in conventional chromatographic separations using microfluidic instruments , diffusive mixing designs with narrow , deep channels , fluids are injected using conventional techniques in the same plane as the mixing channel . the use of shallow , wide mixing channels requires the use of injection systems located in a different vertical plane with respect to the mixing channels such as is illustrated in the schematics shown in fig1 and 2 . in fig1 a - c , the mixer cover 10 is illustrated using a top , a side , and a bottom view . as shown , in fig1 c cover 10 includes a “ t ” shaped channel 14 etched part - way into the thickness of the cover medium ; a first inlet port 11 etched or drilled completely through the thickness of the cover medium such that it is in - line with but spaced apart from “ t ” shaped channel 14 ; and two off - set second inlet ports 13 etched or drilled through the channel thickness so that each intersects at each end of an arm of the “ t ” of channel 14 . finally , cover 10 contains a single outlet port 12 etched or drilled through the thickness of channel 14 such that it intersects the far end of the “ t ” of channel 14 . inlet ports 11 and 13 are used to inject a first and a second fluid , ( labeled “ fluid a ” and “ fluid b ” in fig4 , respectively ), while single port 2 is used to extract a mixed fluid . in fig2 a and b , rectangular mixing channel base 20 is illustrated by showing a top and a side view of the part and is characterized by a single channel 21 etched into the thickness of the base medium . the channel dimensions are chosen for the sake of example only and , in fact , could take on nearly any desired width and height , although shallow channels will work particularly well . the length of channel 21 is designed to be long enough to span the distance between first inlet port 11 and outlet port 12 as well as to run parallel to the length of channel 14 . after completing the fabrication of the features of both cover 10 and channel base 20 , the two are bonded together as shown in fig3 a and 3b . a similar , alternative embodiment is shown in fig5 a and 5b . fluid injection into the present embodiments can be accomplished using any method for driving fluid motion , including electrokinetic pumping , pressure pumping , gravity , magnetic fields , or any other method for moving a fluid . however , the scale of these devices limits flow speeds , which minimizes convective mixing , and the net effect is the laminar flow illustrated in the side view of fig4 a . here fluids a and b are observed to form distinct layers as the result of this laminar flow following vertical injection . layers such as these result only if diffusion is negligible , which takes place when the peclet number ( the dimensionless ratio of diffusion time over convection time ) is infinitely large . fortunately , the peclet number for common microfluidic channels used in practice typically ranges from about 10 to about 100 ; therefore , diffusion will cause fluid a to diffuse into fluid b , and fluid b to diffuse into fluid a as is illustrated in fig4 b . the degree of diffusive mixing will increase as the flow travels downstream into the channel . for ideal electrokinetic flow , therefore , the degree of mixing will be proportional to the length of the mixing section , the applied field ( which results in the desired peclet number ), and the contact surface area between the two layers . the mixing channels illustrated in fig1 - 3 , and 5 , however , have an aspect ratio near unity such that the width and height of the channel are similar . since the surface area of the contact layer between fluids a and b is small , this design is not optimized for diffusive mixing . increased mixing rates are achieved by increasing the channel aspect ratio . as is well known by those skilled in the art of microanalytical design , injection is accomplished preferably in a channel of modest aspect ratio . alternatively , the two fluids can be directed through a channel section in which the channel aspect ratio is altered substantially expanding the channel from a modest width to a large width . unfortunately , haphazard channel expansion has the disadvantage of creating large variations in local fluid speed across the channel resulting in unwanted dispersion and increased probability of channel clogging . a further risk is unwanted recirculation zones that increase the local residence time , effectively increasing the operating time of the microanalytical chip , and complicating chip design , operation , and interpretation of results . fortunately , a method exists for changing channel dimensions while minimizing dispersion . as is described in detail in commonly owned u . s . patent application ser . no . 10 / 456 , 772 , now published application serial number 20030230489 , herein incorporated by reference , a flow channel can be constructed as a set of connected rectangular regions each of which has the geometry of a faceted prism . as described previously , these regions can be designed to change the conductance , or permeability , of the channel by changing the channel size , particularly the depth , and the shape in the region of the facet junctions so that fluid flow is piece - wise uniform . by carefully designing the regional geometries to provide low - and high - permeability regions in the channel , dispersion produced by the junctions is essentially eliminated . the terms “ deep ” and “ shallow ,” as applied in the following paragraphs , are used in a relative sense to describe widely differing channel depths as a means for changing the permeability of the flow channel as illustrated in published application serial number 20030230489 . in general , the ratio of “ deep ” to “ shallow ” will mean a minimum ratio of greater than 10 : 1 . a ratio of such a magnitude minimizes deviations from flow uniformity caused by deviations from perfect designs that can be expected due to variability introduced when using isotropic etching methods . if such effects are not important for a given design , smaller ratios may be employed . for example , consider the etched channel features shown in fig6 a . here three microscope images are compiled to obtain a panoramic view of micro - mixer 100 which is comprised of an injector section 110 at the top of the image that delivers fluid into expansion section 120 and thence into to a wide , shallow central channel region 130 where diffusive mixing can proceed . central channel region 130 is followed again by a constriction section 140 ( the inverse of expansion region 120 ) and outlet section 150 . injector section 110 is itself comprised of inlet ports 111 and 112 for introducing fluids a and b into intersecting channel lengths 113 and 114 . respective expansion / contraction sections 120 and 140 , disposed at either end of shallow , central channel region 130 , are made up of shallow - faceted prism channel portions 121 and 141 , respectively , and deep - faceted prism channel portions 122 and 142 , respectively . expansion section 120 re - directs injected fluids a and b into a shallow , central channel region 130 , in such a way as to minimize or eliminate dispersion in the moving fluids , where diffusive mixing is advantaged due to the relative depth of this region 130 . finally , the mixed fluid is channeled away from the central channel region 130 , through contraction section 140 and into outlet section 150 comprised of channel length 151 and outlet port 152 which delivers the mixed fluids for further processing or disposal . the length of the wide , shallow region can be chosen to have any desired value , limited only by the size of the device substrate employed . after the wide region , the channel can be contracted back to a narrow section , which is desirable in many instances as , for example , when detection using laser - induced fluorescence in a narrow region is of benefit . the respective position of deep and shallow regions is illustrated further in the magnified image of fig6 b . here , the angles between deep and shallow regions correspond to an 85 ° incidence angle for a 10 : 1 permeability ratio for a 3 - interface , 30 × expander without skew compensation , as described in u . s . patent application serial number 20030230489 , herein incorporated by reference . those skilled in the art will recognize that the use of faceted prismatic sections is provided only as an example : the concept presented herein will function properly for any approach that expands the mixer channel . however , methods that retain substantially uniform fields while minimizing dispersion are highly advantageous .