Nonlinear Electrokinetic Effects on Near-Wall Microparticle Transport

12357999 yodammicrofluidic Lab-on-a-Chip （LOC）设备将整个医学实验室缩小到一个几平方英寸的“芯片”上，使得在患者家中几分钟内获得各种血液和尿液测试结果成为可能。LOC测试的一项主要使能技术是控制悬浮在导电水溶液（例如血浆、尿液）中的纳米和微颗粒的传输，其中颗粒溶液的传输由电压梯度或电场驱动，通过直径为几微米到几百微米的微通道。在这样的小通道中，很大一部分颗粒与通道壁相互作用。最近对微通道流动的观察表明，驱动溶液的电场也会产生O（10^-14 N）的排斥力，使悬浮粒子远离壁面，并且这个力与电场大小的平方成正比，也与粒子直径的平方成正比。因此，这项工作的目标是：1)发展我们对粒子和壁面以及溶液的性质如何影响这些粒子-壁面相互作用的基本理解；2)根据纳米粒子和微粒的大小，确定这种排斥力是否可以用于分离，如果可以，则确定最佳分离效率的条件。实验将使用倏逝波粒子测速法，这是一种非常适合观察粒子-壁相互作用动力学的方法，它也足够灵敏，可以检测到这种极小的排斥力的影响，研究由电场驱动的微通道中的流动，以及产生剪切的压力梯度。这些实验将由适度的建模工作来补充。通过确定这种排斥力如何取决于粒子、壁和溶液的性质，这项工作可能会带来以下新技术：a)根据纳米和微粒的大小以及其他性质进行分类；b)在通道壁的不同区域操纵、收集和聚集不同大小（和其他性质）的颗粒。如前所述，这些是为医疗诊断设计LOC的重要技术。它们在新型纳米材料中也很重要，特别是在制造具有负折射率的等离子体超材料方面，这种材料可以用作“隐形斗篷”：这些材料通常是通过在粒子溶液中施加电场，将纳米和微粒组装成大晶体和阵列，放在固体衬底或壁上制成的。这项研究还将在美国最大的机械工程项目中教育不同群体的本科生微流体和纳米技术的应用，并让高中生参与这些领域的暑期研究项目。

1235799YodaMicrofluidic 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.