Patent Publication Number: US-9839908-B2

Title: Micro-chemical mixing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a Divisional of U.S. application Ser. No. 11/319,865 which was filed on Dec. 27, 2005, to Aizenberg, et al, entitled “MICRO-CHEMICAL MIXING,” now granted as U.S. Pat. No. 8,734,003, which in turn is a Continuation-in-Part of U.S. application Ser. No. 11/227,759 filed on Sep. 15, 2005, to Joanna Aizenberg, et al., entitled “FLUID OSCILLATIONS ON STRUCTURED SURFACES,” now granted as U.S. Pat. No. 8,721,161, all of which are commonly assigned with the present invention, and fully incorporated herein by their entirety by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to a device and a method for mixing two or more species within a droplet. 
     BACKGROUND OF THE INVENTION 
     One problem encountered when handling small fluid volumes is to effectively mix different fluids together. For instance, poor mixing can occur in droplet-based microfluidic devices, where the fluids are not confined in channels. In droplet based systems, small droplets of fluid (e.g., fluid volumes of about 100 microliters or less) are moved and mixed together on a surface. In some cases, it is desirable to add a small volume of a reactant to a sample droplet to facilitate the analysis of the sample, without substantially diluting it. In such cases, there is limited ability to mix the two fluids together because there is no movement of the fluids to facilitate mixing. 
     Embodiments of the present invention overcome these problems by providing a device and method that facilitates the movement and mixing of small volumes of fluids. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, the present invention provides a device. The device, without limitation, includes a substrate having a droplet thereover, and an electrical source coupleable to the substrate, the electrical source configured to apply a voltage between the substrate and the droplet using an electrode, wherein the electrode has a first portion and a second portion non-symmetric to the first portion, the first and second portions defined by a plane located normal to a longitudinal axis and through a midpoint of a length of the electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is best understood from the following detailed description when read with the accompanying FIGUREs. It is emphasized that, in accordance with the standard practice in the semiconductor industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A  thru  1 E illustrate cross-sectional views of a device while undergoing a process for mixing two or more species within a droplet in accordance with the principles of the present invention; 
         FIGS. 2A  thru  2 D illustrate different objects, in this embodiment electrodes, that might be used in place of the object illustrated in  FIGS. 1A  thru  1 E; 
         FIG. 3  illustrates an alternative embodiment of an object that might be used with the methodology discussed above with respect to  FIGS. 1A  thru  1 E; 
         FIG. 4  illustrates a cross-sectional view of an alternative embodiment of a device while undergoing a process for mixing two or more species within a droplet in accordance with the principles of the present invention; 
         FIG. 5  illustrates an alternative embodiment of a device in accordance with the principles of the present invention; 
         FIG. 6  illustrates a cross-sectional view of an alternative embodiment of a device while undergoing a process for mixing two or more species within a droplet in accordance with the principles of the present invention; and 
         FIG. 7  illustrates one embodiment of a mobile diagnostic device in accordance with the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention recognizes that the vertical position of a droplet (e.g., a droplet of fluid) can be made to oscillate on certain kinds of substrates. In certain embodiments, the vertical position of the droplet can be made to oscillate on a conductive substrate having fluid-support-structures thereon. The application of a voltage between the substrate and the droplet may cause the droplet to alternate between a state with a high contact angle (e.g., a less flattened configuration or a non-wetted state) and a state with a lower contact angle (e.g., a more flattened configuration or a wetted state). In such embodiments the substrate comprises a pattern of fluid-support-microstructures, the applied voltage causing a surface of the droplet to move between tops of the fluid-support-microstructures and the substrate on which the microstructures are located. Such movements cause the droplet to move between effective more flattened and less flattened states, respectively. 
     As part of the present invention, it was further discovered that repeatedly deforming (e.g., oscillating) the droplet in this manner promotes mixing of two or more species (e.g., chemical species) within the droplet. For instance, the repeated deformation of the droplet can induce motion within the droplet, thereby promoting mixing of the two or more species of fluids. Without being limited to such, it is believed that the movement of the droplet with respect to an object located therein promotes the mixing, the object may for example be an electrode used to provide the voltage. 
     Turning now to  FIGS. 1A  thru  1 E illustrated are cross-sectional views of a device  100  while a droplet undergoes a process for mixing two or more species therein in accordance with the principles of the present invention. The device  100  of  FIGS. 1A  thru  1 E initially includes a substrate  110 . The substrate  110  may be any layer located within a device and having properties consistent with the principles of the present invention. For instance, in one exemplary embodiment of the present invention the substrate  110  is a conductive substrate. 
     Some preferred embodiments of the conductive substrate  110  comprise silicon, metal silicide, or both. In some preferred embodiments, for example, the conductive substrate  110  comprises a metal silicide such as cobalt silicide. However, other metal silicides, such as tungsten silicide or nickel silicide, or alloys thereof, or other electrically conductive materials, such as metal films, can be used. 
     In the embodiment wherein the substrate  110  is a conductive substrate, an insulator layer  115  may be disposed thereon. Those skilled in the art understand the materials that could comprise the insulator layer  115  while staying within the scope of the present invention. It should also be noted that in various embodiments of the present invention, one or both of the substrate  110  or insulator layer  115  has hydrophobic properties. For example, one or both of the substrate  110  or insulator layer  115  might at least partially comprise a low-surface-energy material. For the purposes of the present invention, a low-surface-energy material refers to a material having a surface energy of about 22 dyne/cm (about 22×10 −5  N/cm) or less. Those of ordinary skill in the art would be familiar with the methods to measure the surface energy of such a material. In some preferred embodiments, the low-surface-energy material comprises a fluorinated polymer, such as polytetrafluoroethylene, and has a surface energy ranging from about 18 to about 20 dyne/cm. 
     Located over the substrate  110  in the embodiment shown, and the insulator layer  115  if present, is a droplet  120 . The droplet  120  may comprise a variety of different species and fluid volumes while staying within the scope of the present invention. In one exemplary embodiment of the present invention, however, the droplet  120  has a fluid volume of about 100 microliters or less. It has been observed that the methodology of the present invention is particularly useful for mixing different species located within droplets  120  having fluid volumes of about 100 microliters or less. Nevertheless, the present invention should not be limited to any specific fluid volume. 
     Located within the droplet  120  in the embodiments of  FIGS. 1A  thru  1 E are a first species  130  and a second species  135 . For the purpose of illustration, the first species  130  is denoted as (˜) and the second species is denoted as (*). The first species  130  may be a diluent or a reactant. Similarly, the second species  135  may be a diluent or a reactant. In the exemplary embodiment shown, however, the first species  130  is a first reactant and the second species  135  is a second reactant, both of which are suspended within a third species, such as a diluent. 
     Some preferred embodiments of the device  100  also comprise an electrical source  140  (e.g., an AC or DC voltage source) coupled to the substrate  110  and configured to apply a voltage between the substrate  110  and the droplet  120  located thereover. In the illustrative embodiment of  FIGS. 1A  thru  1 E, the electrical source  140  uses an object  150 , such as an electrode, to apply the voltage. While the embodiment of  FIGS. 1A  thru  1 E illustrates that the object  150  is located above the substrate  110 , other embodiments exist wherein the object  150  contacts the droplet  120  from another location, such as from below the droplet  120 . Those skilled in the art understand how to configure such an alternative embodiment. Moreover, as will be discussed more fully below, the object  150  may take on a number of different configurations and remain within the purview of the present invention. 
     Given the device  100  illustrated in  FIGS. 1A  thru  1 E, the first species  130  and the second species  135  may be at least partially mixed within the droplet  120  using the inventive aspects of the present invention. Turning initially to  FIG. 1A , the droplet is positioned in its less flattened state. For instance, because substantially no voltage is applied between the substrate  110  and the droplet  120 , the droplet is in its natural configuration. It should be noted that the first species  130  and the second species  135  located within the droplet of  FIG. 1A  are substantially, if not completely, separated from one another. 
     Turning now to  FIG. 1B , illustrated is the device  100  of  FIG. 1A , after applying a non-zero voltage between the substrate  110  and the droplet  120  using the electrical source  140  and the object  150 . As would be expected, the droplet  120  moves to a flattened state, and thus is in its deformed configuration. It is the movement of the object  150  within the droplet  120  that is believed to promote the mixing of the first species  130  and the second species  135 . It should be noted, however, that other phenomena might be responsible for at least a portion of the mixing. 
     In some cases, the electrical source  140  is configured to apply a voltage ranging from about 1 to about 50 Volts. It is sometimes desirable for the voltage to be applied as a brief pulse so that the droplet  120  after becoming flattened can bounce back up to its less flattened state. In some cases, the applied voltage is a series of voltage pulses applied at a rate in the range from about 1 to 100 Hertz, and more preferably from about 10 to 30 Hertz. In other cases, the applied voltage is an AC voltage. In some preferred embodiments, the AC voltage has a frequency in the range from about 1 to about 100 Hertz. One cycle of droplet oscillation is defined to occur when the droplet  120  makes a round-trip change from the less flattened state to the more flattened state and back up to the less flattened state, or from the more flattened state to the less flattened state and back down to the more flattened state. Take notice how the first species  130  and the second species  135  in the embodiment of  FIG. 1B  are slightly more mixed within the droplet  120  than the first species  130  and second species  135  in the droplet  120  of  FIG. 1A . 
     Turning now to  FIG. 1C , illustrated is the device  100  of  FIG. 1B  after removing the voltage being applied via the electrical source  140  and object  150 . Thus, the droplet  120  substantially returns to its less flattened state, and has therefore made one complete cycle of movement. As one would expect based upon the disclosures herein, the movement from the more flattened state of  FIG. 1B  to the less flattened state of  FIG. 1C  may promote additional mixing. Accordingly, the first species  130  and second species  135  may be more mixed in the droplet  120  of  FIG. 1C  than the droplet  120  of  FIG. 1B . 
     Moving on to  FIGS. 1D and 1E , the droplet  120  undergoes another cycle of movement, thus further promoting the mixing of the first species  130  and second species  135  therein. In accordance with the principles of the present invention, the droplet  120  may repeatedly be deformed, until a desired amount of mixing between the first species  130  and the second species  135  has occurred. The number of cycles, and thus the amount of mixing between the first species  130  and the second species  135 , may be based upon one or both of a predetermined number of cycles or a predetermined amount of time. In any event, addition mixing typically occurs with each cycle, at least until the first species  130  and second species  135  are completely mixed. 
     Uniquely, the present invention uses the repeated deformation of the droplet  120  having the object  150  therein to accomplish mixing of the first species  130  and second species  135  within the droplet  120 . Accordingly, wherein most methods for mixing the species within the droplet would be based upon the relative movement of the object  150  with respect to the droplet  120 , the present invention is based upon the movement of the droplet  120  with respect to the object  150 . For instance, in most preferred embodiments the object  150  is fixed, and thus stationary, and it is the movement of the droplet  120  using the electrical source  140  that promotes the movement. 
     This being said, the method disclosed herein provides what is believed to be unparalleled mixing for two or more species within a droplet. Namely, the method disclosed herein in capable of easily mixing two or more species that might be located within a droplet having a fluid volume of about 100 microliters or less. Prior to this method, easy mixing of such small volumes was difficult, at best. 
     In various embodiments, the object  150  is positioned asymmetric along the axis of motion of the droplet being physically distorted. For example, the object  150  may be positioned a non-zero angle away from the direction of movement of the droplet during mixing. This non-zero angle might be used to introduce increased mixing. 
     The embodiments of  FIGS. 1A  thru  1 E are droplet based micro fluidic system. It should be noted, however, that other embodiments might consist of micro channel based micro fluidic systems, wherein the droplet might be located within a channel and the mixing occurring within one or more channels, as opposed to that shown in  FIGS. 1A  thru  1 E. Those skilled in the art understand just how the inventive aspects of the present invention could be employed with such a micro channel based micro fluidic system. 
     Turning now to  FIGS. 2A  thru  2 D, illustrated are different objects  200 , in this embodiment electrodes, that might be used in place of the object  150  illustrated in  FIGS. 1A  thru  1 E. Specifically, the objects  200  illustrated in  FIGS. 2A  thru  2 D each have a first portion  210  and a second portion  220  non-symmetric to the first portion  210 . In these embodiments, the first and second portions  210 ,  220 , are defined by a plane  230  located normal to a longitudinal axis  240  and through a midpoint  250  of a length (l) of the object  200 . As is illustrated in  FIGS. 2A  thru  2 D, the first portion  210  located above the plane  230  is non-symmetric to the second portion  220  located below the plane  230 . 
     To accomplish the aforementioned non-symmetric nature of the object  200 , the object  200  may take on many different shapes. For example, the object  200  of  FIG. 2A  comprises an inverted T, or depending on the view, a disk disposed along a shaft. Alternatively, the object  200  of  FIG. 2B  comprises an L, the object  200  of  FIG. 2C  comprises a propeller and the object  200  of  FIG. 2D  comprises a helix. Each of the different shapes of  FIGS. 2A  thru  2 D provide increased mixing when the droplet moves with respect to the object as discussed with respect to  FIGS. 1A  thru  1 E above, at least as compared to the symmetric object  150  illustrated in  FIGS. 1A  thru  1 E. For instance, what might take a first species about 10 minutes to mix with a second species using only simple diffusion, might only take about 1 minute using the object  150  illustrated in  FIGS. 1A  thru  1 E, and further might only take about 15 seconds using an object similar to the object  200  illustrated in  FIG. 2D . Thus, the object  150  of  FIGS. 1A  thru  1 E might provide about 10 times the mixing as compared to passive diffusion, whereas the objects  200  of  FIGS. 2A  thru  2 D might provide about 30 times the mixing as compared to passive diffusion. Obviously, the aforementioned improvements are representative only, and thus should not be used to limit the scope of the present invention. 
     Turning briefly to  FIG. 3 , illustrated is an alternative embodiment of an object  300  that might be used with the methodology discussed above with respect to  FIGS. 1A  thru  1 E. The object  300  of  FIG. 3 , as compared to the objects  150 ,  200  of  FIGS. 1A  thru  1 E and  2 A thru  2 D, respectively, comprises multiple vertical sections  310 . The vertical sections  310  attempt to create a swirling effect within the droplet, thereby providing superior mixing of the two or more species. While each of the vertical sections  310  illustrated in  FIG. 3  are shown as helix structures, similar to the object  200  of  FIG. 2D , other embodiments exist wherein each of the vertical sections  310  are similar to any one of the shapes illustrated in previous FIGUREs, as well as other shapes neither disclosed nor shown. 
     Turning now to  FIG. 4 , illustrated is a cross-sectional view of an alternative embodiment of a device  400  while undergoing a process for mixing two or more species within a droplet in accordance with the principles of the present invention. The device  400  of  FIG. 4  is substantially similar to the device  100  illustrated in  FIGS. 1A  thru  1 E, with the exception that multiple objects  450   a  and  450   b  are positioned at different locations within the droplet  420 . In an exemplary embodiment, each one of the multiple objects  450   a  and  450   b  is an individually addressable electrode. For instance, each one of the multiple objects  450   a  and  450   b  may be connected to different electrical sources  440   a  and  440   b , respectively, thereby providing the ability to address them individually. In an alternative embodiment, each one of the multiple objects  450   a  and  450   b  could be connected to the same electrical source  440 , whether it be a fixed or variable electrical source, and switches could be placed between the electrical source  440  and each one of the multiple objects  450   a  and  450   b . Thus, the switches would allow for the ability to address each one of the multiple objects  450   a  and  450   b  individually. 
     The device  400  of  FIG. 4  might be operated by alternately applying a voltage between the multiple objects  450   a  and  450   b . In such an operation, an additional in-plane oscillation of the droplet  420  between the multiple objects  450   a  and  450   b  might occur. Accordingly, wherein the device  100  of  FIGS. 1A  thru  1 E might only cause the droplet  120  to move normal to the surface on which it rests, the device  400  of  FIG. 4  might cause the droplet  420  to have this additional in-plane movement (e.g., along the surface on which it rests). As those skilled in the art appreciate, this additional in-plane movement may induce increased mixing, at least as compared to the movement created in the droplet  120  of  FIGS. 1A  thru  1 E. 
     As an extension of this point, those skilled in the art could design certain more complex geometries, with numerous addressable objects, to ensure rigorous mixing due to the induced movement of the droplet in the different directions. For example, such rigorous mixing might be induced using a device having its objects positioned as follows: 
                         
By using the combination of these five independent objects (e.g., electrodes A, B, C, D and E) one can either induce normal up and down movement of the droplet by applying a voltage to object C (such as is illustrated with respect to  FIGS. 1A  thru  1 E), induce an in-plane movement of the droplet by applying an alternating voltage between objects A and E or B and D (such as is illustrated with respect to  FIG. 4  above), or induce a spinning movement of the droplet by sequentially applying a voltage to objects A, B, E and D. Obviously, other complex geometries might provide even more significant mixing.
 
     Turning now to  FIG. 5 , illustrated is an alternative embodiment of a device  500  in accordance with the principles of the present invention. The embodiment of the device  500  includes a substrate  510 , an insulator layer  515 , a droplet  520  (in both a less flattened state  520   a  and a more flattened state  520   b ), an electrical source  540  and an object  550 . In this embodiment, the object  550  is both configured to act as a hollow needle, and thus is configured to supply one or more species  560  to the droplet  520 , and well as configured to apply a voltage across the droplet  520 . Thus, in the embodiment shown, the object  550  is an electrode also configured as a hollow needle, or vice-versa. 
     Those skilled in the art understand the many different shapes for the object  550  that might allow the object  550  to function as both the electrode and the needle. For that matter, in addition to a standard needle shape, each of the shapes illustrated in  FIGS. 2A  thru  2 D could be configured as a needle, thus providing both functions. Other shapes could also provide both functions and remain within the purview of the present invention. 
     It should also be noted that rather than the object  550  being configured as a single needle having a single fluid channel to provide a species  560 , the object  550  could comprise a plurality of fluid channels to provide a plurality of different species  560  to the droplet  520 . For example, in one embodiment, the object  550  comprises a cluster of different needles, each different needle having its own fluid channel configured to provide a different species  560 . In another embodiment, however, the object  550  comprises a single needle, however the single needle has a plurality of different fluid channels for providing the different species  560 . Other configurations, which are not disclosed herein for brevity, could nevertheless also be used to introduce different species  560  within the droplet  520 . The above-discussed embodiments are particularly useful wherein there is a desire to keep the different species separate from one another, such as wherein the two species might undesirably react with one another. 
     The device  500  including the object  550  may, therefore, be used to include any one or a collection of species  560  within the droplet  520 . The object  550  may, in addition to the ability to provide one or more species  560  within the droplet  520 , also function as an electrode to move the droplet  520  using electrowetting, mix two or more species within the droplet  520  using the process discussed above with respect to  FIGS. 1A  thru  1 E, or any other known or hereafter discovered process. 
     Turning now to  FIG. 6 , illustrated is a cross-sectional view of an alternative embodiment of a device  600  while undergoing a process for mixing two or more species within a droplet in accordance with the principles of the present invention. The device  600  of  FIG. 6  initially includes a substrate  610 . The device  600  also includes fluid-support-structures  612  that are located over the substrate  610 . Each of the fluid-support-structures  612 , at least in the embodiment shown, has at least one dimension of about 1 millimeter or less, and in some cases, about 1 micron or less. As those skilled in the art appreciate, the fluid-support-structures  612  may comprise microstructures, nanostructures, or both microstructure and nanostructures. 
     In some instances, the fluid-support-structures  612  are laterally separated from each other. For example, the fluid-support-structures  612  depicted in  FIG. 6  are post-shaped, and more specifically, cylindrically shaped posts. The term post, as used herein, includes any structures having round, square, rectangular or other cross-sectional shapes. In some embodiments of the device  600 , the fluid-support-structures  612  form a uniformly spaced array. However, in other cases, the spacing is non-uniform. For instance, in some cases, it is desirable to progressively decrease the spacing between fluid-support-structures  612 . For example, the spacing can be progressively decreased from about 10 microns to about 1 micron in a dimension. 
     In the embodiment shown, the fluid-support-structures  612  are electrically coupled to the substrate  610 . Moreover, each fluid-support-structure  612  is coated with an electrical insulator  615 . One suitable insulator material for the electrical insulator  615  is silicon dioxide. 
     Exemplary fluid-support micro-structures and patterns thereof are described in U.S. Patent Application Publs.: 20050039661 of Avinoam Kornblit et al. (publ&#39;d Feb. 24, 2005), U.S. Patent Application Publ. 20040191127 of Avinoam Kornblit et al. (publ&#39;d Sep. 30, 2004), and U.S. Patent Application Publ. 20050069458 of Marc S. Hodes et al. (publ&#39;d Mar. 31, 2005). The above three published U.S. Patent Applications are incorporated herein in their entirety. 
     The device  600  of  FIG. 6  further includes a droplet  620  located over the substrate  610  and the fluid-support-structures  612 . In the embodiment shown, the droplet  620  is resting on a top surface of the fluid-support-structures  612 . The device  600  may further include an electrical source  640  and an object  650 . The substrate  610 , electrical insulator  615 , droplet  620 , electrical source  640  and object  650  may be similar to their respective features discussed above with regard to previous FIGUREs. 
     As those skilled in the art would expect, at least based upon the aforementioned discussions with respect to  FIGS. 1A  thru  1 E,  FIGS. 2A  thru  2 D, and  FIGS. 3, 4 and 5 , the device  600  may be configured to oscillate the droplet  620  between the tops of the fluid-support-structures  612  and the substrate  610 , when a voltage is applied between the substrate  610  and the droplet  620  using the electrical source  640  and the object  650 . For example, the device  600  can be configured to move the droplet  620  vertically, such that a lower surface of the droplet  620  moves back and forth between the tops of the fluid-support-structures  612  and the substrate  610  in a repetitive manner. 
     Based upon all of the foregoing, it should be noted that the present invention, and all of the embodiments thereof, might be used with, among others, a mobile diagnostic device such as a lab-on-chip or microfluidic device. Turning briefly to  FIG. 7 , illustrated is one embodiment of a mobile diagnostic device  700  in accordance with the principles of the present invention. The mobile diagnostic device  700  illustrated in  FIG. 7  initially includes a sample source region  710  and a chemical analysis region  720 . As is illustrated in  FIG. 7 , the sample source region  710  may include a plurality of droplets  730 , in this instance four droplets  730   a ,  730   b ,  730   c , and  730   d . As is also illustrated in  FIG. 7 , the chemical analysis region  720  may include a plurality of both blank pixels  740  and reactant pixels  750 . 
     The device  700  of  FIG. 7 , as shown, may operate by moving the droplets  730  across the chemical analysis region  720 , for example using electrowetting. As the droplets  730  encounter a reactant pixel  750 , a voltage may be applied across the substrate and the droplet  730 , thereby causing the droplet  730  to move to a more flattened state (e.g., wetted state in certain embodiments), and thus come into contact with the reactant located within that particular reactant pixel. The reactant in the pixel may be of a liquid form or a solid form. For example, the reactant may be in a solid form, and thus dissolved or adsorbed by the droplet  730 . 
     This process is illustrated using the droplet  730   c . For example, the droplet  730   c  is initially located at a position  1 . Thereafter, the droplet  730   c  is moved laterally using any known or hereafter discovered process wherein it undergoes an induced reaction  760  at position  2 . The induced reaction  760 , in this embodiment, is initiated by applying a non-zero voltage between the substrate and the droplet  730   c , thereby causing the droplet  730   c  to move to a more flattened state, and thus come into contact with the reactant in that pixel. Thereafter, as shown, the droplet  730   c  could be moved to a position  3 , wherein it undergoes another induced reaction  770 . 
     It should be noted that while the droplets  730  are located at any particular location, the droplets  730  may be repeatedly deformed in accordance with the principles discussed above with respect to  FIGS. 1A  thru  1 E. Accordingly, the reactant acquired during the induced reactions  760 ,  770 , may be easily mixed using the process originally discussed above with respect to  FIGS. 1A  thru  1 E. 
     In certain embodiments, each of the droplets  730  has its own object, and thus the droplets can be independently repeatedly deformed. In these embodiments, each of the objects could be coupled to an independent AC voltage supply, or alternatively to the same AC voltage supply, to induce the mixing. Each of the mentioned objects could also be configured as a needle, and thus provide additional reactant species to the drops, such as discussed above with respect to  FIG. 5 . Those skilled in the art understand the other ideas that might be used with the device  700 . 
     Although the present invention has been described in detail, those skilled in the art should understand that they could make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.