Patent Publication Number: US-2018036763-A1

Title: Microfluidic device for thermally spraying a liquid containing pigments and/or aroma prone to aggregation or deposition

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to a microfluidic device for thermally spraying a liquid containing pigments and/or aroma prone to aggregation or deposition. 
     Description of the Related Art 
     As is known, for thermal spraying inks and/or aroma, for example in printers and perfumes, the use of microfluidic devices of small dimensions has been proposed, since they can be obtained with microelectronic manufacturing techniques. 
     For instance, U.S. Pat. No. 9,174,445 discloses a microfluidic device designed for thermally spraying ink on paper. 
       FIG. 1  shows a chamber  11  of a microfluidic device  10  for thermally spraying inks and aroma, similar to the one described in the above patent. The chamber  11  illustrated in  FIG. 1  is formed in a chamber layer  12  and is delimited, at the bottom, by a thin layer  13 , of dielectric material, and, at the top, by a nozzle plate  14 . 
     A nozzle  15  is formed through the nozzle plate  14  and has a first portion  15 A, facing the chamber  11 , and a second portion  15 B, facing the opposite direction (towards the outside of the microfluidic device  10 ). The first portion  15 A is significantly wider than the second portion  15 B. A heater  20  is formed in the thin layer  13  so as to be adjacent to the chamber  11  and arranged at the nozzle  15 . The heater  20  may have an area of approximately 40×40 μm 2 , generate, for example, an energy of 3.5 μJ, and is able to reach a maximum temperature of 450° C. in 2 μs. 
     The chamber  11  is moreover provided with a fluidic access  21  enabling inlet and transport of the liquid in the chamber  11 , as indicated by the arrow L. A plurality of columns (not visible in  FIG. 1 ) may be formed in the fluidic access  21 , and have the aim of preventing bulky particles from clogging the fluidic access  21 . 
     In the microfluidic device  10 , the chambers  11  are connected through the fluidic accesses  21  to a supply system (not illustrated). 
     The operation of the chamber  11  is represented schematically in  FIGS. 2A-2E . The liquid L reaches the chamber  11  through the fluidic access  21  ( FIG. 2A ), forming a liquid layer  16  of, for example, 0.3 μm in thickness. The heater  20  heats the liquid layer  16  up to a preset temperature ( FIG. 2B ). This temperature is chosen, according to the liquid used, so as to instantaneously reach boiling point, for example at a temperature close to 300° C. In this situation, the pressure rises to a high level, for example, approximately 5 atm, forming a vapor bubble  17 , which disappears after a few microseconds, for example 10-15 μs, as illustrated in  FIGS. 2C-2D . The pressure thus generated pushes a drop of liquid  18  through the nozzle  15 , after which the liquid layer  16  returns to its initial condition ( FIG. 2E ). 
     The entire cycle is repeated up to ten thousand times per second, supplied by the liquid that continuously arrives through the fluidic access  21 . 
     Many fluids used in thermal-spraying devices contain pigments that tend to aggregate easily, causing, in time, clogging of the supply system and thus failure in the functioning of the thermal-spraying device. 
     To overcome this problem, external liquid-movement means have been proposed. For instance, an external system of pumps and pressure regulators has been proposed, as illustrated in  FIG. 3 . By virtue of the illustrated system, designated as a whole at  25 , the liquid is constantly filtered in a filtering and pressure-control stage  26  in order to prevent clogging of the nozzles  15 . A heater  27  has the aim of keeping the liquid at a constant temperature, for example 40° C., and is kept in continuous circulation by each of the chambers  11  through a pump  28 . 
     Furthermore, the system  25  illustrated in  FIG. 3  generates a “meniscus vacuum”, i.e., a slight negative pressure inside the chambers  11  that keeps the nozzles  15  in optimal conditions and ready to emit the liquid. The system can also continuously remove the bubbles from the liquid by a degasser  29 . 
     However, this solution to the problem of pigment aggregation has the disadvantage of requiring a bulky recirculation system outside the thermal spraying device. It is moreover costly. 
     BRIEF SUMMARY 
     According to one or more embodiments of the present disclosure, a microfluidic device for thermally spraying a liquid is provided, as well as a method for operating a microfluidic device. In one or more embodiments, the microfluidic device provides a simple and effective recirculation system that prevents deposition and aggregation of pigments of particles in the liquid. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the present disclosure, a preferred embodiment thereof is now described, purely by way of non-limiting example, with reference to the attached drawings, wherein: 
         FIG. 1  is a cross-sectional perspective view of a chamber of a known thermal spraying device; 
         FIGS. 2A-2E  illustrate operations of the chamber of  FIG. 1 ; 
         FIG. 3  is a schematic block scheme of an external recirculation system; 
         FIG. 4  is a top plan view, with ghost parts, of an embodiment of the present thermal-spraying device; and 
         FIG. 5  is a cross-section of a portion of the device of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4  shows a thermal spraying device  50  comprising a plurality of chambers  51 , a circulation channel  52 , and liquid movement means  53 . Moreover  FIG. 4  shows (in ghost representation) an upper layer forming a nozzle plate  57  ( FIG. 5 ), whereof only nozzles  58  are visible, as discussed in greater detail hereinafter, with reference to  FIG. 5 . 
     The chambers  51  may be formed as illustrated in  FIG. 1 , to which reference is made, and are each provided with a chamber heater  70 . 
     In the illustrated embodiment, the circulation channel  52  extends along a closed line surrounding the plurality of chambers  51  and is fluidically connected to a supply channel  54 , illustrated only schematically, supplied with a liquid L (of other fluid, such as a gas) in use. Each chamber  51  is connected to the circulation channel  52  by a respective liquid-access channel  59 . 
     The liquid movement means  53  are here formed in the circulation channel  52 . 
     As illustrated in detail in  FIG. 5 , the circulation channel  52  and the plurality of chambers  51  are formed inside a chamber layer  55  and are delimited at the bottom by a thin layer  56  and at the top by a nozzle plate  57 , as may be seen in particular in  FIG. 5 . A substrate  65  is arranged under the thin layer  56  and is made, for example, of semiconductor material, such as monocrystalline silicon. 
     The chamber layer  56 , similar to the chamber layer  12  of  FIG. 1 , is made, for example, of polymeric material, such as dry film, and can be obtained using lamination, reflow, lithographic and/or removal techniques, in a known known manner in the field of microinjectors. Alternatively, it can be molded and bonded onto the thin layer  56 , or made by gluing structures of etched silicon. 
     The thin layer  56 , similar to the thin layer  13  of  FIG. 1 , is made of insulating material, for example dielectric material, such as silicon oxide or/and silicon nitride. 
     The nozzle plate  57 , similar to the nozzle plate  14  of  FIG. 1 , may be formed, for example, by a layer of polymeric material molded and bonded to the chamber layer  55 . 
     As already mentioned and as shown in  FIG. 1 , the thermal spraying device  50  includes a plurality of nozzles  58  is arranged in the nozzle plate  57 , each nozzle at a respective chamber  51 . 
     The liquid movement means  53  are here formed by a channel heater  60  and a plurality of fluidic resistances  61 . 
     The channel heater  60  is made like the chamber heaters  70 . In particular, the channel heater  60  is formed by one or more strips of conductive material, formed in the thin layer  56  under the circulation channel  52 . For instance, the channel heater  60  may be formed by layers of metal materials, such as tantalum, aluminum, tantalum silicon nitride, appropriately machined polymeric materials, and alloys of tantalum and aluminum, silicon chromium, tantalum silicon nitride or tungsten silicon nitride. The channel heater  60  is connected, via contacts  75  and electrical connection lines  72 , to a control and supply unit  76 , including a switching element, for example, a switch  74 , and a power supply generator  73 . The heater  60  and the electrical connection lines  72  may be formed according to the semiconductor technique, through deposition and/or sputtering, masking and etching. 
     The fluidic resistances  61  are here formed by walls  63  extending in the circulation channel  52  adjacent to the channel heater  60 , in particular upstream of the channel heater  60 , in a direction of movement of the liquid, indicated by arrows. The walls  63  may be of the same material as the chamber layer  55  and are defined in the same manufacturing step, via masking and etching, or molding of polymeric material. 
     In particular, the walls  63  extend on two mutually opposite sides  52 A,  52 B of the circulation channel  52  in a direction slanted with respect to a median vertical plane A, longitudinal to the circulation channel  52  in the considered area. The walls  63  here extend throughout the height of the circulation channel  52 . In the embodiment illustrated, the walls  63  face each other two by two, forming pairs of walls  63 , wherein each pair of walls comprises a first wall  63 A and a second wall  63 B, arranged specularly with respect to the median vertical plane A of the circulation channel  52 . In this way, each pair of walls  63 A,  63 B creates a reduction of the liquid passage section so as to block the bubbles forming on the channel heater  60 , as explained in detail hereinafter. 
     In use, the channel heater  60  is activated through the switch  74  during inactivity of the thermal spraying device  50  (and thus of the chamber heaters  70 ) and functions like the heater  20  of the prior art as regards formation of bubbles. In particular, the channel heater  60  heats the liquid layer present in the circulation channel  52  around the channel heater  60  up to a temperature at which a vapor bubble is formed, which subsequently bursts. Bursting of the bubble generates a thrust in the liquid that causes movement thereof into and along the circulation channel  52 . The presence of the fluidic resistances  61  adjacent to the channel heater  60  ensures that the thrust impressed by the bubble on the liquid is exclusively in a direction opposite to fluidic resistances  61 , thus ensuring a stable and continuous circulation. 
     As mentioned, the channel heater  60  is activated by the switch  74  when the process of thermal spraying is inactive, thus maintaining a continuous flow of liquid within the circulation channel  52  and thus preventing stagnation and aggregation of pigments in the liquid when it is not conveyed towards the chambers  51 . 
     The thermal spraying device  50  described herein is advantageous as compared to the known solutions since it enables overcoming the problem of aggregation of the pigments in the liquid without having to resort to a complex and cumbersome system for recirculating the liquid outside the device, but just by adding liquid movement means  53  integrated in the thermal spraying device  50 . 
     Finally, it is clear that modifications and variations may be made to the device and method described and illustrated herein, without thereby departing from the scope of the present disclosure. 
     For instance, the shape and arrangement of the circulation channel  52  may vary with respect to what illustrated. In particular, the circulation channel  52  might not surround the chambers  51  and/or may develop according to more complex lines, for example a labyrinth. The fluidic resistances could be obtained with different solutions, for example via restrictions in the circulation channel  52 , or by appropriately sizing Tesla valves, for example manufactures as taught in U.S. Pat. No. 1,329,559 (see also http://www.epicphysics.com/model-engine-kits/tesla-turbine-kit/the-tesla-valve/). 
     The arrangement of the chambers  51  may differ from the illustrated one. For instance, the chambers  51  may be arranged so as to form an annulus or an S shape, or have some other nonlinear configuration. 
     The circulation channel  52  may be connected just to some of the chambers  51 . For instance, the chambers  51  may be divided into different sectors, and the chambers of different sectors may be connected to different supply channels; for example, they may contain different liquids. In this case, the circulation channel may be connected only to the chambers  51  of one of the sectors. In this case, there is the advantage that recirculation can be dedicated and adapted to some of the chambers  51  instead of to all of them. This makes it possible to have chambers  51  dedicated to fluids with a tendency to aggregation, whereas other chambers  51  may be dedicated to fluids that can be controlled less problematically. For instance, some chambers  51  may be dedicated to perfumes with a tendency to aggregation and thus connected to a dedicated circulation channel  52 , whereas other chambers  51 , dedicated to humidification with water, have no recirculation. 
     The control and supply unit  76  may be integrated in the device  50  or arranged on a separate device. 
     Although the microfluidic devices shown in the Figures are heater actuated, the circulation channel, as well as the channel heater and fluidic resistances, also apply to piezoelectric actuated microfluidic devices. 
     The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.