Micro-total analysis systems, or “labs on a chip”, for detecting chemical and biological threats (agents) at nanomolar (or less) concentrations, transport aqueous samples through microchannels with dimensions of O (10-100 microns). Small channel dimensions greatly reduce the time required to separate and hence analyze high molecular weight biomolecules, increasing sample throughput.  Transport through even smaller “nanopores”--with dimensions of O (1-100 nm)--could potentially lead to new sensors capable of detecting and identifying single molecules.  

Current microfluidic diagnostic techniques such as micro-particle image velocimetry (uPIV) are unsuitable for studying flows in such small channels, however, because:

• Their spatial resolution of 1-2 microns is often greater than the entire channel dimension
• They are best-suited for studying bulk flow, or the flow away from the channel walls.
  

We are developing a new form of PIV--nano-particle image velocimetry (nPIV)--based on total internal reflection fluorescence (TIRF), or evanescent-wave fluorescence, to study flows in the interfacial region next to the channel wall with a spatial resolution normal to the wall of 100-300 nm.  When light undergoes total internal reflection (TIR) at a refractive index interface between, for example, glass and water (where the refractive index of water is less than that of glass), a small amount of light penetrates into the water (Figure 1).  This light, known as the “evanescent wave”, has an intensity that decays exponentially with distance normal to the glass-water interface.  For argon-ion laser light illumination, the “penetration depth” of this evanescent wave is 100-300 nm.

Nano-PIV will be used to measure the velocity profile within the  “electric double layer” (EDL), a flow region analogous to the “boundary layer” in macroscopic channel flows.  In pressure gradient-driven (Poiseuille) flows, the power required to pump flow through a channel of hydraulic diameter D at a given flow rate is proportional to 1/D⁴, so the smaller the channel, the more power is required to pump fluid through the channel.  Electrokinetic pumping, where the flow is driven by electromagnetic, rather than pressure gradient, forces, is therefore the leading technology for transporting and separating fluids  in channels with dimensions of O(10 microns) and less. The velocity profile in “fully developed” electrokinetically driven channel flow is a plug, or uniform, flow over the entire channel except for the region next to the channel walls.  The boundary layer-type flow in this region is driven by the EDL, whose thickness--a function of the fluid ionic strength-- is O(0.1 nm-100 nm) for aqueous solutions with ionic strengths of O(1 M-1 uM).  We will study flows in microchannels with heights ranging from 1-10 microns and widths of about 20 microns in borosilicate glass chips fabricated by Oak Ridge National Laboratories (Figure 2).

Figure 1 Total internal reflection at a glass-water interface with generation of a evanescent wave in the water

Figure 2  “Straight cross” chip with two straight channels intersecting in the center.  Each channel has cross-sectional dimensions of 11.3 microns (depth) by 23.9 microns (width at channel half-depth).   [Chips courtesy of M. Ramsey and S. Jacobsen, ORNL]