Coupling Electrokinetics and Rheology: Novel Flows, Interactions and Particle Motions

大多数流体是非牛顿流体。复杂的，非牛顿流体，包括微观实体，如胶体，聚合物，细胞，或在粘性介质中的囊泡，无处不在。血液、油墨、食品、油漆和个人护理产品只是其中的几个例子。虽然复杂流体在流体动力作用下的变形动力学或流变学已经得到了很好的研究，但相对而言，对外加电场作用下复杂流体的电动力学现象（如电渗透和电泳）知之甚少。考虑到电场通常用于在微纳米流体技术中传输、控制和操纵复杂流体，这是令人惊讶的。该项目确定并量化了复杂流体中的电动力学效应，包括新的流动、相互作用和粒子运动，从而形成了一个新领域的基础：非牛顿介质中的电动力学。这项研究的结果将改变目前利用电动流动和复杂流体的微流体技术，如微毛细管电泳和芯片上的实验室分离，并进一步为设计新的、以前无法想象的技术提供基础。复杂流体中电动力学现象的系统、互补实验和建模，重点是非线性电渗透流动；新型电泳粒子运动和相互作用；现场定向胶体组装正在研究中。关键的实验步骤是制定具有可控流变特性的“非牛顿电解质”，包括粘弹性、剪切减薄和正应力系数。非牛顿流体在微流体通道中的电渗透流动预计具有非线性和时间复杂的依赖于外加电场。重要的是，将实验测量的电渗透流动与计算的流动剖面进行比较，需要谨慎选择流体的适当流变描述。电泳的建模工作表明，非牛顿流变性能产生几个新的、实验上可获得的结果，包括电泳速度对颗粒大小和形状的明确依赖，以及流变性介导的胶体之间的电泳相互作用。至关重要的是，所有这些效应在牛顿流体中都不存在，这说明了复杂流体流变学对电动现象的巨大影响。从这些研究中获得的知识有助于阐明粘弹性在交流电场聚集在电极上的胶体的单粒子动力学和集体行为中的作用。更广泛的影响这项工作将提供对复杂流体中电驱动流动的前所未有的理解，为利用电场传输微结构材料的微/纳米流体技术提供广泛的影响。一个具体的例子是用于分离大分子和生物分子的毛细管电泳：在这里，迄今为止获得的结果表明，连续相的流变学可能为新的无凝胶毛细管电泳方案提供了一条途径，因为在非牛顿流体中，电泳速度与颗粒形状和大小有明确的依赖性。在交流电场中电极上粒子动力学的研究为粘弹性流体中胶体微结构的定向组装提供了新的范例。在教育方面，研究生和本科生研究人员正在接受微流体，电动力学和复杂流体的尖端实验和理论培训。一个新的研究生/上本科水平的课程，微纳米尺度流体物理将开发，以展示中心主题和我们的研究成果。非牛顿流体流动的视觉戏剧性本质非常适合形成拓展活动的科学核心。为此，通过为k -12年级学生设计教育模块，通过与CMU专门的外展项目协商，使其适合年龄和内容，从而实现与更广泛的匹兹堡社区的联系。

Award 1066853PI: KhairMost fluids are non-Newtonian. Complex, non-Newtonian, fluids, comprising of micro-scale entities such as colloids, polymers, cells, or vesicles in a viscous medium, are ubiquitous. Blood, inks, foodstuffs, paints, and personal-care products are just a few examples. While the deformation dynamics, or rheology, of complex fluids under hydrodynamic flows has been well studied, comparatively nothing is known about electrokinetic phenomena (e.g. electro-osmosis and electrophoresis) of complex fluids under applied electric fields. This is surprising, given that electric fields are routinely used to transport, control, and manipulate complex fluids in micro- and nano-fluidic technologies. This project identifies and quantifies electrokinetic effects in complex fluids, including novel flows, interactions, and particle motions, thereby forming the foundation of a new field: electrokinetics in non-Newtonian media. The results of the research will be transformative to current microfluidic technologies that utilize electrokinetic flows and complex fluids, e.g. micro-capillary electrophoresis and lab-on-a-chip separations, and further offer the basis for designing new, previously un-envisioned technologies.Intellectual Merit Systematic, complementary experiments and modeling of electrokinetic phenomena in complex fluids, focusing on non-linear electro-osmotic flows; novel electrophoretic particle motions and interactions; and field-directed colloidal assembly are being examined. A key experimental step is the formulation of "non-Newtonian electrolytes" with controllable rheological properties, including viscoelasticity, shear-thinning, and normal stress coefficients. The electro-osmotic flow of non-Newtonian fluids in microfluidic channels is expected to possess non-linear and temporally complex dependencies on applied electric fields. Importantly, the comparison of experimentally measured electro-osmotic flows against computed flow profiles requires care in choosing an appropriate rheological description of the fluid. Modeling work on electrophoresis suggests several novel, experimentally accessible consequences of non-Newtonian rheology, including an explicit dependence of electrophoretic velocity on particle size and shape, and rheology-mediated electrophoretic interactions between colloids. Crucially, all of these effects are absent in Newtonian fluids, illustrating the dramatic influence of complex fluid rheology on electrokinetic phenomena. The knowledge gained from these investigations aids in elucidating the role of viscoelasticity on the single particle dynamics and collective behavior of colloids assembled above electrodes by AC fields. Broader ImpactThis work will furnish an unprecedented understanding of electrically driven flows in complex fluids, offering broad impacts to micro/nano-fluidic technologies that utilize electric fields to transport micro-structured materials. A specific case is capillary electrophoresis for separation of macro- and bio-molecules: Here, the results obtained to-date suggests that the rheology of the continuous phase may provide a route to novel gel-free capillary electrophoresis protocols, due to the explicit dependence of electrophoretic velocity on particle shape and size in a non-Newtonian fluid. The work on particle dynamics above electrodes in AC fields yields new paradigms for directed assembly of colloidal microstructures in viscoelastic fluids. In education, graduate students and undergraduate researchers are receiving cutting-edge experimental and theoretical training in microfluidics, electrokinetics, and complex fluids. A new graduate/upper-undergraduate level course on Micro- and Nano-Scale Fluid Physics will be developed to showcase central themes and results of our research. The visually dramatic nature of non-Newtonian fluid flow is ideally suited to form the scientific core of outreach activities. To this end, a connection to the wider Pittsburgh community is achieved by designing educational modules for K-12th students, made age- and content-appropriate via consultation with dedicated outreach programs at CMU.