Bubble generating device and bubble generating method

Abstract

To provide a bubble generating device and a bubble generating method that can efficiently extract minute bubbles into a liquid. [Solution] A bubble generator 1 generates microbubbles with a diameter of 100 μm or less in liquid 2 by applying a voltage to liquid 2 and electrolyzing the liquid 2. The device has a plurality of electrodes 3 which are at least partially immersed in liquid 2, and a power supply unit 4 which applies a voltage between the plurality of electrodes 3. At least one of the plurality of electrodes 3 is a specific electrode 30 which has a plurality of electric field concentration parts 5 that generate microbubbles. A surfactant is added to liquid 2.

Description

Technical Field

[0001] The present invention relates to a bubble generator and a bubble generation method for generating minute bubbles in a liquid by electrolyzing the liquid.

Background Art

[0002] Minute bubbles such as fine bubbles and ultrafine bubbles, depending on the diameter of the bubbles, etc., have characteristics such as staying in the liquid without disappearing over a long period of time. Due to their long existence time and small size, such minute bubbles are excellent for activation. Therefore, by selecting the gas species, applications to environmental measures such as soil and water purification, semiconductor manufacturing processes, hydroponics, fish breeding, etc., and productivity improvement measures are expected, and energetic development and application expansion have been carried out.

[0003] As methods for producing minute bubbles, various methods have been proposed. As one of them, there is a method of electrolyzing a liquid such as water to generate minute bubbles. As a device for generating minute bubbles by such electrolysis, there is a bubble generator disclosed in Patent Document 1.

[0004] The bubble generator disclosed in Patent Document 1 is provided with a nanostructure on the electrode and means for applying vibration to the electrode. Thereby, by vibrating the electrode, minute bubbles generated on the electrode are taken out into the liquid. That is, this bubble generator is made in view of the fact that minute bubbles generated on the surface of the nanostructure are difficult to be released from the nanostructure into the liquid. And, in order to release these minute bubbles from the electrode, a technique of applying ultrasonic vibration to the electrode is used.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

[0006] However, the bubble generating device disclosed in Patent Document 1 has the problem of having many components and a complex structure because it includes a vibration application means. Furthermore, even when vibration is applied, microbubbles may not be smoothly released from the nanostructure.

[0007] This invention has been made in view of the above problems, and aims to provide a bubble generating device and a bubble generating method that can efficiently extract minute bubbles into a liquid. [Means for solving the problem]

[0008] One aspect of the present invention is a bubble generator (1) that generates minute bubbles with a diameter of 100 μm or less in a liquid (2) by applying a voltage to the liquid and electrolyzing the liquid, Multiple electrodes (3) are immersed in the above liquid, It has a power supply unit (4) that applies a voltage between the above-mentioned multiple electrodes, At least one of the above-mentioned multiple electrodes is a specific electrode (30) having a number of electric field concentration sections (5) that generate the above-mentioned microbubbles, The above liquid is in a bubble generator to which a surfactant has been added.

[0009] Another aspect of the present invention is a bubble generation method for generating minute bubbles with a diameter of 100 μm or less in a liquid (2) by applying a voltage to the liquid and electrolyzing the liquid, A voltage is applied between a plurality of electrodes (3) that are at least partially immersed in the above liquid, At least one of the above-mentioned multiple electrodes is a specific electrode (30) having a number of electric field concentration sections (5) that generate the above-mentioned microbubbles, The above liquid has a surfactant added to it, and this is the method for generating bubbles. [Effects of the Invention]

[0010] In the bubble generator and bubble generation method described above, a surfactant is added to the liquid. This increases the wettability of the liquid to the electrode surface. Therefore, the microbubbles generated by the electrolysis of the liquid are more easily released from the specific electrode. As a result, the microbubbles can be efficiently extracted into the liquid.

[0011] As described above, the present invention provides a bubble generating device and a bubble generating method that can efficiently extract minute bubbles into a liquid. The reference numerals in parentheses in the claims and the means for solving the problem indicate the correspondence with the specific means described in the embodiments later, and do not limit the technical scope of the present invention. [Brief explanation of the drawing]

[0012] [Figure 1] An explanatory diagram of the bubble generator in Embodiment 1. [Figure 2] A perspective view illustrating a specific electrode in Embodiment 1. [Figure 3] A cross-sectional diagram of a specific electrode in Embodiment 1. [Figure 4] A perspective view illustrating the manufacturing method of a specific electrode in Embodiment 1. [Figure 5] Figure 4 is followed by a perspective diagram illustrating the manufacturing method of a specific electrode. [Figure 6] An electron microscope image showing the state of the silicon substrate during etching in Embodiment 1. [Figure 7] An electron microscope image showing a portion of a specific electrode on which a protrusion is formed in Embodiment 1. [Figure 8] A cross-sectional diagram of a specific electrode in Embodiment 2. [Figure 9] An electron microscope image showing a portion of a specific electrode in Embodiment 2. [Figure 10] A diagram showing an example of the applied voltage profile by the power supply unit in Embodiment 3. [Figure 11] Explanatory drawing of the bubble generator in Embodiment 4. [Figure 12] Explanatory plan drawing of the electrode arrangement as viewed from directly above in Embodiment 4. [Figure 13] Diagram showing the test results in Experimental Example 1. [Figure 14] Photograph showing the state near a specific electrode with or without a surfactant in Experimental Example 1. [Figure 15] Perspective explanatory drawing of a specific electrode in Embodiment 5. [Figure 16] Cross-sectional explanatory drawing of a specific electrode in Embodiment 5. [Figure 17] Perspective explanatory drawing showing the manufacturing method of a specific electrode in Embodiment 5. [Figure 18] Perspective explanatory drawing showing the manufacturing method of a specific electrode following FIG. 17.

Mode for Carrying Out the Invention

[0013] The microbubbles generated by the bubble generator of the above aspect are fine bubbles with a diameter of 100 μm or less. Further, the microbubbles can be ultra-fine bubbles with a diameter of 1 μm or less in particular.

[0014] The numerous electric field concentration portions provided on the specific electrode can be, for example, protruding portions protruding from the electrode surface. Further, the numerous electric field concentration portions provided on the specific electrode can also be, for example, numerous micro-exposed portions exposed from numerous micro-openings provided in an insulating film covering the electrode surface.

[0015] Further, the diameter of the electric field concentration portion when viewed from the normal direction of the electrode surface of the specific electrode can be, for example, 100 μm or less. Further, the diameter of the electric field concentration portion when viewed from the normal direction of the electrode surface can be, for example, 0.1 to 100 μm.

[0016] Furthermore, the surfactants mentioned above can be anionic surfactants, cationic surfactants, amphoteric surfactants, or nonionic surfactants. From the viewpoint of efficiently extracting microbubbles into the liquid, it is preferable that the surfactant be anionic or nonionic. Moreover, considering the effects on the human body and the environment, it is even more preferable that the surfactant be a nonionic surfactant.

[0017] Furthermore, the concentration of the surfactant in the liquid can be set to 0.03 to 4 times the critical micelle concentration. In this case, the release of microbubbles from the specific electrode becomes smoother, and the microbubbles can be efficiently extracted into the liquid.

[0018] Furthermore, the above-mentioned specific electrode may have a hydrophilic film formed on at least a portion other than the electric field concentration area. In this case, the release of microbubbles from the electrode is further promoted. Here, a hydrophilic film means a film that improves the wettability of the liquid on the electrode surface compared to when no hydrophilic film is formed. For example, the hydrophilic film may have a contact angle with the liquid of less than 90°. In other words, for example, a hydrophilic film may have a contact angle of more than 90° for bubbles in the liquid.

[0019] Furthermore, at least a portion of the contact surface in the bubble generator that comes into contact with the liquid, other than the electric field concentration area, can be a hydrophilic surface. In this case, it is possible to further suppress the adhesion of microbubbles released from the electrodes to other parts other than the electrodes. This makes it possible to extract microbubbles more efficiently. Note that "hydrophilic" in "hydrophilic surface" can have the same meaning as "hydrophilic" in "hydrophilic membrane" described above.

[0020] (Embodiment 1) Embodiments of a bubble generating device and a bubble generating method will be explained with reference to Figures 1 to 7. The bubble generator 1 in this embodiment generates minute bubbles with a diameter of 100 μm or less in the liquid 2 by applying a voltage to the liquid 2 and electrolyzing the liquid 2. As shown in Figure 1, the bubble generator 1 has a plurality of electrodes 3 that are at least partially immersed in the liquid 2, and a power supply unit 4 that applies a voltage between the plurality of electrodes 3. In this embodiment, the electrodes 3 are entirely immersed in the liquid 2. At least one of the multiple electrodes 3 is a specific electrode 30 having numerous electric field concentration sections 5 that generate microbubbles, as shown in Figure 2.

[0021] The bubble generator 1 stores liquid 2 in a container 11. A surfactant is added to liquid 2. In this embodiment, the surfactant is a nonionic surfactant. For example, tween 20 can be used as the nonionic surfactant.

[0022] Liquid 2 can be any liquid that generates a gas through electrolysis, such as water, an aqueous solution containing various solutes, or a dispersion of various substances in water. However, from the viewpoint of facilitating electrolysis, it is preferable to use an electrolyte as Liquid 2. Specifically, an aqueous sodium hydroxide solution can be used as Liquid 2. The concentration of sodium hydroxide in the aqueous sodium hydroxide solution can be, for example, about 0.01 to 1.0 mol / L. And, as described above, an appropriate amount of surfactant is added to Liquid 2.

[0023] The concentration of the surfactant in liquid 2 is 0.03 to 4 times the critical micelle concentration. Furthermore, the concentration of the surfactant in liquid 2 can be kept below the critical micelle concentration. That is, it can be 0.03 to 1 times the critical micelle concentration. The critical micelle concentration is the minimum concentration required for the formation of micelles, which are aggregates of surfactant molecules. In other words, by keeping the concentration of the surfactant in liquid 2 below the critical micelle concentration, the formation of micelles in the liquid can be prevented. The critical micelle concentration of surfactant tween20 is 0.95 mmol / L.

[0024] In this embodiment, as shown in Figure 1, the power supply unit 4 consists of a DC power supply. The bubble generator 1 has an anode 3p and a cathode 3n as electrodes 3. The anode 3p and cathode 3n are each formed in a substantially plate shape. The main surface of each electrode 3 is the electrode surface. These electrodes 3 are immersed in the liquid 2 in the container 11 and are arranged with their electrode surfaces facing each other. In addition, a pulse power supply that applies a pulsed current between the electrodes 3 can also be used as the power supply unit 4. In this case, it is easier to promote the release of minute bubbles from the electrodes 3.

[0025] In this embodiment, the cathode 3n is the specific electrode 30, while the anode 3p is not the specific electrode 30. That is, minute bubbles are generated in the cathode 3n, which is the specific electrode 30, and the cathode 3n, which is the specific electrode 30, has a number of electric field concentration areas 5.

[0026] In this embodiment, the specific electrode 30 has a number of electric field concentration portions 5 on the electrode surface 33 facing the other electrode 3 (anode 3p). In this embodiment, the electric field concentration portions 5 can be projections 50 that protrude from the electrode surface 33. As shown in Figures 2 and 3, the projections 50 have pointed tips 51. In this embodiment, the projections 50 have a substantially conical shape. The number of projections 50 are arranged in a two-dimensional alignment on the electrode surface 33.

[0027] Furthermore, in the specific electrode 30, the spacing between adjacent protrusions 50 is equal to or greater than the diameter of the protrusion 50. The diameter of the protrusion 50 when viewed from the direction normal to the electrode surface 33 of the specific electrode 30 can be, for example, 0.5 to 10 μm. The height of the protrusion 50 from the electrode surface 33 can be, for example, 0.5 to 10 μm. The spacing between adjacent protrusions 50 can be, for example, 1 to 20 μm.

[0028] Furthermore, as shown in Figure 2, the specific electrode 30 is protected by a protective member 6 that prevents its edges and the back surface 34, which is the surface opposite to the electrode surface 33, from coming into contact with the liquid 2. The protective member 6 is made of an insulator. The protective member 6 has a backing plate 61 that is positioned on the back side of the specific electrode 30 and a frame 62 that is positioned to surround the outer circumference of the specific electrode 30. An O-ring (not shown) is sandwiched between the backing plate 61 and the frame 62, positioned along the outer edge of the specific electrode 30. The backing plate 61 and the frame 62 can be made of, for example, a resin such as a polyamide resin, or a ceramic such as a ceramic containing SiO2.

[0029] By providing the protective member 6, when the specific electrode 30 is immersed in the liquid 2, parts of the specific electrode 30 other than the electrode surface 33 cannot come into contact with the liquid 2. This prevents the generation of microbubbles on surfaces other than the electrode surface 33 of the specific electrode 30, i.e., the back surface 34 where the protrusions 50 are not formed and the outer edge. Furthermore, it is possible to ensure current density on the electrode surface 33 and efficiently generate microbubbles on the electrode surface 33.

[0030] Furthermore, at least a portion of the contact surface in the bubble generator 1 that comes into contact with the liquid 2, other than the electric field concentration portion 5, is a hydrophilic surface. Examples of hydrophilic surfaces include at least a portion of the inner surface of the container 11 and the surface of the protective member 6 (back plate 61 and frame 62). In addition, in this embodiment, at least a portion of the contact surface other than the electric field concentration portion 5 can be hydrophilic.

[0031] The specific electrode 30 can be made of a semiconductor such as silicon (Si), silicon carbide (SiC), or gallium nitride (GaN), or it can be made of a metal. In this embodiment, the specific electrode 30 is made of an n-type semiconductor. This makes it easy to control the type of gas in the microbubbles generated from the specific electrode 30 by utilizing the rectifying properties of the semiconductor. In addition, since semiconductors have high flatness, semiconductor processes such as resist formation can be easily applied.

[0032] The specific electrode 30 can be made of, for example, n-type Si. That is, in the specific electrode 30 shown in Figure 3, the electrode body 311 and the numerous protrusions 50 can be made of n-type Si. In addition, a metal film 312 is formed on the back surface of the electrode body 311. The metal film 312 can be made of, for example, platinum, gold, nickel, titanium, chromium, aluminum, etc. The specific electrode can also be made of a p-type semiconductor.

[0033] In this embodiment, an example of a method for forming the protrusion 50 on the specific electrode 30 is shown below. As shown in Figures 4 and 5, metal masks M1 and M2 are formed on one main surface of an n-type silicon substrate 3a. In forming the metal masks M1 and M2, first, as shown in Figure 4, a Ni film and an SiO2 film are formed on the main surface of the silicon substrate 3a. These films form the metal mask M1 before patterning. For example, a Ni film approximately 100 nm thick is formed on the main surface of the silicon substrate 3a, and then an SiO2 film approximately 10 nm thick is formed on top of it. The SiO2 film is provided as an oxidation prevention film to avoid unstable etching due to surface oxidation during partial etching of the Ni film. The resistivity of the silicon substrate 3a is, for example, 0.1 Ω·cm or less.

[0034] Next, a photoresist film is deposited on the surface of the metal mask M1 using a spin coater. Then, the photoresist is exposed and developed using a mask aligner to form a dot pattern of photoresist. The diameter of each dot can be, for example, about 4 μm.

[0035] Next, the portion of the metal mask M1 exposed from the photoresist is etched. First, the SiO2 is etched away. BHF (buffered hydrofluoric acid) can be used as the etching solution. After that, the Ni film is etched with, for example, dilute nitric acid. This forms a metal mask M2 with a dot pattern as shown in Figure 5.

[0036] Plasma etching is performed on the silicon substrate 3a on which the dot pattern metal mask M2 is formed. For example, an ICP (inductively coupled plasma) etching apparatus can be used. The conditions can be, for example, as follows:

[0037] CF4 gas and Cl2 gas are introduced into the etching apparatus chamber in a ratio of 37:3, and etching is performed for 2500 seconds at a plasma power of 120W while maintaining a vacuum of 600 torr. Subsequently, CF4 gas and SF6 gas are introduced in a ratio of 45:2, and etching is performed for 3500 seconds at the same plasma power of 120W.

[0038] Figure 6 shows an SEM image of a portion of the substrate surface during the etching process. In the figure, the circular white area is the metal mask M2. After etching is complete, the sample is immersed in a sulfurized aqueous solution to remove any remaining metal mask or foreign matter residue from the surface of the silicon substrate 3a. As a result, a projection 50 is formed as shown in the SEM image in Figure 7.

[0039] Furthermore, a Ni film of approximately 30 nm in thickness is deposited on the electrode surface 33 on which the protrusions 50 are formed. After that, an alloy treatment is performed in a nitrogen atmosphere at 550°C for 30 minutes. In this embodiment, where NaOH is added to the liquid 2, it is effective to use Ni metal as an alkali-resistant metal. However, various metals can be used as the deposited metal depending on the pH of the liquid 2. For example, platinum, gold, nickel, titanium, and chromium can be used as the deposited metal.

[0040] A metal film 312 (see Figure 3) is provided on the back surface of the silicon substrate 3a as an ohmic electrode. For example, a Ti film with a thickness of approximately 20 nm is formed, and then, for example, an Au film with a thickness of approximately 100 nm is deposited on top of it, followed by alloy treatment at 550°C for 30 minutes in a nitrogen atmosphere. This forms the metal film 312. As a result, a specific electrode 30 can be obtained.

[0041] Next, we will explain the effects and benefits of this embodiment. In the bubble generator 1 and bubble generation method described above, a surfactant is added to the liquid 2. This increases the wettability of the liquid 2 to the electrode surface 33. Therefore, the microbubbles generated by the electrolysis of the liquid 2 are more easily released from the specific electrode 30. As a result, the microbubbles can be efficiently extracted into the liquid 2.

[0042] Furthermore, bubbles in a liquid are more easily released from the surface to which they adhere if the liquid has a high wettability to that surface. This is because if liquid 2 is easily wetted to the surface to which it adheres, bubbles are less likely to adhere. Therefore, as described above, the addition of a surfactant to liquid 2 makes it easier for microbubbles in liquid 2 to be released from the specific electrode 30.

[0043] Furthermore, the adhesion of microbubbles can be suppressed not only on the electrode surface 33, but also on the inner surface of the container 11 in the bubble generator 1, the surface of the protective member 6, or the inner surface of the piping connected to the container 11 of the bubble generator 1 (for example, the piping connecting the storage tank and the container 11). This also improves the yield of microbubbles. It is also conceivable to form a hydrophilic film on the inner surface of the container 11, the surface of the protective member 6, the inner surface of the piping, etc., but such a treatment is time-consuming and costly. Moreover, if such a surface treatment is performed, there is a concern that the hydrophilicity may decrease due to deterioration over time, reducing the effect of suppressing the adhesion of microbubbles. In contrast, the present method, which involves adding a surfactant to the liquid 2, eliminates the above concerns.

[0044] Furthermore, the surfactant is a nonionic surfactant. Therefore, microbubbles can be efficiently extracted into liquid 2. An anionic surfactant can also be used. However, from the viewpoint of minimizing impact on the human body and the environment, it is preferable to use a nonionic surfactant.

[0045] Furthermore, the concentration of the surfactant in liquid 2 is 0.03 to 4 times the critical micelle concentration. This allows for smoother release of microbubbles from the specific electrode 30, and enables efficient extraction of microbubbles into liquid 2. Furthermore, if the concentration of the surfactant in liquid 2 falls below 0.03 times the critical micelle concentration, there is a concern that the effect of facilitating the release of microbubbles from the electrode surface 33 may decrease. Also, if the concentration of the surfactant in liquid 2 exceeds 4 times the critical micelle concentration, there is a concern that the surfactant may cover the electrode portion, reducing charge exchange and inhibiting electrolysis.

[0046] As described above, this embodiment provides a bubble generating device and a bubble generating method that can efficiently extract minute bubbles into a liquid.

[0047] (Embodiment 2) As shown in Figure 8, this embodiment is characterized in which a hydrophilic film 32 is formed on at least a portion of the specific electrode 30, excluding the protrusion 50 which is the electric field concentration portion 5. In this embodiment, the hydrophilic film 32 also possesses electrical insulating properties. Furthermore, if the liquid 2 being electrolyzed is water, the hydrophilic film 32 shall be hydrophilic. The hydrophilic film 32 can be composed of, for example, SiO2, Si3N4, or TiO2.

[0048] In this embodiment, the hydrophilic membrane 32 is formed to cover the electrode surface 33 of the specific electrode 30. A portion of the projection 50 protrudes from the hydrophilic membrane 32 at its tip. Therefore, the hydrophilic membrane 32 also covers a portion of the base of the projection 50. Furthermore, of the electrode surface 33 of the specific electrode 30 that is exposed from the protective member 6 (see Figure 3), the portion other than the projection 50 is covered by the hydrophilic membrane 32.

[0049] Furthermore, the surface of the protrusion 50 can also be covered with a hydrophilic film 32. However, in this case, the thickness of the hydrophilic film 32 covering the surface of the protrusion 50 should be made sufficiently small so that tunnel current can pass through, for example, by making it 10 nm or less.

[0050] In forming the specific electrode 30 in this embodiment, for example, after forming the silicon protrusion 50 using the same method as in Embodiment 1 (see Figures 4 to 7), the following steps are performed. Specifically, a hydrophilic film 32 of SiO2 is formed on the electrode surface 33 on which the protrusions 50 are formed by vapor deposition. The thickness of the SiO2 film is such that a portion of the protrusions 50 is exposed, for example, about 4 μm. Vapor deposition can be performed using an EB (electron beam) deposition apparatus. After forming the SiO2 film on the electrode surface 33, a portion of the SiO2 film is etched with buffered hydrofluoric acid to remove the SiO2 film attached to the surface of the protrusions 50, thereby exposing the protrusions 50. Based on the above, a specific electrode 30 can be obtained, as partially shown in the SEM image in Figure 9.

[0051] Otherwise, it is the same as in Embodiment 1. Note that, among the reference numerals used in Embodiment 2 and later, those that are the same as those used in the previously described embodiments represent the same components, etc., as in the previously described embodiments, unless otherwise specified.

[0052] In this embodiment, the release of microbubbles from the specific electrode 30 is further promoted. Furthermore, it has the same effects and advantages as Embodiment 1.

[0053] (Embodiment 3) In this configuration, the voltage applied by the power supply unit 4 to the electrode 3 is a high-frequency pulse voltage, as shown in Figure 10. The pulse voltage profile shown in Figure 10 is obtained by applying an offset voltage of more than half the amplitude of the pulse voltage so that the cathode 3n is always at a lower potential than the anode 3p. The pulse voltage frequency can be, for example, 100 kHz to 10 GHz. The waveform in the same figure shows the time change in the potential of the cathode 3n (specific electrode 30) relative to the anode 3p. Other aspects are the same as in Embodiment 1.

[0054] As in this embodiment, by applying a high-frequency pulse voltage between the electrodes 3, minute bubbles can be efficiently generated in the liquid.

[0055] Furthermore, as described above, by applying an offset voltage so that the cathode 3n is always at a lower potential than the anode 3p, it is possible to limit the minute bubbles generated from the cathode 3n, i.e., the specific electrode 30, to hydrogen only, and prevent the generation of oxygen from the specific electrode 30.

[0056] Furthermore, by using an n-type semiconductor for the specific electrode 30, it is possible to some extent to prevent reverse current from flowing due to its rectifying characteristics. However, it is difficult to raise the breakdown voltage when a reverse voltage is applied, and as a result, it may be difficult to prevent oxygen generation due to the flow of reverse current. Therefore, as in this embodiment, by applying an offset pulse voltage, it becomes easy to generate only hydrogen from the specific electrode 30. Furthermore, it has the same effects and advantages as Embodiment 1.

[0057] (Embodiment 4) This configuration, as shown in Figures 11 and 12, is one in which the anode 3p is arranged along the vertical direction. In this configuration, the anode 3p is positioned so that it does not overlap the cathode 3n when viewed from vertically above. Furthermore, the anode 3p is curved so that it forms a roughly circular arc when viewed from vertically above. In addition, the anode 3p is mesh-like.

[0058] The specific electrode 30, which is the cathode 3n, has its electrode surface 33 facing vertically upward. The specific electrode 30 is formed in a roughly disc shape, and the portion exposed from the protective member 6 is roughly circular. When viewed from vertically above, the anode 3p is arranged on the outer circumference of the exposed portion of the specific electrode 30.

[0059] Furthermore, in this embodiment, the power supply unit 4 is configured to apply a pulse voltage to the electrode 3, similar to the embodiment 3 described above. Otherwise, it is the same as in Embodiment 2.

[0060] In this configuration, the microbubbles generated from the specific electrode 30 can be easily observed from vertically above (see Figure 14 in Experimental Example 1, described later). Furthermore, the amount of microbubbles in the liquid 2 can be easily measured using a sensor or the like. Additionally, the microbubbles extracted into the liquid 2 tend to rise upwards and disperse easily throughout the liquid 2. Furthermore, it has the same effects and advantages as Embodiment 2.

[0061] (Experimental Example 1) This example demonstrates the effect of adding a surfactant to liquid 2 using the bubble generator 1 of Embodiment 3. The basic configuration is the same as in Embodiment 1, except that shown in Embodiment 3, but the configuration of the specific electrode 30 is the same as shown in Embodiment 2 (Figures 8 and 9).

[0062] In this example, liquid 2 was prepared by adding 0.02 mol / L of NaOH to water and tween20 as the surfactant. Comparative tests were then conducted using multiple types of liquid 2, with the surfactant concentration appropriately varied. The surfactant concentration in liquid 2 was varied between 0 and 10.9 as the critical micelle ratio (see Table 1 below). Here, "critical micelle ratio" refers to the ratio to the critical micelle concentration, i.e., the ratio of the surfactant concentration when the critical micelle concentration is set to 1. A critical micelle ratio of 0 means that no surfactant has been added.

[0063] In the test, liquid 2 at various critical micelle ratios was stored in the bubble generator 1 shown in Embodiment 3, and electrolysis was performed by applying a pulse voltage between electrodes 3 under the same conditions. This generated hydrogen from a specific electrode 30. The amount of dissolved hydrogen in liquid 2 was measured at 1 minute, 9 minutes, 18 minutes, and 27 minutes after the start of voltage application. A dissolved hydrogen meter (KM2100DH, manufactured by Kyodo Denshi Kenkyusho) was used for the measurement. The results are shown in Table 1 and Figure 13.

[0064] [Table 1]

[0065] As can be seen from Table 1 and Figure 13, the amount of dissolved hydrogen in liquid 2 is higher in the sample with an appropriate amount of surfactant added (generally a critical micelle ratio of 0.03-4) compared to the sample without surfactant. This is thought to be because the addition of surfactant makes it easier for minute hydrogen bubbles to be released from the specific electrode 30, resulting in a larger amount of dissolved hydrogen in liquid 2. On the other hand, in the sample with a high critical micelle ratio of 10.9, the amount of dissolved hydrogen is actually lower. Therefore, it is thought that an excess of surfactant can have a counterproductive effect. This is thought to be because the surfactant covers part of the electrode surface of the specific electrode 30, reducing conductivity and inhibiting electrolysis.

[0066] Figure 14 shows the electrolysis process without the addition of a surfactant and with the addition of a surfactant at a critical micelle ratio of 3.76. As is clear from the figure, when no surfactant is added, the microbubbles generated at the specific electrode 30 do not become detached but instead aggregate with each other to form larger bubbles. In contrast, this phenomenon is not observed when a surfactant is added.

[0067] The results of this example confirm that by adding an appropriate amount of surfactant to liquid 2, microbubbles can be efficiently extracted into liquid 2.

[0068] (Embodiment 5) As shown in Figures 15 and 16, this embodiment is characterized in that numerous electric field concentration areas 5 provided on a specific electrode 30 are replaced with numerous minute exposed areas 52 that are exposed through numerous minute openings 351 provided in an insulating film 35 covering the electrode surface 33.

[0069] In this embodiment, the insulating film 35 is also a hydrophilic film 32. For example, a metal oxide film such as SiO2, TiO2, or ZrO2 can be used as the insulating film 35. The thickness of the insulating film 35 can be, for example, 30 to 500 nm. The diameter of the minute exposed portion 52 can also be, for example, 30 to 500 nm.

[0070] An example of a method for manufacturing the specific electrode 30 in the bubble generator 1 of this embodiment is described below. For example, an insulating film 35 is formed on one main surface (the main surface that will become the electrode surface 33) of the electrode body 311 made of a silicon substrate (Figure 17). Prior to the formation of the insulating film 35, a Ni metal film is deposited on one main surface of the electrode body 311 to a thickness of approximately 0.1 μm, and then alloy treatment is performed at 550°C for 30 minutes. After that, an insulating film 35 made of SiO2 is deposited on the main surface of the electrode body 311 by sputtering. Also, similar to Embodiment 1, a metal film 312 that will become an ohmic electrode is formed on the back surface of the electrode body 311.

[0071] When sputtering the insulating film 35, the electrode body 311 is placed inside the chamber of the sputtering apparatus, and the temperature inside the chamber is set to 150°C, and 3 × 10 -5 The chamber is evacuated to a vacuum level of approximately Torr. Then, argon gas is introduced into the chamber, and a high-frequency voltage is applied to the SiO2 target inside the chamber to perform sputtering, forming an SiO2 film approximately 30 nm thick on the main surface of the electrode body 311.

[0072] Next, a resist mask RM having dot pattern openings RO is formed on the surface of the insulating film 35, as shown in Figure 18. The resist mask RM can be patterned by photolithography, which is formed by exposing a photosensitive resin, or by nanoimprint lithography. The diameter of each opening RO in the dot pattern is, for example, about 400 nm.

[0073] Next, the insulating film 35 in the RO opening of the resist mask RM is removed to form numerous minute openings 351, as shown in Figure 15. Specifically, the electrode body 311 equipped with the resist mask RM shown in Figure 18 is immersed in a BHF (buffered hydrofluoric acid) solution to partially wet etch the insulating film 35. Alternatively, the insulating film 35 in the RO opening of the resist mask RM is partially removed by ICP dry etching using Cl2 gas. After that, the resist mask RM is removed with oxygen plasma. By doing so, a specific electrode 30 having minute exposed portions 52, as shown in Figure 15, can be obtained. Other aspects are the same as in Embodiment 1.

[0074] In this embodiment, the minute exposed portion 52 functions as the electric field concentration portion 5. This embodiment also has the same effects as Embodiment 1.

[0075] In the above embodiment, an example was shown in which a semiconductor was used as the electrode body 311 of the specific electrode 30, but the electrode body 311 can also be made of metal.

[0076] The present invention is not limited to the embodiments described above, and can be applied to various embodiments without departing from its spirit.

[0077] The features of this invention are as follows. [1] A bubble generator (1) that generates minute bubbles with a diameter of 100 μm or less in a liquid (2) by applying a voltage to the liquid and electrolyzing the liquid, Multiple electrodes (3) are immersed in the above liquid, It has a power supply unit (4) that applies a voltage between the above-mentioned multiple electrodes, At least one of the above-mentioned multiple electrodes is a specific electrode (30) having a number of electric field concentration sections (5) that generate the above-mentioned microbubbles, A bubble generator in which a surfactant is added to the above liquid. [2] The bubble generator according to [1], wherein the surfactant is an anionic surfactant or a nonionic surfactant. [3] The bubble generator according to [1] or [2], wherein the concentration of the surfactant in the liquid is 0.03 to 4 times the critical micelle concentration. [4] The bubble generating device according to any one of [1] to [3], wherein the specific electrode has a hydrophilic film (32) formed on at least a part other than the electric field concentration part. [5] The bubble generating device according to [1] or [2], wherein at least a portion of the contact surface in which the liquid comes into contact with the bubble generating device, other than the electric field concentration portion, is a hydrophilic surface. [6] A bubble generation method comprising applying a voltage to a liquid (2) and electrolyzing the liquid to generate minute bubbles with a diameter of 100 μm or less in the liquid, A voltage is applied between a plurality of electrodes (3) that are at least partially immersed in the above liquid, At least one of the above-mentioned multiple electrodes is a specific electrode (30) having a number of electric field concentration sections that generate the above-mentioned microbubbles, A method for generating bubbles, wherein a surfactant is added to the above liquid. [7] The bubble generation method according to [6], wherein the surfactant is an anionic surfactant or a nonionic surfactant. [8] The bubble generation method according to [6] or [7], wherein the concentration of the surfactant in the liquid is 0.03 to 4 times the critical micelle concentration. [9] The bubble generation method according to any one of [6] to [8], wherein the specific electrode has a hydrophilic film (32) formed on at least a part other than the electric field concentration part.

[10] The bubble generation method according to any one of [6] to [9], wherein at least a portion of the contact surface in the bubble generating device (1) used in the bubble generation method that comes into contact with the liquid, other than the electric field concentration portion, is a hydrophilic surface. [Explanation of Symbols]

[0078] 1. Bubble generator 2 liquid 3 electrodes 30 Specific electrode 4 Power supply section 5. Electric field concentration area

Claims

1. A bubble generator (1) generates minute bubbles with a diameter of 100 μm or less in a liquid (2) by applying a voltage to the liquid and electrolyzing the liquid, Multiple electrodes (3) are immersed in the above liquid, It has a power supply unit (4) that applies a voltage between the above-mentioned plurality of electrodes, At least one of the above-mentioned multiple electrodes is a specific electrode (30) having a number of electric field concentration sections (5) that generate the above-mentioned microbubbles, A bubble generator in which a surfactant is added to the above liquid.

2. The bubble generating device according to claim 1, wherein the surfactant is an anionic surfactant or a nonionic surfactant.

3. The bubble generator according to claim 1 or 2, wherein the concentration of the surfactant in the liquid is 0.03 to 4 times the critical micelle concentration.

4. The bubble generating device according to claim 1 or 2, wherein the above-mentioned specific electrode has a hydrophilic film (32) formed on at least a part other than the electric field concentration portion.

5. The bubble generating device according to claim 1 or 2, wherein at least a portion of the contact surface in which the liquid comes into contact with the bubble generating device, other than the electric field concentration portion, is a surface that is hydrophilic.

6. A bubble generation method comprising applying a voltage to a liquid (2) and electrolyzing the liquid to generate minute bubbles with a diameter of 100 μm or less in the liquid, A voltage is applied between a plurality of electrodes (3) that are at least partially immersed in the above liquid, At least one of the above-mentioned multiple electrodes is a specific electrode (30) having a number of electric field concentration sections that generate the above-mentioned microbubbles, A method for generating bubbles, wherein a surfactant is added to the above liquid.

7. The bubble generation method according to claim 6, wherein the surfactant is an anionic surfactant or a nonionic surfactant.

8. The bubble generation method according to claim 6 or 7, wherein the concentration of the surfactant in the liquid is 0.03 to 4 times the critical micelle concentration.

9. The bubble generation method according to claim 6 or 7, wherein the above-mentioned specific electrode has a hydrophilic film (32) formed on at least a part other than the electric field concentration portion.

10. The bubble generation method according to claim 6 or 7, wherein at least a portion of the contact surface in the bubble generating device (1) used in the bubble generation method that comes into contact with the liquid, other than the electric field concentration portion, is a surface that is hydrophilic.

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