Patent Publication Number: US-2013240073-A1

Title: Microfluidic Device for Altering a Fluid Flow and a Microfluidic System Including the Microfluidic Device

Description:
TECHNICAL FIELD 
     Embodiments relate to a microfluidic device for altering a fluid flow and a microfluidic system including the microfluidic device. 
     BACKGROUND 
     While macroscopic fluidic oscillators may have been well developed, the design options of the microfluidic oscillator may be limited. It may be because the flow behaviors in microfluidic devices may be different with that of macroscopic fluidic devices. Conventional macroscopic fluidic oscillators usually cannot be simply scaled down for microfluidic applications. Microfluidic oscillator designs may require different working principles. 
     Fluidic devices usually can be categorized into active or passive devices. Active devices may refer to devices actuated by external sources, for example, involving piezoelectric elements and magnetic devices. In this regard, active devices may require an external control element and may involve high fabrication cost. 
     Passive devices may refer to devices actuated by the flow of the fluid itself. Passive devices may be preferred over active devices because the devices may be self-contained. Traditional passive fluidic oscillators usually depend on the flow instabilities that occur at high Reynolds number (Re) to operate as desired. They may not be used for microfluidic applications because in microfluidic applications, the fluid flow is generally laminar (generally characterised by low Re). Unfortunately, known fluidic oscillators which are operable at sufficiently low Re for microfluidic applications are characterized by low operating frequencies which may not be desired. 
     SUMMARY 
     In various embodiments, a microfluidic device for altering a fluid flow may be provided. The microfluidic device may include a chamber having a first chamber portion with an inlet configured to receive a fluid flow into the chamber; a second chamber portion with an outlet configured to permit an altered fluid flow out of the chamber, the second chamber portion defining a smaller chamber cross section compared to the first chamber portion; and at least one support structure with at least one support surface defining a division between the first chamber portion and the second chamber portion; and a diaphragm in the first chamber portion, the diaphragm displaceable between a position at the inlet and a position at the at least one support surface by the fluid flow. 
     In various embodiments, a microfluidic system may be provided. The microfluidic system may include a microfluidic device configured to alter a fluid flow including a chamber having a first chamber portion with an inlet configured to receive a fluid flow into the chamber; a second chamber portion with an outlet configured to permit an altered fluid flow out of the chamber, the second chamber portion defining a smaller chamber cross section compared to the first chamber portion; and at least one support structure with at least one support surface defining a division between the first chamber portion and the second chamber portion. The microfluidic device may further include a diaphragm in the first chamber portion, the diaphragm displaceable between a position at the inlet and a position at the at least one supporting surface by the fluid flow. The microfluidic system may further include an input passage connected upstream of the microfluidic device; and an output passage connected downstream of the microfluidic device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings, in which: 
         FIG. 1  shows a cross-sectional view of a microfluidic device including a chamber and a support structure extending from an internal surface of the chamber according to an embodiment; 
         FIG. 2A  shows a top view of a microfluidic device including a chamber having a first chamber portion and a second chamber portion, the second chamber portion defines a cross-sectional dimension smaller than the cross-sectional dimension of the first chamber portion according to an embodiment;  FIG. 2B  shows a cross-sectional view along line A-A of the microfluidic device as shown in  FIG. 2A  according to an embodiment; 
         FIG. 3  shows an exploded view of a microfluidic device including a support structure with a support surface having a plurality of grooves and protrusions, each of the plurality of grooves including a circumferential dimension smaller than each of the plurality of protrusions according to an embodiment; 
         FIG. 4A  shows a perspective view of a portion of a microfluidic device including a chamber and an output passage coupled to the chamber, the chamber including a support structure with a support surface having a plurality of grooves and protrusions, each of the plurality of protrusions including a circumferential dimension smaller than each of the plurality of grooves according to an embodiment;  FIG. 4B  shows a perspective view of a portion of a microfluidic device including a chamber and an output passage coupled to the chamber, the chamber including a support structure with a support surface having a plurality of grooves and protrusions, each of the plurality of protrusions including a circumferential dimension comparable with each of the plurality of grooves according to an embodiment; 
         FIG. 5A  shows an exploded view of a microfluidic device including a diaphragm with a plurality of openings formed along a circumference of the diaphragm according to an embodiment;  FIG. 5B  shows a cross-sectional view along line A-A of the microfluidic device as shown in  FIG. 5A  according to an embodiment; 
         FIGS. 6A to 6D  show respective top views of a diaphragm with a plurality of openings according to an embodiment; 
         FIG. 7  shows a cross-sectional view of a bi-directional design microfluidic device including a chamber having a first chamber portion and a second chamber portion, the first chamber portion including a third chamber portion according to an embodiment; 
         FIG. 8  shows a microfluidic system including a microfluidic device positioned substantially perpendicular to a direction of fluid flow into the microfluidic device according to an embodiment; 
         FIG. 9  shows a microfluidic system including the microfluidic device as shown in  FIG. 2B  positioned in a fluid path of a feeding conduit for an in-line application according to an embodiment; 
         FIG. 10A  shows a microfluidic system including a microfluidic device formed using three substrates according to an embodiment;  FIG. 10B  shows a microfluidic system including a microfluidic device formed using four substrates according to an embodiment; 
         FIG. 11A  shows a microfluidic device including a chamber and a cover having a sealing component configured to seal the chamber according to an embodiment;  FIG. 11B  shows a microfluidic system including the microfluidic device as shown in  FIG. 11A  according to an embodiment; 
         FIG. 12A  shows a photograph of the microfluidic system as shown in  FIG. 11B  according to an embodiment;  FIG. 12B  shows a schematic of a mixing profile of fluids in the microfluidic system as shown in  FIG. 12A  according to an embodiment; 
         FIG. 13  shows an output plot of a microfluidic device showing an oscillation frequency (f) at about 143 Hz according to an embodiment; 
         FIG. 14  shows a mixing profile of fluids in a boxed area of the microfluidic system as shown in  FIG. 12B  according to an embodiment; 
       FIGS.  15 A and  15 A′ shows respective results of mixing of fluids in a microfluidic system in a steady flow without the microfluidic device; FIGS.  15 B and  15 B′ shows respective results of improved mixing of fluids in the microfluidic system in an oscillatory flow according to an embodiment;  FIG. 15C  shows a result of an instant mixing of fluids in a microfluidic system in an oscillatory flow with a larger oscillation magnitude according to an embodiment; 
         FIG. 16  shows a schematic of a mixing profile of fluids in a microfluidic system including an input channel and a sample channel coupled to an inlet of the microfluidic device according to an embodiment; 
         FIG. 17  shows a cross-sectional view of a microfluidic system including a plurality of mixing chambers respectively separated from an output passage of an microfluidic device by a flexible wall according to an embodiment; 
         FIG. 18A  shows a top view of a microfluidic system including four mixing chambers arranged in a configuration according to an embodiment;  FIG. 18B  shows a top view of a microfluidic system including four mixing chambers arranged in an alternative configuration according to an embodiment; 
         FIG. 19A  shows a photograph of the microfluidic system as shown in  FIG. 18B  according to an embodiment;  FIGS. 19B to 19E  show a sequential mixing process of fluids in a mixing chamber according to an embodiment; and 
         FIG. 20  shows a top view of a microfluidic system configured to sequentially or simultaneously mix fluids in selected ones of a multiple number of mixing chambers in a fluidic network according to one embodiment. 
     
    
    
     DESCRIPTION 
     The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. 
     Various embodiments provide an alternative microfluidic device which may overcome or at least alleviate some of the above-mentioned problems. 
       FIG. 1  shows a cross-sectional view of a microfluidic device  102  including a chamber  104  and at least one support structure  106  extending from an internal surface  186  of the chamber  104  according to an embodiment. 
     The microfluidic device  102  may be configured for altering a fluid flow and may be an oscillator, a mixer, a pump or a valve. The microfluidic device  102  may include the chamber  104  having a first chamber portion  108  with an inlet  110  configured to receive a fluid flow into the chamber  104 ; a second chamber portion  112  with an outlet  114  configured to permit an altered fluid flow out of the chamber  104 , the second chamber portion  112  defining a smaller chamber cross-section compared to the first chamber portion  108 ; and the at least one support structure  106  with at least one support surface  116  defining a division between the first chamber portion  108  and the second chamber portion  112 . Further, the microfluidic device  102  may include a diaphragm  118  positioned in the first chamber portion  108 , the diaphragm  118  displaceable between a position at the inlet  110  and a position at the at least one support surface  116  by the fluid flow. The at least one support surface  116  and the diaphragm  118  may be configured to allow a fluid flow from the inlet  110  towards the outlet  114  thereby causing a deformation of the diaphragm  118  and a change in the hydrodynamic forces on the diaphragm  118  to render movement of the diaphragm  118  between the inlet  110  and the outlet  114  creating the altered fluid flow out of the chamber  104 . If the microfluidic device  102  may be perceived or oriented with the inlet  110  above the outlet  114 , the diaphragm  118  may be displaceable and essentially be found to be at a location at or below the inlet  110  and above the at least one support surface  116  in the course of its movement. In other words, the diaphragm  118  may be displaceable between the inlet  110  and the at least one support surface  116 . 
     In an embodiment, the volume of the second chamber portion  112  may or may not be necessarily smaller than that of the first chamber portion  108 . In this regard, the first chamber portion  108  and the second chamber portion  112  should be dimensioned such that the diaphragm  118  may be displaceable between the inlet  110  and the at least one support surface  116 . Further, while the diaphragm  118  bounces back, it may not necessarily reach the inlet  110 . As, an example, the diaphragm  118  may oscillate between the outlet  114  to somewhere below the inlet  110 . 
     The altered fluid flow out of the chamber  104  may be at least one of oscillatory fluid flow and pulsating fluid flow for example. The pulsating fluid flow may typically be associated with cyclical or rhythmic flow in the same direction while the oscillatory fluid flow may be similar to the pulsating fluid flow but the fluid flow may appear intermittent or to be flowing in different directions for two halves of a cycle. Any other suitable fluid flow which may be different from the input fluid flow may also be adopted depending on user and design requirements. 
     In a default position where there is no fluid flow into the chamber  104 , the diaphragm  118  may be configured to be supported onto the at least one support surface  116  and may or may not be permanently connected or attached to any part of the at least one support surface  116  or the chamber  104 . In the presence of fluid flow into the chamber  104 , the diaphragm  118  may be displaceable within the first chamber portion  108  so as to facilitate the fluid flow from the first chamber portion  108  into the second chamber portion  112 . The diaphragm  118  may be deformable under the pressure of the fluid flow on the diaphragm  118 . In other words, the diaphragm  118  may be deformed when there is a difference in pressure between two substantially opposing faces  188 ,  188 ′ of the diaphragm  118 . Deformation of the diaphragm  118  may alter the pattern of the fluid flow in the chamber  104  and change the hydrodynamic forces exerted on the diaphragm  118 . As a result, the lifting force on the diaphragm  118  may increase pushing of the diaphragm  118  away from the second chamber portion  112  and towards the inlet  110 , thus momentarily blocking further fluid flowing into the chamber  104  from the inlet  110 . The displacement of the diaphragm  118  may further change the flow and the hydrodynamic forces exerted on the diaphragm  118 . As a result, the diaphragm  118  tends to restore its original shape. As the pressure on the side  188  of the diaphragm  118  facing the second chamber portion  112  may be reduced, the diaphragm  118  may be pushed downstream and deformed again by the fluid flowing in at the inlet  110 . The cycle may accordingly repeat and thereby generate an altered flow or a pulsed flow downstream of the chamber  104 . Fluid may exit the chamber  104  through the outlet  114  in such a manner where the fluid downstream of the microfluidic device  102  may be characterised by an altered flow or a pulsed flow of a relatively high frequency. At the same time, the microfluidic device  102  may be operable at Re found in most microfluidic applications. 
     As an example, the diaphragm  118  may or may not be hinged or chained at any suitable portion of the at least one support surface  116  or any portion of the chamber  104  as long as the diaphragm  118  may be capable of oscillating in response to the change of pressure between the first chamber portion  108  and the second chamber portion  112 . Conventional practice is inclined towards non-moving or stationary features to generate alteration of fluid flow in passive devices as free components, that is, components unconnected to other components of the device, may introduce undesirable uncontrollable elements into the system. Here, it is boldly proposed to provide a diaphragm  118  that is not connected to the chamber  104 , and harness the somewhat random behavior of a free component to provide a desired result. The absence of a connection, whether in the form of a hinge or chain, further advantageously simplifies fabrication of the device and hence reduces the cost of fabrication, without compromising performance of the device. As a further example, the diaphragm  118  may be substantially flat in shape or planar. The diaphragm  118  may also be configured such that the surface area may be relatively large compared to the thickness of the diaphragm  118 . The diaphragm  118  may also be of any suitable shape or dimension as long as it may be supported onto the at least one support surface  116 . The diaphragm  118  may be of a deformable material such that under the pressure of an incoming flow into the chamber  104 , the diaphragm  118  may deform and become convex downstream. Then deformation of the diaphragm  118  may further change the fluid flow and the hydrodynamic forces such as the lifting force, which may push the diaphragm  118  back. In an embodiment, the diaphragm  118  may include a substantially flat shape so as to facilitate the deformation of the diaphragm  118  and to produce a relatively significant change in the hydrodynamic forces, which may ease the occurrence of the oscillation. Further, the diaphragm  118  may be shaped so as to complement the shape and dimension of the first chamber portion  108  and the second chamber portion  112  so as to optimise the amount of movement within the chamber  104 . In addition, the diaphragm  118  may also be shaped so that the respective volume of the available space between the diaphragm  118  and the inlet  110  and between the diaphragm  118  and the outlet  114  may be optimised when the diaphragm  118  may be displaced towards the inlet  110  or towards the outlet  114  respectively. 
     The microfluidic device  102  may be observed to exhibit three types of behavior depending on the pressure applied to pump fluid into the microfluidic device  102 . Below a first critical pressure, there is no observation of substantial alteration in the fluid flow out of the microfluidic device  102 , and the fluid flow out of the microfluidic device  102  may be described as stable. Above a second critical pressure that is higher than the first critical pressure, the diaphragm  118  is observed to block the outlet  114 . Hence, the microfluidic device  102  may be described as having an operational pressure range between the first critical pressure and the second critical pressure as a fluid flow alteration device, an oscillator, or a mixing promotor. And the same microfluidic device  102  may be described as a valve to close off fluid flow above the second critical pressure. The operational pressure range may be set by varying the depth of the first chamber portion  108 , the depth of the second chamber portion  112 , and the cross-sectional area of the groove  126  (as subsequently shown in  FIG. 3 ) or channels  120  (as subsequently shown in  FIG. 2A ). For example, the first critical pressure may be reduced with the reduction in the depth of the first chamber portion  108 , and/or reduction of the cross-sectional area of the groove  126  or channels  120  as mentioned earlier. The second critical pressure may be reduced with the reduction in the depth of the second chamber portion  112 , and/or reduction in the cross-sectional area of the groove  126  or channels as mentioned earlier. 
     The diaphragm  118  may include a material selected from a group consisting of silicone rubber, natural rubber, latex, nitrile rubber, thermoplastic polyurethane, and elastic metal for example. The diaphragm  118  may also be of a bio-compatible material so as to be suitable for biological applications. As an example, the diaphragm  118  may be a soft, rubber dome with a flexible rim. As a further example, the diaphragm  118  may be a membrane, or a discoid. In some embodiments, the material or dimension of the diaphragm  118  may be chosen to provide a desired degree of stiffness, which in turn may determine the oscillating frequency. 
     The first chamber portion  108  may include a cross-sectional dimension (as denoted by “dchamber 1 ”) in a range of typically about 1 to about 10 mm, for example. As shown in  FIG. 1 , the second chamber portion  112  may define a cross-sectional dimension (denoted by “dchamber 2 ”) smaller than the cross-sectional dimension of the first chamber portion  108 . However, depending on the size or shape of the at least one support structure  106 , the second chamber portion  112  may also define a nominal or average cross-sectional dimension smaller than the cross-sectional dimension of the first chamber portion  108 . The chamber  104  may also include a height (as denoted by “hchamber”) of typically above 1 mm, for example. 
     The at least one support structure  106  may include one support structure or may include a plurality of support structures depending on user and design requirements. In the case of the plurality of support structures, each of the plurality of support structures may be positioned adjacent to each other or may be positioned spaced apart at a fixed or varying predetermined distance away from each other. Further, each of the plurality of support structures may be arranged so as to be at a substantially same level or height along the internal surface  186  of the chamber  104 . However, each of the plurality of support structures may also be arranged at varying heights along the internal surface  186  of the chamber  104  as long as the diaphragm  118  may be supported thereon. 
     The at least one support structure  106  and the diaphragm  118  may be configured in any suitable manner so as to allow a fluid flow from the first chamber portion  108  to the second chamber portion  112 . As an example, the at least one support structure  106  may define at least one channel (not shown) communicating between the first chamber portion  108  and the second chamber portion  112 . As a further example, the at least one channel may be extended from the support surface  116  such that fluid from the first chamber portion  108  may be directed to flow between the at least one channel and a side  188  of the diaphragm  118  as it enters the second chamber portion  112 . In another example, the at least one channel may be formed in any suitable design on the support surface  116 . The number of channels may vary depending on the desired speed or rate of fluid flow or oscillation rate of the diaphragm  118  for example. 
     The microfluidic device  102  may further include an input passage  122  coupled upstream of the inlet  110  such that the input passage  122  is configured to channel the fluid flow into the first chamber portion  108 . The cross-sectional dimension (as denoted by “din”), the height (as denoted by “hin”) and cross-sectional shape of the input passage  122  may vary depending on user and design requirements. The cross-sectional shape of the input passage  122  may be substantially circular, but any other suitable shapes such as square, triangle, rectangle, oval may also be used. 
     The microfluidic device  102  may further include an output passage  124  coupled downstream of the outlet  114  such that the output passage  124  is configured to channel the fluid flow out of the second chamber portion  112 . The cross-sectional dimension (as denoted by “dout”), the height (as denoted by “hout”) and cross-sectional shape of the output passage  124  may vary depending on user and design requirements. The cross-sectional shape of the output passage  124  may be substantially circular, but any other suitable shapes such as square, triangle, rectangle, oval may also be used. 
     The cross-sectional dimension of the input passage  122  may be similar or different from the cross-sectional dimension of the output passage  124  depending on user and design requirements. The height of the input passage  122  may be similar or different from the height of the output passage  124  depending on user and design requirements. Similarly, the cross-sectional shape of the input passage  122  may be similar or different from the cross-sectional shape of the output passage  124  depending on user and design requirements. 
     The microfluidic device  102  may be formed as an integrated device or may be formed from separate portions or substrates. The microfluidic device  102  may be formed from any suitable material or combination of material such as polymeric material or metal materials, for example. 
       FIG. 2A  shows a top view of a microfluidic device  102  including a chamber  104  having a first chamber portion  108  and a second chamber portion  112 , the second chamber portion  112  defines a cross-sectional dimension smaller than the cross-sectional dimension of the first chamber portion  108  according to an embodiment.  FIG. 2B  shows a cross-sectional view along line A-A of the microfluidic device  102  as shown in  FIG. 2A  according to an embodiment. 
     The microfluidic device  102  as shown in  FIGS. 2A and 2B  may be similar to the microfluidic device  102  as shown in  FIG. 1 . 
     In  FIGS. 2A and 2B , the microfluidic device  102  may include the chamber  104  having a first chamber portion  108  with an inlet  110  configured to receive a fluid flow into the chamber  104 ; a second chamber portion  112  with an outlet  114  configured to permit an altered fluid flow out of the chamber  104 , the second chamber portion  112  defining a smaller chamber cross section compared to the first chamber portion  108 ; and the at least one support structure  106 , with the at least one support surface  116  defining a division between the first chamber portion  108  and the second chamber portion  112 . Further, the microfluidic device  102  may include a diaphragm  118  positioned in the first chamber portion  108 , the diaphragm  118  displaceable between a position at the inlet  110  and a position at the at least one support surface  116  by the fluid flow. The at least one support surface  116  and the diaphragm  118  may be configured to allow a fluid flow from the inlet  110  towards the outlet  114  thereby causing a deformation of the diaphragm  118  and a change in the hydrodynamic forces on the diaphragm  118  to render movement of the diaphragm  118  between the inlet  110  and the outlet  114  creating the altered fluid flow out of the chamber  104 . In other words, if the microfluidic device  102  is perceived or oriented with the inlet  110  above the at least one support surface  106 , the diaphragm  118  is displaceable between a position below the inlet  110  and a position above the at least one support surface  116 . The diaphragm  118  is sized such that the diaphragm  118  may be at least supported by the at least one support surface  116  in a default position. 
     The support surface  116  may include four channels  120  communicating between the first chamber portion  108  and the second chamber portion  112 . The higher the number of channels  120 , the higher the flow rate through the microfluidic device  102 . The number of channels  120  may vary depending on user and design requirements. Each of the four channels  120  may include a same or different circumferential dimension (as denoted by “dchannel”). However, the circumferential dimension of each of the four channels  120  may vary depending on user and design requirements. In addition, for each of the four channels  120 , the circumferential dimension may be uniform along the length of the channel  120  as shown in  FIG. 2A  or may vary along the length of the channel  120 , for example, tapered, wavy and others. 
     In a default position where there is no fluid flow into the chamber  104 , the diaphragm  118  may be configured to be supported onto the support surface  116  and may be separated from or unconnected to the chamber  104 . In the presence of fluid flow into the chamber  104 , the diaphragm  118  may be displaceable within the first chamber portion  108  so as to facilitate the fluid flow from the first chamber portion  108  into the second chamber portion  112 . 
     The microfluidic device  102  may further include an input passage  122  coupled to the inlet  110  such that the input passage  122  is configured to channel the fluid flow into the first chamber portion  108 . The microfluidic device  102  may further include an output passage  124  coupled to the outlet  114  such that the output passage  124  is configured to channel the fluid flow out of the second chamber portion  112 . The direction of fluid flow is as shown by the arrows in  FIG. 2B . 
     The dimensions of the wall of the chamber  104  (as denoted by “tchamber”) may vary between about 2 to about 5 mm, for example. The dimensions of the wall of the chamber  104  may vary depending on the material used or may also vary depending on user and design requirements. 
       FIG. 3  shows an exploded view of a microfluidic device  102  including a support structure  106  with a support surface  116  having a plurality of grooves  126  and protrusions  128 , each of the plurality of grooves  126  including a circumferential dimension smaller than each of the plurality of protrusions  128  according to an embodiment. The microfluidic device  102  as shown in  FIG. 3  is similar to the microfluidic device  102  as shown in  FIGS. 2A and 2B . 
     In  FIG. 3 , the microfluidic device  102  may include the chamber  104  having a first chamber portion  108  with an inlet (not shown) configured to receive a fluid flow into the chamber  104 ; a second chamber portion  112  with an outlet  114  configured to permit an altered fluid flow out of the chamber  104 , the second chamber portion  112  defining a smaller chamber cross section compared to the first chamber portion  108 ; and the support structure  106  with the support surface  116  defining a division between the first chamber portion  108  and the second chamber portion  112 . Further, the microfluidic device  102  may include a diaphragm  118  positioned in the first chamber portion  108 , the diaphragm  118  displaceable between a position at the inlet and a position at the support surface  116  by the fluid flow. 
     The diaphragm  118  may be configured to be supported onto the support surface  116  such that the diaphragm  118  may be in contact with the chamber  104  or may also be separated from or unconnected to the chamber  104 . In the presence of fluid flow into the chamber  104 , the diaphragm  118  may be displaceable within the first chamber portion  108  so as to facilitate the fluid flow from the first chamber portion  108  into the second chamber portion  112 . 
     Like in  FIGS. 2A and 2B , the second chamber portion  112  as shown in.  FIG. 3 , may define a cross-sectional dimension substantially smaller than the cross-sectional dimension of the first chamber portion  108  thereby forming the support structure  106 . The support structure  106  may define a plurality of channels  120  communicating between the first chamber portion  108  and the second chamber portion  112 . As an example, each of the plurality of channels  120  may be formed on the support surface  116  of the support structure  106 . The number of channels  120  and the arrangement of the channels  120  may vary depending on the desired speed or rate of fluid flow or desired oscillation rate of the diaphragm  118  for example. 
     The microfluidic device  102  may further include an input passage  122  coupled to the inlet such that the input passage  122  may be configured to channel the fluid flow into the first chamber portion  108 . The dimension and cross-sectional shape of the input passage  122  may vary depending on user and design requirements. The microfluidic device  102  may further include an output passage  124  coupled to the outlet  114  such that the output passage  124  may be configured to channel the fluid flow out of the second chamber portion  112 . The dimension and cross-sectional shape of the output passage  124  may also vary depending on user and design requirements. 
     The microfluidic device  102  may further include a cover  130  disposed over the inlet and configured to at least substantially cover the diaphragm  118  within the chamber  104 . The cover  130  may include a cover opening (not shown), the cover opening may be positioned to align with the input passage  122  so as to allow the fluid flow into the chamber  104  through the cover opening and the input passage  122 . The cover  130 , the diaphragm  118  and the chamber  104  may be formed using same or different materials. 
       FIG. 4A  shows a perspective view of a portion of a microfluidic device  102  including a chamber  104  and an output passage  124  coupled to the chamber  104 , the chamber  104  including a support structure  106  with a support surface  116  having a plurality of grooves  126  and protrusions  128 , each of the plurality of protrusions  128  including a circumferential dimension smaller than each of the plurality of grooves  126  according to an embodiment. 
     As shown in  FIG. 4A , the support surface  116  may include four grooves  126  and four protrusions  128 . However, the number of grooves  126  and protrusions  128  may vary depending on user and design requirements. Each of the grooves  126  and protrusions  128  may be positioned in an alternating manner around the circumference of the support surface  116 . These combinations of grooves  126  and protrusions  128  defines the plurality of channels  120  communicating between the first chamber portion  108  and the second chamber portion  112 . 
     The preferred number of protrusions  128  or grooves  126  is above 2. The protrusions  128  or grooves  126  may be distributed evenly or unevenly around the circumference of the support surface  116 . The circumferential dimension (as denoted by “dprotrusion”) of each of the protrusions  128  may be equal to, or smaller, or larger than each of the circumferential dimension (as denoted by “dgroove”) of each of the grooves  126 . For example, the circumferential dimension of each of the protrusions  128  may vary between about 1 to about 89 degree in the case of four protrusions  128 .  FIG. 4A  shows an example that the circumferential dimension of each protrusion  128  is smaller than that of each groove  126 .  FIG. 4B  shows an example that the circumferential dimension of each protrusion  128  is about the same with that of each groove  126 . The grooves  126  and protrusions  128  may be formed such that respective portions of the support surface  116  may be removed. The extent of the removal of the portions of the support surface  116  may vary depending on user and design requirements. The removal may be done in one step or may be done in many steps, removing different amount of portions of the support surface  116  at each time. 
     The circumferential dimension of each of the four protrusions  128  or four grooves  126  may be substantially uniform along the entire length of the grooves  126  or protrusions  128  as shown in  FIG. 4B  or may vary along the length of the grooves  126  or protrusions  128 , for example tapered or patterned. The design of each of the four protrusions  128  or four grooves  126  may vary depending on user and design requirements. 
       FIG. 5A  shows an exploded view of a microfluidic device  102  including a diaphragm  118  with a plurality of openings  134  formed along a circumference of the diaphragm  118  according to an embodiment and  FIG. 5B  shows a cross-sectional view along line A-A of the microfluidic device  102  as shown in  FIG. 5A  according to an embodiment. 
     The microfluidic device  102  as shown in  FIGS. 5A and 5B  may be similar to the microfluidic device  102  as shown in  FIG. 3  except that in  FIG. 5 , the support surface  116  may be substantially even and does not include any groove or protrusion and that the diaphragm  118  may include a plurality of openings  134 . The support surface  116  and the diaphragm  118  may be configured in any suitable manner as long as there may be at least one channel (not shown) communicating between the first chamber portion  108  and the second chamber portion  112 . 
     As shown in  FIG. 5A , the diaphragm  118  includes six openings  134  formed along the circumference of the diaphragm  118 . However, any suitable number of openings  134  may be formed along the circumference of the diaphragm  118  or within the diaphragm  118 . Each of the six openings  134  may appear to be substantially square in shape. Each of the six openings  134  may include the same or different shape from each other. In this regard, each of the six openings  134  may include any other suitable shape or as long as the support structure  106  and the diaphragm  118  may be configured to allow a fluid flow from the first chamber portion  108  to the second chamber portion  112 . Each of the six openings  134  may also be spaced apart from each other at a fixed or varying distance depending on user and design requirements. The six openings may also be formed in a combination along the circumference and within the diaphragm  118 . 
     In  FIGS. 5A and 5B , the microfluidic device  102  may include the chamber  104  having the first chamber portion  108  with an inlet  110  configured to receive a fluid flow into the chamber  104 ; the second chamber portion  112  with an outlet  114  configured to permit an altered fluid flow out of the chamber  104 , the second chamber portion  112  defining a smaller chamber cross section compared to the first chamber portion  108 ; and the support structure  106  with a support surface  116  defining a division between the first chamber portion  108  and the second chamber portion  112 . Further, the microfluidic device  102  may include the diaphragm  118  positioned in the first chamber portion  108 , the diaphragm  118  displaceable between a position at the inlet  110  and a position at the at least one support surface  116  by the fluid flow. 
     The diaphragm  118  may be configured to be supported onto the support surface  116  and may be separated from or unconnected to the chamber  104 . In the presence of fluid flow into the chamber  104 , the diaphragm  118  may be displaceable within the first chamber portion  108  so as to facilitate the fluid flow from the first chamber portion  108  into the second chamber portion  112 . 
     As shown in  FIGS. 5A and 5B , the second chamber portion  112  may define a cross-sectional dimension substantially smaller than the cross-sectional dimension of the first chamber portion  108 , thereby forming the support structure  106  with the support surface  116  defining the division between the first chamber portion  108  and the second chamber portion  112 . 
     The microfluidic device  102  may further include an input passage  122  coupled to the inlet  110  such that the input passage  122  is configured to channel the fluid flow into the first chamber portion  108 . The microfluidic device  102  may further include an output passage  124  coupled to the outlet  114  such that the output passage  124  is configured to channel the fluid flow out of the second chamber portion  112 . 
     The microfluidic device  102  may further include a cover  130  disposed over the inlet  110  and configured to at least substantially cover the diaphragm  118  within the chamber  104 . The cover  130  may include a cover opening (not shown), the cover opening is positioned to align with the input passage  122  so as to allow the fluid flow into the chamber  104  through the cover opening and the input passage  122 . 
       FIGS. 6A to 6D  show respective top views of a diaphragm  118  with a plurality of openings  134  according to an embodiment. 
       FIG. 6A  shows a diaphragm  118  which is substantially circular in shape. The diameter of the diaphragm  118  may be at least larger than the diameter of the second chamber portion  112  so that the diaphragm  118  may be supported on a substantially even support surface formed by the difference in cross-section between the first chamber portion (shown in the foreground) and the second chamber portion  112  (faintly shown in the background). Three substantially rectangular openings or cutouts  134  may be formed along a circumference of the diaphragm  118  and sized so as to allow fluid flow from the first chamber portion to the second chamber portion  112 . The number, shape and positioning of the openings  134  may vary depending on user and design requirements. 
     Like in  FIG. 6A ,  FIG. 6B  shows a diaphragm  118  which is also substantially circular in shape. Six substantially rectangular openings  134  or cutouts may be formed along a circumference of the diaphragm  118  so as to allow fluid flow from the first chamber portion (shown in the foreground) to the second chamber portion  112  (faintly shown in the background). The circumferential dimension of each of the openings  134  may be smaller than that as shown in  FIG. 6A . The circumferential dimension, number and shape of the openings  134  may vary depending on user and design requirements. 
     Like in  FIG. 6B ,  FIG. 6C  shows a diaphragm  118  which is substantially circular in shape. Unlike the openings  134  formed along a circumference of the diaphragm  118  as shown in  FIG. 6B , the six substantially rectangular openings  134  as shown in  FIG. 6C  may be formed within the diaphragm  118  so as to allow fluid flow from the first chamber portion (shown in the foreground) to the second chamber portion  112  (faintly shown in the background). Each of the six substantially rectangular openings  134  may be positioned within the diaphragm  118  such that a portion of each of the six substantially rectangular openings  134  are aligned with an edge  190  of the second chamber portion  112  so as to allow the fluid flow from the first chamber portion to the second chamber portion  112 . The number, positioning, dimension and shape of each of the openings  134  may vary depending on user and design requirements. 
     Like in  FIG. 6C ,  FIG. 6D  shows a diaphragm  118  which is substantially circular in shape. Instead of six substantially rectangular openings  134  formed within the diaphragm  118  as shown in  FIG. 6C , six substantially circular openings  134  may be formed within the diaphragm  118  so as to allow fluid flow from the first chamber portion (shown in the foreground) to the second chamber portion  112  (faintly shown in the background). Each of the six substantially circular openings  134  may be positioned within the diaphragm  118  such that the openings  134  are aligned with an edge  190  of the second chamber portion  112  so as to allow the fluid flow from the first chamber portion  108  to the second chamber portion  112 . The dimension and shape of each of the openings  134  may vary depending on user and design requirements. 
       FIGS. 6A to 6D  shows that the diaphragm  118  may be substantially circular in shape. However, the diaphragm  118  may also include any other suitable shapes for example non-circular depending on user and design requirements. In addition, openings  134  on the diaphragm  118  may be formed as perforations or cut-outs by any suitable method. 
       FIG. 7  shows a cross-sectional view of a bi-directional design microfluidic device  102  including a chamber  104  having a first chamber portion  108  and a second chamber portion  112 , the first chamber portion  108  including a third chamber portion  136  according to an embodiment; 
     The microfluidic device  102  as shown in  FIG. 7  is a modification of the microfluidic device  102  as shown in  FIG. 2B . In the microfluidic device  102  as shown in  FIG. 7 , the first chamber portion  108  may further include the third chamber portion  136  leading from the inlet  110 , the third chamber portion  136  defining a smaller chamber cross section compared to the rest of the first chamber portion  108 . 
     As an example of the bi-directional design microfluidic device  102  as shown in  FIG. 7 , the third chamber portion  136  may include a chamber volume substantially similar to that of the second chamber portion  112 . And the cross-sectional dimension of the third chamber portion  136  may be substantially the same or different as the cross-sectional dimension of the second chamber portion  112 . In addition, the first chamber portion  108  may include a cross-sectional dimension larger than each of the respective second chamber portion  112  or third chamber portion  136 . In this regard, this may provide symmetry to the microfluidic device  102  such that it may be possible to orientate the microfluidic device  102  in any direction as the diaphragm  118  may be positioned either on the support surface  116  defining a division between the first chamber portion  108  and the second chamber portion  112  as shown in  FIG. 7  or on a further support surface  117  defining a division between the third chamber portion  136  and the first chamber portion  108 . Advantageously, the microfluidic device  102  may be provided with a non-symmetrical design, for example, by having the cross-sectional dimension of the third chamber portion  136  different with the cross-sectional dimension of the second chamber portion  112 , such that the operating pressure range and oscillation frequency may be different for the same microfluidic device  102  operating in different directions. In other words, the microfluidic device  102  advantageously enable a microfluidic system to be designed such that it may create a first altered fluid flow for fluid flow in one direction and a second altered fluid flow for fluid flow in a second direction, in which the first altered fluid flow and the second fluid flow are different, for example, in their respective oscillation frequencies. Advantageously, the microfluidic device  102  may be configured to provide an altered fluid flow in one direction and to provide a valving effect in another direction. 
     The bi-directional design microfluidic device  102  as shown in  FIG. 7  may be formed by aligning two separate substrates or by a single substrate. In the case of two separate substrates, the input passage  122 , the third chamber portion  136  and a part of the first chamber portion  108  may be formed in a first substrate  138  and the output passage  124 , the second chamber portion  112  and the remaining part of the first chamber portion  108  may be formed in a second substrate  140 . The first substrate  138  and the second substrate  140  may be of the same material or different material. The diaphragm  118  may be positioned within the first chamber portion  108  such that the diaphragm  118  may be supported on the support surface  116  or on the further support surface  117 . 
       FIG. 8  shows a microfluidic system  142  including a microfluidic device  102  positioned substantially perpendicular to a direction of fluid flow into the microfluidic device  102  according to an embodiment. 
     In  FIG. 8 , the microfluidic device  102  may be shown to be embedded within the microfluidic system  142 . The microfluidic device  102  may include a chamber  104  having a first chamber portion  108  with an inlet  110  configured to receive a fluid flow into the chamber  104 ; a second chamber portion  112  with an outlet  114  configured to permit an altered fluid flow out of the chamber  104 , the second chamber portion  112  defining a smaller chamber cross section compared to the first chamber portion  108 ; and a support structure  106  with a support surface  116  defining a division between the first chamber portion  108  and the second chamber portion  112 . The microfluidic device  102  may further include a diaphragm  118  positioned in the first chamber portion  108 , the diaphragm  118  displaceable between a position at the inlet  110  and a position at the support surface  116  by the fluid flow. Further, the microfluidic system  142  may include an input passage  122  connected upstream of the microfluidic device  102 ; and an output passage  124  connected downstream of the microfluidic device  102 . The direction of the fluid flow may be as shown by the arrows in  FIG. 8 . 
       FIG. 9  shows a microfluidic system  142  including the microfluidic device  102  as shown in  FIG. 2B  positioned in a fluid path of a feeding conduit or feeding duct  144  for an in-line application according to an embodiment. 
     The microfluidic device  102  or microfluidic oscillator may be used as a part of a microfluidic system  142  or may also be used as a standalone, plug-and-play device. As shown in  FIG. 9 , the microfluidic device  102  may be mounted in line with the feeding duct  144  to provide an oscillatory flow. The direction of the fluid flow may be as shown by the arrows in  FIG. 9 . The microfluidic device  102  may be connected at any suitable or desired position along any fluid path within the microfluidic system  142  to provide the altered fluid flow. 
       FIGS. 10A and 10B  respectively shows a microfluidic device  102  integrated into a microfluidic system  142 .  FIG. 10A  shows a microfluidic system  142  including a microfluidic device  102  formed using three substrates or layers  138 ,  140 ,  148  according to an embodiment. 
     In  FIG. 10A , the oscillation chamber  104  may be fabricated on a first substrate  138  using any suitable method such as injection molding, end-milling for example. An output channel  146  may be fabricated on a second substrate  140  and a third substrate  148  may be used as the cover  130 . An input passage  122  upstream of the chamber  104  may be formed on the third substrate  148 . Then the diaphragm or membrane  118  may be put into the chamber  104 . After alignment of the respective first substrate  138 , second substrate  140  and third substrate  148 , all the first substrate  138 , second substrate  140  and third substrate  148  may be bonded together to seal the chamber  104 . The direction of the fluid flow may be as shown by the arrows in  FIG. 10A . 
     Each of the first substrate  138 , the second substrate  140  and the third substrate  148  may include the same or different material. Each of the first substrate  138 , the second substrate  140  and the third substrate  148  may include a material selected from a group consisting of polymeric material or metal materials for example. In an embodiment, the microfluidic device  102  may be fabricated with common polymeric materials such as polycarbonate (PC), poly(methylmethacrylate) (PMMA), cyclic olefin copolymer (COC), for example. Injection molding may be used for mass production. For metal materials, micro-milling may be used to machine the microfluidic device  102 . 
     The diaphragm or membrane  118  may be made of an elastic material such as silicon rubber. The diaphragm  118  or membrane may be easily cut using punching method or using a carbon dioxide (CO 2 ) laser. Depending on the user requirements, the support surface  116  and the diaphragm  118  may be configured in any manner according to any one of  FIG. 1 ,  2 B or  5 B as long as at least one channel (not shown) may be provided for fluid communication between the first chamber portion  108  and the second chamber portion  112 . 
       FIG. 10B  shows a microfluidic system  142  including a microfluidic device  102  formed using four substrates or layers  138 ,  140 ,  148 ,  150  according to an embodiment. The microfluidic system  142  as shown  FIG. 10B  may be similar to the microfluidic system  142  as shown in  FIG. 10A  except for a difference in the number of substrates or layers. In  FIG. 10B , the oscillation chamber  104  and output passage  124  may be fabricated on a first substrate  138  using any suitable method such as injection molding, end-milling for example. An output channel  146  may be fabricated on a second substrate  140  and an input channel  152  and an input passage  122  may be fabricated on a third substrate  148 . Further, a fourth substrate  150  may be used as the cover  130 . Then the diaphragm or membrane  118  may be put into the chamber  104 . After alignment of the respective first substrate  138 , second substrate  140 , third substrate  148  and fourth substrate  150 , all the first substrate  138 , second substrate  140 , third substrate  148  and fourth substrate  150  may be bonded up to seal the chamber  104 . 
     Similar to  FIG. 10A , each of the first substrate  138 , the second substrate  140 , the third substrate  148  and the fourth substrate  150  may include the same or different material. Each of the first substrate  138 , the second substrate  140 , the third substrate  148  and the fourth substrate  150  may include a material selected from a group consisting of polymeric material or metal materials. 
       FIG. 11A  shows a microfluidic device  102  including a chamber  104  and a cover  130  having a sealing component  154  configured to seal the chamber  104  according to an embodiment. 
       FIG. 11A  shows the microfluidic device  102  or microfluidic oscillator being configured in a way that may allow a flexible adjustment of the depth of the first chamber portion or the upstream chamber portion  108  of the oscillation chamber  104  which may influence the flow rate and frequency. The side wall of the oscillation chamber  104  may be extended and an internal thread track may be fabricated on an internal surface  186  of the oscillation chamber  104 . A screw  192  together with an inlet tubing  184  may be used as the sealing component  154  to seal the oscillation chamber  104 . One advantage of the design as shown in  FIGS. 11A and 11B  may be that it may be relatively convenient to open the chamber  104  and replace the diaphragm or membrane  118 . It may also allow the flexible adjustment of the depth of the first chamber portion or the upstream chamber portion or cavity  108  to control the operational flow rate. 
       FIG. 11B  shows a microfluidic system  142  including the microfluidic device  102  as shown in  FIG. 11A  according to an embodiment. When used in the microfluidic system  142 , a part of the microfluidic device  102  or microfluidic oscillator may be formed in a first substrate  138  and an output channel  146  may be formed in a second substrate  140 . Altered fluid may flow out of the microfluidic device  102  via the output passage  124  into the output channel  146  of the microfluidic system  142  as shown by the arrows as shown in  FIG. 11B . 
       FIG. 12A  shows a photograph  1200  of the microfluidic system  142  as shown in  FIG. 11B  according to an embodiment. 
     The microfluidic system  142  as shown in  FIG. 12A  may be fabricated using micro-milling machine and thermal bonding technique. The oscillation chamber  104  and the input passage or vertical orifice (not clearly shown) may be fabricated on a first PMMA plate (about 3 mm thick) using a micro-milling machine. An input microchannel  152  and an output microchannel  146  may be fabricated on a second PMMA plate (about 1.5 mm thick). M6 thread track may be fabricated in a PMMA block with a depth of about 6 mm. Then, the first PMMA plate and the second PMMA plate, the PMMA block may be aligned and bonded together using a thermal bonding method. An M6 screw  192  may be drilled through to allocate the inlet tubing  184 , and the M6 screw  192  and the inlet tubing  184  may be permanently glued together. The diaphragm or elastic membrane (not clearly shown) may be made of silicone rubber and may be cut using a carbon dioxide (CO 2 ) laser. A sample microchannel  156  is fabricated at an immediate position downstream of the microfluidic device or oscillator  102 , so that the oscillator  102  can work as a mixer. In some embodiments, the microfluidic system  142  may include a microfluidic chip. 
     As an example, some of the main parameters of the microfluidic device  102  may be as follows: (unit: mm):
         Diameter/depth of the upstream cavity: 6/1.0   Diameter/depth of the downstream cavity: 4/0.5   Depth/width, (and number) of the grooves on stair: 0.15/0.5, (4)   Diameter of the vertical outlet channel: 0.8   Width/depth of the microchannel in the bottom layer: 0.5/0.5   Depth of the PMMA block: 6   Diameter/thickness of the membrane: 5.5/0.5
 
This would provide a microfluidic device  102  having an operational pressure range of around 1.1 bar to 5 bar, such that the microfluidic device  102  is configured to provide altered fluid flow when fluid is pumped into the microfluidic device  102  at a pressure ranging from about 1.1 bar to 5 bar. Advantageously, this microfluidic device  102  may at the same time provide a microfluidic valving effect of around 5 bar so that if fluid is pumped into the microfluidic device  102  above the operational pressure range, the microfluidic device  102  would block fluid flow out of the outlet or output microchannel  146 . Since a single microfluidic device  102  may serve multiple functions, the overall microfluidic system  142  may be designed with fewer devices, which would mean lower cost and less assembly processes would be involved.
       

       FIG. 12B  shows a schematic of a mixing profile of fluids in the microfluidic system  142  as shown in  FIG. 12A  according to an embodiment. 
       FIG. 12B  shows an input channel  152  coupled to an inlet  110  of the microfluidic device  102 . Further, a sample channel  156  is coupled to an outlet  114  of the microfluidic device  102 . Fluid flowing in from the input channel  152  may be altered by the microfluidic device  102  so as to provide an altered fluid flow out of the microfluidic device  102 . This altered fluid flow may be mixed with a sample fluid flowing in from the sample channel  156 , thereby producing a mixed output fluid flow out of an output channel  146  of the microfluidic system  142 . 
     When the microfluidic device  102  or oscillator operates at a high flow rate, the microfluidic device  102  produces sound. The frequency f may be detected using a microsensor. Results show that oscillatory frequency ranges from several tens Hz to around 400 Hz.  FIG. 13  shows an output plot  1300  of a microfluidic device  102  showing an oscillation frequency (f) at about 143 Hz according to an embodiment. 
       FIG. 14  shows a mixing profile of fluids in a boxed area  1200  of the microfluidic system  142  as shown in  FIG. 12B  according to an embodiment. 
     The altered fluid flow or oscillatory flow (or termed “liquid  1 ”) flowing out from the microfluidic device (not shown) may be mixed with a sample fluid (or termed “liquid  2 ”), thereby producing a mixture flowing out of the microfluidic system  142 . The extent of the mixing depends on the magnitude and frequency of the oscillatory flow. 
     FIGS.  15 A and  15 A′ shows respective results  1500 ,  1502  of mixing of fluids in a microfluidic system in a steady flow without the microfluidic device. FIGS.  15 B and  15 B′shows respective results  1504 ,  1506  of improved mixing of fluids in a microfluidic system  142  in an oscillatory flow according to an embodiment.  FIG. 15C  shows a result  1508  of an instant mixing of fluids in a microfluidic system  142  in an oscillatory flow with a larger oscillation magnitude or at an increased or a higher flow rate according to an embodiment. 
     Two fluids, namely aqueous alkaline solution (0.5 wt. % NaOH solution) and 1% phenolphthalein solution may be used for mixing test. When the two fluids come into contact, their color will change from colorless to red (or seen as shaded). Relevant results  1500 ,  1502 ,  1504 ,  1506 ,  1508  are shown in  FIGS. 15A ,  15 A′,  15 B,  15 W and  15 C.  FIG. 15A  shows the result  1500  without the microfluidic device  102  or oscillator and the flow at Re of about 70 is stable. The fluid interface is clearly observed. As shown in the result  1502  in FIG.  15 A′, after a channel length of about 3 cm downstream of the microfluidic device  102 , the mixing is poor. The fluid interface just slightly smeared out through diffusion.  FIG. 15B  shows the result  1504  after the microfluidic device  102  or oscillator is added. At about the same Re, an oscillatory flow is produced. The material interface cannot be identified. The fluids become globally pink (or slightly shaded as shown in  FIG. 15 ) which is the sign of mixing. As shown in the result  1506  in FIG.  15 B′, after about 3 cm downstream of the microfluidic device  102  or oscillator, the fluids have been well mixed (or fully shaded as shown in FIG.  15 B′).  FIG. 15C  shows the result  1508  with the microfluidic device  102  or oscillator at Re of about 150. As the Re is increased, the pulsating flow becomes much stronger with an increased magnitude. As a result, the fluids instantly mixed (shown as fully shaded in  FIG. 15C ) once the fluids come into contact. 
       FIG. 16  shows an alternative mixing profile of fluids in a microfluidic system  142  from that as shown in  FIG. 12B .  FIG. 16  shows a schematic of a mixing profile of fluids in a microfluidic system  142  including an input channel  152  and a sample channel  156  coupled to an inlet  110  of the microfluidic device  102  according to an embodiment. Both the input channel  152  and the sample channel  156  may be coupled to the inlet  110  of the microfluidic device  102 . After passing through the microfluidic device  102 , both the fluid initially flowing in the input channel  152  and the sample fluid initially flowing in the sample channel  156  may be mixed, thereby producing a mixed output fluid flow out of the outlet  114  of the microfluidic device  102  into an output channel  146  out of the microfluidic system  142 . 
       FIG. 17  shows a cross-sectional view of a microfluidic system  142  including a plurality of mixing chambers  160  respectively separated from an output passage  124  of an microfluidic device (not shown) by a flexible wall  158  according to an embodiment. 
     The output passage  124  may be coupled downstream of the microfluidic device. The microfluidic system  142  may include a sample fluid channel  156  separated from the output passage  124  via the flexible wall  158 . 
     The microfluidic system  142  may further include three mixing chambers  160 , each of the three mixing chambers  160  with at least one sample fluid and the flexible wall  158  separating each of the three mixing chambers  160  from the output passage  124 , in which the flexible wall  158  may be configured to allow a transfer of energy from the altered fluid flow to the at least one sample fluid within each of the three mixing chambers  160 . 
     As shown in  FIG. 17 , the sample fluid channel  156  may include two sample inlets  162  and one sample outlet  164 . However, the sample fluid channel  156  may include any suitable number of the sample inlet  162  and the sample outlet  164  depending on user and design requirements. The direction of the sample fluid flow along the sample fluid channel  156  and the direction of the altered fluid flow out of the microfluidic device  102  and along the output passage  124  may be respectively shown in  FIG. 17 . 
     The flexible wall  158  may be of any suitable material, for example, an elastic film The thickness of the flexible wall  158  may also vary depending on user and design requirements. The flexible wall  158  may extend along the entire length of the overlap between the output passage  124  and the sample fluid channel  156  or may only be present within each of the three mixing chambers  160  depending on user and design requirements. 
       FIG. 18A  shows a top view of a microfluidic system  142  including four mixing chambers  170 ,  172 ,  174 ,  176  arranged in a configuration according to an embodiment. 
     The microfluidic system  142  may include two sample input channels, i.e. a sample input channel  156  and a further sample input channel  166 . A sample fluid and a further sample fluid may flow along the respective sample input channel  156  and the further sample input channel  166  so as to be mixed in a first mixing chamber  170 . The microfluidic system  142  may further include a microfluidic device  102  coupled upstream of the first mixing chamber  170  so as to provide an altered fluid flow through the first mixing chamber  170 , separated from the sample fluid and the further sample fluid by a flexible wall  158  (not shown in  FIG. 18A , but as shown in  FIG. 17 ). The flexible wall  158  is configured to allow a transfer of energy from the altered fluid flow to the sample fluid and the further sample fluid so as to enhance mixing of the sample fluid and the further sample fluid within the first mixing chamber  170 . 
     The two sample fluids may be first fed into all the four mixing chambers  170 ,  172 ,  174  and  176 . Then the oscillator or microfluidic device  102  is switched on and thus providing an altered fluid flow in the output microchannel  146  which simultaneously transmits energy at respective flexible walls of the mixing chambers  170 ,  172 ,  174  and  176  to the two sample fluids in these mixing chambers  170 ,  172 ,  174  and  176 . In this manner, liquids in all the four mixing chambers  170 ,  172 ,  174  and  176  will be mixed at the same time, such that a fluidic network is effected. 
     At the same time, the altered fluid flow out of the microfluidic device  102  may flow out of the first mixing chamber  170  to the second mixing chamber  172 , then to the third mixing chamber  174  and then the fourth mixing chamber  176  before flowing out of an altered fluid output channel  146 . 
       FIG. 18B  shows a top view of a microfluidic system  142  including four mixing chambers  170 ,  172 ,  174 ,  176  arranged in an alternative configuration according to an embodiment. 
     In both the microfluidic systems  142  as shown in  FIG. 18A  and  FIG. 18B , the altered flow is provided to the four mixing chambers  170 ,  172 ,  174 ,  176  at the same time. The difference between that as shown in  FIG. 18A  and  FIG. 18B  is that in the in-series design as shown in  FIG. 18A , the oscillation magnitude may decrease in the sequential four chambers  170 ,  172 ,  174 ,  176 . 
       FIG. 19A  shows a photograph  1900  of the microfluidic system  142  as shown in  FIG. 18B  according to an embodiment. 
     Similar to that as shown in  FIG. 18B , the microfluidic system  142  may include a microfluidic device  102 , an input passage  122  to the microfluidic device  102 , a first sample channel  156 , a further sample channel  166 , four mixing chambers  170 ,  172 ,  174 ,  176 , a sample output channel  168  and an altered fluid output channel  146 . 
       FIGS. 19B to 19E  show a sequential mixing process of fluids in a mixing chamber  160  according to an embodiment. The mixing chamber  160  may be any one of the four mixing chambers  170 ,  172 ,  174 ,  176  as shown in  FIG. 18A  or  FIG. 18B . In  FIG. 19B , two different fluids  159  and  157  flow into the mixing chamber  160  in parallel. Without the oscillation, the mixing is slow and the fluid interface  158  can be clearly observed.  FIG. 19C  to  FIG. 19E  show the mixing after around 0.2, 0.4 and 0.6s with the device  102  in operation. The homogeneity has been achieved after around 0.6s. 
       FIG. 20  shows a top view of a microfluidic system  142  configured to sequentially or simultaneously mix fluids in selected ones of a multiple number of mixing chambers  170 ,  172 ,  174 ,  176  in a fluidic network according to one embodiment. 
     In  FIG. 20 , the microfluidic system  142  includes micro-valves  202 ,  204 ,  206  and  208  positioned in the respective fluid passages for selective provision of altered fluid flow to different mixing chambers  170 ,  172 ,  174 ,  176  in the fluidic network. For example, when all the micro-valves  202 ,  204 ,  206  and  208  are open, the microfluidic system  142  is similar to that of  FIG. 18B . When the micro-valves  202  and  204  are closed and the micro-valves  206  and  208  are open, only the liquids in mixing chambers  174  and  176  are mixed with the aid of the altered fluid flow. If all the micro-valves  202 ,  204 ,  206  and  208  are closed, only the liquids in chamber  174  are mixed. The micro-valves  202 ,  204 ,  206  and  208  may be pneumatic valves or mechanical valves involving the use of solenoids, for example. 
     In some embodiments, the microfluidic system  142  or microfluidic device  102  may be used in different potential industrial applications. One example may be in mixing or heat transfer enhancement in a microchannel. Though converting a steady laminar flow to an unstable oscillatory flow, the microfluidic oscillator may improve mass and heat transfer. The microfluidic oscillator may work as a microfluidic mixer. Another example may be in channel or device cleaning and recovery. For reusable microfluidic devices, channel cleaning is required to remove the residuals and contaminants after each use. The microfluidic oscillator  102  may provide a pulsating flow to improve the efficiency of the cleaning process. A further example is in chemical or biochemical reaction enhancement. The oscillator  102  can be used in micro reactor systems to improve the mixing between the chemical reactants, so as to improve the chemical or bio-chemical reaction. Yet another example is in fouling prevention or, reduction. In micro reactor or micro-heat exchanger systems, an oscillatory flow helps to reduce deposition of solids on the inner surface of the channels, and hence prevent or reduce the fouling. A further example is in filtration enhancement. An oscillatory flow also helps to prevent the fouling of a filter to improve the filtration rate. Another example is in emulsion formation. The microfluidic oscillator  102  may also be used to generate small droplets of liquid in another immiscible liquid to form emulsions. 
     In some embodiment, a passive microfluidic oscillator  102  that may operate at low Re range (e.g. Re&lt;100) may be disclosed. Stable oscillations may be achieved at Re of about 50, thereby rendering the microfluidic oscillator an ideal choice for microfluidic applications. 
     The microfluidic oscillator  102  may also realize a relatively high frequency of up to several hundred hertz. A higher frequency may provide better performance for many applications, such as fluid mixing and heat transfer enhancement, fouling prevention, for example. 
     The microfluidic oscillator  102  may involve a passive design. Though a moving diaphragm  118  or membrane is used, the oscillation is realized in a passive way. It is autoinitiated, self-sustained and operates constantly under flow intake. In comparison with known active design, there is no need of the external control systems such as required for Lead Zirconate-Titanate (PZT) agitator. The structure is simple, cheap and more reliable. 
     The microfluidic oscillator  102  may be robust. For example, the oscillation may be strong and stable. Further, the oscillation may not be sensitive to the disturbance from surrounding environment. 
     While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.