« FöregåendeFortsätt »
METHOD OF FORMING NANOBUBBLES
 The present invention relates to a method of forming nanobubbles which have potential utility in every industrial application and impart special functions, especially to water.
 It has been known that bubbles (microbubbles) having a diameter of not more than 50 um have a nature different from larger bubbles and are utilized in various fields.
 For example, Patent Reference 1 proposes an invention utilizing a nature of microbubbles wherein the presence of microbubbles promotes a physiological activity in creatures and increases metabolism, as a result of which ontogenetic growth is enhanced.
 Recently, bubbles (bubbles having a diameter of not more than 1 um, hereinafter referred to as nanobubbles) with a diameter smaller than that of microbubbles are said to have beneficial effects also from an industrial point of view and have become a focus of attention.
 However, there is no method of forming nanobubbles. At the present state of the art, nanobubbles can momentarily exist only at the time of natural disappearance or collapse of microbubbles. Some nanobubbles with a diameter of the order of 1 um or less can be present in a stable state by the use of a surfactant or an organic substance. Such nanobubbles, however, are encapsulated in a strong shell made up of the surfactant or organic substance, so that they are isolated from the surrounding water. These nanobubbles have not functions such as an activational effect and a bactericidal effect on organisms.
DISCLOSURE OF THE INVENTION
 The present invention has been made in view of the aforementioned circumstances and an object of the invention is to provide a method of forming nanobubbles that remain in a solution for a long time and continue to impart the solution with a function such as an activational effect or a bactericidal effect on organisms.
 The present invention is directed to a method of forming nanobubbles remaining in a solution for a long time. The aforementioned object is achieved by applying physical irritation to microbubbles contained in a liquid so that the microbubbles are abruptly reduced in size.
 The aforementioned object is achieved more effectively by the fact that in the step of abruptly reducing microbubbles in size, when the diameter of the microbubble is reduced to 200 nm or less, the charge density on the surface of the microbubble increases and an electrostatic repulsive force is produced, whereby the size reduction of the microbubble stops; or in the step of abruptly reducing microbubbles in size, due to ions adsorbed on the gas-liquid interface and an electrostatic attraction, both ions in the solution having opposite charges to each other and attracted to the vicinity of the interface are concentrated in a high concentration so as to serve as a shell surrounding the microbubble and inhibit dissolution of a gas within the
microbubble into the solution whereby the microbubble is stabilized, or the ions adsorbed on the gas-liquid interface are hydrogen ions and hydroxide ions and electrolytic ions within the solution are used as the ions attracted to the vicinity of the interface whereby the microbubble is stabilized; or in the step of abruptly reducing microbubbles in size, the temperature within the microbubble sharply rises by adiabatic compression so that a physicochemical change in association with the ultrahigh temperature is applied around the microbubble whereby the microbubble is stabilized.
 The aforementioned object is achieved more effectively when the physical irritation is to discharge static electricity through the microbubbles using a discharge device; when the physical irritation is to apply ultrasonic irradiation to the microbubbles using an ultrasonic generator; when the physical irritation is to cause a solution to flow by driving a rotor mounted in a vessel containing therein the solution and use compression, expansion and vortex flow which are produced during the flow; or when the physical irritation in the case of having a circulating circuit in the vessel to cause compression, expansion and vortex flow of the solution by passing the solution through an orifice or perforated plate having a single hole or a plurality of holes after receiving the solution that contains the microbubbles.
BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 shows the particle size frequency distribution of nanobubbles formed according to the methods of forming nanobubble of the present invention (even distribution: about 140 nm, standard deviation: about 30 nm);
 FIG. 2 shows the relationship between the surface potential of a microbubble and the pH of an aqueous solution;
 FIG. 3 shows the rise in zeta potential associated with the reduction in size of microbubbles;
 FIG. 4 is a schematic view showing a mechanism wherein nanobubbles are present and stable;
 FIG. 5 is a side view of an apparatus for forming nanobubbles using a discharge device;
 FIG. 6 is a side view of an apparatus for forming nanobubbles using an ultrasonic generator;
 FIG. 7 is a side view of an apparatus for forming nanobubbles by causing vortex flow; and
 FIG. 8 is a side view of an apparatus for forming nanobubbles by causing vortex flow by a rotator.
 5 circulating pump
 6 orifice plate (perforated plate)
 7 rotator
BEST MODE FOR CARRYING OUT THE
 Nanobubbles formed by the present invention are characterized by remaining in a solution for a long time; as long as one or more months. Depending upon the nature of the gas within nanobubbles, the solution containing nanobubbles therein provides a physiological activation effect on organisms; a killing or antiproliferative effect on microorganisms such as bacteria and viruses; or a chemical reaction with an organic or inorganic substance.
 The nature of nanobubbles and a method of forming nanobubbles will be described below. For illustrative convenience, descriptions are given only for the case of an aqueous solution.
 The nanobubbles formed in accordance with the methods of forming nanobubbles of the present invention have a particle size or bubble diameter of not more than 200 nm. The nanobubbles formed in accordance with the methods of forming nanobubbles of the present invention remain in an aqueous solution for a long time; as long as one or more months. A preservation method of the aqueous solution containing nanobubbles therein is not particularly limited. Even when such a solution is stored in an ordinary vessel, the nanobubbles will not disappear for one or more months.
 The physical property of a microbubble is to have a surface potential depending on the pH of the aqueous solution as shown in FIG. 2. This is because a hydrogenbonding network of water at the gas-liquid interface requires more hydrogen ions and hydroxide ions as configuration factors. Since the electric charge keeps the equilibrium condition with respect to the surrounding water, it is constant regardless of the bubble diameter. Furthermore, an electrostatic force acts due to the static electrification on the surface, so that ions having the opposite electric charge are attracted to the vicinity of the gas-liquid interface.
 While the equilibrium charge state of a microbubble is maintained, if the microbubble is reduced in size within a short time, electric charges are concentrated. FIG. 3 shows the change of surface potential when the bubble diameter is reduced from about 25 |xm to about 5 (m for 10 seconds. It can be seen from FIG. 3 that reduction in bubble diameter causes deviation from the normal equilibrium condition, which results in the concentration of the electric charges. When this size-reduction speed is made higher increased and the bubble diameter is made smaller reduced, the charge amount per unit area increases inversely with the square of the bubble diameter.
 Since the microbubble is surrounded by a gasliquid interface, the interior of the microbubble is subjected to self-pressurization under the influence of a surface tension. The pressure rise in a micro bubble with respect to an environmental pressure can be evaluated through the YoungLaplace equation.
AP=4a/D (Eq. 1)
 Wherein AP is a the pressure rise variation, a is a the surface tension, and D is a the bubble diameter. In the
case of distilled water at room temperature, for a microbubble with a diameter of 10 (im, its internal pressure rises to about 0.3 atmospheric pressures, and for a microbubble with a diameter of 1 (im, its internal pressure rises to about 3 atmospheric pressures. The gas within the self-pressurized microbubble dissolves in water according to the Henry's law. Thus, the bubble diameter is gradually reduces reduced, which increases the internal pressure of the bubble so that the bubble diameter reduction rate is accelerated. As a result, bubbles with a diameter of not more than 1 |xm are completely dissolved almost instantly. That is, nanobubbles can be present only for an instant moment.
 In contrast, according to the methods of forming nanobubbles of the present invention, microbubbles having a diameter of 10 |xm to 50 |xm are abruptly reduced by physical irritation. When the aqueous solution containing microbubbles therein is mixed with electrolytes of ferrous ions, manganese ions, calcium ions, sodium ions, magnesium ions or any other mineral ion such that the electrical conductivity in the aqueous solution containing microbubbles therein becomes not less than 300 |xS/cm, the reduction in size of the bubbles is inhibited by electrostatic repulsive force. As used herein, the electrostatic repulsive force is an electrostatic force that acts on ions having the same charge and located in a diametrically opposed relationship to one another with respect to a spherical microbubble due to the increase in curvature of the sphere caused by the reduction in size of the microbubble. Since the microbubble reduced in size is subjected to pressure from surface tension, the tendency to reduce in size increases with the reduction in size of the microbubble. However, when the bubble diameter becomes smaller than 500 nm, the electrostatic repulsive force becomes evident and reduction in size of the bubble stops.
 When the aqueous solution is mixed with electrolytes of ferrous ions, manganese ions, calcium ions, sodium ions, magnesium ions or any other mineral ion such that the electrical conductivity in the aqueous solution becomes not less than 300 |xS/cm, the electrostatic repulsive force sufficiently acts such that the force reducing the bubble in size and the electrostatic repulsive force are balanced, as a result of which the bubble is stabilized. While the diameter of the so stabilized bubble (nanobubble diameter) differs depending upon the concentration and type of the electrolytic ion, it becomes as small as 200 nm or less as shown in FIG. 1.
 The characteristics of the nanobubble are not only to keep gas therewithin in a pressurized state, but also to form a significantly strong electric field through the concentrated surface electric charges. This strong electric field exerts great influence upon the gas within the bubble and the aqueous solution around the bubble, which imparts the aqueous solution with a physiological activational effect, a bactericidal effect on organisms, chemical reactivity, etc.
 FIG. 4 shows a mechanism where nanobubbles are present and stable. In the case of a nanobubble, electric charges are present on the gas-liquid interface in a significantly concentrated manner, so that the electrostatic repulsive force, which acts between the ions located in a diametrically opposed relationship to one another with respect to the sphere, inhibits the sphere (bubble) from being contracted. The concentrated high electric field serves to form an inorganic shell mainly composed of electrolytic ions