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

奖项1066853PI：凯尔大多数流体都是非牛顿流体。在粘性介质中，由胶体、聚合物、细胞或囊泡等微尺度实体组成的复杂的非牛顿流体无处不在。血液、墨水、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流动下的变形动力学或流变学已经得到了很好的研究，但对于复杂流体在外加电场下的电动现象(如电渗透和电泳法)却知之甚少。这很令人惊讶，因为在微纳流体技术中，电场经常被用来运输、控制和操纵复杂的流体。该项目确定并量化了复杂流体中的电动效应，包括新的流动、相互作用和颗粒运动，从而形成了一个新领域的基础：非牛顿介质中的电动。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控技术，如微毛细管电泳和芯片实验室分离，并进一步为设计以前未被设想的新技术提供基础。复杂流体中电动现象的系统、补充实验和建模的智力价值，重点是非线性电渗透流动；新的电泳性粒子运动和相互作用；以及场定向胶体组装。一个关键的实验步骤是配制具有可控流变性的“非牛顿电解液”，包括粘弹性、剪切稀化和法向应力系数。非牛顿流体在微流控通道中的电渗流动具有非线性和复杂的电场依赖性。重要的是，将实验测量的电渗流与计算的流型进行比较时，需要注意选择适当的流体流变学描述。对电泳的建模工作表明，非牛顿流变学的几个新的、可通过实验获得的结果，包括电泳速度与颗粒大小和形状的明确依赖，以及流变学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是不存在的，这说明了复杂的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下组装在电极上方的胶体的单颗粒动力学和集体行为中的作用。更广泛的影响这项工作将对复杂流体中的电驱动流动提供前所未有的理解，对利用电场传输微结构材料的微/纳米流体技术产生广泛影响。一个具体的例子是毛细管电泳法分离大分子和生物分子：在这里，迄今获得的结果表明，连续相的流变学可能提供一种新的无凝胶毛细管电泳法的途径，因为在非牛顿流体中，电泳速度明显依赖于颗粒的形状和大小。交流电场中电极上方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，研究生和本科生正在接受微流体、电动力学和复杂流体方面的尖端实验和理论培训。将开发一门关于微纳尺度流体物理的新的研究生/本科生水平课程，以展示我们研究的中心主题和结果。非牛顿流体流动的视觉戏剧性非常适合形成外展活动的科学核心。为此，与更广泛的匹兹堡社区的联系是通过为K-12学生设计教育模块实现的，通过咨询CMU专门的外展计划，使其适合年龄和内容。

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.