Nonlinear Electrokinetic Effects on Near-Wall Microparticle Transport

1235799 Yoda微流控芯片实验室（Microfluidic Lab-on-a-Chip，简称QF）设备将整个医学实验室缩小到一个几平方英寸的“芯片”上，使患者在家中几分钟内就可以获得各种血液和尿液检测结果。 一种主要的使能微流控测试的技术是控制悬浮在导电水溶液（例如血浆、尿液）中的直径范围为约0.5至5纳米的纳米颗粒和微米颗粒的输送，其中颗粒溶液的输送由电压梯度或电场驱动通过直径为几微米至几百微米的微通道。在这样的小通道中，大部分颗粒与通道壁相互作用。 最近在微通道流动中的观察表明，驱动溶液的电场也会产生一个O（10^-14 N）的斥力，将悬浮颗粒驱离壁面，而且这个力与电场大小的平方成正比，也与颗粒直径的平方成正比。 因此，这项工作的目标是：1）发展我们对颗粒和壁表面的性质以及溶液的性质如何影响这些颗粒-壁相互作用的基本理解; 2）确定这种排斥力是否可以用于根据其尺寸分离纳米颗粒和微粒，如果可以，优化分离效率的条件。 实验将使用渐逝波粒子测速法，这种方法非常适合于可视化粒子-壁相互作用的动力学，也足够敏感，可以检测到这种极小排斥力的影响，研究通过电场驱动的微通道的流动，以及压力梯度来产生剪切。 这些实验将通过适度的建模工作来补充。 通过确定这种排斥力如何取决于颗粒，壁和溶液的性质，这项拟议的工作可能会导致新的技术：a）基于它们的大小和其他性质对纳米和微米颗粒进行分类;和B）操纵，收集和组装不同尺寸（和其他性质）的颗粒在通道壁的不同区域。 如前所述，这些是设计用于医疗诊断的RFID的重要技术。 它们在新的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时，可以用作“隐形斗篷”：这些材料通常通过将纳米和微米颗粒组装成固体基底或墙壁上的大晶体和阵列来制造，通过向颗粒溶液施加电场。 这项研究还将教育不同群体的本科生在机械工程在美国最大的程序微流体和纳米技术的应用，并涉及高中生在这些领域的夏季研究项目。

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.