal plane.  
  It should be understood that the control 18 for the modulator I7 and the deflection signal source 21 must be synchronized so that the deflection signal source 21 provides the desired charge to the droplet 19 when it is within the deflection means 20. This desired charge results in the desired deflection of the droplet 19.  
  Referring to FIG. 2. there is shown another embodiment in which the ink stream 12 is subjected to a light beam 30 from a high intensity light source 3I such as a light emitting diode or an injection laser. which is a solid state laser. for example. The light beam from the high intensity light source 31 is modulated by a control 32. which is connected to the high intensity light source 31. Thus. the control 32 controls the period of time that the high intensity light source 31 applies the light beam 30 to a segment of the ink stream 12. which cannot be transparent to the light beam 30.  
  Accordingly. the high intensity light source 31 is a source of energy for creating a thermal disturbance or change in segments of the ink stream [2. This conversion of the energy of the light beam 30 to heat in the ink stream 12 produces the desired increase in temperature in spaced segments of the ink stream 12 to decrease the surface tension of the spaced segments of the stream 12 so that synchronous formation of the droplets 19 again occurs. The remainder of the operation is the same as described when the high intensity light source 16 of FIG. 1 is employed including synchronization ofthe control 32 with the deflection signal source 2|.  
  Because the high intensity light source 31 can be relatively small and does not require a modulator between it and the ink stream 12 but uses only the control 32 to regulate it. the high intensity light source 3] can be placed fairly close to the ink stream 12 such as about one diameter of the ink stream 12 from the ink stream 12. As a result. there is no need for any lens with the high intensity light source 31 as is necessary with the high intensity light source 16.  
  Referring to H08. 3 and 4. there is shown another form of the invention in which an electric heater 35 such as a thin film resistive heater or an inductive heater. is employed. The heater 35 is disposed within an opening or passage 36 of a nozzle 37. which is connected to the ink supply 10 in the same manner as the nozzle H. and adjacent the exit of the opening 36.  
  Electrical energy is supplied to the heater 35 through contacts 38 and 39. which are connected to a control 40. The control regulates when the heater 35 is turned on and off to control for how long the heat is applied to the segment of the ink stream 12.  
  When the heater 35 is a thin film resistive heater. the heat transfer to the ink stream 12 via conduction re sults in the surface of the segment of the ink stream 12 becoming hotter than the interior of the ink stream 12 in the segment to which heat is applied. Since the heat is applied to the surface of the ink stream 12. this aids in reducing the surface tension of the segment of the ink stream 12.  
  When the heater 35 is formed of a thin film of resistive material. it is formed of any suitable material. For example. it could be formed of copper or Nichrome.  
  When the heater 35 is an inductive heater. the heat transfer to the ink stream 12 is by induction. As a result. the thermal disturbance or change in the segment of the ink stream [2 to which the inductive heat is applied is produced by conversion in the same manner as when light is applied.  
  The nozzle 37 is preferably formed of an electrically insulating material such as quartz. for example. If the nozzle 37 is formed of metal. then the heater 35 must be electrically insulated from the nozzle 37. In such an arrangement. a layer of insulating material such as silicon dioxide, for example. would be disposed around the portions of the heater 35 and the contacts 38 and 39 in engagement with the nozzle 37.  
  The ink stream 12 can be transparent when used with this modification. The remainder of the o eration of this modification is the same as that described with re spect to FIG. I including the synchronization of the control 40 with the deflection signal source 2].  
  Referring to FIGS. 5 and 6. there is shown a modification of the structure of FIGS. 3 and 4 in that a heater 45. which can be a thin film resistive heater or an inductive heater. does not completely surround an opening or passage 46 of a nozzle 47 but only partially surrounds it. Furthermore. the heater 45 is disposed within the nozzle 47 to form a streamline with the surface of the opening 46 as shown in FIG. 5.  
  The heater 45 is connected through contacts 48 and 49 to a control 50. The nozzle 47 is preferably formed of an electrically insulating material such as quartz. for example. If the nozzle 47 is formed of metal. then a layer of insulating material such as silicon dioxide. for example. must be used to electrically insulate the heater 45 and the contacts 48 and 49 from the nozzle 47.  
  The control 50 functions in the same manner as the control 40 to determine when the heater 45 is on and off. The control 50 is synchronized with the deflection signal source 21. The remainder of the operation of this modification is the same as described with respect to FIG. 1.  
  The heater 45 requires a smaller quantity of power than that required when the heater 35 is employed. By heating a portion of the surface of the segment of the ink stream [2 when the heater 45 is a thin film resisti\ e heater. the temperature of the portion of the segment of the ink stream [2 to which the heat is applied is increased sufficiently to decrease the surface tension of the segment of the ink stream 12.  
  While the heater 45. which only partially surrounds the stream 12. has been shown as being disposed to form a streamline with the surface of the opening 46. it should be understood that the heater 45 could be disposed within the opening 46 in the nozzle 47 as the heater 35 of FIGS. 3 and 4. Similarly. while the heater 35 has been shown as being disposed within the opening 36. it should be understood that the heater 35 could be disposed to form a streamline with the opening 36 as the heater 45 of FIGS. 5 and 6.  
  Instead of disposing the heater 45 within the nozzle 47 to form a streamline with the surface of the opening 46. it should be understood that the high intensity light source 31 could be disposed within the nozzle 47 rather than the heater 45. Thus. it is not necessary for the high intensity light source 3] to be exterior of the nozzle as shown in FIG. 2.  
  To show how the reduction in surface tension of a liquid stream can form droplets with substantially uniform spacing and of substantially uniform size. a stream of water to which a thin film resistive heater of 809? nickel and 20% chrome (Nichrome) applies heat will be considered as an example. When the temperature of water is heated from 20 C. to 30 C. its surface tension decreases from 72.75 dynes/cm. to 7|.l8 dynes/cm. for a decrease of 2.4%. Since the internal pressure of a jet stream of liquid is directly proportional to the surface tension of the stream and inversely proportional to the diameter of the stream. a temperature rise of l0 C. at the surface of the water stream has the same effect on internal pressure as an increase in the diameter of the stream of 2.47:.  
  11&#39; the stream is assumed to have a diameter of 1 mil. a 2.45; variation in the surface tension of a 1 mil section or segment at 4 mil intervals produces a diameter increase of 2.40 within approximately 5 microseconds since it is known. as explained hereinafter. that the time constant of the stream instability causes a disturbance to double in magnitude in about 5 microseconds and since the initial 2.4; reduction in surface tension is equivalent to an initial diameter increase of 2.4  
  As explained in Breakup of a Laminal Capillary Jet of a Viscoelastic Fluid&#34; by M. Goldin et al. on pages 689-71 1 of \&#39;ol. 38. Part -1 of the Journal of Fluid Mechanies l 1969 I. the growth rate of a diameter disturbance is given by (ll 1) 11,,1 where d is the size of the disturbance when the time 1 and 1(1) is the size of the disturbance when time is equal to or greater than 0. Ignoring viscosity. the coefficient a.,* is given in equation 1 19) on page 693 ofthe aforesaid article by where 0&#39; is the surface tension and is approximately 70 dyne cm for water. p is the density and is l gm cm for water. and a is the radius of the stream. For a jet diameter of 1 mil. a 0.5 mil 12.5 X 10 cm. Thus.  
 a,,* 170/: (12.5 X lu 0.134 x 10&#39;.  
  A disturbance of size 11,, doubles when a 2. Accordingly. a,,*! ln 2 so that I 1n 2/a,,*. With a 0.134 X 10&#34; r 5.17 X 10&#34; sec. Thus. for a 1 mil diameter jet of water. a diameter disturbance doubles in size in about 5 X sec.  
  Since the random breakup of a jet stream until it has droplet formation is about 100 microseconds after leaving a nozzle. it is necessary that the breakup of the stream into the controlled droplet formation occur before this time. Therefore. if the disturbance in the diameter of the stream were to be reduced front the point of droplet formation back to its exit from the nozzle by one-half every 5 microseconds since this is the opposite of doubling the disturbance every 5 microseconds from the nozzle to the point ofdroplet formation. the reduction of the diameter of the stream adjacent the outlet of the nozzle to obtain controlled droplet formation in 100 microseconds can be ascertained. Thus. /a 10 stream diameters so that a perturbation of 10 mil in a stream having a diameter of 1 mil is adequate to produce droplets of uniform spacing in 100 microseconds from this perturbation. Accordingly. a 3 X 10&#34; mil perturbation would completely dominate random disturbances and cause droplet formation in about 75 microseconds.  
  To ascertain ifthere will be sufficient heat flow in the water within this period of time. the 1 mil diameter nozzle. which supplies the jet stream of 1 mil diameter. is approximated by two plates being l mil apart and utilizing the temperature distribution in a plate as shown in FIG. 104 on page of Mathematical and Physical Principles of Engineering Analysis by Walter C. Johnson (First Edition. fourth impression). FIG. 104 shows the temperature distribution in a plate as a function of time starting with a uniform temperature in the plate and maintaining the surfaces of the plate thereafter at a constant temperature. which is different from the ini tial uniform temperature. There is a plurality of curves shown with each being for a different value of Br in which B is a constant and l is the time with B 1r k/L rp. In this equation. k is the thermal conductivity and is (1.04 X10 watt cm. K&#34; at 20 C. for water. L is the spacing between the plates so it is l mil. c represents the specific heat and is 4.18 watt sec gm T. for water. and p is the density and is equal to 1 gm emf for water. Thus. for water at 20 C. B equals 2.3 X 10 sec&#34;. When B! 10 the temperature from FIG. 10-1 of Mathematical Physical Principles of Engineering Analysis may be approximated by a straight line connecting the surface temperature 6 at 0 and .v/L 0.1. Thus. with B: 10*. r =4.35 X l0 see.  
  If the surface temperature of the water is raised C. above the interior. the heat energy put into a 1 mil length of a water stream. which has a diameter of l mil with the heat being applied to only the outer layer of the stream with a thickness of0.l mil. is the product of [20C X 0.5). (2.5 X10 cm). (1r X 2.5 X l0 cm). (2.5 X 10&#34; cm). and (4.18 watt sec gnf C&#34;&#39;) with (20C X 0.5) being the average temperature in the 0.1 mil layer of water. (2.5 X 10* cm) being the thickness of the outer layer of the water to which the heat is applied. (1r X 2.5 X 10 cm) being the width of the stream of water and is its circumference. (2.5 X 10 cm) being the 1 mil length of the stream of water to which the heat is applied. and (4.18 watt sec gm&#34; C &#39;1 being the specific heat of water. This produces a heat energy of 2.05 X 10&#39; watt sec. With this heat energy of 2.05 X 10&#39; watt sec being applied for 4.35 X 10&#34; seconds. the power input to the water during this time period is approximately 0.05 watt. with a 505i duty cycle and 50% efficiency in transferring heat from a thin film resistive heater to the water. a power input of 0.05 watt to the stream seems adequate.  
  Thus. sufficient heat energy can be supplied to the water stream to reduce its surface tension 2.4% within the necessary time. If a thin film of Nichrome (8091 nickel. 20V: chrome) with a thickness of l micron is used as the heater. of the heat therein is removed when B: equals 1.0 according to FIG. 104 of the Mathematical and Physical Principles of Engineering Analysis by Johnson. For Nichromc. B is approximately 0.3 X 10&#39;&#34; since k is equal to 0.12 watt cm. K. L is equal to 10* cm. (1 micron). and up is equal to approximately 4 watt sec emf. With B X 0.3 X 10 .115 33 X l0 sec. so that 70% 0f the heat would be removed from a 1 micron thick heater of Nichrome in 33 nanoseconds.  
  The resistance of the thin film resistive heater is obtained from the equation of R i&#39;l/A. In the equation. r is the resistivity of Nichrome and is X 10 ohm/meter. l is the length of the heater. and A is the area of the resistive heating element of the heater.  
  The resistive heater 35 in FIG. 4 can be considered as two resistive heaters of length rr 11/2 electrically in parallel where (I is the diameter of the nozzle. The nozzle diameter is 1 mil or 25 microns since it surrounds the stream of 1 mil diameter. Thus. each heater is about 37.5 microns long.  
  Since the heater should extend for the length of the segment of the stream to which the heat is applied. it would be 1 mil or 25 microns. and this is considered to be its width insofar as determining its resistance. The thickness of the heater is 1 micron since this is the thickness utilized in calculating B in Br for Nichrome. Thus. from R NH. the resistance R of each halfof the thin film resistive heater would be 1.5 ohms.  
  Accordingly. the thin film resistive heater is capable of supplying sufficient heat in 33 nanoseconds to reduce the surface tension of the water stream 2.4&#34; Since this is a very small portion of time in comparison with the available period of 5 microseconds to reduce the surface tension of the water stream 2.4%. the thin film resistive heater can be utilized effectively through modulating its supply of heat.  
  Additionally. with the resistance of 0,75 ohm of the heater 35 of FIG. 4. an application of \oltage of about 0.2 volt could he used and require only 267 milliamps to produce the necessary heat energy of 0.05 watt. Thus. no large current or voltage is required.  
  It should be understood that the calculations of the foregoing example are an approximation and could he an order of magnitude different. Thus. the heat required could be an order of magnitude different from that calculated but this still would not require a large current or voltage.  
  While the foregoing has discussed the stream as being formed of water. it should he understood that an ink stream could have a different surface tension. However. the same type of calculations would be made to obtain the necessary size of a thin film resistive heater such as the heater 35. for example.  
  While the period oftime for applying a source of heat or light to the stream can be for the same period oftime as it is not applied. it should be understood that such is not a requisite for satisfactory operation. Thus. the period oftime during which the energy is applied could he shorter or longer than the period of time during which the energy is not applied to the stream. it is only necessary that the formation of the droplets occur substantially before random break up of the droplets would occur and that the disturbance produced be greater than any natural disturbance of the stream.  
  The diameter of each formed droplet is a function of both the velocity and the diameter of the stream and the frequency with which the energy is applied to the stream. However. the quantity ofenergy does not affect the diameter of the droplet.  
  The quantity of the energy only determines the hreakoff point of the droplet from the stream. That is. as the quantity of the energy is increased. the hreakoff point of the droplet occurs closer to the opening of the nozzle. Of course. the quantity of energy must be suffi cient to produce synchronization of droplet formation.  
  It should be understood that the wave form of the modulated power applied to the stream can be of varying shapes such as a square wave pulse or a sine wave. for example. The specific shape of the power applied to the stream influences the formation of the droplet. and a particular power wave form can be chosen to minimize satellites during droplet formation.  
  An advantage of this invention is that substantially uniform spacing and breakoff of the droplets of an ink stream is obtained so that each droplet can he controlled as to whether it is applied to a recording surface and the area to which it is applied. Another advantage of this invention is that it has no effect on the jet stream of any adjacent nozzle whereby a plurality of ink streams can be disposed close to each other.  
  While the invention has been particularly shown and described with reference to preferred embodiments thereof. it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.  
 What is claimed is:  
  l. A method of forming droplets at a substantially constant breakoff point and with substantially uniform distances from each other from a liquid stream exiting from an opening or the like including:  
 selectively altering the surface tension of spaced segments of the stream by initially reducing the surface tension of each of the spaced segments of the stream as it passes a predetermined portion of its path and before random break up of the stream into droplets would occur after the stream leatcs the opening;  
 and controlling the amount of initial reduction of the surface tension of each of the spaced segments of the stream to control the breakot&#39;f point of the droplets from the stream at a substantially constant breakoff point and with substantially uniform distances from each other.  
 2. The method according to claim 1 including:  
 applying a source of energy directly to each of the spaced segments of the liquid stream to produce a thermal change in the spaced segments of the stream before random break up of the stream into droplets would occur after the stream leaves the opening:  
 periodically applying the source of energy to each of the spaced segments of the stream as it passes the predetermined portion of its path for a constant period of time to initially reduce the surface tension of the stream in the spaced segments of the stream to form droplets at substantially uniform distances from each other;  
 and controlling the quantity of energy applied from the source of energy during each constant period of time to control the breakoff point.  
  3. The method according to claim 2 including applying the source ofenergy to the spaced segments of the stream after the stream leaves the opening.  
  4. The method according to claim 3 including applying light from a high intensity light source as the source of energy to produce the thermal change in the stream.  
  5. The method according to claim 2 including controlling the quantity of energy applied during the constant period of time and controlling the time period of the constant period of time to obtain the desired synchronization and breakoff point of the droplets from the stream.  
  6. The method according to claim 2 including applying the source of energy to the spaced segments of the stream just before the stream exits from the opening.  
  7. The method according to claim 6 including applying the source of energy to the spaced segments of the stream around the entire periphery of the stream.  
  8. The method according to claim 7 including applying heat to the spaced segments of the stream as the source of energy.  
  9. The method according to claim 6 including applying the source of energy to the spaced segments of the stream around only a portion of the periphery of the stream.  
  10. The method according to claim 9 including applying heat to the spaced segments of the stream as the source of energy.  
  ll. An apparatus for forming droplets at a substantially constant breakoff point and with substantially uniform distances from each other from a liquid stream including:  
 means to supply the liquid stream through an opening or the like;  
 and means to selectively alter the surface tension of spaced segments ol the stream to form droplets at substantially uniform distances from each other and of substantially uniform size. said selecti\el altering means being applied to each of the spaced segments of the stream as it passes a predetermined portion of its path to initially reduce the surface tension of each of the spaced segments l el&#39;ore random break up of the stream into droplets would occur after the stream exits from the opening.  
  l2. The apparatus according to claim ll in which said selectively altering means includes means to periodically apply a source of energy directly to each ol&#39;the spaced segments ol the liquid stream for a constant period of time at the predetermined portion of its path to produce a thermal change in the stream to reduce the surface tension of each of the spaced segments of the stream to which the energy is applied.  
  IS. The apparatus according to claim T2 in which said applying means comprises energy supply means disposed exterior of the opening to apply energy to the spaced segments ofthe stream before random break up of the liquid stream into droplets would occur.  
 14. The apparatus according to claim U in which:  
 . 12 said energy supply means includes:  
 a high intensity light source: and means to modulate said high intensity light source to apply the light of said high intensity light source periodically for the constant period of time.  
  [5. The apparatus according to claim 14 in which said high intensity light source is a continuous light source.  
  [6. The apparatus according to claim [2 in which said applying means includes heating means disposed within the opening [7. The apparatus according to claim 16 in which said heating means is disposed adjacent the exit of the opening.  
  18. The apparatus according to claim 16 in which said heating means completely surrounds the liquid stream to apply heat to the entire periphery of each of the spaced segments of the liquid stream.  
  19. The apparatus according to claim [6 in which said heating means only partially surrounds the liquid stream to apply heat to only the surrounded portion of the periphery of each of the spaced segments of the liquid stream.