Patent Publication Number: US-2009236269-A1

Title: Classification method and classification apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2008-073773 filed Mar. 21, 2008. 
     BACKGROUND 
     1. Technical Field 
     This invention relates to a classification method and a classification apparatus. 
     2. Related Art 
     Methods of classifying fine particles are divided into dry methods and the wet methods. Some of the dry methods can achieve high accuracy owing to a large difference in specific gravity between a fluid and fine particles. Although a difference in specific gravity between a liquid and fine particles is small in wet methods, high classification accuracy can be achieved thereby in the case of a fine powder since fine particles can be easily dispersed in the liquid. By a classification apparatus which usually comprises a rotor in a rotary unit and a stator in a stationary unit, classification is carried out due to the balance between centrifugal force and inertial force. In the dry methods, there have been marketed rotational unit-free classification devices in which “Coanda effect” is employed. In recent years, on the other hand, various studies have been on methods of conducting chemical reactions and unit operations in the micro area and examinations have been made on methods and apparatuses for effectively classifying fine particles without causing contamination and so on. 
     SUMMARY 
     According to an aspect of the invention, there is provided a classification apparatus, including: 
     a dispersion liquid inlet channel that introduces a dispersion liquid containing particles; 
     a classification channel that classifies the particles; and 
     at least one discharge channel that discharges the classified particles, 
     wherein the classification channel is provided inclinedly to a direction of gravity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic sectional view which shows the first exemplary embodiment of a classification apparatus that is appropriately usable in the present embodiment; 
         FIG. 2  is a schematic sectional view which shows the second exemplary embodiment of a classification apparatus that is appropriately usable in the present embodiment; 
         FIGS. 3A to 3D  are model views which show a case where large and small particles are sent to the classification apparatus of the second exemplary embodiment; 
         FIGS. 4A and 4B  are perspective model views which show a third exemplary embodiment of a classification apparatus that is appropriately usable in the present embodiment; 
         FIGS. 5A to 5F  are a flow chart which shows an exemplary example of the method of producing a classification apparatus that is appropriately usable in the third exemplary embodiment; and 
         FIG. 6  shows particle size distributions of the dispersion liquid A and the dispersion liquids (B 1  and B 3 ) discharged from the discharge channels  4   a  and  4   c.    
     
    
    
     DETAILED DESCRIPTION 
     The classification apparatus in the preset embodiment comprises: a dispersion liquid inlet channel for introducing a dispersion liquid containing particles; a classification channel for classifying the particles; and a discharge channel for discharging the particles having been classified, wherein the classification channel is provided inclinedly to the direction of gravity. 
     The classification method in the present embodiment comprises: a dispersion liquid inlet step of introducing a dispersion liquid containing particles into a dispersion liquid inlet channel; a classification step of classifying the dispersion liquid by passing through a classification channel that is provided inclinedly to the direction of gravity; and a discharge step of discharging the particles having been classified from a discharge channel. 
     In the embodiment, the dispersion medium of the dispersion liquid containing particles will be simply called “dispersion medium” too. 
     &lt;Classification Channel&gt; 
     In the classification apparatus of the embodiment, the classification channel is provided inclinedly to the direction of gravity. The classification method of the embodiment involves the classification step of classifying the dispersion liquid by passing through the classification channel that is provided inclinedly to the direction of gravity. 
     In the case where the particles have a larger specific gravity than the dispersion medium of the dispersion liquid containing particles, the particles sediment at a velocity proportional to the square of the particle diameter. Particles having larger particle diameter sediment quickly and collide with the inclined face (bottom face) of the classification channel. In the case of a laminar flow, the flow velocity around the wall face is almost zero. Thus, the particles having collided with the bottom face drop along the inclined face due to gravity followed by discharge from a discharge channel provided in the lower part of the classification channel. On the other hand, particles having smaller particle diameter are discharged as such front a discharge channel provided in the upper part of the classification channel without colliding with the inclined face. In this embodiment, one or more discharge channels may be provided. The discharge channel in the lower part of the classification channel can be substituted by a particle reservoir. 
     The inclined angle of the classification channel in the embodiment can be appropriately determined so long as being larger than 0°. From the viewpoint that particles drop along the inclined face, an angle of 15° or larger is preferred. From the viewpoint of ensuring a sufficient classification efficiency, an angle not larger than 75° is preferred. 
     The inclined angle of the classification channel is more preferably 20° or larger but not larger than 70° and still preferably 30° or larger but not larger than 60°. 
     The incline of the classification channel means the incline of the classification channel bottom face to the direction of gravity. For example, a horizontal channel has an inclined angle of 0° C. In this embodiment, the inclined angle of the top face of the classification channel is not specifically restricted. 
     In this embodiment, it is preferable that the classification channel is a microchannel. When the classification channel is a microchannel, the sedimentation distance is short so that the sedimentation time toward the inclined wall is drastically shortened, which contributes to an increase in the efficiency. In this case, furthermore, the laminar flow can be maintained even at a high flow velocity, which makes it possible to prevent a lowering in the classification ability caused by a turbulent flow. Also, a microchannel is preferable from the viewpoint that the flow velocity of the particles is almost zero around the wall of the classification channel in the case of the laminar flow, which contributes to the improvement in the classification efficiency. 
     As the microchannel, use may be preferably made of a channel having a width of several to several thousand μm. The classification apparatus in this embodiment may be a reactor having a plurality of microscale classification channels. 
     Because of being in the microscale, a microchannel has a small size (representative length) and a small flow velocity and the Reynolds number thereof is 2,300 or less. Therefore, the apparatus having such a microscale channel is not turbulent flow dominant just as a regular apparatus but laminar layer dominant. 
     The Reynolds number (Re) is determined as follows. When it is 2,300 or less, the apparatus is laminar layer dominant. 
     The Reynolds number (Re) is proportional to flow velocity (u(m/s)) and representative length (L(m)). 
     
       
         
           
             
               
                 
                   
                     R 
                     e 
                   
                   = 
                   
                     
                       uL 
                       v 
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In this formula, v stands for the coefficient of kinetic viscosity (m 2 /s) of a fluid. 
     In the case of a channel having a rectangular cross-section, the representative length (L(m)) is defined by the following formula. 
     
       
         
           
             
               
                 
                   L 
                   = 
                   
                     
                       
                         4 
                          
                         
                             
                         
                          
                         S 
                       
                       
                         l 
                         p 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In this formula, S stands for the cross-section area (m 2 ), and l p  stands for the peripheral length (m). 
     Referring the width in the cross-section of the rectangular channel as to x (m) and the height thereof as to t (m), the following formula (3) is established 
         S=tx l   p =2( x+t )   (3) 
     Referring the flow amount of a fluid as to a (m 3 /s), the following formula (4) is established. 
     
       
         
           
             
               
                 
                   u 
                   = 
                   
                     a 
                     S 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     By assigning the formulae (2), (3) and (4) to the formula (1), the following formula (5) is derived. 
     
       
         
           
             
               
                 
                   
                     R 
                     e 
                   
                   = 
                   
                     
                       
                         2 
                          
                         a 
                       
                       v 
                     
                     · 
                     
                       1 
                       
                         x 
                         + 
                         t 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Next, it will be supposed that purified water is sent into the rectangular channel at a constant flow velocity (for example, 10 ml/h). The coefficient of kinetic viscosity of purified water at 25° C. is 0.893×10 −7  m 2 /s. 
     Provided that the channel height t is constant while the channel width x is variable, the Reynolds number is in inverse proportion to the channel width. 
     Thus, a channel having a Reynolds number of 2,300 or less can be designed. So long as the height t is sufficiently small, a laminar flow can be maintained though the channel width x increases. 
     Next, the classification principle in the classification apparatus of the present embodiment will be illustrated. 
     &lt;Classification Principle&gt; 
     To recover particles having a desired particle diameter or less from a dispersion liquid in the case where a dispersion liquid is sent into a classification channel upward, i.e., in the direction opposite to the direction of gravity, particles having a terminal velocity lower than the upward flow velocity move with the upward flow and thus sent to the upper part of the classification channel. On the other hand, particles having a terminal velocity higher than the upward flow velocity sediment in the direction of gravity. By providing a discharge channel in the upper part of the classification channel, the particles having the desired particle diameter or less can be recovered. By providing a discharge channel in the lower part of the classification channel, the particles having the desired particle diameter or more can be recovered. By inclining the classification channel to the direction of gravity, the upward flow velocity can be lowered, which enables efficient classification. 
     The classification apparatus and classification method according to an aspect of the invention are an apparatus and a method for classifying fine particles whereby particles in a dispersion liquid are classified with the use of a classification channel (preferably a microchannel) by taking advantage of difference in sedimentation velocity between the particles. The classification apparatus includes a dispersion liquid inlet channel, a classification channel and a discharge channel optionally together with a transportation liquid inlet channel. In this embodiment, it is preferable that the liquids are sent as laminar flows in all channels. 
     In the present embodiment, the particles coming into contact with the inclined face due to the sedimentation sediment along the bottom face of the classification channel (i.e., the inclined wall face), because the bottom face of the classification channel is inclined. Since the flow velocity at the wall face is almost zero in the laminar flow as described above, the particles coming into contact with the bottom face of the classification channel are scarcely affected by the upward flow. Owing to the difference in specific gravity from the dispersion medium, these particles are sedimented by gravity. As a result, the classification can be completed by using a channel having a shorter length compared with the existing sedimentation-type classification apparatus and the particles can be classified within a shorter time. 
     In the case where the specific gravity of particles exceeds the specific gravity of the dispersion medium, the particles sediment and the sedimentation velocity varies depending on the specific gravity or particle diameter of the particles. The particles are classified by taking advantage of this difference in sedimentation velocity. Since the sedimentation velocity is in proportion to the square of the particle diameter, particles having a larger particle diameter sediment at the higher velocity then there are particles differing in particle diameter. 
     In this embodiment, it becomes possible to broaden the range of particles to which the classification method is applicable by loading an external force being in proportion to particle diameter in addition to the difference in sedimentation velocity. As such an external force, an electric field or a magnetic field may be cited. 
     &lt;Substitute Fluid&gt; 
     When particles sediment, the fluid flows into the positions at which the particles once exist, which results in the generation of a micro-scale upward flow. This phenomenon, which is called Boycott effect, causes agitation of the particles even in the laminar flow, thereby lowering the classification efficiency. 
     In the classification apparatus according to an aspect of the present embodiment, in contrast thereto, particles are separated in the upward flow and, therefore, not affected by Boycott effect. Therefore, the separation can be highly effectively carried out. 
     &lt;Particles&gt; 
     Although the particles to be classified in this embodiment are not specifically restricted in size, it is preferable that the diameter (the diameter or the maximum diameter) of the particles is 0.1 μm or more but not more than 1,000 μm. The classification apparatus and classification method of the embodiment are suitable for classifying particles of 1 μm or more but not more than 100 μm in particle diameter, and more suitable for particles of 5 μm or more but not more than 20 μm in particle diameter. 
     It is preferable that the particle diameter is 1,000 μm or less, since the channel can be prevented from clogging in this case. On the other hand, it is preferable that the particle diameter is 0.1 μm or more, since such particles would hardly stick to the wall face. 
     The kind of the particles to be classified is not specifically restricted. Namely, they may be resin fine particles, inorganic fine particles, metal fine particles, ceramic fine particles, cells (for example, lymphocytes, leucocytes, erythrocytes, and so on), etc. without restriction. It is also possible to use a biological sample (whole blood) having been appropriately diluted if needed may be used as the dispersion medium. 
     Moreover, it is possible to classify high molecular-weight fine particles, crystals or aggregates of an organic matter such as a pigment, crystals or aggregates of an inorganic matter, a metal oxide, a metal nitride, fine particles of a metal compound such as a metal nitride, toner particles, and so on. 
     The particles may have arbitrary shapes such as spheres, spheroids, irregular shapes, needles, etc. without specific restriction. Among them, spherical and/or spheroidal particles are preferred. The ratio of the major axis length to the minor axis length (major axis length/minor axis length) is preferably 1 or more but not more than 50 and still preferably 1 or more but not more than 20. 
     Specific examples of the high molecular-weight fine particles include fine particles of a polyvinyl butyral resin, a polyvinyl acetal resin, a polyallylate resin, a polycarbonate resin, a polyester resin, a phenoxy resin, a polyvinyl chloride resin, a polyvinyl acetate resin, a polystyrene resin, an acrylic resin, a methacrylic resin, a styrene-acryl resin, a styrene-methacryl resin, a polyacrylamide resin, a polyamide resin, a polyvinylpyridine resin, a cellulose-based resin, a polyurethane resin, an epoxy resin, a silicone resin, a polyvinyl alcohol resin, casein, a vinyl chloride-vinyl acetate copolymer, a denatured vinyl chloride-vinyl acetate copolymer, a vinyl chloride-vinyl acetate-maleic anhydride copolymer, a styrene-butadiene copolymer, a vinylidene chloride-acrylonitrile copolymer, a styrene-alkyd resin, a phenol-formaldehyde resin and so on. 
     Examples of the metals or the fine particles of a metal compound include carbon black, a metal such as zinc, aluminum, copper, iron, nickel, chromium or titanium or an alloy of the same, a metal oxide such as TiO 2 , SnO 2 , Sb 2 O 3 , In 2 O 3 , ZnO, MgO or iron oxide or a compound containing the same, a metal nitride such as silicon nitride or fine particles containing a combination of the same. 
     Although these fine particles are produced by various methods, it has been a common practice to produce fine particles via synthesis in a liquid medium (dispersion medium) followed by the classification of the fine particles as such. In some cases, a product in the form of a mass is mechanically pulverized and the fine particles thus obtained are dispersed in a liquid medium followed by classification. In such a case, the pulverization is frequently carried out in a liquid medium (dispersion medium) and the obtained particles are classified as such. 
     In the case of classifying a powder (fine particles) produced by the dry method, on the other hand, the fine particles should be preliminarily dispersed in a liquid medium. The methods of dispersing a dry pulverized powder in a liquid medium include a method using a sand mill, a colloid mill, an atoraita, a ball mill, a daino mill, a high-pressure homogenizer, a supersonic disperser, a coball mill, a roll mill or the like. It is preferable that this method be carried out under such conditions that the primary particles are not pulverized by the dispersing treatment. 
     &lt;Dispersion Medium and Transportation Liquid&gt; 
     As the dispersion medium of the dispersion liquid containing the particles and the transportation liquid, any solvent may be used without specific restriction. However, use is made of a solvent having a specific gravity that is smaller than the specific gravity of at least one kind of particles in the dispersion liquid. It is also preferred to use a solvent having a specific gravity that is smaller than the specific gravities of all particles in the dispersion liquid. The transportation liquid means a particle-free solvent that is to be sent to the classification channel. 
     Also, it is preferable that the difference obtained by subtracting the specific gravity of the dispersion medium or transportation liquid from the specific gravity of the particles is each 0.01 or more. Although a larger difference in specific gravity is preferred since the higher sedimentation velocity of the particles can be thus obtained, it is preferred that this difference is not more than 20. It is more preferable that the difference in specific gravity is from 0.05 to 11 and still preferably from 0.05 to 4. It is preferable that the difference obtained by subtracting the specific gravity of the dispersion medium or transportation liquid from the specific gravity of the particles is each 0.01 or more, since the particles can be thus sedimented. On the other hand, it is also preferable that this difference is not more than 20, since an appropriate sedimentation velocity can be thus achieved and clogging hardly occurs. 
     The dispersion medium and the transportation liquid may be either the same or different. 
     As the dispersion medium or the transportation liquid, use may be preferably made of one showing a difference in specific gravity of 0.01 to 20 from the specific gravity of the particles, as described above. Examples thereof include water, an aqueous medium, an organic solvent-based medium and so on. 
     Examples of the water include ion exchange water, distilled water, electrolytic ionic water and the like. Specific examples of the organic solvent-based solvent includes methanol, ethanol, n-propanol, n-butanol, benzyl alcohol, methyl cellosolve, ethyl cellosolve, acetone, methyl ethyl ketone, cyclohexanone, methyl acetate, n-butyl acetate, dioxane, tetrahydrofuran, methylene chloride, chloroform, chlorobenzene, toluene, xylene etc. and a mixture of two or more kinds of the same. 
     In the present embodiment, a preferable liquid medium and a preferable transportation liquid differ depending upon the kind of the particles to be classified. The liquid media and transportation liquids that are preferable for respective kinds of the particles are as follows. Examples of the liquid medium to be combined with the high molecular-weight particle (generally, having specific gravity of about 1.05 to about 1.6) include aqueous solvents, alcohols, organic solvents such as xylene, acid or alkali water etc in which the particles are insoluble. 
     Examples of the liquid medium to be combined with the metal or metal compound particle (generally, having specific gravity of about 2 to about 10) include water, alcohols, organic solvents such as xylene and oils which cause no damage on a metal due to oxidation or reduction. 
     Next, the present embodiment will be illustrated by referring to the drawings. 
     In the following description, the same symbol represents the same subject. 
     First Exemplary Embodiment 
     Next, the first exemplary embodiment of the present embodiment will be illustrated by referring to  FIG. 1 . 
       FIG. 1  is a schematic sectional view showing the first exemplary embodiment of a classification apparatus that is appropriately usable in the present embodiment. 
     A classification apparatus  1  includes a dispersion liquid inlet channel  2  for introducing a solution containing particles (dispersion liquid) A, a classification channel  3  that is continuing the dispersion liquid inlet channel  2  and inclined at an angle θ to the direction of gravity (45° in  FIG. 1 ), and a discharge channel  4  for discharging the classified particles which is continuing the classification channel  3 . 
     In  FIG. 1 , a plate-like microchannel having a depth of 500 μm is formed in the depth direction perpendicular to the page. The arrow in  FIG. 1  indicates the direction of gravity. 
     In the classification apparatus  1  shown in  FIG. 1 , the dispersion liquid A is fed from the dispersion liquid inlet  2 ′ that is formed in the lower part of the classification apparatus  1  and sent in the direction opposite to the direction of gravity (vertical direction) via the dispersion liquid inlet channel  2 . The method of introducing the dispersion liquid A is not specifically restricted and an appropriate method can be selected from among publicly known ones. It is preferable to inject the dispersion liquid A with a microsyringe, a rotary pump, a screw pump, a centrifugal pump, a piezo pump, etc. 
     In  FIG. 1 , the dispersion liquid A is sent to the direction opposite to the vertical direction. However, the invention is not restricted thereto so long as the dispersion liquid inlet channel  2  is provided in the lower part of the classification apparatus  1 . That is, the dispersion liquid inlet channel  2  may be provided horizontally to the lower part of the classification channel  3  or the dispersion liquid inlet channel  2  may extend from the classification channel  3  in an inclined state. It is particularly preferable to send the liquid in the direction of gravity, since sedimentation of the particles can be thus prevented. Namely, the constitution is not specifically restricted so long as the dispersion liquid A is sent from the lower part of the classification channel  3 . 
     Next, the classification apparatus  1  of the first exemplary embodiment will be illustrated by presenting as an example a case where the dispersion liquid A contains particles having three kinds (small, medium and large) of diameters (particles a (small), particles b (medium) and particles c (large)). 
     Concerning the small particles a among the particles contained in the dispersion liquid A in the classification apparatus as shown in  FIG. 1 , the rising velocity of the dispersion liquid A is higher than the terminal velocity of the small particles a. Thus, the small particles a are discharged as a discharged liquid B from the discharge channel  4  provided in the upper part of the classification channel  3 . 
     Terminal velocity means the kinetic velocity of a single particle that moves in a static flow under an external force in the state where the external force applied to the particle equals the resistance from the fluid. In  FIG. 1 , the external force applied to the particles is gravity. When the dispersion liquid is sent at a flow velocity higher than the terminal velocity, the particles rise. When the dispersion liquid is sent at a flow velocity lower than the terminal velocity, on the contrary, the particles drop. 
     On the other hand, the large particles c sediment due to gravity and come into contact with the bottom face  5  of the classification channel  3 , since the terminal velocity thereof is higher than the flow velocity of the dispersion liquid A. The flow velocity at the wall face is almost zero in a microchannel. When the large particles c sediment and thus come into contact with the bottom face  5  of the classification channel  3 , they sediment along the wall face at a higher flow velocity than in the case of sedimenting in the fluid. In the microchannel apparatus of  FIG. 1 , therefore classification can be quickly carried out compared with the case where no inclined classification channel is formed. 
     In  FIG. 1 , the large particles  3  having sedimented along the bottom face  5  of the classification channel  3  are recovered into a particle reservoir  7 . To maintain the sending pressure of the dispersion liquid at a high level, it is preferable to minimize channels opened outward such as the discharge channel. In the case of treating a definite amount of the dispersion liquid, therefore, use can be preferably made of the system wherein the large particles c are recovered into the particle reservoir  7  to prevent a lowering in the sending pressure of the dispersion liquid. 
     However, the present embodiment is not restricted to the above case. That is, a second discharge channel may be provided as a substitute for the particle reservoir. In the case of continuously treating the dispersion liquid, it is preferable that a second discharge channel is provided as a substitute for the particle reservoir. 
     Concerning the particles b having a particle diameter intermediate between the small particles a and the large particles c in  FIG. 1 , the flow velocity of the dispersion liquid is set a slightly lower than the terminal velocity of the particles b. The particles b gradually sediment and come into contact with the bottom face  5  in the downstream (upper) side of the classification channel  3 . Then, the particles b sediment along the bottom face of the classification channel  3  and recovered into the particle reservoir  7 . 
     Although the flow velocity of the dispersion liquid is set lower than the terminal velocity of the particles b in  FIG. 1 , the present embodiment is not restricted thereto. Namely, it is also possible that the flow velocity of the dispersion liquid is set higher (faster) than the terminal velocity of the particles b so that the particles b are discharged from the discharge channel  4  and recovered into the discharged liquid B. 
     In  FIG. 1 , the particles are classified into two portions, i.e., the particles discharged from the discharge channel  4  and those collected into the particle reservoir  7 . It is also possible to classify the particles in three or more portions by providing additional discharge channel(s) in the course of the classification channel  3 . 
     Next, a method of producing the classification apparatus according to the present embodiment will be illustrated. 
     The classification apparatus of the embodiment can be formed on a solid substrate by using a microfabricating technique. 
     Examples of the material usable in the solid substrate include metals, silicon, Teflon™, glass, ceramics, plastics, and so on. Among them, metals, silicon, Teflon™, glass and ceramics are preferred from the viewpoints of heat resistance, pressure resistance, solvent resistance, and light transmission, and glass is particularly preferred. 
     The microfabrication techniques for forming the channels are described in, for example, Maikuroriakuta-Shin&#39;Jidai no Gosei Gijyutu ( Microreactor—A Synthetic Technique in the New Age ), 2003, published by CMC, supervised by Junichi YOSHIDA; Bisai Kako Gijutsu: Oyo-hen: Fotonikusu Erekutoronikus e no Oyo ( Microprocessing Techniques: Application: Application to Photonics Electronics Mechatronics ), 2003, published by NTS, edited by Event Committee, The Society of Polymer Science, Japan; and so on. 
     Typical examples of these methods include the LIGA technique using X-ray lithography, the high-aspect-ratio photolithography using EPON SU-8, the microelectric discharge machining (μ-EDM), the high-aspect-ratio machining of silicon by Deep RIE, the hot emboss machining, the stereo lithography, the laser machining, the ion beam machining, the mechanical microcutting using a microtool made of a hard material such as diamond, and so on. These techniques may be used either alone or in combination. The preferred microfabrication techniques are the LIGA technique using X-ray lithography, the high-aspect-ratio photolithography using EPSON SU-8, the microelectric discharge machining (μ-EDM), and the mechanical microcutting. 
     The channels used in the present embodiment can also be formed by using a pattern formed by using a photoresist on a silicon wafer as a mold and pouring a resin thereinto followed by hardening (the molding method). In the molding method, use can be made of a silicon resin typified by dimethylpolysiloxane (PDMS) or its derivatives. 
     In fabricating the classification apparatus according to the present embodiment, a joining technique can be used. Usual joining techniques are roughly classified into the solid-phase joining and the liquid-phase joining. Typical examples of the joining methods commonly used include the solid-phase joining such as the pressure joining and the diffusion joining and the liquid-phase joining such as the welding, the eutectic bonding, the soldering, the adhesion, etc. 
     In the joining, it is desirable to employ a highly accurate joining method by which the dimension accuracy can be kept without the destruction of the microstructure of the channel, etc., caused by deterioration or large deformation of a material due to high-temperature heating. Examples of such a technique include the silicon direct joining, the anodic joining, the surface activation joining, the direct joining using hydrogen bond, the joining using an aqueous HF solution, the Au—Si eutectic joining, the void-free joining, the diffusion joining, etc. 
     The classification apparatus according to the present embodiment can be formed by laminating pattern elements (film pattern elements). The thickness of the pattern elements preferably ranges from 5 to 50 μm and more preferably from 10 to 30 μm. The classification apparatus according to the embodiment may be a classification apparatus that is formed by laminating pattern elements carrying definite two-dimensional patterns formed thereon. It is preferable that the pattern elements are laminated in such a manner as the faces thereof being in direct contact. More specifically, reference can be made to the production method that is disclosed in JP-A-2006-187684. 
     In the classification apparatus shown in  FIG. 1 , the channels have a rectangular cross-section. The width of the dispersion liquid inlet channel  2  is 500 μm, the width of the classification channel  3  is 1,000 μm and the width of the discharge channel  4  is 500 μm. Each of these channels has a depth (the depth direction perpendicular to the page) of 500 μm. 
     The length of the dispersion liquid inlet channel  2  is 20 mm, the length of the classification channel  3  is 50 mm and the length of the discharge channel  4  is 10 mm. 
     In the first exemplary embodiment, particles of a desired particle diameter can be classified by appropriately selecting the specific gravity and diameter of the particles contained in the dispersion liquid, the specific gravity of the dispersion medium, the sending velocity of the dispersion liquid, etc. Although the classification ability is improved with an increase in the length of the classification channel, a longer channel brings about an increase in the capacity required for the classification apparatus. Therefore, it is preferable to select an appropriate channel length depending on the purpose. 
     Second Exemplary Embodiment 
     Next, the second exemplary embodiment of the present embodiment will be illustrated by referring to  FIG. 2 . 
       FIG. 2  is a schematic sectional view showing the second exemplary embodiment of a classification apparatus that is appropriately usable in the present embodiment. In a classification apparatus  1  shown in  FIG. 2 , a plate-like microchannel having a depth of 500 μm is formed in the depth direction perpendicular to the page. The arrow in  FIG. 2  indicates the direction of gravity. 
     In  FIG. 2 , the classification apparatus  1  comprises a dispersion liquid inlet channel  2  for introducing a solution containing particles (dispersion liquid) A, a classification channel  3  that is continuing the dispersion liquid inlet channel  2  and inclined at an angle θ to the direction of gravity, and discharge channels  4   a,    4   b  and  4   c  for discharging the classified particles that are continuing the classification channel  3 . In the second exemplary embodiment, the classification apparatus  1  also has a transportation liquid inlet channel  6 . 
     In the second exemplary embodiment, the dispersion liquid A is fed from the dispersion liquid inlet  2 ′ that is formed in the upper part of the classification apparatus  1  and sent in the direction of gravity. 
     In the second exemplary embodiment, the classification channel  3  is provided in such a manner that the cross-section area thereof increases along the traveling direction of the dispersion liquid A. 
     By providing the classification channel in such a manner that the cross-section area thereof increases along the traveling direction of the dispersion liquid, the following advantages can be obtained. Namely, when the dispersion liquid is sent at a low velocity, clogging sometimes arises in the dispersion liquid inlet channel and/or the classification channel. Therefore, it is required to send the dispersion liquid at such a velocity as not causing the clogging. When the dispersion liquid is sent at an excessively high velocity on the other hand, its velocity exceeds the terminal velocity of the particles. As a result, the particles cannot be sufficiently classified. 
     In the case where the cross-section area thereof increases along the traveling direction of the dispersion liquid, the flow velocity decreases as the liquid is sent toward the downstream. Therefore, the classification can be sufficiently carried out even though the dispersion liquid is sent at a high speed in the upstream. Thus, the classification efficiency can be improved while avoiding clogging. 
     In the second exemplary embodiment, the dispersion liquid A is fed from the dispersion liquid inlet  2 ′ that is formed in the upper part of the classification apparatus  1  and sent in the direction of gravity via the dispersion liquid inlet channel  2 . On the other hand, the transportation liquid C is fed from the transportation liquid inlet  6 ′ that is formed in the lower part of the classification apparatus  1  and sent in the direction opposite to the direction of gravity via the transportation liquid inlet channel  6 . The methods of introducing the dispersion liquid A and the transportation liquid C are not specifically restricted and appropriate methods can be selected from among publicly known ones as in the first exemplary embodiment. Also, the invention is not restricted to the embodiment as shown in  FIG. 2  so long as the liquids are fed into the dispersion liquid inlet channel  2  and the transportation liquid inlet channel  6  respectively from the upper and lower parts of the classification apparatus  1 . 
     By sending the dispersion liquid A in the direction of gravity in the dispersion liquid inlet channel  2  formed in the upper part of the classification apparatus  1  as shown in  FIG. 2 , the dispersion liquid A can be prevented from clogging. By sending the transportation liquid  6  in the direction opposite to gravity in the transportation liquid inlet channel  6  formed in the lower part of the classification apparatus  1 , there arises a laminar flow that is send upward from the lower part of the classification channel  3 . 
     The dispersion liquid A sent from the upper part and the transportation liquid C sent from the lower part join together in the classification channel  3  and form an interface, thereby forming a laminar flow. 
     Next, the classification apparatus  1  of the second exemplary embodiment will be illustrated by presenting as an example a case where the dispersion liquid A contains particles having three kinds (small, medium and large) of diameters (particles a (small), particles b (medium) and particles c (large)). 
     Concerning the small particles a among the particles contained in the dispersion liquid A in the classification apparatus as shown in  FIG. 2 , the small particles mostly remain in the dispersion liquid A because of the low sedimentation velocity. Thus, these particles exist in the discharged liquid B 1  that is discharged from the discharge channel  4   a.    
     In contrast, the large particles c sediment downward due to gravity. When coming into contact with the bottom face  5  of the classification channel  3  wherein the flow velocity of the liquid is almost zero, these particles quickly sediment downward along the bottom face. They are found in the discharged liquid B 3  that is discharged from the discharge channel  4   c.    
     The particles b having a particle diameter intermediate between the small particles a and the large particles c sediment due to gravity. Until coming into contact with the bottom face  5  of the classification channel  3 , these particles are sent to the discharge channel  4   b.  Therefore, they are found in the discharged liquid B 2 . 
     Although three discharge channels in total (i.e.,  4   a,    4   b  and  4   c ) are formed in  FIG. 2 , the present embodiment is not restricted thereto and any number of discharge channels may be provided. Moreover, particles of a desired particle diameter can be classified by optionally altering the position of a discharge channel. Needless to say, the dispersion medium, the transportation liquid, the flow velocity, the channel width, the channel length, etc. may be appropriately changed too. 
       FIGS. 3A to 3D  are model views showing a case where large and small particles are sent to the classification apparatus of the second exemplary embodiment. 
     In  FIGS. 3A to 3D , the dispersion liquid A sent from the upper part of the classification apparatus  1  and the transportation liquid C sent from the lower part thereof join together in the classification channel  3  and form an interface  10  (indicated by a solid line in  FIGS. 3A to 3D ). The dispersion liquid A contains the small particles a and the large particles c. 
     Because of being largely affected by gravity, the large particles c gradually sediment ( FIGS. 3B and 3C ). When coming into contact with the bottom face  5  of the classification channel  3 , they sediment downward along the bottom face  5  ( FIG. 3D ). Then, the large particles c are discharged as the discharged liquid B 3  from the discharge channel  4   c  provided in the lower part of the classification channel  3 . 
     On the other hand, the small particles a are sent together with the laminar flow of the dispersion liquid A and then discharged as the discharged liquid B 1  from the discharge channel  4   a  provided in the upper part of the classification channel  3 . 
     This classification apparatus of the second exemplary embodiment can be made of the same material as in the classification apparatus of the first exemplary embodiment. Also, it can be produced by the same method as in the classification apparatus of the first exemplary embodiment. 
     Third Exemplary Embodiment 
       FIGS. 4A and 4B  is a perspective model view which shows a third exemplary embodiment of a classification apparatus that is appropriately usable in the present embodiment. A classification apparatus  1  shown in  FIG. 4A  or  4 B has a cone shape. 
     In  FIGS. 4A and 4B , the classification apparatus  1  comprises a dispersion liquid inlet channel  2  for introducing a solution containing particles (dispersion liquid) A, a classification channel  3  that is continuing the dispersion liquid inlet channel  2  and inclined at an angle θ to the direction of gravity, and discharge channels (not shown in the figure) for discharging the classified particles that are continuing the classification channel  3 . In  FIGS. 4A and 4B , through holes  8   a  and  8   b  that are formed in the classification channel  3  and continue the discharge channels are merely showed. 
     In the third exemplary embodiment, the dispersion liquid inlet  2 ′ is formed in the lower part of the classification apparatus  1  and the dispersion liquid A is fed into the dispersion liquid inlet channel  2  in the direction opposite to the direction of gravity (vertical direction). The method of introducing the dispersion liquid A is not specifically restricted and an appropriate method can be selected as in the first exemplary embodiment. 
     Although the dispersion liquid A is sent in the opposite direction to the direction of gravity in  FIGS. 4A and 4B , the invention is not restricted thereto so long as the dispersion liquid inlet channel  2  is formed in the lower part of the classification apparatus  1 . That is, the dispersion liquid inlet channel  2  may be provided horizontally to the lower part of the classification channel  3  so long as the dispersion liquid A is sent in the lower part of the classification channel  3 . 
     In the third exemplary embodiment, the classification channel  3  is provided in such a manner that the cross-section (cross-section area) thereof increases along the traveling direction of the dispersion liquid A. As the cross-section (cross-section area) of the classification channel  3  increases along the traveling direction of the dispersion liquid A, the classification efficiency can be improved while avoiding clogging, as discussed above. 
     As  FIG. 4A  shows, the classification channel  3  may have a constant channel thickness. As  FIG. 4B  shows, alternatively, the classification channel  3  may have a cone shape. The classification channel  3  as shown in  FIG. 4B  shows a larger increase in the cross-section area in the traveling direction of the dispersion liquid A. 
     In the third exemplary embodiment, the small particles among the particles contained in the dispersion liquid A have a terminal velocity lower than the sending velocity (rising velocity) of the dispersion liquid A. Thus, the small particles are discharged from the through hole  8   a  formed above the classification channel  3  into the discharge channel (not shown in the figure). 
     On the other hand, the large particles among the particles contained in the dispersion liquid A sediment due to gravity since the sending velocity (rising velocity) of the dispersion liquid A is lower than the terminal velocity of the larger particles. When coming into contact with the bottom face  5  of the classification channel  3 , these large particles sediment along the wall face and discharged from the through hole  8   b  formed below the classification channel  3  into the discharge channel (not shown in the figure). 
     In fabricating the classification apparatus according to the third exemplary embodiment, a joining technique can be used. Usual joining techniques are roughly classified into the solid-phase joining and the liquid-phase joining. Typical examples of the joining methods commonly used include the solid-phase joining such as the pressure joining and the diffusion joining and the liquid-phase joining such as the welding, eutectic bonding, the soldering, the adhesion, etc. 
     In the joining, it is desirable to employ a highly accurate joining method by which the dimension accuracy can be kept without the destruction of the microstructure of the channel, etc., caused by deterioration or large deformation of a material due to high-temperature heating. Examples of such a technique include the silicon direct joining, the anodic joining, the surface activation joining, the direct joining using hydrogen bond, the joining using an aqueous HF solution, the Au—Si eutectic joining, the void-free joining, the diffusion joining, etc. 
     Since the classification apparatus according to the present embodiment has channels in a three-dimensional shape, it is preferably formed by laminating pattern elements (film pattern elements). The thickness of the pattern elements preferably ranges from 5 to 50 μm and more preferably from 10 to 30 μm. 
     The classification apparatus according to the embodiment is preferably a classification apparatus that is formed by laminating pattern elements carrying definite two-dimensional patterns formed thereon. It is preferable that the pattern elements are laminated in such a manner as the faces thereof being in direct contact. 
     It is preferable that a plurality of pattern elements corresponding to the horizontal cross-section shapes of the classification apparatus are laminated to form the classification apparatus, since the classification apparatus can be conveniently fabricated thereby. 
     As a preferable example of the method of producing the classification apparatus according to the present embodiment, there can be cited a method of producing a classification apparatus which comprises: (i) the step of forming a plurality of pattern elements corresponding to the horizontal cross-section shapes of the target classification apparatus on a first substrate (the donor substrate-forming step); and (ii) the step of repeatedly joining and separating the first substrate having a plurality of pattern elements formed thereon as described above and a second substrate to transfer the plurality of pattern elements on the first substrate onto the second substrate (the joining step). More specifically, reference can be made to the production method that is disclosed in JP-A-2006-187684. 
     Next, the method of producing the classification apparatus according to the present embodiment will be described in greater detail. 
     [Donor Substrate-Forming Step] 
     In this embodiment, it is preferable to fabricate the donor substrate by using the electrocasting method. The donor substrate means a substrate wherein a plurality of pattern elements corresponding to the horizontal cross-section shapes of the target classification apparatus are formed on a first substrate. It is preferable that the first substrate is made of a metal, ceramics or silicon and a metal such as stainless steel is appropriately usable therefore. 
     After preparing the first substrate, a thick photo resist is applied on the first substrate. Next, it is exposed via photo masks corresponding to the respective horizontal cross-section shapes of the target classification apparatus and the photo resist is developed. Thus, positive/negative reversal resist patterns of the respective horizontal cross-section shapes are formed. Then, the substrate having these resist patterns is dipped in a plating bath and, for example, nickel plating is developed in the photo resist-uncoated areas of the surface of the metal substrate. It is preferable that the pattern elements are made of copper or nickel with the use of the electrocasting method. 
     After removing the resist patterns, the pattern elements corresponding to the respective horizontal cross-section shapes of the classification apparatus are formed on the first substrate. 
     [Joining Step] 
     The joining step means a step wherein the first substrate (donor substrate) having the plurality of pattern elements formed thereon and a second substrate (target substrate) are repeatedly joined and separated to thereby transfer the pattern elements from the donor substrate to the target substrate. It is preferable that joining is carried out by the joining at ordinary temperature or the surface activation joining. 
       FIGS. 5A to 5F  are a flow chart showing an example of the method of producing a classification apparatus that is appropriately usable in the third exemplary embodiment. 
     As  FIG. 5A  shows, a plurality of pattern elements ( 401 A and  401 B) corresponding to the respective cross-section shapes of the target classification apparatus are formed on a metal substrate  400  serving as a first substrate of a donor substrate  405 . This donor substrate  405  is located on a lower stage (not shown in the figure) in a vacuum tank, while a target substrate  410  is located on an upper stage (not shown in the figure) in the vacuum tank. Next, the vacuum tank is evacuated to achieve high- or ultrahigh-vacuum conditions. Then the lower stage is relatively moved to the upper stage and a pattern element  401 A that is the first layer of the donor substrate  405  is positioned immediately below the target substrate  410 . Subsequently, the surface of the target substrate  410  and the surface of the pattern element  401 A (i.e., the first layer) are cleaned by irradiating with an argon atom beam. 
     As  FIG. 5B  shows, the upper stage is moved downward and the target substrate  410  and the donor substrate  405  are pressed by applying a definite load (for example, 10 kgf/cm 2 ) to thereby join the target substrate  410  and the pattern element  401 A (i.e., the first layer) at ordinary temperature (the surface activation joining). In this embodiment, pattern elements  401 A,  401 B and so on are laminated in this order. 
     As  FIG. 5C  shows, the upper stage is then moved upward and thus the donor substrate is separated from the target substrate. Thus, the pattern element  401 A (i.e., the first layer) is peeled off from the metal substrate (the first substrate)  400  and transferred onto the target substrate  410  side. This is because the adhesion force between the pattern element  401 A and the donor substrate  405  is larger than the adhesion force between the pattern element  401 A and the metal substrate (the first substrate)  400 . 
     As  FIG. 5D  shows, the lower stage is moved so that a pattern element  401 B (i.e., the second layer) on the donor substrate  405  is positioned immediately below the target substrate  410 . Then, the surface of the pattern element  401 A (i.e., the first layer) having been transferred onto the target substrate  410  side (the face being in contact with the metal substrate  400 ) and the surface of the pattern element  401 B (i.e., the second layer) are cleaned in the above-described manner. 
     As  FIG. 5E  shows, the upper stage is moved downward and thus the pattern element  401 A (i.e., the first layer) and the pattern element  401 B (i.e., the second layer) are joined together. When the upper stage is moved upward as shown in  FIG. 5F , the pattern element  401 B (i.e., the second layer) is separated from the metal substrate (the first substrate)  400  and transferred onto the target substrate  410  side. 
     Other pattern elements are also repeatedly subjected to the positioning between the donor substrate  405  and the target substrate  410 , joining and separation in the same manner. Thus, a plurality of pattern elements is transferred onto the target substrate. Then, the laminate having been transferred onto the target substrate  410  is taken out from the upper stage and the target substrate  410  is removed. Thus, a classification apparatus can be obtained. 
     Although the donor substrate is fabricated by using the electrocasting method in the above embodiment, use can be made of a semiconductor process therefore. For example, a donor substrate can be fabricated by preparing a substrate comprising an Si wafer, forming a polyimide releasing layer on this substrate by the spin coating method, forming an Al film that is a component of a classification apparatus on the surface of the releasing layer by the sputtering method, and then patterning the Al film by the photolithography. 
     EXAMPLE 
     Next, the present embodiment will be illustrated in greater detail by referring to Example. However, it is to be understood that the present embodiment is not restricted to the following Example. 
     In this Example, classification was carried out by using the classification apparatus  1  shown in  FIG. 2 . 
     In  FIG. 2 , the microchannels (dispersion liquid inlet channel, transportation liquid inlet channel, classification channel and discharge channels) are all in a plate-shape having a depth of 0.5 mm. 
     The dispersion liquid inlet channel  2 , the transportation liquid inlet channel  6  and the discharge channels  4  ( 4   a,    4   b  and  4   c ) have all a width of 0.5 mm. It is also preferable that the joint part of the classification channel  3  to the discharge channel  4   a  is rounded to avoid clogging. To introduce the dispersion liquid A and the transportation liquid C, a microsyringe was employed. 
     The classification channel  3  is in an isosceles triangle shape having a side length of 50 mm. 
     In  FIG. 2 , the arrow indicates the direction of gravity. 
     As the dispersion liquid A, a 10 wt % aqueous dispersion of spherical particles of polymethyl methacrylate (PMMA) (TECHNOPOLYMER manufactured by SEKISUI PLASTICS Co.) was employed. As the transportation liquid C, purified water was employed. 
     The dispersion liquid A and the transportation liquid C were fed both at 10 ml/h. 
     The particles contained in the dispersion liquid A dropped halfway in the classification channel  3 . In this step, particles having large diameter, which had a high sedimentation velocity, were discharged as such from the discharge channel  4   c  or, after coming into contact with the bottom face  5  of the classification channel  3 , dropped along the wall face and then discharged from the discharge channel  4   c.    
     On the other hand, particles having small diameter did not collide with the bottom face (inclined face)  5  but were discharged as such from the discharge channel  4   a.    
       FIG. 6  shows particle size distributions of the dispersion liquid A and the dispersions (B 1  and B 3 ) discharged from the discharge channels  4   a  and  4   c.  In the bar graphs in  FIG. 6 , a 5 μm bar, for example, shows the frequency of particles being larger than 3 μm but not larger than 5 μm. 
     Thus, no particle with a diameter exceeding 15 μm was detected in the dispersion B 1  having been classified, which proved that classification of micrometer-size particles had been completed. 
     After performing the treatment continuously for 8 hours, the classification efficiency was not lowered and no particle with a diameter exceeding 15 μm was detected in the dispersion B 1 . After performing the treatment continuously for additional 8 hours, no clogging arose in the channels. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments are chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various exemplary embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.