Patent Publication Number: US-11388808-B2

Title: Biphasic plasma microreactor and method of using the same

Description:
TECHNICAL FIELD 
     The present invention relates to a gas-liquid plasma microreactor and to a method using such a device for generating a plasma and to perform chemical synthesis. The microreactor according to the invention aims at generating plasma in a gas flowing along a liquid by applying an appropriate electric field so that the reactive species or molecules formed in the plasma can be efficiently transferred into the liquid. 
     BACKGROUND OF THE INVENTION 
     Plasma discharges are known to generate reactive species in gas phase (such as radicals, neutrals in different excited states, photons and electrons), at room temperature and pressure. The use of plasma discharges is known to perform chemical synthesis, particularly in gas. However, the use of gas limits the amount of matter involved in the synthesis. 
     Therefore, biphasic gas-liquid plasma reactors have been developed. The effectiveness of those reactors is limited by the transfer of active species formed by plasma discharge from gas to liquid. The effectiveness of the different types of plasma reactors for transferring active species has been evaluated by Malik et al. [1]. These authors compared 27 different versions of plasma reactors using the energy required to obtain 50% discoloration of dyes in water (G50 expressed in g/kWh). Their conclusion was that the most efficient reactors are pulsed powered reactors in which the liquid (i) is sprayed in the plasma zone or (ii) flows down the inner wall of a cylindrical electrode as a thin film. The improvement of efficiency was explained by the large surface-to-volume ratio of the liquid, resulting in a faster rate of transfer of reactive species from gas to liquid and the short distance through which a pollutant molecule in liquid needs to diffuse in order to reach the liquid surface. 
     Particularly, Yano et al. [2] describe a wetted wall plasma reactor, wherein a thin film of liquid flows down the inner walls of a cylindrical electrode in the presence of a discharge in the gas between wire-to-cylinder electrodes. The problem of such a reactor is that uniform liquid films are difficult to obtain, especially thin liquid films (&lt;1 mm). 
     Matsui et al. [3] describe a two-phase flow reactor to decompose acetic acid in water. Pulsed dielectric barrier discharge was generated in oxygen bubbles flowing in water. The reactor consisted in two coaxial glass tubes, the gap between the inner tube and the outer tube being 1 mm. A bubbler at the bottom of the reactor was used to create gas bubbles. Good results were obtained in terms of acetic acid decomposition. The drawback of this reactor is however the large size of the bubbles (from 0.1 cm to 1 cm) and the poor control of the bubble size leading to a large size distribution of the bubbles. The consequence is that some parts of the liquid may absorb too much radicals while other parts may lack radicals. 
     Therefore, microfluidic channels have been used to enhance the control of the size distribution. By using microfluidic tools, Zhang et al. [4], [5], have developed a plasma microreactor wherein micro-sized bubbles are generated using a traditional flow-focusing geometry. Plasma discharges are then generated repeatedly in the micro-sized dispersed gas bubbles moving in the continuous liquid phase thanks to high-voltage electrodes positioned on each side of the channel. Due to the high surface over volume ratio of gas bubbles, the reaction products can be extracted very rapidly, for example in milliseconds, from the plasma phase. Together with the development of the microreactor, experimental (Electron Spin Resonance spectroscopy, fluorescence) and theoretical (plasma modelling) methods aiming to quantify and qualify the generated radical species have been implemented. In the case of a water/argon diphasic flow, the production of dissolved OH° radicals has been evidenced and quantified, making hydroxylation reactions possible. However, working with bubbles limits the amount of gas seen by a given portion of the liquid, which hinders the use of the gas constituents as reagent for the liquid phase. Besides, the residence times of both phases are strongly correlated, and limited by the flow rates necessary for the production of bubbles. 
     SUMMARY OF THE INVENTION 
     A plasma microreactor has been developed to respond at least partially to the above-mentioned issues of the prior art. The plasma microreactor comprises:
         a support, made at least partially of a dielectric material, the support comprising a gas inlet, adapted to be connected to a gas reservoir, a liquid inlet, adapted to be connected to a liquid reservoir and at least a fluid outlet adapted to be connected to a receiver containing gas and/or liquid,   a liquid microchannel in the support adapted to allow a liquid flow from the liquid inlet to the fluid outlet,   a gas channel, in the support adapted to allow a gas flow from the gas inlet to the fluid outlet,   at least a ground electrode,   at least a high voltage electrode, separated from the gas channel by the dielectric material of the support,       

     wherein said ground electrode and said high voltage electrode are arranged on opposite sides of the gas channel so as to be able to create an electric field inside the gas channel, 
     wherein the liquid microchannel and the gas channel are contiguous and at least an opening is arranged between the liquid microchannel and the gas channel so as to form a fluid channel and to cause the liquid flow contact the gas flow and wherein the liquid flow is retained within the liquid microchannel by capillarity action. 
     In further optional aspects of the invention: 
     at least one, and notably one, opening is arranged between the liquid microchannel and the gas channel on at least 80%, notably 90%, such as about 100%, of the length of the fluid channel. 
     the opening is partially defined by a convex bended portion arranged between the liquid microchannel and the gas channel, said convex bended portion extending continuously along the length of the liquid microchannel and along the length of the gas channel, 
     the convex bended portion has a radius of curvature being less than 20 μm, 
     the convex bended portion defines an edge along the length of the liquid microchannel and along the length of the gas channel, 
     the plasma microreactor comprises a high voltage source electrically connected to the ground electrode(s) and to the high voltage electrode(s), 
     at least one of the ground electrode and the high voltage electrode is embedded in the support, 
     the length of the gas channel and the length of the liquid microchannel are over 2 cm, notably over 20 cm and preferably over 100 cm, 
     the support is made of a UV-cured polymer such as a polymer obtained by photopolymerisation of a thiol-ene based resin, a poly(tetramethylene succinate), a cyclic olefin copolymer (COC) such as a copolymer of ethylene and norbornene or tetracyclododecene, glass, a ceramic material, or a combination thereof, 
     two liquid microchannels are arranged on opposite sides of the gas channel, so that the liquid microchannel formed by said liquid microchannels and said gas channel has a T-shaped section, 
     a fluid channel is formed by the liquid microchannel(s) and by the gas channel(s), said fluid channel being arranged following a main plane, the design of the fluid channel allowing to have a surface density of the fluid channel in said plane higher than 0.3 over a square of at least 20 mm 2 , 
     at least a portion of the fluid channel is arranged in a serpentine pattern, 
     the liquid microchannel has a smaller height than the gas channel so as to form a step between the liquid microchannel and the gas channel, 
     the support is entirely made of glass or in ceramic material or at least made of a polymeric layer comprised between two glass layers, 
     the height of the liquid microchannel is more than 1 μm, notably more than 10 μm, preferably more than 40 μm and is less than 200 μm, notably less than 100 μm, preferably less than 50 μm. 
     the height of the gas channel is comprised between the height of the liquid microchannel and 1 mm, notably between the height of the liquid microchannel and 200 μm and preferably between the height of the liquid microchannel and 100 μm. 
     Another aspect of the present invention is a method for generating a plasma in a plasma microreactor, comprising the steps of:
     (a) providing the above described plasma microreactor,   (b) providing a liquid and making the liquid flow through the liquid microchannel(s) in a given direction,   (c) providing a gas and making the gas flow through the gas channel in said direction,   (d) applying a high voltage between the high voltage electrode(s) and the ground electrode so as to generate a plasma in the gas channel.   

     In further optional aspects of the invention: 
     the gas flow rate through the gas channel is higher than the liquid flow rate through the liquid microchannel, preferably between than 5 times and 10000 times the liquid flow rate in the liquid microchannel, notably comprised between 50 times and 2000 times the liquid flow rate in the liquid microchannel, and preferably comprised between 80 times and 1000 times the liquid flow rate in the liquid microchannel, 
     the gas is selected from air, argon, helium, oxygen, hydrogen, nitrogen, water vapor, ammoniac, carbon dioxide, carbon monoxide, volatile hydrocarbons such as methane, volatile organic compounds and a mixture thereof, 
     the liquid is selected from solvents, more particularly from organic or aqueous solvents, such as water, an aliphatic hydrocarbon, an aromatic hydrocarbon, an alcohol, an ether, an ester, a ketone, a halogenated solvent, dimethylsulfoxide (DMSO), acetonitrile, dimethylformamide (DMF), an ionic liquid or a mixture thereof; and reagents such as methyl methacrylate (MMA) or phenol; or a mixture thereof, 
     the high voltage is comprised between 250 V and 30 kV, notably between 1 kV and 20 kV, preferably between 1 kV and 10 kV, 
     the high voltage is a variable, such as sinusoidal, high voltage with a frequency comprised advantageously between 100 Hz and 1 MHz, in particular comprised between 100 Hz and 100 kHz, preferably comprised between 100 Hz and 10 kHz, or the high voltage is a pulsed voltage with a frequency comprised advantageously between 100 Hz and 1 MHz, in particular comprised between 100 Hz and 100 kHz, preferably comprised between 100 Hz and 10 kHz, 
     a compound present in the liquid, such as a solvent or a reagent, is submitted to at least a chemical reaction such as cleaving, oxidation, hydrogenation, dehydrogenation, amination or carbonylation. 
     Definitions 
     The term “length” of a channel will be used herein to designate the size of a channel according to the main flow direction of a fluid through the channel. 
     The term “height” of a channel will be used herein to designate the minimum size of a channel in a first direction transverse to the main flow direction of a fluid through the channel. 
     The term “width” of a channel will be used herein to designate the maximum size of a channel in a direction transverse to the main flow direction of a fluid through the channel and transverse to the first direction. 
     The term “microchannel” or “microfluidic channel” will be used therein to designate a channel comprising at least one inlet and at least one outlet, the height of which is comprised between 100 nm and 1 mm. A microchannel has at least a wall in a material adapted for flowing a liquid and/or a gas. 
     The term “microreactor” will be used therein to designate a device comprising a housing which minimum size is comprised between 100 nm and 1 mm. More precisely, the term “microreactor” will be used therein to designate a device comprising at least a microchannel. 
     The term “convex” will be used herein to define a surface or the section of a surface defining a bump. More specifically, a “convex bended portion” of the wall of a fluidic channel is a portion of the wall wherein, for two points chosen on the convex bended portion of the wall, the segment between the two points is not comprised by the fluidic channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described by way of example, with reference to the accompanying drawings in which: 
         FIG. 1  diagrammatically shows a cross section view of a plasma microreactor according to a possible embodiment of the invention, 
         FIG. 2  diagrammatically shows a cross section view of a plasma microreactor according to a possible embodiment of the invention, 
         FIG. 3A  is an optical profilometer image of two parallel portions of a fluid channel, 
         FIG. 3B  diagrammatically shows a cross section view of a plasma microreactor according to a possible embodiment of the invention, 
         FIG. 3C  diagrammatically shows a three-dimensional view of a plasma microreactor according to a possible embodiment of the invention, 
         FIG. 3D  diagrammatically shows a cross section view of a plasma microreactor according to a possible embodiment of the invention, 
         FIG. 4A  and  FIG. 4B  illustrate a top view of a fluid channel in the plasma microreactor, 
         FIG. 5  diagrammatically shows a general setup comprising the plasma microreactor, 
         FIG. 6  illustrates a double dielectric barrier discharge configuration of the electrodes, 
         FIG. 7  illustrates a single dielectric barrier discharge configuration of the electrodes, 
         FIG. 8  and  FIG. 9  illustrate the designs of masks for the manufacture of two layers of the plasma microreactor according to a possible embodiment of the invention, 
         FIG. 10A ,  FIG. 10B ,  FIG. 10C ,  FIG. 10D  and  FIG. 10E  illustrate the manufacture of a part the plasma microreactor according to a possible embodiment of the invention, 
         FIG. 11A ,  FIG. 11B ,  FIG. 11C ,  FIG. 11D  and  FIG. 11E  illustrate the manufacture of electrode-bearing coverslips by a lift-off process according to a possible embodiment of the invention, 
         FIG. 12A  illustrates the manufacture of the plasma microreactor according to a possible embodiment of the invention, 
         FIG. 12B  illustrates a cross section view of the plasma microreactor according to a possible embodiment of the invention, 
         FIG. 13A ,  FIG. 13B ,  FIG. 13C  and  FIG. 13D  illustrate the manufacture of the plasma microreactor according to a possible embodiment of the invention, 
         FIG. 14A  and  FIG. 14B  illustrate the manufacture of the plasma microreactor according to a possible embodiment of the invention, 
         FIG. 15A  and  FIG. 15B  illustrate the plasma microreactor according to a possible embodiment of the invention, 
         FIG. 16  illustrate an electrical characterization of the plasma microreactor made by the Lissajous method, 
         FIG. 17  is a top view image of the plasma microreactor, 
         FIG. 18A  and  FIG. 18B  are top view images of the plasma microreactor, 
         FIG. 19  illustrates a gas flow and a liquid flow in a plasma microreactor according to a possible embodiment of the invention, and in a flat fluid channel, 
         FIG. 20  illustrates the molar fraction of compounds in the output liquid, in percentage, 
         FIGS. 21, 22, 23 and 24  illustrate a gas flow and a liquid flow in a plasma microreactor according to a possible embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED ASPECTS OF THE INVENTION 
     Architecture of the Microreactor 
     Referring to  FIG. 1 , the plasma microreactor  1  includes a support  3 . The support  3  comprises a gas channel  10  and two liquid microchannels  9 , arranged on opposite sides of the gas channel  10  so as to form a fluid channel  2  having a T-shape cross section. A ground electrode  4  and a high voltage electrode  5  are arranged on opposite sides of the gas channel  10 . The high voltage electrode  5  and the ground electrode  4  are electrically connected to a high voltage source  6  and separated from the fluid channel  2  by a dielectric material of the support  3 . 
     Dielectric Material 
     The support  3  according to the present invention is at least partially made of a dielectric material in order to isolate the gas channel  10 , since a high-voltage electric field can be controlled inside the support  3  between the electrodes  4 , 5 . The dielectric material used in the present invention can be any material known for its dielectric properties. However, the dielectric material will be advantageously impervious to gas and liquid which have to flow through the fluid channel comprised in the support  3 . Moreover, the dielectric material should also advantageously allow the formation of the support  3  comprising at least a liquid microchannel  9  and a gas channel  10 , and for example embedded electrodes. The dielectric material can thus be a UV-cured polymer (i.e. a polymer obtained by photopolymerisation of monomers or prepolymers), such as a polymer obtained by photopolymerisation of a thiol-ene based resin (for example a Norland Optical Adhesive® (NOA), such as NOA-81 or NOA-61, preferably NOA-81), a poly(tetramethylene succinate) (PTMS), a cyclic olefin copolymer (COC) such as a copolymer of ethylene and norbornene or tetracyclododecene, glass, a ceramic material, or a combination thereof. In a preferred embodiment of the invention, the support  3  is made of a polymeric layer comprised between two glass layers, for example two glass coverslips. In another preferred embodiment of the invention, the support  3  is entirely made of glass or of ceramic material. In order to form the liquid microchannel(s)  9  and the gas channel(s)  10 , glass can be micromachined by chemical wet etching (using HF selective etching for example) and/or laser engraving. 
     Liquid Microchannel(s) and Gas Channel(s) 
     In all the embodiments of the invention, the support  3  comprises at least a gas channel  10 , preferably at least gas microchannel, and at least a liquid microchannel  9  so as to form at least a fluid channel  2 , each liquid microchannel  9  being contiguous with a gas channel  10 , an opening being arranged between each liquid microchannel  9  and at least a gas channel  10  to cause a liquid  8  in said liquid microchannel  9  to contact a gas  12  in said gas channel  10 , the liquid  8  flow being retained within the liquid microchannel  9  by capillarity action. 
     In reference to  FIG. 2 , the support  3  comprises a gas channel  10  and two liquid microchannels  9  so as to form a fluid channel  2  having a T-shape cross section. Each liquid microchannel  9  is contiguous with the gas channel  10 . An opening  11  is arranged between each liquid microchannel  9  and the gas channel  10  to cause the liquid  8  in the liquid microchannel  9  to contact a gas  12  in the gas channel  10 , the liquid  8  flow being retained within the liquid microchannel  9  by capillarity action. It is known to adapt the shape and the dimensions of a fluid channel  2  formed by the liquid microchannels  9  and by the gas channel  10 , to retain a liquid  8  in the liquid microchannel  9  by capillarity action. For example, it is possible to pin a triple line (i.e. a line where a gas phase, a liquid phase and a solid phase coexist) at a convex bended portion  7 , specifically a sharp edge of the wall of the fluid channel  2 . This convex bended portion  7  may partially define the opening  11 . Knaust et al. [6] for example describe a fluid channel comprising a 10 μm high guide as a convex bended portion, at the bottom along the middle of the channel. The high guide is adapted to pin a triple line in order to separate a liquid microchannel in which a water flow is stabilized, and a gas channel in which a supercritical CO 2  flow is stabilized, as the water and the CO 2  are in contact. 
     The convex bended portion  7  partially defines the opening  11 , arranged between the liquid microchannel  9  and the gas channel  10 . The convex bended portion  7  is extending continuously along the length of the liquid microchannel  9  and of the gas channel  10 . Preferably, the radius of curvature of the convex bended portion  7  is less than 50 μm, notably less than 20 μm and preferably less than 5 μm. Therefore, said convex bended portion  7  is adapted to pin a triple line when a liquid  8  is injected in the liquid microchannel, and to enable the stabilization of a liquid flow in a liquid microchannel  9 , and of a gas flow in the gas channel  10 , the liquid flow contacting the gas flow at the opening  11 . 
     The height h l  of the liquid microchannel  9  is comprised between 500 nm and 500 μm, notably between 2 μm and 200 μm and preferably between 5 μm and 100 μm. The height h g  of the gas channel  10  is comprised between 500 nm and 1 mm, notably between 2 μm and 200 μm and preferably between 5 μm and 100 μm. 
     In a preferred embodiment, the liquid microchannel  9  can have a smaller height than the gas channel  10 , so as to form a step between the liquid microchannel  9  and the gas channel  10 . The convex bended portion  7  is, in this case, the edge of the step between the liquid microchannel  9  and the gas channel  10 . Therefore, the liquid  8  can be maintained within the liquid microchannel  9  by capillarity action. In reference to  FIG. 2 , the fluid channel  2  can be made in the support  3  by micromachining a layer  18  in which the fluid channel  2  is designed, said layer  18 , advantageously made of a polymer, being sandwiched between two support layers  17 , such as glass layers, optionally comprising the electrodes. The layer  18  can be therefore formed of a first layer L 1  wherein a part of the gas channel  10  is designed, and a second layer L 2  wherein another part of the gas channel  10  and the liquid microchannel  9  are designed. In this embodiment, the height of the gas channel  10  is preferably comprised between the height of the liquid microchannel  9  and 200 μm, notably between the height of the gas channel  10  and 100 μm. 
       FIG. 3A  is an optical profilometer image of two parallel portions of fluid channel  2 . In each fluid channel  2 , the gas channel  10  is surrounded by two liquid microchannels  9  arranged at the opposite side of the gas channel  10 . Therefore, the contact between the gas phase and the liquid phase is optimized compared to an arrangement comprising only one liquid microchannel  9  and a gas channel  10  contiguous to the liquid microchannel  9 . The different fluid channels  2  can be parallelized, and separated by the support  3 . Therefore, the amount of species transferred to the liquid phase can be increased. 
       FIG. 3B  illustrates schematically a different possible embodiment of the invention. The support  3  can comprise a gas channel  10  and two liquid microchannels  9  so as to form a fluid channel  2  having a cross-shape cross section. Each liquid microchannel  9  is contiguous with the gas channel  10 . An opening  11  is arranged between each liquid microchannel  9  and the gas channel  10  to cause the liquid  8  in the liquid microchannel  9  to contact a gas  12  in the gas channel  10 , the liquid  8  flow being retained within the liquid microchannel  9  by capillarity action. Each liquid microchannel  9  and the gas channel  10  can be arranged so that two convex portions  7  define the opening  11 . Each of the convex portions  7  of an opening  11  is extending continuously along the length of the liquid microchannel  9  and of the gas channel  10 . Therefore, the capillary action retaining the liquid  8  flow can be increased in comparison with an opening  11  having one convex portion  7 . 
     The height h l  of the liquid microchannel  9  is advantageously comprised between 500 nm and 500 μm, notably between 2 μm and 200 μm and preferably between 5 μm and 100 μm. The height h g  of the gas channel  10  is advantageously comprised between 500 nm and 1 mm, notably between 2 μm and 200 μm and preferably between 5 μm and 100 μm. 
     The liquid microchannel  9  can have a smaller height than the gas channel  10 , so as to form two steps between the liquid microchannel  9  and the gas channel  10 . The convex bended portions  7  are, in this case, the edges of the steps between the liquid microchannel  9  and the gas channel  10 . 
       FIG. 3C  is a schematic view of the embodiment illustrated in  FIG. 3B , wherein each opening  11  is defined by two convex portions  7 . The fluid channel  2  can have a layout forming a serpentine in the support  3 . Therefore, the surface of the opening(s) in the support  3  of constant size can be increased, compared to typical linear layouts of microfluidic channels. 
       FIG. 3D  illustrates schematically a different possible embodiment of the invention. The fluidic channel  2  can comprise a gas channel  10 , at least partially surrounded by an array of liquid microchannels  9 . Each liquid microchannel  9  is contiguous with the gas channel  10 . An opening  11  is arranged between each liquid microchannel  9  and the gas channel  10  to cause the liquid  8  in the liquid microchannel  9  to contact the gas  12  in the gas channel  10 , the liquid  8  flow being retained within the liquid microchannel  9  by capillarity action. The liquid microchannel  9  can be arranged so that at least one, and preferably two convex portions  7  define each opening  11  (the two liquid microchannels  9  at the extremities could be arranged with only one convex portion  7  defining the opening  11  as illustrated on  FIG. 3D ). Each of the convex portions  7  of an opening  11  is extending continuously along the length of the liquid microchannel  9  and of the gas channel  10 . 
     The liquid microchannels  9  can form a two-dimensional array of liquid microchannels  9 , and preferably of parallels liquid microchannels  9 , each liquid microchannel being contiguous with the gas channel  10 . The array can be arranged underneath the gas channel  10 . Therefore, it is possible to parallelize the layout of the openings  11 , so as to simplify the fabrication of the plasma reactor  1 . 
     The height h l  of the liquid microchannel  9  is advantageously comprised between 500 nm and 500 μm, notably between 2 μm and 200 μm and preferably between 5 μm and 100 μm. The height h g  of the gas channel  10  is advantageously comprised between 500 nm and 1 mm, notably between 2 μm and 200 μm and preferably between 5 μm and 100 μm. 
     The width w l  of the liquid microchannel  9  is advantageously comprised between 500 nm and 500 μm, notably between 2 μm and 200 μm and preferably between 5 μm and 100 μm. The width w g  of the gas channel  10  is advantageously comprised between 2 μm and 10 mm, notably between 2 μm and 1 mm. 
       FIG. 4A  and  FIG. 4B  illustrates a top view of a fluid channel  2  in the plasma microreactor  1 . In  FIG. 4 , the liquid microchannels  9  and the liquid inlet  14  are illustrated in gray as the gas channel  10  and the gas inlet  13  are illustrated in black. The support  3  comprises a gas inlet  13 , adapted to be connected to a gas reservoir, and a liquid inlet  14  adapted to be connected to a liquid reservoir. In a preferred embodiment, the liquid inlet can be divided into two channels, for example of equal hydrodynamic resistance, each of the two channels being connected to one of the liquid microchannels  9 . Therefore, the liquid inlet  14  is adapted to provide the liquid  8  at equal flow rates into the liquid microchannels  9  arranged on opposite sides of the gas channel  10 . 
     In a different embodiment of the invention (not shown), the support  3  can comprise a plurality of liquid inlets  14 . For example, the support  3  can comprise two liquid inlets  14 , adapted to be connected to two liquid reservoirs and respectively to two liquid microchannels  9 . Therefore, different liquids  8  can be injected in two different liquid microchannels  9  in contact with the same gas channel  10 . 
     The fluid channel  2  can be connected to a receiver, containing gas and/or liquid, by the means of a common fluid outlet (not illustrated) of the fluid channel  2 . In a different embodiment, illustrated in  FIG. 4 , a liquid outlet  15   a  is connecting the liquid microchannel  9  to a liquid receiver, and a gas outlet  15   b  is connecting the gas channel  10  to a gas receiver. Therefore, the pressures at the end of the liquid microchannel  9  and at the end of the gas channel  10  can be tuned. 
     The fluid channel  2  is arranged following a main plan in a serpentine pattern. Therefore, the density of fluid channel  2  for a given surface can be optimized for a predefined microreactor surface. Particularly, the fluid channel  2  length, hence the liquid microchannel  9  length and/or the gas channel  10  length, can be higher than 2 cm, notably higher than 20 cm and preferably higher than 100 cm. This fluid channel  2  length can be higher than 1 m for example in a microreactor made of standard typical glass coverslips of 60 mm by 24 mm. Therefore, it is possible to increase the contact surface between the liquid  8  and the gas  12  for a predefined surface, and therefore to increase the efficiency of the plasma microreactor  1 . Other patterns of fluid channel  2  can be used, in order to increase the density of fluid channel  2  in the main plane. Multiple parallel fluid channels  2  can be designed for instance. More generally, the surface density of fluid channel  2  in the main plane can be higher than 0.3 over a square of 20 mm 2  or more. 
     General Setup 
       FIG. 5  diagrammatically shows a plasma microreactor  1  according to an embodiment of the present invention comprising a support  3  according to the embodiment illustrated on  FIG. 2 . The gas inlet  13  of the support  3  is connected to a gas reservoir  19 , whereas the liquid inlet  14  of the support  3  is connected to a liquid reservoir  20 . The liquid reservoir  20  can be for example a syringe. Other liquid reservoirs  20  can be used, depending on the type of liquid flow actuator coupled with the liquid reservoir  20 . A liquid reservoir  20  comprising a pressurized gaseous phase can be used when using a pressure controller (for example a Fluigent MFCS pressure controller). A liquid reservoir  20  can also be fluidically connected to a peristaltic pump. 
     The liquid outlet  15   b  can be directly connected to an analyzer, in order to analyze the chemical species at the output of the plasma microreactor  1 . The liquid outlet  15   b  can also be connected to another plasma microreactor  1  according to the present invention or not, so as to perform another synthesis step on the liquid issued from the plasma microreactor  1  for example. In another embodiment, the gas outlet  15   a  and the liquid outlet  15   b  can be connected to a common receiver. 
     The plasma microreactor  1  can be monitored by a computer, specifically adapted to monitor:
     pressure controllers  23 , which control the pressure of gas  12  upstream the plasma microreactor  1 ,   a pressure controller and/or an automated syringe driver  20 , which controls the pressure and/or the flow rate of the liquid  8  upstream the plasma microreactor  1 ,   a high voltage source  6  which can be composed of various apparatuses such as a function generator, a signal amplifier, a fast high voltage switch, and an oscilloscope, and   optionally an imaging system (not illustrated), such as an iCCD camera or a CCD camera, if need be, to visualize the flow shape or the electric discharges in the gas channel  10 .   

     Flow Control 
     The plasma microreactor  1  is adapted to handle a liquid phase and a gas phase. For both of them, it is essential for the chemistry to control the flow rates and have access to their residence times. Thus, in a preferred embodiment, the liquid  8  flow is monitored by a syringe driver (kdScientific Legato  180 ). Other liquid flow actuator can be used, as pressure controller or a peristaltic pump. Mass flow rate controllers (Bronkhorst EL-FLOW) can be used to control the gas flow rate and realize gas mixtures. In a preferred embodiment, the gas flow rate through the gas channel  10  is higher than the liquid flow rate through the liquid microchannel  9 , preferably between 5 times and 10000 times the liquid flow rate in the liquid microchannel  9 , notably comprised between 50 times and 2000 times the liquid flow rate in the liquid microchannel, and preferably comprised between 80 times and 1000 times the liquid flow rate in the liquid microchannel. Therefore, it is possible to tune the residence time of the reactive species independently from the liquid flow rate in the liquid microchannel  9 : the residence time of each phase is not correlated. The liquid  8  comprises at least a solvent and optionally at least a reagent. The flow rate of liquid  8  monitored by the syringe driver can be comprised in the range from 0.5 μL/min to 100 μL/min, notably from 1 μL/min to 50 μL/min and preferably from 4 μL/min to 30 μL/min. The gas used can be an inert gas (for example Ar, He, N 2 ) or an active gas (O 2 , H 2 , CO 2 , CO, NH 3 , volatile hydrocarbons such as CH 4 ) or a mixture thereof. The flow rate of gas  12  can be comprised in the range from 0.1 mL/min to 50 mL/min, notably from 0.5 mL/min to 10 mL/min. 
     Electrodes 
     The plasma microreactor  1  according to the invention comprises at least one ground electrode  4  and at least a high voltage electrode  5 . 
     By “ground electrode” (also called earth electrode) is meant an electrode which is connected to the ground. 
     By “high voltage electrode” is meant an electrode which is connected to a high voltage source  6 , the high-voltage source  6  being advantageously a source of high voltage between 250 V and 30 kV, notably between 1 kV and 20 kV, preferably between 1 kV and 10 kV. 
     The ground electrode(s)  4  and the high-voltage electrode(s)  5  are made with an electrical conductor material, such as indium (In), tin (Sn), copper (Cu), gold (Au), oxides and/or alloys thereof or mixtures thereof, in particular indium tin oxide (ITO), copper (Cu), gold (Au), chromium (Cr) or indium-tin alloy (In-Sn) or mixture thereof, more particularly ITO optionally in mixture with gold. The ground electrode(s) and the high-voltage electrode(s) can be made with an identical or different electrical conductor material. 
     The ground electrode(s) can be constituted with one or several ground electrode(s). Each ground electrode can have various forms, and extend over various parts of the fluid channel  2 . 
     In the same way, the high-voltage electrode(s) can be constituted with one or several high-voltage electrode(s), extending over various parts of the fluid channel  2 . The ground electrode(s)  4  and the high voltage electrode(s)  5  can have identical or different patterns. Therefore, as the electrode arrangement defines precisely where the plasma occurs in the gas channel  10 , a chemical reaction can be initiated in a part of the fluid channel  2  surrounded by the ground electrode  4  and the high voltage electrode  5 , and can occur in the other part of the fluid channel  2 . Particular spatial pattern of the electrode  4 . 5  can therefore also be useful to alternate parts of the fluid channel  2  wherein the plasma can occur and other parts wherein reactive species created in the plasma can diffuse and react the liquid phase. 
     The design of a specific mask layer can also be drawn (not illustrated), corresponding to the ground electrode  4  or to the high voltage electrode  5 , before the manufacture of the electrode. This mask can define for example a rectangular area adapted to cover the part of the gas channel  10  where a plasma discharge occurs. Therefore, uncontrolled discharge upstream or downstream is avoided, and a repeatable discharge volume in all the reactors is allowed. 
       FIG. 5  diagrammatically shows an upper view of the high-voltage electrode  5  and the ground electrode  4  extending respectively over and under the main plane in which is arranged the fluid channel  2  according to various embodiments of the invention. Other forms could however be envisaged. In a preferred embodiment, the ground electrode  4  extends over a plane on one side of said main plane, and the high voltage electrode  5  extends over another plane on the other side of said main plane. Therefore, the discharge in the gas channel  10  can be homogeneous, and the reactivity of the gas phase can also be homogeneous. 
     In a preferred embodiment, the high voltage electrode  5  and the ground electrode  4  are respectively deposited on opposite faces of the support  3 , for example over and under the support  3 . In another preferred embodiment, at least one of the ground electrode  4  and the high voltage electrode  5  can be embedded in the support  3 . In all the embodiment of the invention, at least one of the ground electrode  4  and the high voltage electrode  5  is separated from the fluidic channel  2  by a dielectric material. 
     In a preferred embodiment, the distance between the high-voltage electrode(s)  5  and the ground electrode(s)  4  is comprised between 10 μm and 10 mm, notably between 50 μm and 5 mm and preferably between 500 μm and 2 mm. Smaller distances are preferred, notably between 500 μm and 1000 μm, in order to have a low breakdown voltage which is less energy-consuming. This distance depends also on the height of the gas channel  10 , which is present between the ground electrode  4  and the high voltage electrode  5 . 
     In reference to  FIG. 6 , in a preferred embodiment, the ground electrode  4  and the high voltage electrode  5  can be arranged in a double dielectric barrier discharge configuration (double-DBD). Therefore, electrode sputtering by the plasma can be avoided. 
     In reference to  FIG. 7 , the ground electrode  4  and the high voltage electrode  5  can be arranged in a single dielectric barrier discharge configuration (mono-DBD). 
     Electrical Connections 
     The high voltage electrode  5  and the ground electrode  4  are respectively connected to a high voltage source  6  and to the ground. In preferred embodiments, the high voltage source  6  can provide sinusoidal waves of potential or a pulsed potential. 
     Sinusoidal waves are generated by a function generator and magnified by a signal amplifier (Trek, 20/20C) to several thousands of volts, optionally in the range from 500 V peak to peak to 20 000 V peak to peak, preferably in the range from 1 000 V peak to peak to 10 000 V peak to peak. The frequency of a wave can vary from 50 Hz to 5 kHz. This range can be limited by the slew rate of the amplifier and/or by the behavior of the discharge in the plasma microreactor  1  (transport of species, diffusion, kinetics of the targeted chemical reactions, etc.). 
     High voltage pulses are produced by a pulsed generator, made of a DC high voltage generator (Spellman UM Series) coupled with a fast high voltage switch (Behlke HTS Series). It produces 100 ns-wide pulses of up to ±10 kV. 
     Manufacture of the Plasma Microreactor 
     In reference to  FIG. 8  and  FIG. 9 , the fluid channel  2  is designed as a serpentine-patterned channel whose cross-section is illustrated in the abovementioned embodiments, for example in  FIG. 2 .  FIG. 8  and  FIG. 9  illustrate the designs of the masks for the manufacture of respectively the first layer L 1  and the second layer L 2  of the plasma microreactor  1 . The designs are drawn with a 2D computer-assisted design software (Clewin5). 
     In reference to  FIG. 10A ,  FIG. 10B ,  FIG. 10C ,  FIG. 10D  and  FIG. 10E , a PDMS mold  103  is manufactured. 
     In reference to  FIG. 10A , negative photoresist  101  (SU 8   2035 , purchased from MicroChem®) is spin-coated at 3500 RPM during 30 s over a silicon wafer  102 , to obtain a 30 μm thick layer. 
     In reference to  FIG. 10B , the photoresist layer is insolated for 5.5 sec at 40 mW/cm 2  in a mask aligner (UV-KUB3 Mark Aligner from KLOE® company) through a mask  105  corresponding to the first layer L 1  (as illustrated notably in  FIG. 8 ), then soft baked for 1 min 65° C. and then for 6 min at 95° C. The insolated parts of the photoresist layer undergo a chemical reaction which toughens them. After the soft bake, a second layer of photoresist is spin-coated onto the first layer at 2500 RPM during 30 s to yield a 50 μm thick layer. The same procedure as before is repeated with a mask  105  corresponding to the second layer L 2  (as illustrated notably in  FIG. 8 ), aligned with the distinguishable shapes of the first layer. After the second soft bake, the substrate  102  and photoresist layers are developed during 7 min in a bath of 1-methoxy-2-propanol acetate (Microposit®) to remove the unreacted parts of the photoresist layer. 
     In reference to  FIG. 10C , this procedure makes possible to provide a positive mold  104  having at least the shape of the forecasted fluid channel  2 . 
     In reference to  FIG. 10D , a mixture of liquid PDMS (RTV, room temperature vulcanizing polydimethylsiloxane) and cross-linking agent is poured over the positive mold  104 , and cross-linked at 70° C. for 1 hour. 
     In reference to  FIG. 10E , the crosslinked PDMS layer is peeled off the positive mold  104  to provide the PDMS mold  103 . The positive PDMS mold  103  is then used to fabricate the plasma microreactor  1 , comprising two 150 μm-thick D 263 M glass coverslips (Menzel-Glaser, 24×60 mm), between which NOA-81 (Norland Optical Adhesive) resist will build the walls of the fluid channel  2 . 
       FIG. 11A ,  FIG. 11B ,  FIG. 11C ,  FIG. 11D  and  FIG. 11E  illustrate diagrammatically the manufacture of electrode-bearing coverslips by a lift-off process. 
     In reference to  FIG. 11A , positive photoresist  111  (S 1818 , Microposit®) is spin-coated over the coverslip  112 . 
     In reference to  FIG. 11B , the photoresist  111  is UV-insolated for 11 sec at 23 mW/cm 2  through a mask  115  corresponding to an electrode, then baked 1 min at 65° C. and 2 min at 115° C. 
     In reference to  FIG. 11C , the photoresist  111  is developed in a bath of photoresist developer (MF-319 Microposit®). The photoresist  111  stays only on the parts which were not insolated, protecting the underlaying glass where the electrode should not be. 
     In reference to  FIG. 11D , the coverslip is introduced in a magnetron sputtering deposition system (Plasmionique®) operating with one target of ITO and one target of gold. After an oxygen plasma cleaning step, the deposition process follows the steps of: pure ITO deposition, mixed Au-ITO deposition, pure ITO deposition tso form the Au-ITO layer  113 . The addition of gold in the ITO film enhances its conductivity by a factor of  100  after thermal annealing 2h at 400° C. in air. The final thickness of the deposited thin film is approximatively 70 nm. 
     In reference to  FIG. 11E , the photoresist  111  is removed together with parts of the target material covering it, for example using a bath of acetone. 
     Referring to  FIG. 12A , the electrode side  121  of the first coverslip  122  is electrically connected to a band of aluminum foil through silver paint (not illustrated), then glued, for example with NOA, to a 1 mm thick glass slide substrate to ensure the robustness of the chip. Inlet and outlet bores  124  are drilled all the way through this assembly. 
       FIG. 12B  illustrates a cross view section of the assembly made of the glass slide substrate  123  glued to the first coverslip  122  on this electrode side  121  with bores  124 . 
     Referring to  FIG. 13A , the negative PDMS mold  103  is laid onto the bare side of the first coverslip  122 . Therefore, the electrode (for example the ground electrode  4 ) can be separated from the fluid channel  2  by the glass of the coverslip  122  which is a dielectric material. 
     Referring to  FIG. 13B , liquid NOA-81 drops are deposited around the mold  103 . The NOA-81 is a thiol-ene based, UV-curable resin. It has been chosen as a micromachining material over traditionally used PDMS (polydimethylsiloxane) for its good physical, chemical, electrical and optical properties, most of them being detailed in [7]. Therefore, unlike PDMS microreactors, NOA-81 microreactors are impermeable to gas such as air and water vapor, which ensures a closed environment for plasma and chemical reactions. Moreover, cured NOA-81 has a high elastic modulus (typically 1 GPa), which avoids sagging effects. NOA-81 appears less sensitive to solvent swelling effects than PDMS. The dielectric constant of NOA-81 being 4.05 at 1 MHz and 6.5 at 1 kHz, it is as a performant insulating material than PDMS. With a high transmittance in the visible and near-UV range, NOA-81 makes it possible for in situ discharge diagnostics (Optical Emission Spectroscopy (OES) or ultra-rapid camera measurements). Owing to these properties, NOA-81 is a good candidate for the manufacture of the plasma microreactor  1 . 
     The liquid is driven between the glass of the first coverslip  122  and the PDMS mold  103  by capillary forces until it fills completely the open parts of the microstructure. During that steps, the array of circular shapes seen on the masks  105 , illustrated in  FIG. 8  and in  FIG. 9 , act as pillars to prevent the PDMS mold  103  from collapsing where there is no channel part to put it in contact with the rigid glass. 
     Referring to  FIG. 13C , a short treatment with UV light is then used to obtain a partially insulated NOA-81 microstructure  125  which keeps a thin layer of uncured liquid on its surface. 
     Referring to  FIG. 13D , the PDMS mold  103  is then carefully removed from the microstructure  125 . 
     Referring to  FIG. 14A  and  FIG. 14B , the bare side of a second glass coverslip  142 , is homogeneously pressed on the structure. A treatment with UV light ends the curing process. 
     Referring to  FIG. 15A  and  FIG. 15B , an electric connection with the electrode of the second coverslip (for example the high voltage electrode  5 ) is made with an aluminum foil, and connectors (for example Nanoports® connectors) are glued to the bores  124 . The electrodes  4 . 5  face each other in a plane-to-plane configuration. Particularly, the ground electrode  4  and the high voltage electrode  5  are arranged in a double-DBD configuration: both electrodes are separated from the plasma zone, i.e. from the gas channel  10  by an insulating layer. In another embodiment, a single-DBD can be fabricated by fabricating the ground electrode  4  directly one the glass slide substrate  123  and fabricating the microstructure  125  directly on the surface of the substrate  123  where the ground electrode  4  is deposited. 
     An optical diagnostic can be performed with an optical transmission imaging setup by using a transparent ITO ground electrode  4  and a transparent ITO high voltage electrode  5 . In another embodiment, the ground electrode  4  can be made in an opaque material, for example in Cr/Au. Imaging the light emitted by a plasma discharge is therefore possible using one transparent electrode. If none of these diagnostics are required, both electrodes can be made out of regular, cheap and/or opaque metals. 
     Power Assessment 
     In reference to  FIG. 16 , the electrodes  4 , 5  can be polarized by a sine wave potential. The electrical characterization is made by the Lissajous method. A 3.2 nF capacitor is connected in series between the ground electrode  4  and the ground, and the voltage drop across both the high voltage electrode  5  and across the capacitor are measured with adapted voltage probes (LeCroy PPE 20 kV, 1000:1, 100 MHz; Teledyne LeCroy PP024 10:1 500 MHz) with an oscilloscope (Teledyne Lecroy WaveSurfer10, 1 GHz) connected to a computer. 
     In the absence of any plasma discharge, the plasma microreactor  1  acts as a capacitance. Therefore, the measured voltages are in phase because the equivalent electrical circuit is equivalent to two capacitances in series. This behavior is measured by a straight line displayed by the oscilloscope in XY mode. 
     When plasma discharges occur, small amounts of charges come across the plasma in the gas channel  10  and are deposited on the capacitance. This deviation from the pure capacitive behavior of the reactor proves the presence of discharges and can be measured when the oscilloscope displays a parallelogram-shaped XY image. 
     Moreover, since the voltage drop across the capacitance is proportional to its charge, the area of the parallelogram readily gives us the energy transferred to the reactor during one period, as illustrated in  FIG. 16 . 
     In another embodiment, the electrodes  4 , 5  can be polarized by a pulsed high voltage. A high voltage probe (Keysight 10076 C voltage probe, 4 kV, 500 MHz or LeCroy PPE 20 kV, 1000:1, 100 MHz) provides with the voltage drop across the plasma microreactor  1  while a current probe (Pearson current probe, model 2877, 200 MHz) placed between the micro-reactor and the ground gives the total electrical current (capacitive+discharge) flowing through the plasma microreactor  1 . The capacitive current being the derivative of the voltage, the discharge current can be extracted from the total current and the energy deposited into the plasma microreactor  1  can be calculated. 
     Optical Detection 
     A charge-coupled device (CCD) camera (Pixelink PL-B781U) can be mounted on a macroscope (Leica Z16 APO) and image the fluid flow inside the plasma microreactor  1 . The plasma microreactor  1  can be back-illuminated by a diffuse LED lamp. Therefore, the stability of the collinear gas/liquid flow can be evaluated and the liquid  8  and gas  12  flow rates can be tuned. 
       FIG. 17  is a top view image of the plasma microreactor  1 . An intensified charge-coupled device (iCCD) camera (Pimax 4 , Princeton Instruments) was triggered by the function generator to collect images of plasma discharges through a macroscope (Leica Z16 APO). A dark environment is necessary for the detection of weak optical emission of electrical discharges, due to the small volume concerned.  FIG. 17  illustrates the light emitted by the plasma homogeneously all along the gas channel  10  when a discharge is triggered while cyclohexane and argon are flowing contiguously: cyclohexane liquid  8  in the liquid microchannel  9  and argon gas  12  in the gas channel  10 . 
       FIG. 18A  and  FIG. 18B  are top view images of the plasma microreactor  1 . In reference to  FIG. 18A , plasma discharges are imaged in a T-shaped fluid channel  2 . A gas  12  flows in the entire fluid channel  2 , i.e. both in the liquid microchannel  9  and in the gas channel  10 . Therefore, the difference of height between the liquid microchannel  9  and the gas channel  10  can be measured through a difference of intensity in the plasma discharges image. 
     In reference to  FIG. 18B , a liquid  8  flows in the liquid microchannel  9  and a gas flows in the gas channel  10 . Therefore, plasma discharges are only visible in the gas channel  10 . 
     Plasma Enhanced Chemical Synthesis 
     The above described plasma microreactor  1  is adapted to generate a plasma in contact with a fluid phase. A method for generating a plasma in a plasma microreactor (1), comprises the steps of:
     (a) providing the plasma microreactor  1 ,   (b) providing a liquid  8  and making the liquid flow through the liquid microchannel(s)  9  in a given direction,   (c) providing a gas  12  and making the gas flow through the gas channel  10  in said direction,   (d) applying a high voltage between the high voltage electrode(s)  5  and the ground electrode(s)  4  so as to generate a plasma in the gas channel  10 . As the control of the liquid flow and the gas flow are independent, it is possible to tune the residence time of the liquid phase and of the gas phase, and for example having different a liquid flow rate and a different gas flow rate.   

     EXAMPLES 
     Example 1 
     Stabilization of the Flows 
       FIG. 19  illustrates a flow of water and a flow of argon, controlled in two different types of fluid channels: a fluid channel  2  comprising two convex bended portions  7  according to the embodiment illustrated in  FIG. 2  (“with a step”), and in a flat fluid channel different the invention (“without a step”), for varying gas flow rates from 0.5 mL/min to 2 mL/min. Therefore, without convex bended portions  7 , no stable biphasic flow can be achieved, and the flow is a plug flow or scattered water droplets which randomly move onwards. The fluid channel  2  comprising convex bended portion  7  allows gas  12  to flow through the liquid  8 . 
     More generally, the behaviors of cyclohexane and water flows are dependent on the respective affinities of cyclohexane and water for glass and NOA. Whereas cyclohexane has a similar affinity for glass (contact angle) ˜10° and NOA (contact angle) ˜6° which are the two materials of the fluid channel  2  walls, water only barely wets NOA (contact angle with water) ˜65°. These contact angles were measured with a drop shape analyzer (Kriiss analyzer). The wettability of the plasma microreactor  1  material has to be taken in consideration when designing the plasma microreactor  1  for chemical reaction comprising a predefined liquid phase. 
     Example 2 
     Oxidation of Cyclohexane 
     Liquid cyclohexane (VWR, HiPerSolv CHROMANORM for HPLC) was introduced at three different flow rates (6 μL/min, 12 μL/min and 24 μL/min) in the plasma microreactor  1  together with pure O 2  gas at respectively 0.5 mL/min, 1.5 mL/min and 2 mL/min (sccm). A high voltage, a 2 kHz sine wave potential triggers atmospheric pressure dielectric barrier discharges in the gas channel  10 . The amplitude of the potential was adapted to get a constant power of 500 mW, as measured thanks to the aforementioned electrical setup, and was controlled to be measured being between 6 kV and 7 kV peak to peak. 
     The outflowing mixture of liquid  8  and gas  12  was led together though a PTFE tube into a vial immersed in an 8° C. water bath. A second tube connected to the vial served as an exit for the gas  12 . The cold bath reduces the stripping of the volatile cyclohexane, which affects the assessment of final conversion. Experiments with known amounts of pure cyclohexane flowing at 6 μL/min and argon gas at 1 mL/min in the same recovering conditions indeed showed a loss of 14±5% of cyclohexane, whereas a loss of sensibly 30% cyclohexane was measured without the cold bath. The temperature of 8° C. was chosen to be slightly above the melting temperature of cyclohexane (T m =6.47° C.). Indeed, when a 0° C. ice bath is used, the liquid flowing out of the tube inside the vial freezes and clogs the end of the tube. Without this temperature control, pressure rises inside the plasma microreactor  1 , owing to the inflowing gas rate until the stress limits of the reactor which then debonds itself, leaks and is definitely unusable. 
     The analysis of the liquid  8  was performed by GC-MS/FID (Agilent, 7890B GC+5977B MSD). The chromatographs showed three main products, one secondary product and a lot of traces. Two of the main products were positively identified as cyclohexanol and cyclohexanone, and the secondary product as being cyclohexene. The third major product was thought to be cyclohexyl hydroperoxide, which was indirectly proven by two elements. Firstly, band test of the hydroperoxide function was carried out on the cyclohexane before and after passing through the plasma microreactor  1 , and gave positive result only in the latter case. Secondly, NaBH 4  reduction of the outflowing liquid has been carried out, and the comparison of the corresponding chromatographs showed a disappearance of the unknown peak, while the cyclohexanol peak was increased. This is coherent with the reduction of cyclohexyl hydroperoxide into cyclohexanol. 
     The three products cyclohexanol, cyclohexanone and cyclohexyl hydroperoxide actually stem from the same reaction chain and can be easily turned into one product (cyclohexanol, by reduction). The products are usable for industrial purposes. The mixture cyclohexanol/cyclohexanone is indeed known as “KA oil” and used as precursor of nylon. Therefore, it was chosen to count these products together when calculating the selectivity of the reaction. 
       FIG. 20  illustrates the molar fraction of compounds in the output liquid  8 . For each reaction time (30 sec, 1 min and 2 min), the portions (a) of the diagram illustrate the proportion of unknow compounds, the portions (b) of the diagram illustrate the proportion of cyclohexyl hydroperoxide, the portions (c) of the diagram illustrate the proportion of cyclohexanone, the portions (d) of the diagram illustrate the proportion of cyclohexanol and the portions (e) of the diagram illustrate the proportion of cyclohexene. Therefore, a partial oxidation of cyclohexane was performed in the plasma microreactor  1 , and no over-oxidation was detected. 
     Table 1 illustrates the selectivity of the conversion and the proportion of converted cyclohexane (total conversion) for three different reaction times in the plasma microreactor  1 . 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Reaction time 
                 Selectivity 
                 Total conversion 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 30 
                 sec 
                 71% 
                  7% 
               
               
                 1 
                 min 
                 81% 
                 14% 
               
               
                 2 
                 min 
                 67% 
                 32% 
               
               
                   
               
            
           
         
       
     
     Example 3 
     Variation of the Reactants 
     Other gases  12  can be used to perform a chemical reaction as described in the example 2. The use of pure hydrogen gas  12  on cyclohexane in the plasma microreactor  1  produces cyclohexene as the main product, which is unexpected since it proves a dehydrogenation reaction. Cyclohexene selectivity varies from 17% to 35%, decreasing with the increasing overall conversion. A significant amount of the by-products is thought to stem from the overreaction of cyclohexene which, owing to its double bond, is more reactive than cyclohexane. Table 2 illustrates possible reactions to be performed in the plasma microreactor  1  according to the provided type of gas. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Type of gas 
                 Possible reactions 
               
               
                   
                   
               
             
            
               
                   
                 Ar/He 
                 Bond breaking (not selective) 
               
               
                   
                   
                 Enhancing the reactivity of active 
               
               
                   
                   
                 gases (by helping energy transfers) 
               
               
                   
                 O 2   
                 Partial oxidation 
               
               
                   
                   
                 Total oxidation 
               
               
                   
                 H 2   
                 Hydrogenation 
               
               
                   
                   
                 Dehydrogenation 
               
               
                   
                 NH 3   
                 Amination 
               
               
                   
                 N 2   
                 Bond breaking (not selective) 
               
               
                   
                   
                 Amination (with H 2 ) 
               
               
                   
                 CO/CO 2   
                 Carbonylation 
               
               
                   
                   
               
            
           
         
       
     
     Example 4 
     Variation of the Design of the Liquid and Gas Microchannels 
     A microreactor was manufactured in Borofloat®  33  glass according to the geometry depicted on  FIG. 3D . The fabrication steps included plasma etching of two glass plates and their subsequent bonding. In a globally serpentine-shaped channel, the liquid flow was lead by a dozen of 40 μm×40 μm parallel grooves on one of the 1 mm-wide sides of the channel, and the gas flow allowed to flow in the above 300 μm-high channel space. 
       FIG. 21  shows pictures of the channel taken in transmission for various flow rates. When empty ( FIG. 21 d   ), the channel appears dark because of the granularity of the plasma-etched glass surface which scatters light. When covered by a liquid layer, the surface is smoothed, the light scattering is reduced and the picture becomes lighter. The presence of a liquid flow is then assessed by the brightness of the picture. In  FIGS. 21 a , 21 b  and 21 c    the flow rates were the following: 20 ml/min of oxygen gas, and 25, 30 and 35 μl/min of liquid cyclohexane, respectively. 
     Although the groove-like structure of the channel remains unseen with this setup, one can clearly see the difference between two areas: the centre of the channel and its edge (brighter, only on  FIGS. 21 b  and 21 c   ). The latter reveals a liquid flow taking the full channel height. In contrast, the first is therefore only a thin layer of liquid which only wets the bottom of the channel, thanks to the groove-like structure. Zooming onto the channel allows one&#39;s careful eye to guess this structure ( FIG. 22 , with 10 ml/min oxygen flow rate and 20 μl/min cyclohexane flow rate). 
     Two comb-shaped copper electrodes were then deposited on both sides of the chip to allow direct optical visualisation. A plasma was triggered by a 2 kHz 20 kV peak-to-peak sine high voltage between these electrodes. 
       FIGS. 23 and 24  show a 20 ml/min oxygen and 300/min cyclohexane biphasic flow, with the plasma enabled. The liquid flow is well confined inside the grooves. An GC-FID analysis of the out-flowing liquid gave a 5.9% conversion and a 83% selectivity in (cyclohexanol+cyclohexanone+cyclohexyl hydroperoxide). 
     REFERENCES 
     
         
         [1] Malik, M. A. ( 2010 ). Water purification by plasmas: Which reactors are most energy efficient?,  Plasma Chemistry and Plasma Processing,  30(1), 21-31. 
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         [3] Matsui, Y., Takeuchi, N., Sasaki, K., Hayashi, R., &amp; Yasuoka, K. (2011). Experimental and theoretical study of acetic-acid decomposition by a pulsed dielectric-barrier plasma in a gas-liquid two-phase flow. Plasma Sources Science and Technology, 20(3), 034015. 
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