This invention claims priority of U.S. Provisional Patent Appl. Ser. No. 60/209,051, filed Jun. 2, 2000, incorporated herein by reference.
 This invention was made in part with Government support under Grant No. AF F 30602-97-2-0102, awarded by the Defense Advanced Research Projects Agency. The Government may have certain rights in this invention.
FIELD OF THE INVENTION
 The present invention provides an active microfluidic mixer for mixing of liquid samples using electrohydrodynamic (EHD) convection for applications in microfluidic-based biochemical analysis systems and biochips. A new active micro-mixer for liquid/liquid mixing has been designed, fabricated, and demonstrated by flowing two liquid samples through the microchannel. The device can be used in the nano- or pico-liter range of liquid volumes by applying a low voltage across the microchannel.
 The present invention also pertains to methods of using such devices for the separation and analysis of biological materials for immunoassays, DNA sequencing, protein analysis and biochemical detection applications.
BACKGROUND OF THE INVENTION
 In microfluidic-based biochemical analysis systems, mixing of the liquid samples is considered as one of the most challenging tasks in order to achieve an appropriate reaction in a short period of time.
 Mixing of the liquid samples is frequently required to increase reaction probability so as to improve the detection and analysis capability of micro total analysis systems for detection of biological molecules or for analyzing DNA in microfluidic systems. There are, however, some difficulties in realizing reliable micro mixing devices, because the fluid in microchannels shows as a laminar flow characteristic in most cases due to low Reynolds number. Mechanical stirring or agitating of the liquid samples usually achieves mixing of the liquid samples in macro- scale systems, but these methods are not feasible for micro-scale devices due to its small size and fabrication compatibility. For these reasons, several micro mixing devices have been recently developed and reported. Most of them are passive micromixers, but a few semi- active micro-mixers with enough mixing capabilities have been reported. Passive mixing devices can also be useful in micro total analysis systems (m-tas) and biochip applications, but they have limitations when precise control of mixing performances concerning mixing volume and time is required.
 The electrohydrodynamic and magnetohydrodynamic (MHD) phenomena have been explored since early 1960's and there have been studies to realize the EHD and MHD micromixers. Both EHD and MHD phenomena are attractive when scaled down to micro levels, specifically for microfluidic control because of their simple structure in micro- and nano-scale fluidic control. In addition, since these EHD and MHD devices do not include mechanically moving parts, they provide more reliable mixing. Since the
microfluidic mixer in this work has numerous advantages such as: simple structure, an active mixing characteristic and no mechanically moving parts, it has significant potential in microfluidic analysis systems and biochip applications.
 The use of micromachining techniques to fabricate such analysis systems is often in silicon. Silicon provides the practical benefit of enabling mass production of such systems. A number of established techniques developed by the microelectronics industry using micromachining exist and provide accepted approaches to miniaturization. Examples of the use of such micromachining techniques are found in U.S. Pat. Nos. 5,194,133, 5,132,012, 4,908,112, and 4,891, 120 incorporated herein by reference in their entirety.
BRIEF SUMMARY OF THE INVENTION
 The present invention provides a novel active micro-mixer using electrohydrodynamic (EHD) convection. At least two fluid samples are introduced into a microchannel device wherein the surface charges are induced at the interface of the liquid samples that have different electric conductivities, and these surface charges react with applied electric fields to generate electric shear forces. By applying electric fields, the separate flow streams get mixed passing the electrodes. The micro mixing device invented in this work has simple structure and no mechanical moving part, which can provide a reliable mixing function on biochips.
 Schematic illustration of the proposed active micro-mixer shown in FIG. 1. A metal electrode was deposited and patterned on a silicon wafer that was anisotropically etched. Another metal electrode was also patterned on a Pyrex glass wafer and bonded to silicon wafer using polymer bonding technique. After fabrication, two liquid samples, which have different electric conductivities, have been injected into the microchannel. The cross sectional view and basic mixing principle is shown in FIG. 2. at and a2 denote electric conductivities of each liquid sample. From the electromagnetic theory, surface charges are induced and accumulated on the boundary of dielectric materials, which are the liquid samples in this case. When an external electric field is applied over the surface charges, the charges will be moved with liquids due to a shear stress generated at the interface layer between the liquids to be mixed. These phenomena can continuously occur and thus the convection of the liquid samples will continue until the liquid samples get fully mixed to eliminate the interfacial shear stress. The electric force profile over the interface, which causes convection of the liquids, is plotted in FIG. 3 based on analytical analyses. The mixing speed is governed by the parameters of applied electric fields, electric properties of the liquid samples, and geometry of the electrodes. As described in FIGS. 1 and 2, the invented active micro-mixer has very simple structure without any mechanical moving part so it provides more reliable mixing performance.
 In order to demonstrate the proposed mixing concepts, two different liquid samples have been chosen: one is DI water (low conductivity) and the other is saltine water (high conductivity) which was dyed for the optical monitoring. Two liquid samples have been injected through the fabricated device as shown in FIG. 4(a). With no applied electric fields, the two injected liquid samples were not mixed in the microfluidic channel as clearly showing two separate liquid streams along the microchannel. By applying