Microfluidic Lab-on-a-Chip (LOC) devices shrink an entire medical laboratory onto a single "chip" of a few square inches, making it possible to obtain the results of various blood and urine tests within a few minutes at the patient's home. A major enabling technology for LOC tests is controlling the transport of nano- and microparticles with diameters ranging from about 0.5 to 5 nanometers suspended in a conducting aqueous solution (e.g. blood plasma, urine) where the transport of the particle solution is driven by a voltage gradient, or electric field, through microchannels with diameters of a few micrometers to a few hundred micrometers. In such small channels, a large fraction of the particles interact with the channel walls. Recent observations in microchannel flows suggest that the electric field that drives the solution also gives rise to a repulsive force of O(10^-14 N) that drives the suspended particles away from the wall, and that this force, which is proportional to the square of the electric field magnitude, also scales as the square of the particle diameter. The objectives of this work are therefore: 1) to develop our fundamental understanding of how the properties of the particle and wall surfaces, as well as those of the solution, affect these particle-wall interactions; and 2) to determine whether this repulsive force can be used to separate nano- and microparticles based upon their size and if so, the conditions that optimize separation efficiency. The experiments will use evanescent-wave particle velocimetry, a method that is uniquely suited to visualizing the dynamics of particle-wall interactions that is also sensitive enough to detect the effects of this extremely small repulsive force, to study flows through microchannels driven by an electric field, as well as a pressure gradient to create shear. The experiments will be complemented by a modest modeling effort. By determining how this repulsive force depends upon the properties of the particle, the wall, and the solution, this proposed work could lead to new technologies for: a) sorting nano- and microparticles based on their size, among other properties; and b) manipulating, collecting, and assembling particles of different sizes (and other properties) in different regions of the channel wall. As noted earlier, these are important technologies in designing LOC for medical diagnostics. They are also important in new nanomaterials, specifically in making plasmonic metamaterials with a negative refractive index that can be used as "invisibility cloaks": these materials are typically fabricated by assembling nano- and microparticles into large crystals and arrays on a solid substrate, or wall, by applying an electric field to a particle solution. This research will also educate a diverse group of undergraduate students in the largest program in Mechanical Engineering in the U.S. to the applications of microfluidics and nanotechnology, and involve high school students in summer research projects in these areas.
