Patent Publication Number: US-2020276511-A1

Title: Ultrafine bubble generating apparatus, and ultrafine bubble generating method

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an ultrafine bubble generating method and an ultrafine bubble generating apparatus for generating ultrafine bubbles smaller than 1.0 μm in diameter, and an ultrafine bubble-containing liquid. 
     Description of the Related Art 
     Recently, there have been developed techniques for applying the features of fine bubbles such as microbubbles in micrometer-size in diameter and nanobubbles in nanometer-size in diameter. Especially, the utility of ultrafine bubbles (hereinafter also referred to as “UFBs”) smaller than 1.0 μm in diameter have been confirmed in various fields. 
     Japanese Patent No. 6118544 discloses a fine air bubble generating apparatus that generates fine bubbles by ejecting from a depressurizing nozzle a pressurized liquid in which a gas is pressurized and dissolved. Japanese Patent No. 4456176 discloses an apparatus that generates fine bubbles by repeating separating and converging of flows of a gas-mixed liquid with a mixing unit. 
     SUMMARY OF THE INVENTION 
     The present invention is made to solve the above-described problems. Therefore, an object of the present invention is to provide an ultrafine bubble generating apparatus and an ultrafine bubble generating method capable of efficiently generating a UFB-containing liquid with high purity. 
     The ultrafine bubble generating apparatus of the present invention is an ultrafine bubble generating apparatus for generating ultrafine bubbles, including an element substrate that includes a plurality of heaters that generate the ultrafine bubbles by heating a liquid and a wiring connected with the heaters, wherein a restriction member that restricts a growth of a bubble generated by an action of each heater is included at least in a part of surroundings of the heater, and a first region having a predetermined area is provided between the restriction member and the heater. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an example of a UFB generating apparatus; 
         FIG. 2  is a schematic configuration diagram of a pre-processing unit; 
         FIGS. 3A and 3B  are a schematic configuration diagram of a dissolving unit and a diagram for describing the dissolving states in a liquid; 
         FIG. 4  is a schematic configuration diagram of a T-UFB generating unit; 
         FIGS. 5A and 5B  are diagrams for describing details of a heating element; 
         FIGS. 6A and 6B  are diagrams for describing the states of film boiling on the heating element; 
         FIGS. 7A to 7D  are diagrams illustrating the states of generation of UFBs caused by expansion of a film boiling bubble; 
         FIGS. 8A to 8C  are diagrams illustrating the states of generation of UFBs caused by shrinkage of the film boiling bubble; 
         FIGS. 9A to 9C  are diagrams illustrating the states of generation of UFBs caused by reheating of the liquid; 
         FIGS. 10A and 10B  are diagrams illustrating the states of generation of UFBs caused by shock waves made by disappearance of the bubble generated by the film boiling; 
         FIGS. 11A to 11C  are diagrams illustrating a configuration example of a post-processing unit; 
         FIGS. 12A and 12B  are diagrams illustrating a chamber; 
         FIGS. 13A and 13B  are diagrams illustrating an element substrate; 
         FIG. 14  is diagram illustrating an element substrate on which the walls are formed; 
         FIGS. 15A and 15B  are diagrams illustrating a lid substrate; 
         FIGS. 16A and 16B  are diagrams illustrating a supply pipe connected to a supply port and a discharge pipe connected to a discharge port; 
         FIGS. 17A and 17B  are diagrams illustrating a state where an element substrate and flexible wiring substrates are electrically connected with each other; 
         FIGS. 18A to 18C  are diagrams illustrating a heater part provided with a heating element; 
         FIGS. 19A to 19C  are diagrams illustrating a state in which a liquid bubbles when a heating element is driven over time; 
         FIGS. 20A to 20K  are diagrams illustrating a process of forming an element substrate in order of process; 
         FIGS. 21A to 21H  are diagrams illustrating a process of forming an element substrate in order of process; 
         FIGS. 22A to 22D  are diagrams illustrating a process of forming a chamber in order of process; 
         FIGS. 23A to 23C  are diagrams illustrating a heater part on an element substrate; and 
         FIGS. 24A to 24C  are diagrams illustrating a heater part on an element substrate. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Both the apparatuses described in Japanese Patent Nos. 6118544 and 4456176 generate not only the UFBs of nanometer-size in diameter but also relatively a large number of milli-bubbles of millimeter-size in diameter and microbubbles of micrometer-size in diameter. However, because the milli-bubbles and the microbubbles are affected by the buoyancy, the bubbles are likely to gradually rise to the liquid surface and disappear during long-time storage. 
     On the other hand, the UFBs of nanometer-size in diameter are suitable for long-time storage since they are less likely to be affected by the buoyancy and float in the liquid with Brownian motion. However, when the UFBs are generated with the milli-bubbles and the microbubbles or the gas-liquid interface energy of the UFBs is small, the UFBs are affected by the disappearance of the milli-bubbles and the microbubbles and decreased over time. That is, in order to obtain a UFB-containing liquid in which the concentration reduction of the UFBs can be suppressed even during long-time storage, it is required to generate highly pure and highly concentrated UFBs with large gas-liquid interface energy when generating a UFB-containing liquid. 
     &lt;&lt;Configuration of UFB Generating Apparatus&gt;&gt; 
       FIG. 1  is a diagram illustrating an example of a UFB generating apparatus applicable to the present invention. A UFB generating apparatus  1  of this embodiment includes a pre-processing unit  100 , dissolving unit  200 , a T-UFB generating unit  300 , a post-processing unit  400 , and a collecting unit  500 . Each unit performs unique processing on a liquid W such as tap water supplied to the pre-processing unit  100  in the above order, and the thus-processed liquid W is collected as a T-UFB-containing liquid by the collecting unit  500 . Functions and configurations of the units are described below. Although details are described later, UFBs generated by utilizing the film boiling caused by rapid heating are referred to as thermal-ultrafine bubbles (T-UFBs) in this specification. 
       FIG. 2  is a schematic configuration diagram of the pre-processing unit  100 . The pre-processing unit  100  of this embodiment performs a degassing treatment on the supplied liquid W. The pre-processing unit  100  mainly includes a degassing container  101 , a shower head  102 , a depressurizing pump  103 , a liquid introduction passage  104 , a liquid circulation passage  105 , and a liquid discharge passage  106 . For example, the liquid W such as tap water is supplied to the degassing container  101  from the liquid introduction passage  104  through a valve  109 . In this process, the shower head  102  provided in the degassing container  101  sprays a mist of the liquid W in the degassing container  101 . The shower head  102  is for prompting the gasification of the liquid W; however, a centrifugal and the like may be used instead as the mechanism for producing the gasification prompt effect. 
     When a certain amount of the liquid W is reserved in the degassing container  101  and then the depressurizing pump  103  is activated with all the valves closed, already-gasified gas components are discharged, and gasification and discharge of gas components dissolved in the liquid W are also prompted. In this process, the internal pressure of the degassing container  101  may be depressurized to around several hundreds to thousands of Pa (1.0 Torr to 10.0 Torr) while checking a manometer  108 . The gases to be removed by the pre-processing unit  100  includes nitrogen, oxygen, argon, carbon dioxide, and so on, for example. 
     The above-described degassing processing can be repeatedly performed on the same liquid W by utilizing the liquid circulation passage  105 . Specifically, the shower head  102  is operated with the valve  109  of the liquid introduction passage  104  and a valve  110  of the liquid discharge passage  106  closed and a valve  107  of the liquid circulation passage  105  opened. This allows the liquid W reserved in the degassing container  101  and degassed once to be resprayed in the degassing container  101  from the shower head  102 . In addition, with the depressurizing pump  103  operated, the gasification processing by the shower head  102  and the degassing processing by the depressurizing pump  103  are repeatedly performed on the same liquid W. Every time the above processing utilizing the liquid circulation passage  105  is performed repeatedly, it is possible to decrease the gas components contained in the liquid W in stages. Once the liquid W degassed to a desired purity is obtained, the liquid W is transferred to the dissolving unit  200  through the liquid discharge passage  106  with the valve  110  opened. 
       FIG. 2  illustrates the degassing unit  100  that depressurizes the gas part to gasify the solute; however, the method of degassing the solution is not limited thereto. For example, a heating and boiling method for boiling the liquid W to gasify the solute may be employed, or a film degassing method for increasing the interface between the liquid and the gas using hollow fibers. A SEPAREL series (produced by DIC corporation) is commercially supplied as the degassing module using the hollow fibers. The SEPAREL series uses poly(4-methylpentene-1) (PMP) for the raw material of the hollow fibers and is used for removing air bubbles from ink and the like mainly supplied for a piezo head. In addition, two or more of an evacuating method, the heating and boiling method, and the film degassing method may be used together. 
       FIGS. 3A and 3B  are a schematic configuration diagram of the dissolving unit  200  and a diagram for describing the dissolving states in the liquid. The dissolving unit  200  is a unit for dissolving a desired gas into the liquid W supplied from the pre-processing unit  100 . The dissolving unit  200  of this embodiment mainly includes a dissolving container  201 , a rotation shaft  203  provided with a rotation plate  202 , a liquid introduction passage  204 , a gas introduction passage  205 , a liquid discharge passage  206 , and a pressurizing pump  207 . 
     The liquid W supplied from the pre-processing unit  100  is supplied and reserved into the dissolving container  201  through the liquid introduction passage  204 . Meanwhile, a gas G is supplied to the dissolving container  201  through the gas introduction passage  205 . 
     Once predetermined amounts of the liquid W and the gas G are reserved in the dissolving container  201 , the pressurizing pump  207  is activated to increase the internal pressure of the dissolving container  201  to about 0.5 MPa. A safety valve  208  is arranged between the pressurizing pump  207  and the dissolving container  201 . With the rotation plate  202  in the liquid rotated via the rotation shaft  203 , the gas G supplied to the dissolving container  201  is transformed into air bubbles, and the contact area between the gas G and the liquid W is increased to prompt the dissolution into the liquid W. This operation is continued until the solubility of the gas G reaches almost the maximum saturation solubility. In this case, a unit for decreasing the temperature of the liquid may be provided to dissolve the gas as much as possible. When the gas is with low solubility, it is also possible to increase the internal pressure of the dissolving container  201  to 0.5 MPa or higher. In this case, the material and the like of the container need to be the optimum for safety sake. 
     Once the liquid Win which the components of the gas G are dissolved at a desired concentration is obtained, the liquid W is discharged through the liquid discharge passage  206  and supplied to the T-UFB generating unit  300 . In this process, a back-pressure valve  209  adjusts the flow pressure of the liquid W to prevent excessive increase of the pressure during the supplying. 
       FIG. 3B  is a diagram schematically illustrating the dissolving states of the gas G put in the dissolving container  201 . An air bubble  2  containing the components of the gas G put in the liquid W is dissolved from a portion in contact with the liquid W. The air bubble  2  thus shrinks gradually, and a gas-dissolved liquid  3  then appears around the air bubble  2 . Since the air bubble  2  is affected by the buoyancy, the air bubble  2  may be moved to a position away from the center of the gas-dissolved liquid  3  or be separated out from the gas-dissolved liquid  3  to become a residual air bubble  4 . Specifically, in the liquid W to be supplied to the T-UFB generating unit  300  through the liquid discharge passage  206 , there is a mix of the air bubbles  2  surrounded by the gas-dissolved liquids  3  and the air bubbles  2  and the gas-dissolved liquids  3  separated from each other. 
     The gas-dissolved liquid  3  in the drawings means “a region of the liquid W in which the dissolution concentration of the gas G mixed therein is relatively high.” In the gas components actually dissolved in the liquid W, the concentration of the gas components in the gas-dissolved liquid  3  is the highest at a portion surrounding the air bubble  2 . In a case where the gas-dissolved liquid  3  is separated from the air bubble  2  the concentration of the gas components of the gas-dissolved liquid  3  is the highest at the center of the region, and the concentration is continuously decreased as away from the center. That is, although the region of the gas-dissolved liquid  3  is surrounded by a broken line in  FIG. 3  for the sake of explanation, such a clear boundary does not actually exist. In addition, in the present invention, a gas that cannot be dissolved completely may be accepted to exist in the form of an air bubble in the liquid. 
       FIG. 4  is a schematic configuration diagram of the T-UFB generating unit  300 . The T-UFB generating unit  300  mainly includes a chamber  301 , a liquid introduction passage  302 , and a liquid discharge passage  303 . The flow from the liquid introduction passage  302  to the liquid discharge passage  303  through the chamber  301  is formed by a not-illustrated flow pump. Various pumps including a diaphragm pump, a gear pump, and a screw pump may be employed as the flow pump. In in the liquid W introduced from the liquid introduction passage  302 , the gas-dissolved liquid  3  of the gas G put by the dissolving unit  200  is mixed. 
     An element substrate  12  provided with a heating element  10  is arranged on a bottom section of the chamber  301 . With a predetermined voltage pulse applied to the heating element  10 , a bubble  13  generated by the film boiling (hereinafter, also referred to as a film boiling bubble  13 ) is generated in a region in contact with the heating element  10 . Then, an ultrafine bubble (UFB)  11  containing the gas G is generated caused by expansion and shrinkage of the film boiling bubble  13 . As a result, a UFB-containing liquid W containing many UFBs  11  is discharged from the liquid discharge passage  303 . 
       FIGS. 5A and 5B  are diagrams for illustrating a detailed configuration of the heating element  10 .  FIG. 5A  illustrates a closeup view of the heating element  10 , and  FIG. 5B  illustrates a cross-sectional view of a wider region of the element substrate  12  including the heating element  10 . 
     As illustrated in  FIG. 5A , in the element substrate  12  of this embodiment, a thermal oxide film  305  as a heat-accumulating layer and an interlaminar film  306  also served as a heat-accumulating layer are laminated on a surface of a silicon substrate  304 . An SiO2 film or an SiN film may be used as the interlaminar film  306 . A resistive layer  307  is formed on a surface of the interlaminar film  306 , and a wiring  308  is partially formed on a surface of the resistive layer  307 . An Al-alloy wiring of Al, Al—Si, Al—Cu, or the like may be used as the wiring  308 . A protective layer  309  made of an SiO2 film or an Si3N4 film is formed on surfaces of the wiring  308 , the resistive layer  307 , and the interlaminar film  306 . 
     A cavitation-resistant film  310  for protecting the protective layer  309  from chemical and physical impacts due to the heat evolved by the resistive layer  307  is formed on a portion and around the portion on the surface of the protective layer  309 , the portion corresponding to a heat-acting portion  311  that eventually becomes the heating element  10 . A region on the surface of the resistive layer  307  in which the wiring  308  is not formed is the heat-acting portion  311  in which the resistive layer  307  evolves heat. The heating portion of the resistive layer  307  on which the wiring  308  is not formed functions as the heating element (heater)  10 . As described above, the layers in the element substrate  12  are sequentially formed on the surface of the silicon substrate  304  by a semiconductor production technique, and the heat-acting portion  311  is thus provided on the silicon substrate  304 . 
     The configuration illustrated in the drawings is an example, and various other configurations are applicable. For example, a configuration in which the laminating order of the resistive layer  307  and the wiring  308  is opposite, and a configuration in which an electrode is connected to a lower surface of the resistive layer  307  (so-called a plug electrode configuration) are applicable. In other words, as described later, any configuration may be applied as long as the configuration allows the heat-acting portion  311  to heat the liquid for generating the film boiling in the liquid. 
       FIG. 5B  is an example of a cross-sectional view of a region including a circuit connected to the wiring  308  in the element substrate  12 . An N-type well region  322  and a P-type well region  323  are partially provided in a top layer of the silicon substrate  304 , which is a P-type conductor. AP-MOS  320  is formed in the N-type well region  322  and an N-MOS  321  is formed in the P-type well region  323  by introduction and diffusion of impurities by the ion implantation and the like in the general MOS process. 
     The P-MOS  320  includes a source region  325  and a drain region  326  formed by partial introduction of N-type or P-type impurities in a top layer of the N-type well region  322 , a gate wiring  335 , and so on. The gate wiring  335  is deposited on a part of a top surface of the N-type well region  322  excluding the source region  325  and the drain region  326 , with a gate insulation film  328  of several hundreds of A in thickness interposed between the gate wiring  335  and the top surface of the N-type well region  322 . 
     The N-MOS  321  includes the source region  325  and the drain region  326  formed by partial introduction of N-type or P-type impurities in a top layer of the P-type well region  323 , the gate wiring  335 , and so on. The gate wiring  335  is deposited on a part of a top surface of the P-type well region  323  excluding the source region  325  and the drain region  326 , with the gate insulation film  328  of several hundreds of A in thickness interposed between the gate wiring  335  and the top surface of the P-type well region  323 . The gate wiring  335  is made of polysilicon of 3000 Å to 5000 Å in thickness deposited by the CVD method. A C-MOS logic is constructed with the P-MOS  320  and the N-MOS  321 . 
     In the P-type well region  323 , an N-MOS transistor  330  for driving an electrothermal conversion element (heating resistance element) is formed on a portion different from the portion including the N-MOS  321 . The N-MOS transistor  330  includes a source region  332  and a drain region  331  partially provided in the top layer of the P-type well region  323  by the steps of introduction and diffusion of impurities, a gate wiring  333 , and so on. The gate wiring  333  is deposited on a part of the top surface of the P-type well region  323  excluding the source region  332  and the drain region  331 , with the gate insulation film  328  interposed between the gate wiring  333  and the top surface of the P-type well region  323 . 
     In this example, the N-MOS transistor  330  is used as the transistor for driving the electrothermal conversion element. However, the transistor for driving is not limited to the N-MOS transistor  330 , and any transistor may be used as long as the transistor has a capability of driving multiple electrothermal conversion elements individually and can implement the above-described fine configuration. Although the electrothermal conversion element and the transistor for driving the electrothermal conversion element are formed on the same substrate in this example, those may be formed on different substrates separately. 
     An oxide film separation region  324  is formed by field oxidation of 5000 Å to 10000 Å in thickness between the elements, such as between the P-MOS  320  and the N-MOS  321  and between the N-MOS  321  and the N-MOS transistor  330 . The oxide film separation region  324  separates the elements. A portion of the oxide film separation region  324  corresponding to the heat-acting portion  311  functions as a heat-accumulating layer  334 , which is the first layer on the silicon substrate  304 . 
     An interlayer insulation film  336  including a PSG film, a BPSG film, or the like of about 7000 Å in thickness is formed by the CVD method on each surface of the elements such as the P-MOS  320 , the N-MOS  321 , and the N-MOS transistor  330 . After the interlayer insulation film  336  is made flat by heat treatment, an Al electrode  337  as a first wiring layer is formed in a contact hole penetrating through the interlayer insulation film  336  and the gate insulation film  328 . On surfaces of the interlayer insulation film  336  and the Al electrode  337 , an interlayer insulation film  338  including an SiO2 film of 10000 Å to 15000 Å in thickness is formed by a plasma CVD method. On the surface of the interlayer insulation film  338 , a resistive layer  307  including a TaSiN film of about 500 Å in thickness is formed by a co-sputter method on portions corresponding to the heat-acting portion  311  and the N-MOS transistor  330 . The resistive layer  307  is electrically connected with the Al electrode  337  near the drain region  331  via a through-hole formed in the interlayer insulation film  338 . On the surface of the resistive layer  307 , the wiring  308  of Al as a second wiring layer for a wiring to each electrothermal conversion element is formed. The protective layer  309  on the surfaces of the wiring  308 , the resistive layer  307 , and the interlayer insulation film  338  includes an SiN film of 3000 Å in thickness formed by the plasma CVD method. The cavitation-resistant film  310  deposited on the surface of the protective layer  309  includes a thin film of about 2000 Å in thickness, which is at least one metal selected from the group consisting of Ta, Fe, Ni, Cr, Ge, Ru, Zr, Ir, and the like. Various materials other than the above-described TaSiN such as TaN0.8, CrSiN, TaAl, WSiN, and the like can be applied as long as the material can generate the film boiling in the liquid. 
       FIGS. 6A and 6B  are diagrams illustrating the states of the film boiling when a predetermined voltage pulse is applied to the heating element  10 . In this case, the case of generating the film boiling under atmospheric pressure is described. In  FIG. 6A , the horizontal axis represents time. The vertical axis in the lower graph represents a voltage applied to the heating element  10 , and the vertical axis in the upper graph represents the volume and the internal pressure of the film boiling bubble  13  generated by the film boiling. On the other hand,  FIG. 6B  illustrates the states of the film boiling bubble  13  in association with timings  1  to  3  shown in  FIG. 6A . Each of the states is described below in chronological order. The UFBs  11  generated by the film boiling as described later are mainly generated near a surface of the film boiling bubble  13 . The states illustrated in  FIG. 6B  are the states where the UFBs  11  generated by the generating unit  300  are resupplied to the dissolving unit  200  through the circulation route, and the liquid containing the UFBs  11  is resupplied to the liquid passage of the generating unit  300 , as illustrated in  FIG. 1 . 
     Before a voltage is applied to the heating element  10 , the atmospheric pressure is substantially maintained in the chamber  301 . Once a voltage is applied to the heating element  10 , the film boiling is generated in the liquid in contact with the heating element  10 , and a thus-generated air bubble (hereinafter, referred to as the film boiling bubble  13 ) is expanded by a high pressure acting from inside (timing  1 ). A bubbling pressure in this process is expected to be around 8 to 10 MPa, which is a value close to a saturation vapor pressure of water. 
     The time for applying a voltage (pulse width) is around 0.5 μsec to 10.0 μsec, and the film boiling bubble  13  is expanded by the inertia of the pressure obtained in timing  1  even after the voltage application. However, a negative pressure generated with the expansion is gradually increased inside the film boiling bubble  13 , and the negative pressure acts in a direction to shrink the film boiling bubble  13 . After a while, the volume of the film boiling bubble  13  becomes the maximum in timing  2  when the inertial force and the negative pressure are balanced, and thereafter the film boiling bubble  13  shrinks rapidly by the negative pressure. 
     In the disappearance of the film boiling bubble  13 , the film boiling bubble  13  disappears not in the entire surface of the heating element  10  but in one or more extremely small regions. For this reason, on the heating element  10 , further greater force than that in the bubbling in timing  1  is generated in the extremely small region in which the film boiling bubble  13  disappears (timing  3 ). 
     The generation, expansion, shrinkage, and disappearance of the film boiling bubble  13  as described above are repeated every time a voltage pulse is applied to the heating element  10 , and new UFBs  11  are generated each time. 
     The states of generation of the UFBs  11  in each process of the generation, expansion, shrinkage, and disappearance of the film boiling bubble  13  are further described in detail with reference to  FIGS. 7A to 10B . 
       FIGS. 7A to 7D  are diagrams schematically illustrating the states of generation of the UFBs  11  caused by the generation and the expansion of the film boiling bubble  13 .  FIG. 7A  illustrates the state before the application of a voltage pulse to the heating element  10 . The liquid W in which the gas-dissolved liquids  3  are mixed flows inside the chamber  301 . 
       FIG. 7B  illustrates the state where a voltage is applied to the heating element  10 , and the film boiling bubble  13  is evenly generated in almost all over the region of the heating element  10  in contact with the liquid W. When a voltage is applied, the surface temperature of the heating element  10  rapidly increases at a speed of 10° C./μsec. The film boiling occurs at a time point when the temperature reaches almost 300° C., and the film boiling bubble  13  is thus generated. 
     Thereafter, the surface temperature of the heating element  10  keeps increasing to around 600 to 800° C. during the pulse application, and the liquid around the film boiling bubble  13  is rapidly heated as well. In  FIG. 7B , a region of the liquid that is around the film boiling bubble  13  and to be rapidly heated is indicated as a not-yet-bubbling high temperature region  14 . The gas-dissolved liquid  3  within the not-yet-bubbling high temperature region  14  exceeds the thermal dissolution limit and is vaporized to become the UFB. The thus-vaporized air bubbles have diameters of around 10 nm to 100 nm and large gas-liquid interface energy. Thus, the air bubbles float independently in the liquid W without disappearing in a short time. In this embodiment, the air bubbles generated by the thermal action from the generation to the expansion of the film boiling bubble  13  are called first UFBs  11 A. 
       FIG. 7C  illustrates the state where the film boiling bubble  13  is expanded. Even after the voltage pulse application to the heating element  10 , the film boiling bubble  13  continues expansion by the inertia of the force obtained from the generation thereof, and the not-yet-bubbling high temperature region  14  is also moved and spread by the inertia. Specifically, in the process of the expansion of the film boiling bubble  13 , the gas-dissolved liquid  3  within the not-yet-bubbling high temperature region  14  is vaporized as a new air bubble and becomes the first UFB  11 A. 
       FIG. 7D  illustrates the state where the film boiling bubble  13  has the maximum volume. As the film boiling bubble  13  is expanded by the inertia, the negative pressure inside the film boiling bubble  13  is gradually increased along with the expansion, and the negative pressure acts to shrink the film boiling bubble  13 . At a time point when the negative pressure and the inertial force are balanced, the volume of the film boiling bubble  13  becomes the maximum, and then the shrinkage is started. 
     In the shrinking stage of the film boiling bubble  13 , there are UFBs generated by the processes illustrated in  FIGS. 8A to 8C  (second UFBs  11 B) and UFBs generated by the processes illustrated in  FIGS. 9A to 9C  (third UFBs  11 C). It is considered that these two processes are made simultaneously. 
       FIGS. 8A to 8C  are diagrams illustrating the states of generation of the UFBs  11  caused by the shrinkage of the film boiling bubble  13 .  FIG. 8A  illustrates the state where the film boiling bubble  13  starts shrinking. Although the film boiling bubble  13  starts shrinking, the surrounding liquid W still has the inertial force in the expansion direction. Because of this, the inertial force acting in the direction of going away from the heating element  10  and the force going toward the heating element  10  caused by the shrinkage of the film boiling bubble  13  act in a surrounding region extremely close to the film boiling bubble  13 , and the region is depressurized. The region is indicated in the drawings as a not-yet-bubbling negative pressure region  15 . 
     The gas-dissolved liquid  3  within the not-yet-bubbling negative pressure region  15  exceeds the pressure dissolution limit and is vaporized to become an air bubble. The thus-vaporized air bubbles have diameters of about 100 nm and thereafter float independently in the liquid W without disappearing in a short time. In this embodiment, the air bubbles vaporized by the pressure action during the shrinkage of the film boiling bubble  13  are called the second UFBs  11 B. 
       FIG. 8B  illustrates a process of the shrinkage of the film boiling bubble  13 . The shrinking speed of the film boiling bubble  13  is accelerated by the negative pressure, and the not-yet-bubbling negative pressure region  15  is also moved along with the shrinkage of the film boiling bubble  13 . Specifically, in the process of the shrinkage of the film boiling bubble  13 , the gas-dissolved liquids  3  within a part over the not-yet-bubbling negative pressure region  15  are precipitated one after another and become the second UFBs  11 B. 
       FIG. 8C  illustrates the state immediately before the disappearance of the film boiling bubble  13 . Although the moving speed of the surrounding liquid W is also increased by the accelerated shrinkage of the film boiling bubble  13 , a pressure loss occurs due to a flow passage resistance in the chamber  301 . As a result, the region occupied by the not-yet-bubbling negative pressure region  15  is further increased, and a number of the second UFBs  11 B are generated. 
       FIGS. 9A to 9C  are diagrams illustrating the states of generation of the UFBs by reheating of the liquid W during the shrinkage of the film boiling bubble  13 .  FIG. 9A  illustrates the state where the surface of the heating element  10  is covered with the shrinking film boiling bubble  13 . 
       FIG. 9B  illustrates the state where the shrinkage of the film boiling bubble  13  has progressed, and a part of the surface of the heating element  10  comes in contact with the liquid W. In this state, there is heat left on the surface of the heating element  10 , but the heat is not high enough to cause the film boiling even if the liquid W comes in contact with the surface. A region of the liquid to be heated by coming in contact with the surface of the heating element  10  is indicated in the drawings as a not-yet-bubbling reheated region  16 . Although the film boiling is not made, the gas-dissolved liquid  3  within the not-yet-bubbling reheated region  16  exceeds the thermal dissolution limit and is vaporized. In this embodiment, the air bubbles generated by the reheating of the liquid W during the shrinkage of the film boiling bubble  13  are called the third UFBs  11 C. 
       FIG. 9C  illustrates the state where the shrinkage of the film boiling bubble  13  has further progressed. The smaller the film boiling bubble  13 , the greater the region of the heating element  10  in contact with the liquid W, and the third UFBs  11 C are generated until the film boiling bubble  13  disappears. 
       FIGS. 10A and 10B  are diagrams illustrating the states of generation of the UFBs caused by an impact from the disappearance of the film boiling bubble  13  generated by the film boiling (that is, a type of cavitation).  FIG. 10A  illustrates the state immediately before the disappearance of the film boiling bubble  13 . In this state, the film boiling bubble  13  shrinks rapidly by the internal negative pressure, and the not-yet-bubbling negative pressure region  15  surrounds the film boiling bubble  13 . 
       FIG. 10B  illustrates the state immediately after the film boiling bubble  13  disappears at a point P. When the film boiling bubble  13  disappears, acoustic waves ripple concentrically from the point P as a starting point due to the impact of the disappearance. The acoustic wave is a collective term of an elastic wave that is propagated through anything regardless of gas, liquid, and solid. In this embodiment, compression waves of the liquid W, which are a high pressure surface  17 A and a low pressure surface  17 B of the liquid W, are propagated alternately. 
     In this case, the gas-dissolved liquid  3  within the not-yet-bubbling negative pressure region  15  is resonated by the shock waves made by the disappearance of the film boiling bubble  13 , and the gas-dissolved liquid  3  exceeds the pressure dissolution limit and the phase transition is made in timing when the low pressure surface  17 B passes therethrough. Specifically, a number of air bubbles are vaporized in the not-yet-bubbling negative pressure region  15  simultaneously with the disappearance of the film boiling bubble  13 . In this embodiment, the air bubbles generated by the shock waves made by the disappearance of the film boiling bubble  13  are called fourth UFBs  11 D. 
     The fourth UFBs  11 D generated by the shock waves made by the disappearance of the film boiling bubble  13  suddenly appear in an extremely short time (1 μS or less) in an extremely narrow thin film-shaped region. The diameter is sufficiently smaller than that of the first to third UFBs, and the gas-liquid interface energy is higher than that of the first to third UFBs. For this reason, it is considered that the fourth UFBs  11 D have different characteristics from the first to third UFBs  11 A to  11 C and generate different effects. 
     Additionally, the fourth UFBs  11 D are evenly generated in many parts of the region of the concentric sphere in which the shock waves are propagated, and the fourth UFBs  11 D evenly exist in the chamber  301  from the generation thereof. Although many first to third UFBs already exist in the timing of the generation of the fourth UFBs  11 D, the presence of the first to third UFBs does not affect the generation of the fourth UFBs  11 D greatly. It is also considered that the first to third UFBs do not disappear due to the generation of the fourth UFBs  11 D. 
     As described above, it is expected that the UFBs  11  are generated in the multiple stages from the generation to the disappearance of the film boiling bubble  13  by the heat generation of the heating element  10 . The first UFBs  11 A, the second UFBs  11 B, and the third UFBs  11 C are generated near the surface of the film boiling bubble generated by the film boiling. In this case, near means a region within about 20 μm from the surface of the film boiling bubble. The fourth UFBs  11 D are generated in a region through which the shock waves are propagated when the air bubble disappears. Although the above example illustrates the stages to the disappearance of the film boiling bubble  13 , the way of generating the UFBs is not limited thereto. For example, with the generated film boiling bubble  13  communicating with the atmospheric air before the bubble disappearance, the UFBs can be generated also if the film boiling bubble  13  does not reach the disappearance. 
     Next, remaining properties of the UFBs are described. The higher the temperature of the liquid, the lower the dissolution properties of the gas components, and the lower the temperature, the higher the dissolution properties of the gas components. In other words, the phase transition of the dissolved gas components is prompted and the generation of the UFBs becomes easier as the temperature of the liquid is higher. The temperature of the liquid and the solubility of the gas are in the inverse relationship, and the gas exceeding the saturation solubility is transformed into air bubbles and appeared in the liquid as the liquid temperature increases. 
     Therefore, when the temperature of the liquid rapidly increases from normal temperature, the dissolution properties are decreased without stopping, and the generation of the UFBs starts. The thermal dissolution properties are decreased as the temperature increases, and a number of the UFBs are generated. 
     Conversely, when the temperature of the liquid decreases from normal temperature, the dissolution properties of the gas are increased, and the generated UFBs are more likely to be liquefied. However, such temperature is sufficiently lower than normal temperature. Additionally, since the once generated UFBs have a high internal pressure and large gas-liquid interface energy even when the temperature of the liquid decreases, it is highly unlikely that there is exerted a sufficiently high pressure to break such a gas-liquid interface. In other words, the once generated UFBs do not disappear easily as long as the liquid is stored at normal temperature and normal pressure. 
     In this embodiment, the first UFBs  11 A described with  FIGS. 7A to 7C  and the third UFBs  11 C described with  FIGS. 9A to 9C  can be described as UFBs that are generated by utilizing such thermal dissolution properties of gas. 
     On the other hand, in the relationship between the pressure and the dissolution properties of liquid, the higher the pressure of the liquid, the higher the dissolution properties of the gas, and the lower the pressure, the lower the dissolution properties. In other words, the phase transition to the gas of the gas-dissolved liquid dissolved in the liquid is prompted and the generation of the UFBs becomes easier as the pressure of the liquid is lower. Once the pressure of the liquid becomes lower than normal pressure, the dissolution properties are decreased instantly, and the generation of the UFBs starts. The pressure dissolution properties are decreased as the pressure decreases, and a number of the UFBs are generated. 
     Conversely, when the pressure of the liquid increases to be higher than normal temperature, the dissolution properties of the gas are increased, and the generated UFBs are more likely to be liquefied. However, such pressure is sufficiently higher than the atmospheric pressure. Additionally, since the once generated UFBs have a high internal pressure and large gas-liquid interface energy even when the pressure of the liquid increases, it is highly unlikely that there is exerted a sufficiently high pressure to break such a gas-liquid interface. In other words, the once generated UFBs do not disappear easily as long as the liquid is stored at normal temperature and normal pressure. 
     In this embodiment, the second UFBs  11 B described with  FIGS. 8A to 8C  and the fourth UFBs  11 D described with  FIGS. 10A to 10C  can be described as UFBs that are generated by utilizing such pressure dissolution properties of gas. 
     Those first to fourth UFBs generated by different causes are described individually above; however, the above-described generation causes occur simultaneously with the event of the film boiling. Thus, at least two types of the first to the fourth UFBs may be generated at the same time, and these generation causes may cooperate to generate the UFBs. It should be noted that it is common for all the generation causes to be induced by the volume change of the film boiling bubble generated by the film boiling phenomenon. In this specification, the method of generating the UFBs by utilizing the film boiling caused by the rapid heating as described above is referred to as a thermal-ultrafine bubble (T-UFB) generating method. Additionally, the UFBs generated by the T-UFB generating method are referred to as T-UFBs, and the liquid containing the T-UFBs generated by the T-UFB generating method is referred to as a T-UFB-containing liquid. 
     Almost all the air bubbles generated by the T-UFB generating method are 1.0 μm or less, and milli-bubbles and microbubbles are unlikely to be generated. That is, the T-UFB generating method allows dominant and efficient generation of the UFBs. Additionally, the T-UFBs generated by the T-UFB generating method have larger gas-liquid interface energy than that of the UFBs generated by a conventional method, and the T-UFBs do not disappear easily as long as being stored at normal temperature and normal pressure. Moreover, even if new T-UFBs are generated by new film boiling, it is possible to prevent disappearance of the already generated T-UFBs due to the impact from the new generation. That is, it can be said that the number and the concentration of the T-UFBs contained in the T-UFB-containing liquid have the hysteresis properties depending on the number of times the film boiling is made in the T-UFB-containing liquid. In other words, it is possible to adjust the concentration of the T-UFBs contained in the T-UFB-containing liquid by controlling the number of the heating elements provided in the T-UFB generating unit  300  and the number of the voltage pulse application to the heating elements. 
     Reference to  FIG. 1  is made again. Once the T-UFB-containing liquid W with a desired UFB concentration is generated in the T-UFB generating unit  300 , the UFB-containing liquid W is supplied to the post-processing unit  400 . 
       FIGS. 11A to 11C  are diagrams illustrating configuration examples of the post-processing unit  400  of this embodiment. The post-processing unit  400  of this embodiment removes impurities in the UFB-containing liquid W in stages in the order from inorganic ions, organic substances, and insoluble solid substances. 
       FIG. 11A  illustrates a first post-processing mechanism  410  that removes the inorganic ions. The first post-processing mechanism  410  includes an exchange container  411 , cation exchange resins  412 , a liquid introduction passage  413 , a collecting pipe  414 , and a liquid discharge passage  415 . The exchange container  411  stores the cation exchange resins  412 . The UFB-containing liquid W generated by the T-UFB generating unit  300  is injected to the exchange container  411  through the liquid introduction passage  413  and absorbed into the cation exchange resins  412  such that the cations as the impurities are removed. Such impurities include metal materials peeled off from the element substrate  12  of the T-UFB generating unit  300 , such as SiO2, SiN, SiC, Ta, Al2O3, Ta2O5, and Ir. 
     The cation exchange resins  412  are synthetic resins in which a functional group (ion exchange group) is introduced in a high polymer matrix having a three-dimensional network, and the appearance of the synthetic resins are spherical particles of around 0.4 to 0.7 mm. A general high polymer matrix is the styrene-divinylbenzene copolymer, and the functional group may be that of methacrylic acid series and acrylic acid series, for example. However, the above material is an example. As long as the material can remove desired inorganic ions effectively, the above material can be changed to various materials. The UFB-containing liquid W absorbed in the cation exchange resins  412  to remove the inorganic ions is collected by the collecting pipe  414  and transferred to the next step through the liquid discharge passage  415 . In this process in the present embodiment, not all the inorganic ions contained in the UFB-containing liquid W supplied from the liquid introduction passage  413  need to be removed as long as at least a part of the inorganic ions are removed. 
       FIG. 11B  illustrates a second post-processing mechanism  420  that removes the organic substances. The second post-processing mechanism  420  includes a storage container  421 , a filtration filter  422 , a vacuum pump  423 , a valve  424 , a liquid introduction passage  425 , a liquid discharge passage  426 , and an air suction passage  427 . Inside of the storage container  421  is divided into upper and lower two regions by the filtration filter  422 . The liquid introduction passage  425  is connected to the upper region of the upper and lower two regions, and the air suction passage  427  and the liquid discharge passage  426  are connected to the lower region thereof. Once the vacuum pump  423  is driven with the valve  424  closed, the air in the storage container  421  is discharged through the air suction passage  427  to make the pressure inside the storage container  421  negative pressure, and the UFB-containing liquid W is thereafter introduced from the liquid introduction passage  425 . Then, the UFB-containing liquid W from which the impurities are removed by the filtration filter  422  is reserved into the storage container  421 . 
     The impurities removed by the filtration filter  422  include organic materials that may be mixed at a tube or each unit, such as organic compounds including silicon, siloxane, and epoxy, for example. A filter film usable for the filtration filter  422  includes a filter of a sub-μm-mesh (a filter of 1 μm or smaller in mesh diameter) that can remove bacteria, and a filter of a nm-mesh that can remove virus. The filtration filter having such a fine opening diameter may remove air bubbles larger than the opening diameter of the filter. Particularly, there may be the case where the filter is clogged by the fine air bubbles adsorbed to the openings (mesh) of the filter, which may slowdown the filtering speed. However, as described above, most of the air bubbles generated by the T-UFB generating method described in the present embodiment of the invention are in the size of 1 μm or smaller in diameter, and milli-bubbles and microbubbles are not likely to be generated. That is, since the probability of generating milli-bubbles and microbubbles is extremely low, it is possible to suppress the slowdown in the filtering speed due to the adsorption of the air bubbles to the filter. For this reason, it is favorable to apply the filtration filter  422  provided with the filter of 1 μm or smaller in mesh diameter to the system having the T-UFB generating method. 
     Examples of the filtration applicable to this embodiment may be a so-called dead-end filtration and cross-flow filtration. In the dead-end filtration, the direction of the flow of the supplied liquid and the direction of the flow of the filtration liquid passing through the filter openings are the same, and specifically, the directions of the flows are made along with each other. In contrast, in the cross-flow filtration, the supplied liquid flows in a direction along a filter surface, and specifically, the direction of the flow of the supplied liquid and the direction of the flow of the filtration liquid passing through the filter openings are crossed with each other. It is preferable to apply the cross-flow filtration to suppress the adsorption of the air bubbles to the filter openings. 
     After a certain amount of the UFB-containing liquid W is reserved in the storage container  421 , the vacuum pump  423  is stopped and the valve  424  is opened to transfer the T-UFB-containing liquid in the storage container  421  to the next step through the liquid discharge passage  426 . Although the vacuum filtration method is employed as the method of removing the organic impurities herein, a gravity filtration method and a pressurized filtration can also be employed as the filtration method using a filter, for example. 
       FIG. 11C  illustrates a third post-processing mechanism  430  that removes the insoluble solid substances. The third post-processing mechanism  430  includes a precipitation container  431 , a liquid introduction passage  432 , a valve  433 , and a liquid discharge passage  434 . 
     First, a predetermined amount of the UFB-containing liquid W is reserved into the precipitation container  431  through the liquid introduction passage  432  with the valve  433  closed, and leaving it for a while. Meanwhile, the solid substances in the UFB-containing liquid W are precipitated onto the bottom of the precipitation container  431  by gravity. Among the bubbles in the UFB-containing liquid, relatively large bubbles such as microbubbles are raised to the liquid surface by the buoyancy and also removed from the UFB-containing liquid. After a lapse of sufficient time, the valve  433  is opened, and the UFB-containing liquid W from which the solid substances and large bubbles are removed is transferred to the collecting unit  500  through the liquid discharge passage  434 . The example of applying the three post-processing mechanisms in sequence is shown in this embodiment; however, it is not limited thereto, and the order of the three post-processing mechanisms may be changed, or at least one needed post-processing mechanism may be employed. 
     Reference to  FIG. 1  is made again. The T-UFB-containing liquid W from which the impurities are removed by the post-processing unit  400  may be directly transferred to the collecting unit  500  or may be put back to the dissolving unit  200  again to form a circulation system. In the latter case, the gas dissolution concentration of the T-UFB-containing liquid W that is decreased due to the generation of the T-UFBs can be increased. Preferably the gas dissolution concentration of the T-UFB-containing liquid W that is decreased the reduced gas dissolution concentration of the T-UFB-containing liquid W can be compensated to the saturated state again by the dissolving unit  200 . If new T-UFBs are generated by the T-UFB generating unit  300  after the compensation, it is possible to further increase the concentration of the UFBs contained in the T-UFB-containing liquid with the above-described properties. That is, it is possible to increase the concentration of the contained UFBs by the number of circulations through the dissolving unit  200 , the T-UFB generating unit  300 , and the post-processing unit  400 , and it is possible to transfer the UFB-containing liquid W to the collecting unit  500  after a predetermined concentration of the contained UFBs is obtained. This embodiment shows a form in which the UFB-containing liquid processed by the post-processing unit  400  is put back to the dissolving unit  200  and circulated; however, it is not limited thereto, and the UFB-containing liquid after passing through the T-UFB generating unit may be put back again to the dissolving unit  200  before being supplied to the post-processing unit  400  such that the post-processing is performed by the post-processing unit  400  after the T-UFB concentration is increased through multiple times of circulation, for example. 
     The collecting unit  500  collects and preserves the UFB-containing liquid W transferred from the post-processing unit  400 . The T-UFB-containing liquid collected by the collecting unit  500  is a UFB-containing liquid with high purity from which various impurities are removed. 
     In the collecting unit  500 , the UFB-containing liquid W may be classified by the size of the T-UFBs by performing some stages of filtration processing. Since it is expected that the temperature of the T-UFB-containing liquid W obtained by the T-UFB method is higher than normal temperature, the collecting unit  500  may be provided with a cooling unit. The cooling unit may be provided to a part of the post-processing unit  400 . 
     The schematic description of the UFB generating apparatus  1  is given above; however, it is needless to say that the illustrated multiple units can be changed, and not all of them need to be prepared. Depending on the type of the liquid W and the gas G to be used and the intended use of the T-UFB-containing liquid to be generated, a part of the above-described units may be omitted, or another unit other than the above-described units may be added. 
     For example, when the gas to be contained by the UFBs is the atmospheric air, the degassing unit as the pre-processing unit  100  and the dissolving unit  200  can be omitted. On the other hand, when multiple kinds of gases are desired to be contained by the UFBs, another dissolving unit  200  may be added. 
     The units for removing the impurities as described in  FIGS. 11A to 11C  may be provided upstream of the T-UFB generating unit  300  or may be provided both upstream and downstream thereof. When the liquid to be supplied to the UFB generating apparatus is tap water, rain water, contaminated water, or the like, there may be included organic and inorganic impurities in the liquid. If such a liquid W including the impurities is supplied to the T-UFB generating unit  300 , there is a risk of deteriorating the heating element  10  and inducing the salting-out phenomenon. With the mechanisms as illustrated in  FIGS. 11A to 11C  provided upstream of the T-UFB generating unit  300 , it is possible to remove the above-described impurities previously. 
     &lt;&lt;Liquid and Gas Usable for T-UFB-Containing Liquid&gt;&gt; 
     Now, the liquid W usable for generating the T-UFB-containing liquid is described. The liquid W usable in this embodiment is, for example, pure water, ion exchange water, distilled water, bioactive water, magnetic active water, lotion, tap water, sea water, river water, clean and sewage water, lake water, underground water, rain water, and so on. A mixed liquid containing the above liquid and the like is also usable. A mixed solvent containing water and soluble organic solvent can be also used. The soluble organic solvent to be used by being mixed with water is not particularly limited; however, the followings can be a specific example thereof. An alkyl alcohol group of the carbon number of 1 to 4 including methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, and tert-butyl alcohol. An amide group including N-methyl-2-pyrrolidone, 2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, N,N-dimethylformamide, and N,N-dimethylacetamide. A keton group or a ketoalcohol group including acetone and diacetone alcohol. A cyclic ether group including tetrahydrofuran and dioxane. A glycol group including ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,2-hexanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, diethylene glycol, triethylene glycol, and thiodiglycol. A group of lower alkyl ether of polyhydric alcohol including ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, and triethylene glycol monobutyl ether. A polyalkylene glycol group including polyethylene glycol and polypropylene glycol. A triol group including glycerin, 1,2,6-hexanetriol, and trimethylolpropane. These soluble organic solvents can be used individually, or two or more of them can be used together. 
     A gas component that can be introduced into the dissolving unit  200  is, for example, hydrogen, helium, oxygen, nitrogen, methane, fluorine, neon, carbon dioxide, ozone, argon, chlorine, ethane, propane, air, and so on. The gas component may be a mixed gas containing some of the above. Additionally, it is not necessary for the dissolving unit  200  to dissolve a substance in a gas state, and the dissolving unit  200  may fuse a liquid or a solid containing desired components into the liquid W. The dissolution in this case may be spontaneous dissolution, dissolution caused by pressure application, or dissolution caused by hydration, ionization, and chemical reaction due to electrolytic dissociation. 
     &lt;&lt;Effects of T-UFB Generating Method&gt;&gt; 
     Next, the characteristics and the effects of the above-described T-UFB generating method are described by comparing with a conventional UFB generating method. For example, in a conventional air bubble generating apparatus as represented by the Venturi method, a mechanical depressurizing structure such as a depressurizing nozzle is provided in a part of a flow passage. A liquid flows at a predetermined pressure to pass through the depressurizing structure, and air bubbles of various sizes are generated in a downstream region of the depressurizing structure. 
     In this case, among the generated air bubbles, since the relatively large bubbles such as milli-bubbles and microbubbles are affected by the buoyancy, such bubbles rise to the liquid surface and disappear. Even the UFBs that are not affected by the buoyancy may also disappear with the milli-bubbles and microbubbles since the gas-liquid interface energy of the UFBs is not very large. Additionally, even if the above-described depressurizing structures are arranged in series, and the same liquid flows through the depressurizing structures repeatedly, it is impossible to store for a long time the UFBs of the number corresponding to the number of repetitions. In other words, it has been difficult for the UFB-containing liquid generated by the conventional UFB generating method to maintain the concentration of the contained UFBs at a predetermined value for a long time. 
     In contrast, in the T-UFB generating method of this embodiment utilizing the film boiling, a rapid temperature change from normal temperature to about 300° C. and a rapid pressure change from normal pressure to around a several megapascal occur locally in a part extremely close to the heating element. The heating element is a rectangular shape having one side of around several tens to hundreds of μm. It is around 1/10 to 1/1000 of the size of a conventional UFB generating unit. Additionally, with the gas-dissolved liquid within the extremely thin film region of the film boiling bubble surface exceeding the thermal dissolution limit or the pressure dissolution limit instantaneously (in an extremely short time under microseconds), the phase transition occurs and the gas-dissolved liquid is precipitated as the UFBs. In this case, the relatively large bubbles such as milli-bubbles and microbubbles are hardly generated, and the liquid contains the UFBs of about 100 nm in diameter with extremely high purity. Moreover, since the T-UFBs generated in this way have sufficiently large gas-liquid interface energy, the T-UFBs are not broken easily under the normal environment and can be stored for a long time. 
     Particularly, the present invention using the film boiling phenomenon that enables local formation of a gas interface in the liquid can form an interface in a part of the liquid close to the heating element without affecting the entire liquid region, and a region on which the thermal and pressure actions performed can be extremely local. As a result, it is possible to stably generate desired UFBs. With further more conditions for generating the UFBs applied to the generation liquid through the liquid circulation, it is possible to additionally generate new UFBs with small effects on the already-made UFBs. As a result, it is possible to produce a UFB liquid of a desired size and concentration relatively easily. 
     Moreover, since the T-UFB generating method has the above-described hysteresis properties, it is possible to increase the concentration to a desired concentration while keeping the high purity. In other words, according to the T-UFB generating method, it is possible to efficiently generate a long-time storable UFB-containing liquid with high purity and high concentration. 
     &lt;&lt;Specific Usage of T-UFB-Containing Liquid&gt;&gt; 
     In general, applications of the ultrafine bubble-containing liquids are distinguished by the type of the containing gas. Any type of gas can make the UFBs as long as an amount of around PPM to BPM of the gas can be dissolved in the liquid. For example, the ultrafine bubble-containing liquids can be applied to the following applications.
         A UFB-containing liquid containing air can be preferably applied to cleansing in the industrial, agricultural and fishery, and medical scenes and the like, and to cultivation of plants and agricultural and fishery products.   A UFB-containing liquid containing ozone can be preferably applied to not only cleansing application in the industrial, agricultural and fishery, and medical scenes and the like, but to also applications intended to disinfection, sterilization, and decontamination, and environmental cleanup of drainage and contaminated soil, for example.   A UFB-containing liquid containing nitrogen can be preferably applied to not only cleansing application in the industrial, agricultural and fishery, and medical scenes and the like, but to also applications intended to disinfection, sterilization, and decontamination, and environmental cleanup of drainage and contaminated soil, for example.   A UFB-containing liquid containing oxygen can be preferably applied to cleansing application in the industrial, agricultural and fishery, and medical scenes and the like, and to cultivation of plants and agricultural and fishery products.   A UFB-containing liquid containing carbon dioxide can be preferably applied to not only cleansing application in the industrial, agricultural and fishery, and medical scenes and the like, but to also applications intended to disinfection, sterilization, and decontamination, for example.   A UFB-containing liquid containing perfluorocarbons as a medical gas can be preferably applied to ultrasonic diagnosis and treatment. As described above, the UFB-containing liquids can exert the effects in various fields of medical, chemical, dental, food, industrial, agricultural and fishery, and so on.       

     In each of the applications, the purity and the concentration of the UFBs contained in the UFB-containing liquid are important for quickly and reliably exert the effect of the UFB-containing liquid. In other words, unprecedented effects can be expected in various fields by utilizing the T-UFB generating method of this embodiment that enables generation of the UFB-containing liquid with high purity and desired concentration. Here is below a list of the applications in which the T-UFB generating method and the T-UFB-containing liquid are expected to be preferably applicable. 
     (A) Liquid Purification Application 
     
         
         
           
             With the T-UFB generating unit provided to a water clarification unit, enhancement of an effect of water clarification and an effect of purification of PH adjustment liquid is expected. The T-UFB generating unit may also be provided to a carbonated water server. 
             With the T-UFB generating unit provided to a humidifier, aroma diffuser, coffee maker, and the like, enhancement of a humidifying effect, a deodorant effect, and a scent spreading effect in a room is expected. 
             If the UFB-containing liquid in which an ozone gas is dissolved by the dissolving unit is generated and is used for dental treatment, burn treatment, and wound treatment using an endoscope, enhancement of a medical cleansing effect and an antiseptic effect is expected. 
             With the T-UFB generating unit provided to a water storage tank of a condominium, enhancement of a water clarification effect and chlorine removing effect of drinking water to be stored for a long time is expected. 
             If the T-UFB-containing liquid containing ozone or carbon dioxide is used for brewing process of Japanese sake, shochu, wine, and so on in which the high-temperature pasteurization processing cannot be performed, more efficient pasteurization processing than that with the conventional liquid is expected. 
             If the UFB-containing liquid is mixed into the ingredient in a production process of the foods for specified health use and the foods with functional claims, the pasteurization processing is possible, and thus it is possible to provide safe and functional foods without a loss of flavor. 
             With the T-UFB generating unit provided to a supplying route of sea water and fresh water for cultivation in a cultivation place of fishery products such as fish and pearl, prompting of spawning and growing of the fishery products is expected. 
             With the T-UFB generating unit provided in a purification process of water for food preservation, enhancement of the preservation state of the food is expected. 
             With the T-UFB generating unit provided in a bleaching unit for bleaching pool water or underground water, a higher bleaching effect is expected. 
             With the T-UFB-containing liquid used for repairing a crack of a concrete member, enhancement of the effect of crack repairment is expected. 
             With the T-UFBs contained in liquid fuel for a machine using liquid fuel (such as automobile, vessel, and airplane), enhancement of energy efficiency of the fuel is expected. 
           
         
       
    
     (B) Cleansing Application 
     Recently, the UFB-containing liquids have been receiving attention as cleansing water for removing soils and the like attached to clothing. If the T-UFB generating unit described in the above embodiment is provided to a washing machine, and the UFB-containing liquid with higher purity and better permeability than the conventional liquid is supplied to the washing tub, further enhancement of detergency is expected.
         With the T-UFB generating unit provided to a bath shower and a bedpan washer, not only a cleansing effect on all kinds of animals including human body but also an effect of prompting contamination removal of a water stain and a mold on a bathroom and a bedpan are expected.   With the T-UFB generating unit provided to a window washer for automobiles, a high-pressure washer for cleansing wall members and the like, a car washer, a dishwasher, a food washer, and the like, further enhancement of the cleansing effects thereof is expected.   With the T-UFB-containing liquid used for cleansing and maintenance of parts produced in a factory including a burring step after pressing, enhancement of the cleansing effect is expected.   In production of semiconductor elements, if the T-UFB-containing liquid is used as polishing water for a wafer, enhancement of the polishing effect is expected. Additionally, if the T-UFB-containing liquid is used in a resist removal step, prompting of peeling of resist that is not peeled off easily is enhanced.   With the T-UFB generating unit is provided to machines for cleansing and decontaminating medical machines such as a medical robot, a dental treatment unit, an organ preservation container, and the like, enhancement of the cleansing effect and the decontamination effect of the machines is expected. The T-UFB generating unit is also applicable to treatment of animals.       

     (C) Pharmaceutical Application 
     
         
         
           
             If the T-UFB-containing liquid is contained in cosmetics and the like, permeation into subcutaneous cells is prompted, and additives that give bad effects to skin such as preservative and surfactant can be reduced greatly. As a result, it is possible to provide safer and more functional cosmetics. 
             If a high concentration nanobubble preparation containing the T-UFBs is used for contrasts for medical examination apparatuses such as a CT and an MRI, reflected light of X-rays and ultrasonic waves can be efficiently used. This makes it possible to capture a more detailed image that is usable for initial diagnosis of a cancer and the like. 
             If a high concentration nanobubble water containing the T-UFBs is used for a ultrasonic wave treatment machine called high-intensity focused ultrasound (HIFU), the irradiation power of ultrasonic waves can be reduced, and thus the treatment can be made more non-invasive. Particularly, it is possible to reduce the damage to normal tissues. 
             It is possible to create a nanobubble preparation by using high concentration nanobubbles containing the T-UFBs as a source, modifying a phospholipid forming a liposome in a negative electric charge region around the air bubble, and applying various medical substances (such as DNA and RNA) through the phospholipid. 
             If a drug containing high concentration nanobubble water made by the T-UFB generation is transferred into a dental canal for regenerative treatment of pulp and dentine, the drug enters deeply a dentinal tubule by the permeation effect of the nanobubble water, and the decontamination effect is prompted. This makes it possible to treat the infected root canal of the pulp safely in a short time. 
           
         
       
    
     Hereinafter, characteristics of the present application of the invention are described. 
       FIG. 12A  is a diagram illustrating a chamber  301  that is a part of the T-UFB generating unit  300  in this embodiment, and  FIG. 12B  is a cross-sectional view taken along the XIIb-XIIb line in  FIG. 12A . The chamber  301  of this embodiment is formed by providing walls  352  on the element substrate  12  formed of a silicon substrate in the form of a wafer on which the later-described heating elements  10  (see  FIG. 13B ) and wirings  308  (see  FIGS. 13A and 13B ) are formed, and by attaching a lid substrate  351  on the tops of the walls  352 . Specifically, the chamber  301  forms and provides a space in which the heating elements  10  are located (see  FIG. 13B ). The silicon substrate in the form of a wafer is a substrate of a silicon wafer formed by slicing a single crystal ingot of silicon and is a substrate on which no cutting by dicing or the like is performed after slicing. 
     In the chamber  301 , electrode pads  350  used for supplying power from outside to the element substrate  12  are provided at a distance from the chamber  301  by the walls  352 . As described above, the chamber  301  of this embodiment has a simple configuration in which the walls  352  are formed on the element substrate  12  and the lid substrate  351  is attached on the tops of the walls  352 . 
       FIG. 13A  is a diagram illustrating the element substrate  12 , and  FIG. 13B  is an enlarged view of  FIG. 13A . In the element substrate  12  of this embodiment, there are formed the multiple heating elements  10 , the wirings  308  supplying power to the heating elements  10 , and the electrode pads  350  for connecting the wirings  308  with external wirings. In this embodiment, as described above, the element substrate  12  is not made into a chip and is used in the form of a wafer. 
     Two electrode pads including an electrode pad  3501  and an electrode pad  3502  are provided as the electrode pads  350  on the element substrate  12 , while the electrode pad  3501  is provided in one end portion of the element substrate  12  and the electrode pad  3502  is provided in the other end portion opposing the one end portion. The wirings  308  are each connected with the corresponding heating elements  10  and either one of the two electrode pads  350 . In such arrangement of the electrode pads  350 , the lengths of the wirings from the heating elements  10  to the electrode pads  350  are different from each other. That is, the heating element  10  connected to one of the electrode pads  350  by a long wiring and the heating element  10  connected to one of the electrode pads  350  by a short wiring are provided on the element substrate  12 . In this case, the wiring resistances between the electrode pads  350  and the heating elements  10  differ from each other depending on the difference between the lengths of the wirings, and voltage drops according to the lengths of the wirings occur during energization. 
     To deal with this, in this embodiment, the widths of the wirings differ from each other taking into consideration the differences between the distances from the electrode pads  350  to the heating elements  10 . That is, the wirings are formed to have wider wiring widths as the distance from the electrode pad  350  to the heating element  10  is longer. This allows the configuration in which the voltage drops among the wirings to be substantially equal. 
       FIG. 14  is a diagram illustrating the element substrate  12  on which the walls  352  are formed. The walls  352  are formed by photolithography to form a part of the chamber  301 . The walls  352  make a distance between the chamber  301  and the electrode pads  350  in the end portions of the element substrate  12 . Since the capacity of the chamber  301  is determined according to the heights of the walls  352 , it is desirable to determine the heights of the walls  352  based on the flow rate of the liquid flowing through the chamber  301  as necessary. The chamber  301  is formed by attaching the lid substrate  351  on the tops of these walls  352 . With the lid substrate  351  attached to the walls  352 , a supply port  355  for supplying the liquid to the chamber  301  and a discharge port  356  for discharging the liquid from the chamber  301  are formed. The liquid flowed in the chamber  301  from the supply port  355  flows over the element substrate  12  between the walls  352  and is discharged from the discharge port  356 . 
       FIG. 15A  is a front view illustrating the lid substrate  351 , and  FIG. 15B  is a cross-sectional view taken along the XVb-XVb line in  FIG. 15A . The lid substrate  351  is formed of a substrate made of silicon and is attached on the tops of the walls  352  to form the chamber  301 . Although a substrate made of silicon is employed as the lid substrate  351  in this embodiment, the embodiment is not limited thereto, and a substrate formed of a material other than silicon may be adopted. 
       FIG. 16A  is a diagram illustrating a supply pipe  353  and a discharge pipe  354  connected to the supply port  355  and the discharge port  356  of the chamber  301 , respectively, and  FIG. 16B  is a cross-sectional view taken along the XVIb-XVIb line in  FIG. 16A . The supply port  355  and the discharge port  356  are provided to oppose each other in the end portions of the element substrate  12 . The supply port  355  and the discharge port  356  are openings formed by the two walls  352  and the element substrate  12  and the lid substrate  351 , while the supply port  355  is provided to be able to supply the liquid to the chamber  301  and the discharge port  356  is provided to be able to discharge the liquid from the chamber  301 . The supply pipe  353  is connected with the supply port  355 , and the discharge pipe  354  is connected with the discharge port  356 . Since the pair of the supply port  355  and the discharge port  356  and the pair of the supply pipe  353  and the discharge pipe  354  respectively have the same configurations, they are interchangeable. 
       FIG. 17A  is a diagram illustrating the state where the element substrate  12  and flexible wiring substrates  357  are electrically connected with each other, and  FIG. 17B  is a cross-sectional view taken along the XVIIb-XVIIb line in  FIG. 17A . The supply pipe  353  and the discharge pipe  354  are omitted in  FIGS. 17A and 17B . The element substrate  12  is electrically connected with the flexible wiring substrates  357  through the electrode pads  350 , and the electrode pads  350  and wirings of the flexible wiring substrates  357  are connected with each other through wire bondings  358 . 
     With the chamber  301  formed as described above, it is possible to generate the film boiling bubble  13  in a region of the liquid in contact with the heating element  10  by applying a predetermined voltage pulse to the heating element  10  on the bottom surface of the chamber  301 , and to generate the ultrafine bubbles along with the expansion and shrinkage of the film boiling bubble  13 . 
       FIGS. 18A to 18C  are diagrams illustrating a heater part  250  provided with the heating element  10  in the element substrate  12  of this embodiment. In  FIGS. 18A and 18B , illustration of the heating element  10  is omitted.  FIG. 18A  is a diagram illustrating a recess portion  180  of the heater part  250 , and  FIG. 18B  is a cross-sectional view taken along the XVIIIb-XVIIIb line in  FIG. 18A . In the element substrate  12  of this embodiment, the recess portion  180  is formed in a portion in which the heating element  10  is provided. The recess portion  180  of the element substrate  12  is a rectangular recess portion  180  as illustrated in  FIGS. 18A and 18C , and a cross section thereof is in a trapezoidal shape having inclined surfaces at an angle of θ=54.7° as illustrated in  FIG. 18B . The recess portion  180  is formed by anisotropic etching a silicon substrate &lt;1.0.0&gt;. In the recess portion  180 , the heating element  10  is provided in a position displaced in a −y direction from the center of the recess portion  180  as illustrated in  FIG. 18C . 
     The heating element  10  is not put in contact with the inclined surfaces of the recess portion  180  in the −y direction, and between one of the inclined surfaces of the recess portion  180  and the heating element  10 , there is a bubble-disappearance position  181  in which the bubble  13  generated by driving the heating element  10  disappears. 
       FIGS. 19A to 19C  are diagrams illustrating states of bubbling in the liquid over time while driving the heating element  10  of this embodiment.  FIG. 19A  illustrates a state before driving the heating element  10 , and no bubbling occurs in the liquid yet.  FIG. 19B  illustrates a state where the bubbling occurs in the liquid by driving the heating element  10 , and during the bubbling, a direction in which the bubble  13  expands is restricted by the recess portion  180 , and the bubble  13  is formed while expanding largely in the y direction. This is because the inclined surfaces of the recess portion  180  in positions in an x direction, a −x direction, and the −y direction from the heating element  10  are provided closely to the heating element  10  as walls, but the inclined surface of the recess portion  180  in a position in they direction from the heating element  10  is provided at a predetermined distance from the heating element  10 . 
     With this configuration, during the bubbling, the bubble  13  is less likely to be affected by the inclined surface of the recess portion  180  in the y direction, but is likely to be affected by the inclined surfaces of the recess portion  180  positioned in the x, −x, and −y directions from the heating element  10 . Thus, the bubble  13  grows while expanding largely in the y direction in which the bubble  13  is less likely to be affected by the inclined surface. Thereafter, in the process of bubble disappearance, the bubble  13  is affected by the recess portion  180  and quickly shrinks in the −y direction, which is opposite to the case of bubbling, as illustrated in  FIG. 19C . Consequently, the bubble  13  disappears in the bubble-disappearance position  181  in a region having a predetermined area, which is a position displaced in the −y direction from the heating element  10 . 
     In general, in the case where the bubble generated by driving the heating element disappears, the surrounding liquids collide with each other in the center of the bubble at the moment of the disappearance, and so-called cavitation that generates a tiny but strong pressure wave (shock wave) thus occurs. If the cavitation occurs repeatedly on the heating element while using the apparatus continuously, the heating element may be damaged by being hit by the shock waves repeatedly. 
     To deal with this, in the UFB generating apparatus of this embodiment, the shape of the bubble  13  generated by driving the heating element  10  is restricted by the recess portion  180 , and the bubble-disappearance position  181  in which the bubble  13  disappears is in the position displaced in the −y direction from the heating element  10 . The bubble-disappearance position  181  has a predetermined area, and the area is the maximum area that can keep the effect of the cavitation within that area during the bubble disappearance, which prevents the cavitation during the bubble disappearance from affecting the adjacent heating element  10 . This area varies depending on the size of the heating element and the driving voltage to be used, and thus the area may be set appropriately to be fit for the apparatus. 
     This makes it possible to suppress the effect of the cavitation on the heating element  10  during the bubble disappearance. Thus, it is possible to obtain a reliable UFB generating apparatus that suppresses the damage of the heating element  10  by setting the bubble-disappearance position  181  in which the bubble  13  disappears in a position other than where the heating element  10  is positioned. 
     Although the heating element  10  is arranged in the position displaced in the −y direction from the center of the recess portion  180  in this embodiment, the embodiment is not limited thereto. That is, the heating element  10  may be arranged in any position as long as the position in which the heating element  10  is arranged is displaced from the center of the recess portion  180  and the position is other than the bubble-disappearance position  181  in which the bubble  13  disappears, the bubble-disappearance position  181  being displaced from the heating element  10 . 
       FIGS. 20A to 20K  and  FIGS. 21A to 21H  are diagrams illustrating steps of forming the element substrate  12  in the order of the steps. Hereinafter, a method of forming the element substrate  12  in this embodiment is described in the order of the steps. The method of mounting the heating element  10  and so on on the substrate described herein is similar to the conventional method. First, as illustrated in  FIG. 20A , a silicon substrate &lt;1.0.0&gt;  210  in the form of a wafer to be used as the element substrate  12  is prepared. As illustrated in  FIG. 20B , an oxide film  211  of 1 μm is formed as a heat storage layer for the top surface of the silicon substrate  210  and as a protection film for the back surface thereof by processing at a temperature of 1200° C. for 70 minutes under an oxidation atmosphere condition using water vapor by a thermal oxidation furnace. Thereafter, as illustrated in  FIG. 20C , a photoresist  212  manufactured by TOKYO OHKAKOGYO CO., LTD. is applied in a thickness of 2 μm by spin coating. Then, a glass mask for exposure in a predetermined shape is used to perform exposure with an i-line stepper FPA-3000i5 manufactured by Canon, and the resist  212  in a region corresponding to the portion in which the heating element  10  is to be arranged is removed to form an opening. 
     Then, as illustrated in  FIG. 20D , the oxide film  211  in the opening is removed by dry etching using a mixed gas of CF4 and O2. As illustrated in  FIG. 20E , the substrate is immersed in a resist delamination liquid remover  1112 A manufactured by Rohm and Haas Company to delaminate and remove the resist  212 . Subsequently, as illustrated in  FIG. 20F , the substrate in the state of  FIG. 20E  is immersed in an alkali etching liquid at 80° C. as a TMAH22% aqueous solution for 40 minutes to form a recess portion  215  with a depth of 3 μm from an opening  213  including a surface  214 . The recess portion  215  includes flat and smooth surfaces having tapers at an angle of 54.7 degrees from the opening of the oxide film  211  and the surface  214  as a bottom surface, and the surface  214  is formed as a flat surface on which the heating element  10  can be arranged. 
     Thereafter, the substrate in the state of  FIG. 20F  is processed for 300 minutes under an oxidation atmosphere condition using water vapor to form an oxide film  216  of 2 μm in thickness on the silicon top surface of the recess portion  215  as illustrated in  FIG. 20G . Then, a TaSiN resistance layer  217  of 30 nm in thickness is formed on the substrate in the state of  FIG. 20G  and subsequently an Al-wiring layer  218  of 500 nm in thickness as a wiring material is formed on the TaSiN resistance layer  217  by spattering (see  FIG. 20H ). Thereafter, the TaSiN resistance layer  217  and the Al-wiring layer  218  are formed in predetermined shapes by photolithography. Specifically, first, as illustrated in  FIG. 20I , a photoresist  219  manufactured by TOKYO OHKA KOGYO CO., LTD. is applied in a thickness of 2 μm by spin coating (see  FIG. 20I ), and then, a glass mask for exposure in a predetermined shape is used to perform exposure with an i-line stepper FPA-3000i5 manufactured by Canon. Thereafter, development is performed, and the resist  219  is left in a shape of the wiring  308  as illustrated in  FIG. 13A  (see  FIG. 20J ). 
     Subsequently, the Al-wiring layer  218  and the TaSiN resistance layer  217  are etched simultaneously by reactive-ion etching using a BCl3 gas and a Cl2 gas to form a wiring portion. In this process, the Al-wiring layer  218  and the TaSiN resistance layer  217  are arranged in a position displaced from the center of the recess portion  215 . Thereafter, as a step of forming a heater, the heating element  10  is formed by partially removing the Al-wiring layer  218  and exposing the TaSiN resistance layer  217  by wet etching using phosphate in the recess portion  215  (see  FIG. 20K ). This makes it possible to displace the position in which the bubble  13  generated by driving the heating element  10  disappears from the top of the heating element  10 . Thereafter, as illustrated in  FIG. 21A , the substrate in the state of  FIG. 20K  is immersed in the resist delamination liquid remover  1112 A to delaminate and remove the resist  219 . 
     Next, a protection layer and a cavitation-resistant film for insulating the heating element  10  and the wiring  308  from the liquid and protecting them from heat and impact of the bubbling are formed. As illustrated in  FIG. 21B , a film  220  of silicon nitride (hereinafter, written as SiN) is formed in a thickness of 500 nm by plasma CVD. Then, a metal Ir film  221  is formed in a thickness of 200 nm on the SiN film  220  by spattering as illustrated in  FIG. 21C . The SiN film  220  is the protection layer for electric insulation from the liquid, and the metal Ir film  221  has particularly a function of the cavitation-resistant film protecting the heating element  10  from heating in the heating element  10  and from an impact from bubbling and bubble disappearance, that is, cavitation. 
     Thereafter, the SiN film  220  and the metal Ir film  221  are formed in predetermined shapes by photolithography. Specifically, as illustrated in  FIG. 21D , a photoresist  222  manufactured by TOKYO OHKA KOGYO CO., LTD. is applied in a thickness of 2 μm by spin coating, and then, a glass mask for exposure in a predetermined shape is used to perform exposure with the i-line stepper FPA-3000i5. Thereafter, development is performed, and the resist  222  is partially removed as illustrated in  FIG. 21E  to leave the resist  222  in a predetermined shape. Subsequently, the metal Ir film  221  in the portion from which the resist  222  is removed is etched by reactive-ion etching using CF4 as illustrated in  FIG. 21F . The SiN film  220  is etched subsequently as illustrated in  FIG. 21G , and the electrode pad  350  for the connection with the external wiring is formed. At last, the substrate in the state of  FIG. 21G  is immersed in the resist delamination liquid remover  1112 A to delaminate and remove the resist  222 , and the element substrate  12  including the heating element  10 , the electrode pad  350 , and the recess portion  180  is completed as illustrated in  FIG. 21H . 
       FIGS. 22A to 22D  are diagrams illustrating steps of forming the chamber  301  in the order of the steps. Hereinafter, a method of forming the chamber  301  in this embodiment is described in the order of the steps. The element substrate  12  as illustrated in  FIG. 7A  is prepared, and thereafter, as illustrated in  FIG. 22B , a member  223  to be the walls  352  is applied to have a predetermined thickness by spin coating, and then the walls  352  are formed by photolithography as illustrated in  FIG. 22C . Thereafter, the chamber  301  is formed by attaching the lid substrate  351  to the walls  352 . 
     In this embodiment, although the recess portion is formed on the element substrate by etching and the heating element is formed on the bottom surface of the recess portion, the embodiment is not limited thereto. That is, the heating element may be formed to be exposed on the top surface of the substrate, and then walls of a predetermined height may be formed around the heating element by a method such as laminating films. 
     As described above, the growth of the bubble during the bubbling is partially restricted by forming the walls around the heating element, and the position in which the bubble disappears is displaced from the heating element. This makes it possible to efficiently generate a UFB-containing liquid with high purity and to provide an ultrafine bubble generating apparatus and an ultrafine bubble generating method that can extend the lifetime of the apparatus additionally. 
     Second Embodiment 
     Hereinafter, a second embodiment of the present invention is described with reference to the drawings. Since the basic configuration of this embodiment is similar to the configuration of the first embodiment, only characteristic configurations are described below. 
       FIGS. 23A to 23C  are diagrams illustrating the heater part  250  in the element substrate  12  of this embodiment. As illustrated in  FIG. 23A , the heater part  250  in this embodiment is provided with walls  232 , which are restriction members that restrict the growth of the bubble  13  in three directions (y direction, −y direction, −x direction) around the heating element  10  during the bubbling. Although the recess portion  180  is formed by digging the substrate by etching in the first embodiment, there is no recess portion formed by digging the substrate in this embodiment. The walls  232  are formed in the three directions around the heating element  10  as illustrated in  FIG. 23A  by laminating films on the substrate having the top surface on which the heating element  10  is exposed. A region having a predetermined area is formed adjacent to the heating element in a direction other than the three directions. 
     With the walls  232  provided in the three directions around the heating element  10  as described above, the growth in the surrounding three directions of the bubble  13  generated by driving the heating element  10  is restricted by the walls  232 , and the bubble  13  grows while expanding largely in the x direction in which the walls  232  are not provided. The bubble  13  grown while expanding largely in the x direction as illustrated in  FIG. 23B  then starts shrinking, and disappears in the bubble-disappearance position  181  displaced in the −x direction from the heating element  10  as illustrated in  FIG. 23C . With the bubble  13  disappearing in the bubble-disappearance position  181  that is a position displaced from the heating element  10  as described above, it is possible to suppress the effect of the cavitation on the heating element  10  during the bubble disappearance, and thus the damage on the heating element can be suppressed. Consequently, it is possible to obtain a reliable UFB generating apparatus. 
     Although the method of forming the walls  232  in the three direction around the heating element by laminating the films is described in this embodiment, the embodiment is not limited thereto. That is, the wall may be formed in at least a part of the surroundings of the heating element  10 . With the wall formed in at least a part of the surroundings of the heating element  10  like this, the growth of the bubble in the direction in which the wall is formed is restricted during the bubbling, and a bubble that grows while expanding in a direction opposite to the direction in which the wall is provided is formed. In the shrinkage, the bubble that has grown while expanding in the direction opposite to the direction in which the wall is provided quickly shrinks in the direction from the heating element toward the wall, which is opposite to the case of bubbling. Consequently, the bubble-disappearance position is displaced from the top of the heating element, and the bubble disappears in the bubble-disappearance position between the wall and the heating element. This makes it possible to suppress the effect of the cavitation on the heating element  10  during the bubble disappearance. 
     Although no wall is provided in the x direction from the heating element  10  in this embodiment, the wall in the x direction may be formed with a region (second region) provided between the heating element  10  and the wall in the x direction, the second region being wider than a region (first region) between the heating element  10  and the wall  232  in the −x direction. With the wall in the x direction provided like this, the bubble to be generated is likely to be affected more by the wall in the −x direction than the wall in the x direction, and thus it is possible to control the bubble to grow while expanding in the x direction. 
     Third Embodiment 
     Hereinafter, a third embodiment of the present invention is described with reference to the drawings. Since the basic configuration of this embodiment is similar to the configuration of the first embodiment, only characteristic configurations are described below. 
       FIGS. 24A to 24C  are diagrams illustrating the heater part  250  in the element substrate  12  of this embodiment. The heater part  250  of this embodiment is provided with walls  240  around the heating element  10  to form a recess portion  242  in which a flow passage  241  and the heating element  10  are provided. Like the second embodiment, the substrate is not dug in this embodiment as well, and the walls  240  as illustrated in  FIG. 24A  are formed by laminating films on the substrate having the top surface on which the heating element  10  is exposed. The walls in the three directions (−x direction, −y direction, x direction) around the heating element  10  are formed integrally. With the walls  240  formed like this, the liquid that has flowed through the flow passage  241  flows into the recess portion  242 , and the bubbling occurs by the heating element  10  heating the liquid. In this process, since there is the recess portion  242  formed by the walls  240  in the three directions (−x direction, −y direction, x direction) around the heating element  10 , the growth in the three directions of the bubble  13  is restricted, and the bubble  13  grows while expanding toward the flow passage  241 . As illustrated in  FIG. 24B , the grown bubble  13  largely expanding in a flow passage direction (y direction) then starts shrinking, and disappears in the bubble-disappearance position  181  displaced in the −y direction from the heating element  10  as illustrated in  FIG. 24C . With the bubble  13  disappearing in the bubble-disappearance position  181  displaced from the heating element  10  as described above, it is possible to suppress the effect of the cavitation on the heating element  10  during the bubble disappearance, and thus the damage on the heating element  10  can be suppressed. Consequently, it is possible to obtain a reliable UFB generating apparatus. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2019-036508 filed Feb. 28, 2019, which is hereby incorporated by reference wherein in its entirety.