Abstract:
A magnetohydrodynamic force fluid logic controller is provided including a solid substrate, a fluid chamber disposed above the substrate, containing a fluid under test. It also includes an active magnet such that the active magnet is disposed to control north and south poles on the fluid under test or on the carrier fluid and a two-dimensional distribution of magnetized domains disposed on a surface of the solid substrate. The magnetized domains have poles activated by the above the active magnet, where the different parts of the above the fluid under test self-interact and split, merge or propagate along different directions on the above the substrate subject to their placement on the magnetized domains, whereby the active magnet regulates the self-interaction. The results of the self-interactions are equivalent to physical binary logic operations where “1” is the presence and “0” is the absence of a fluidic volume at a given location.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Patent Application 62/164,323 filed May 20, 2015, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to microfluidics. 
       BACKGROUND OF THE INVENTION 
       [0003]    Droplet based microfluidics is a rapidly growing interdisciplinary field of research with numerous applications ranging from fast analytical systems or synthesis of advanced materials to protein crystallization and biological assays for living cells. What is needed is a device and method for the precise and reliable control of multiple droplet volumes simultaneously with a control mechanism of minimal complexity. 
       SUMMARY OF THE INVENTION 
       [0004]    In particular, the current invention relates to a microfluidic device capable of propagating, merging or splitting microfluidic volumes of fluids using self-interactions between these volumes based on magnetohydrodynamic forces. 
         [0005]    The current invention is based on the physical principle that a given fluid immersed in a carrier fluid (forming an immisciple solution between the two) is subject to self-repulsive forces on the double condition that 1) either the given fluid under test or the carrier fluid has magnetic properties and 2) there is an external magnetic field that magnetizes the magnetic component of the fluid solution. The self-repulsive force exerted on the fluid solution is of magnetohydrodynamic nature. 
         [0006]    We use this self-repulsive magnetohydrodynamic force to cause microfluidic volumes of fluids (comprising of the fluid under test and the carrier fluid) to split, merge or propagate in different directions inside a fluid chamber. The fluid chamber is built on top of the solid substrate. The solid substrate has a two-dimensional distribution of magnetized domains that get activated by the magnetic fields generated by an active magnet. The active magnet also magnetizes the microfluidic volumes of fluids that obtain north and south poles. Therefore, the microfluidic volumes of fluids self-interact based on a magnetostatic force that tends to repel them from each other. Using different designs of the two-dimensional distribution of magnetized domains, we control if 1) the self-interacting fluidic volumes are split in smaller volumes, 2) if self-interacting fluidic volumes are merged in bigger volumes, 3) if the self-interacting fluidic volumes maintain a constant volume and but the diverted and propagate along a given path. As far as the 3 rd  case is concerned, diverting a fluid volume along a given path can be equivalent to a binary logic operation, where the result “1” is the presence of a fluidic volume at a given path and “0” is the absence of a fluidic volume at a path. According to one main aspect of the invention the fluid under test is droplets of micro- to nano-liter volumes. The invention from now will be termed as droplet logic controller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  shows a high-level description of the six components of the droplet controller using an isometric orientation and zoom views on individual components. 
           [0008]      FIG. 2  shows the principle of operation of the droplet controller: how a single droplet propagates on a track of metallic bars under the influence of two external magnetic fields. 
           [0009]      FIGS. 3 a - q    shows different embodiments for designs of metallic bars 
           [0010]      FIGS. 4 a - d    show a NOT logic gate of the droplet controller. 
           [0011]      FIGS. 5 a - e    show an AND-OR logic gate of the droplet controller. 
           [0012]      FIG. 6 a - e    show different embodiments for the design of the solid substrate and fluid chamber of the droplet controller. 
           [0013]      FIG. 7 a - d    show different embodiments for the fluid solution of the droplet controller. 
           [0014]      FIG. 8 a - c    show different embodiments for the system of electromagnetic coils/permanent magnets of the droplet controller. 
           [0015]      FIG. 9 a - c    show an arrangement of bars to merge two droplets into a single droplet. 
           [0016]      FIG. 10 a - c    show an arrangement of bars to break-up a single droplet into two droplets. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The Droplet Logic Controller includes six components (shown in  FIG. 1 ): 1) a solid substrate that bears the magnetic bars, 2) a fluid chamber that is mounted on top of the solid substrate and contains the droplets, 3) the fluidic solution (fluid under test with the magnetic beads and a carrier fluid) that is inside the fluid chamber, 4) a system of electromagnetic coils and/or permanent magnets that provide the external magnetic fields which magnetize both the bars on the substrate and the magnetic beads within the droplets, 5) electronic equipment to drive the electromagnetic coils 6) a video camera and a microscope system that are used to monitor the droplets inside the fluid chamber. 
         [0018]    We describe in detail each of the six components of your device mentioned above, and we present multiple low-level embodiments for each. 
       1. Solid Substrate 
       [0019]    The substrate is made of non-magnetic material (e.g. glass) and bears soft magnetic thin rectangular bars on its surface. The flow chamber containing the droplets is mounted on top of the substrate such that the droplets can be manipulated by the fields generated by the substrate&#39;s thin rectangular bars. We will explain the principle of operation, using one embodiment ( FIG. 2 ) for the substrate and the droplets. The bars are polarized using an in-plane rotating field B xy  ( FIG. 2 ), thus forming north and south poles. The droplets, shown as black ellipsoids in  FIG. 2  are made of ferrofluid (water-based solution of 10 nm magnetic beads hold together by surfactants) and are polarized out-of-plane by a static normal magnetic field B z . Each droplet is set in motion to match its south pole to the north pole of the bar. For the shape of the bars, we can use a periodic track of “T-bars” and “I-bars”. When placed in a rotating field B xy , the “T &amp; I-bar” arrangement will produce a cascade, from left to right, of four north poles per full rotation of B xy  creating a propagation pathway for the droplet (see  FIG. 2 ). 
         [0020]    Note that by using only those two magnetic fields B xy , B z  and a continuous track of “T-bars” and “I-bars” populated with droplets, we can set all these droplets in motion without the need to control each of them individually. In addition, all the droplets move in synchronous fashion since they follow the north poles on the bars that are created by the rotating magnetic field B xy . 
         [0021]    The “T &amp; I-bar” arrangement represents only one embodiment of the device. In fact, the shape of the bars does not need to have necessarily the “T” and “I” letter shape. A wide range of shapes can produce distinct north and poles, creating propagation pathways for the droplets. In  FIGS. 3 a - q    we show various shapes for the designs of the bars: a) T shape b) double T shape c) I shape d) S shape e) serpentine shape f) zig-zag shape g) chevron shape h) curved shape i) unequal chevron shape with protrusion j) crossed I bars k) I shape with protrusions l) circular shape m) unequal double T shape n) unequal double T shape with protrusions o) T shape with protrusions p) slanted T shape q) double slanted T shape. 
         [0022]    The solid substrate can be made of a non-magnetic material that is silica, SiO 2 , silicon wafer, plastic or non-magnetic metal. The metallic bars can be made of permalloy material or any soft-magnetic material. Note that the material of the bars needs to be magnetic and preferably soft magnetic (exhibiting negligible hysteresis and coercivity) so that it can respond instantaneously to the rotating field B xy  without any delay in the formation of magnetic poles. 
         [0023]    Apart from the propagation mechanism, the combination of bars enables us to build physical logic gates utilizing interactions between two or more droplets. Here we describe how we can build the three physical logic gates 1) AND, 2) OR, 3) NOT. These types of logic gates are very important because—through their combination—we can create any logic gate (universal logic). Contrary to logic gates of electronic circuits, the physical logic gates we describe here are conservative because no droplets can be generated or destroyed. The interactions are based on a magnetic repulsive force between two adjacent droplets that have parallel magnetizations (magnetized by the external field Bz). We can create these logic gates using intersecting “T” and “I” bar tracks (though other shapes of bars, like those in  FIG. 3  can be similarly used) where two droplets attempt to occupy the same north pole on a bar on a droplet junction. Because of the repulsive force, one or both of them can be diverted to different pathways. 
         [0024]    In  FIGS. 4 a - d    we show a top view of a droplet junction that performs a NOT logic operation. The field Bxy is rotating clockwise in the sequence 1-2-3-4 obtaining the angular orientations shown in the white circles on the top left corner of each one of  FIGS. 4   b,c,d . For every position 1-4 of Bxy, north poles are created on the bars (marked as 1-4). The droplets (shown as transparent circles) move following the sequence 1-2-3-4. In  FIG. 4 a    we show the truth table for the NOT gate and in  FIG. 4 b    we show a general schematic of the gate. The NOT gate is a junction with an input A, an logic NOT output A and a control droplet C. Droplets at positions 3 before the junction will repel each other (repulsion force indicated by line with two arrow ends). In  FIG. 4 c    we show the case A=1, where the droplet is prevented from propagating rightwards (A=0). Dashed and Solid droplet lines indicate past and present droplet positions. In  FIG. 4 d    we show the case A=0 where the droplet C, creates a A=1 rightwards. 
         [0025]    In  FIGS. 5 a - e    we show a top view of a droplet junction that performs a double AND/OR logic operation. The field Bxy is rotating clockwise in the sequence 1-2-3-4 in the angular orientations shown in the white circles on the top left corner of each one of  FIGS. 5   b,c,d  (similar notation as in the  FIGS. 4   b,c,d ). For every position 1-4 of Bxy, north poles are created on the bars (marked as 1-4). The droplets (shown as transparent circles) move following the sequences 1-2-3-4. In  FIG. 5 a    we show the truth table for the AND/OR gate and in  FIG. 5 b    we show a general schematic of the AND/OR gate. The AND/OR gate is a junction with two inputs A,B and two outputs, where one output is the logic AND (A·B) and the second output is the logic OR (A+B). Droplets at positions 1 will repel each other (repulsion force indicated by line with two arrow ends). As in  FIGS. 4 a - d   , dashed and solid droplet lines indicate past and present droplet positions. In  FIG. 5 c    we show the case A=1, B=1 where the both outputs are equal to 1. In  FIG. 5 d    we show the case A=1, B=0, while in In  FIG. 5 e    we have the case A=0, B=1. 
       2. Fluid Chamber 
       [0026]    The fluid chamber is mounted on top of the substrate. Here, present different low-level embodiments for the assembly between the fluid chamber and the solid substrate based on standard microfabrication methods (photolithography) used in microfluidics.  FIGS. 6 a - e    show cross-section views of four different embodiments ( FIGS. 6 a - d   ) obtained from the high-level assembly of the fluid chamber and the solid substrate ( FIG. 6 e   ). All four embodiments of  FIGS. 6 a - d    include a solid substrate, fluid input and output port, arrays of metallic bars disposed on a surface of the solid substrate and a fluid chamber mounted on top that includes a top cover. The top cover isolates the droplets and prevents unwanted evaporation of the fluids. The top cover can be made of glass, acrylic or Polydimethylsiloxane (PDMS), a widely used silicon-based organic polymer used extensively in microfluidics. In essence, any non-magnetic material, could be used for the top cover as long it is inert, non-toxic, and non-flammable. Preferably the top cover should be optically clear to allow monitoring of the droplets by a video camera (see  FIG. 1 ). All of the embodiments have a non-wetting layer deposited in the surfaces that come in contact with the droplet, i.e. the bottom side of the top cover and the top side of the solid substrate. The non-wetting layer allows the smooth movement of the droplet in the fluid chamber avoiding pinning effects, where the droplet could unexpectedly get stuck on the surface. The composition of the non-wetting surface depends on the type of the droplet. For example, if the droplets are water-based, the non-wetting layers need to be hydrophobic like Teflon, fluorosilanes, PDMS, silicon based spray and other types of superhydrophobic materials that have been used successfully in the field of microfluidics. If the droplets are oil-based, oleophobic layers are needed, e.g. fluoropolymer-based solids. 
         [0027]      FIG. 6 a    shows an embodiment where the metallic bars are coated with a non-wetting layer. The droplet (we used the term “test fluid” to account for the different embodiments for the fluidic solution that we will present later) thus flows in a closed top channel immersed in a carrier liquid. The embodiment shown in  FIG. 6 b    is an extension to  FIG. 6 a    where we have added an additional flattening step: a filling material is deposited in excess on top of the metallic bars (like SiO 2 , polymers, photoresists like Su-8) and then flattened to provide for a flat flow channel for the fluid under test. The embodiment shown in  FIG. 6 c    is similar to the one shown in  FIG. 6 b    but includes an additional geometric structures in the providing hydrodynamic resistance/restrictions on the fluid chamber. A wide variety of geometric blocks can be applied like such as walls, channels, grooves, protrusions, and channels.  FIG. 6 d    shows another embodiment where an inner layer of electrically conductive material lines like copper, gold or graphite can be added below the metallic bars domains. Applying a voltage difference across these lines results in a electric current which generates local magnetic fields. These local magnetic fields create local perturbations on the effective magnetic fields on the fluid channel that can enhance or diminish the effect of the polarized magnetic bars. The role of these conductive wires is to provide additional external control by the user. 
       3. Fluidic Solution 
       [0028]    The fluidic solution consists of 1) a fluid under test, that has been assumed to be a water or oil-based droplet containing magnetic beads and 2) a carrier fluid. While the device can potentially work without the carrier fluid, the addition of the latter is important as it creates a lubrication film that reduces the drag resistance force exerted on the droplet as it flows in the fluid chamber. The carrier fluid also prevents other unwanted phenomena such as the drying of the droplet. To prevent the droplet from dissolving into the carrier liquid, they must be immiscible: if the droplet is water-based, the carrier fluid must be oil-based and vice versa. 
         [0029]    The fluidic solution can also have different embodiments according to  FIGS. 7 a - d   . Despite the fact that both the principle of operation in  FIG. 2  and the logic gates in  FIGS. 4 a - d , 5 a - e    were described using a droplet as a fluid under test, other embodiments can also work as long as there is at least one element in the fluidic solution with magnetic properties. In  FIG. 7 a    we present the first embodiment, which is basically the embodiment seen in  FIGS. 1, 2, 4   a - d ,  5   a - e ,  6   a - e : a water or oil-based droplet carrying magnetic beads immersed in a immiscible carrier fluid. In the embodiment of  FIG. 7 b    there is a stable emulsion of a droplet inside another droplet. The outer droplet contains the magnetic particles and thus is subject to the magnetic forces exerted by the magnetic bars of the solid substrate. The inner droplet is the fluid under test that may contain other particles of interest for potential applications. The two droplets can be kept distinct one from another by using surfactants at their interface enabling both of them to move together without mixing. The embodiment of a double droplet in  FIG. 7 b    is based on the removal of the magnetic beads from the fluid under test, thus enabling it to carry any additional components that could not co-exist with the magnetic beads (incompatible or reactive). In  FIG. 7 c    the fluidic solution is the inverse of the one in  FIG. 7 a   : the droplet (fluid under test) is non-magnetic and the carrier fluid is magnetic containing the magnetic beads. Displacing the magnetic carrier fluid using the metallic bars of the solid substrate can cause the displacement of the non-magnetic droplet. Similar to the embodiment shown in  FIG. 7 b   , the fluid under test is not magnetic and can therefore carry any components that could be incompatible with magnetic beads or perform processes (such as chemical reactions) that could not be achieved in the close vicinity of magnetic elements. In  FIG. 7 d   , both the droplet (as the fluid under test) and the carrier fluid are non-magnetic but their interface is populated by magnetic surfactants that can make the droplet propagate. 
       4. System of Electromagnetic Coils/Permanent Magnets to Provide External Magnetic Fields 
       [0030]    The system of electromagnetic coils or permanent magnets generates the external magnetic fields that magnetize the metallic bars on the substrate and the droplet (magnetic fields B xy , B z  correspondingly in  FIG. 2 ). The magnetic field B xy  is exerted on the plane of the solid substrate and it is rotating with constant magnitude. The magnetic field B z  is exerted perpendicular to the plane of the solid substrate, it has fixed a direction and a constant magnitude. 
         [0031]      FIGS. 8 a - c    show different embodiments for the system of coils/magnets. The embodiment in  FIG. 8 a    includes two orthogonal Helmholtz coil pairs for the generation of the rotational field B xy . The coil pairs are stationary and have winded copper wire with insulation coating. The fluidic chip (solid substrate and flow chamber) is placed at the geometric center of those two coil pairs. To generate a rotational field at the center of the coils where the fluidic chip is, each coil pair has an alternating current (AC) with 90° phase difference one from another. Therefore in  FIG. 8 a   , the first coil pair generates a magnetic field B x  along its symmetry axis (x) with a sinusoidal magnitude. The second coil pair generates a magnetic field B y  along its symmetry axis (y) with a sinusoidal magnitude (we will generalize the term “sinusoidal” later) that has 90° phase difference in comparison to the first coil pair. The vector summation of those two orthogonal vectors, B x , B y  produces a rotational field B xy  on the plane xy that has a frequency equal to the frequency of the AC current that flows in the wire. According to the embodiment in  FIGS. 8 a - c   , the coil pairs are Helmholtz-type (mean radius of coil is equal to the distance between the two coils compiling the pair) because this particular type provides optimal magnetic field uniformity in the center of the coil arrangement where the fluidic chip is placed. To generate a perpendicular field B z  at the fluidic chip, the chip is placed in the center of an additional single coil. This single coil has a direct current (DC) that generates the field B z  perpendicular to the fluidic chip, as shown in  FIG. 8 a   . The use of a single coil allows 1) the easy ON/OFF activation of the field B z  and 2) changing the magnitude of B z  for experimentation purposes. However, the single coil with the DC current can also be substituted by a square or circular type of permanent magnet that will provide a constant B z  field, even though this is not shown in  FIG. 8 a   . In this case, the fluidic chip would have to be placed on top of the north or south pole of such a magnet. 
         [0032]      FIG. 8 b    shows an embodiment similar to the one shown in  FIG. 8 a   . The difference in  FIG. 8 b    is that the coils are not Helmholtz-type. If the working area on the fluidic chip (the area where droplets propagate) is much smaller in comparison to the radii of the coils, then the uniformity of magnetic fields of this arrangement may be sufficient without the need for Helmholtz-type coil pairs.  FIG. 8 c    shows an embodiment where the whole coil system can be shrunk in the form of a small chip. In this embodiment, instead of using coil pairs similar to  FIGS. 8 a,b    (much larger that the fluidic chip), we wind two wirings around the fluidic chips. The wirings are orthogonal to each other and have AC currents with 90° phase difference, in order to generate a rotating field B xy  similar to the one in  FIGS. 8 a,b   . The perpendicular field B z  is provided by a permanent magnet that is placed below the fluidic chip as shown in  FIG. 8   c.    
       5. Electronic Equipment to Drive the Electromagnetic Coils 
       [0033]    Standard electronics, used extensively in prior art, can be used to drive the electromagnetic coils. In either of the three embodiments in  FIGS. 8 a - c   , the AC current that needs to flow into each of the two coil pairs with a 90° phase difference (in order to generate the rotating field B xy ), requires electronic equipment that should include: 1) a double-output waveform generator to generate two AC current signals with 90° phase difference and 2) a double-input/double output power amplifier that will receive the two AC signals from the waveform generator, amplify them and output them to the coils. The power amplifier can be either an audio amplifier (simplest solution) or a more sophisticated laboratory-setting AC power amplifier. The DC current needed to power the single coil (in order to generate the perpendicular field B z ) can be provided by a DC power supply. Additional DC signals needed to power the electrically conductive wires of the solid substrate shown in  FIG. 6 d    can be provided by a microcontroller. The output pins of the microcontroller can be wired to the conductive wires of the solid substrate using standard wiring connection techniques. 
         [0034]    As far as the rotating field B xy  field is concerned, it can assume any type of rotation. If for example the B x , B y  fields are sinusoidal with the same frequency and 90° phase difference, their vector sum B xy  will be a rotating field with the same frequency. In this first case, the orientation angle of B xy  in the plane xy will change linearly. Another embodiment includes B x , B y  fields that have the same frequency and a phase difference of 90° but they are step waves. In this second case, the resulting B xy  will be a rotating field with a rotation angle that will obtain only four discrete values at 0°, 90°, 180°, 270° with each of them lasting for one quarter of the period. Potentially, other combinations of the rotating B xy  can work as long as B xy  remains periodic. 
       6. Video Camera and Microscope System 
       [0035]    The device includes a video camera that is mounted above the fluidic chip to monitor the operation of the system. A microscope system will be coupled to the camera to provide that appropriate magnification objective lens. The microscope system may include light source, filters, beamsplitters, mirrors and other optical components known from prior art. 
       Description of Additional Features of the Invention 
       [0036]    The goal of this section is to describe additional features of the invention, in particular the capacity to merge droplets or separate (break-up) a single droplet into two smaller droplets. This capacity is very useful for a lot of applications as it can mix different components together (supposing that two droplets carrying different components merge into a single droplet) or distribute the content of a single droplet in two different directions (through the break-up of a single droplet). 
         [0037]    In  FIGS. 9 a - c    we show an arrangement of “T” and “I” bars that merges two droplets into a single one. In  FIGS. 9 a - c   , we use similar notation to  FIGS. 4 a - d , 5 a - e   . The two droplets, named A and B in  FIG. 9 a    are occupying two different bars that have the orientation “1”. Once the magnetic field B xy  rotates to the position “2” as shown in  FIG. 9 b   , both droplets A,B are drawn on the same double “I” letter bar with orientation “2”. Stacking two “I” letter bars together, amplifies the attractive force exerted by the bars to the droplets, overcoming the droplet-to-droplet magnetostatic repulsion force. Once the droplets merge, the resulting single droplet can move to the position “3”, as shown in  FIG. 9 c    and keep propagating rightwards as a single output. 
         [0038]    Similarly, in  FIGS. 10 a - c    we show an arrangement of “T” and “I” bars that causes a single droplet A to break up into two droplets A 1 , A 2  (we use similar notation to  FIGS. 4,5 ). In  FIG. 10 a    the single droplet is occupying a bar that has the orientation “3”. Once the magnetic field rotates to the position 4 as shown in FIG.  10   b , the single droplet is simultaneously attracted to two different parallel “I” bars that cause the droplet to stretch and start breaking-up. When the magnetic field B xy  rotates to the position 1, as shown in  FIG. 10 c   , the droplet breakes-up into two droplets A 1 , A 2  that occupy different bars with orientations “1”. After breaking-up, the droplets A 1 , A 2  propagate in different directions. 
         [0039]    Similar merging or break-up mechanisms can be made using different arrangements and/or different shapes for the bars. 
         [0040]    In the current invention, either the fluid under test or the carrier liquid can include: water-based ferrofluid, oil-based ferrofluid, fluid with magnetic beads, magnetic nanoparticles dispensed in a fluid, and fluid with magnetic surfactant on the surface silicone oil, hydrocarbon oil, fluoroinert oil, water, all optionally coated with surfactants. Aspects of the invention include all multiple combinations between the test and the carrier fluid where i) the carrier fluid is non-magnetic and the fluid under test is magnetic, or ii) the carrier fluid magnetic and the fluid under test is non-magnetic, or iii) the carrier fluid non-magnetic and the fluid under test is a multi-phase emulsion of magnetic fluid. 
         [0041]    In one aspect of the invention, the fluid under test consists of droplets of micro- to nano-liter volumes. 
         [0042]    According to another aspect of the invention, the magnetized domain includes permalloy bars, or soft magnetic material. 
         [0043]    In another aspect of the invention, the active magnet can generate a dynamic magnetic field can be a rotating magnetic field, a varying magnitude magnetic field, an x-direction oscillating magnetic field, a y-direction oscillating magnetic field, an ON-OFF magnetic field in the z-direction, a clocked magnetic field, or a periodically varying magnetic field profile. In a further aspect of the invention, the active magnet is disposed external to the solid substrate or the active magnet is embedded on the solid substrate. 
         [0044]    In another aspect, the invention further includes a micro-coil or current wire disposed above the fluid chamber and disposed to apply an external magnetic field external to the droplet controller. 
         [0045]    Other embodiments and/or example are taught in U.S. Provisional Patent Application 62/164,323 filed May 20, 2015, which is incorporated herein by reference in its entirety.