Mechanistic Understanding and Control over Electrokinetic Assembly and Separation of Colloids in pH Gradients

分离液体中的微米级颗粒是许多化工过程中的一项重要操作。基于组成或表面性质差异的颗粒分离尤其具有挑战性，因为它不能用传统的基于过滤器的方法来实现。然而，这些分离在许多环境和工业应用中都很重要，包括去除废水中的颗粒，评估水道中微塑料的水平，以及使用全血进行细胞分析。电场是分离胶体的一种很有前途的解决方案，因为它们允许对大型和小型设备中的颗粒进行空间控制，并且能源效率高。这项研究项目研究了同时使用电场和pH梯度来操纵微米级胶体颗粒并控制它们的组装和分离。当胶体通过pH梯度时，胶体表面化学、pH梯度和电场之间的相互作用导致胶体表面的动态电荷。这种动态性质导致了一系列新的现象，如颗粒悬浮，基于尺寸、密度和表面化学的胶体分离，以及双层胶体膜的形成。该项目的研究人员将进行实验，以确定粒子的组成、大小、形状和表面化学如何影响胶体粒子在电场和pH梯度中的行为。研究团队将接待高中生进行为期9个月的研究实习。该小组还将开发和管理实验室模块，向马里兰大学夏季STEM夏令营的中学生演示胶体和界面科学现象。该研究项目的各个方面将被整合到一门研究生级别的胶体和界面科学课程中。该研究项目旨在测量微米级胶体在pH梯度和低频(1 KHz)振荡电场中的磷化和平流运动，并建立潜在的电动动力。在电极表面附近，电活性的苯醌分子发生氧化还原反应，在电极-电解液界面消耗或产生质子，从而在电极表面附近产生微观的pH梯度。用光学显微镜和共聚焦显微镜观察胶体在平行板电化学池中的行为。胶体将经历各种竞争和协同的磷化和平流力，包括电泳力、电渗力、电流体流动和沉降力，这些共同决定了胶体在电极表面的组装状态和悬浮高度。作用在胶体上的电动作用力的标度模型将揭示胶体表面的动态电荷和偶极电场协同作用，控制粒子的组装状态，并将胶体悬浮到电极上方的特定位置，这取决于粒子和电场的性质。这些现象将被用来证明不同形状和不同表面化学成分的大小相似的颗粒的混合物可以被分离和组装成双层胶体晶体。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Separating micron-sized particles in liquids is an important operation in many chemical processes. The separation of particles based on differences in composition or surface properties is especially challenging because it cannot be achieved with conventional filter-based methods. Nevertheless, these separations are important in many environmental and industrial applications, including particle removal from wastewater, assessing levels of microplastics in waterways, and cell assays using whole blood. Electric fields represent a promising solution for separating colloids because they allow spatial control over particles in large- and small-scale devices and are energy efficient. This research project investigates the concurrent use of electric fields and pH gradients to manipulate micron scale colloidal particles and to control their assembly and separation. Interactions between the colloid surface chemistry, the pH gradient, and the electric field lead to a dynamic surface charge on the colloids as they move through the pH gradient. This dynamic property results in a wide range of new phenomena such as particle levitation, separation of colloids based on size, density, and surface chemistry, and formation of bilayer colloidal films. The project researchers will conduct experiments to determine how particle composition, size, shape, and surface chemistry affect the behavior of colloidal particles in electric fields and pH gradients. The research team will host high school students for 9-month research internships. The group also will develop and administer laboratory modules demonstrating colloid and interfacial science phenomena to middle school students at a summer STEM camp at the University of Maryland. Aspects from the research project will be integrated into a graduate-level course on colloid and interfacial science.This research project aims to measure the phoretic and advective motion of micron-scale colloids in pH gradients and low frequency ( 1 kHz) oscillatory electric fields and to establish the underlying electrokinetic forces. Microscopic pH gradients will be generated electrochemically near the electrode surface by redox reactions of electroactive quinone molecules, which consume or generate protons at the electrode-electrolyte interface. The behavior of the colloids in a parallel plate electrochemical cell will be observed with optical and confocal microscopy. The colloids will experience various competitive and synergistic phoretic and advective forces, including electrophoresis, electroosmosis, electrohydrodynamic fluid flow, and sedimentation, which together will determine the assembly state and levitation height of colloids above the electrode surface. A scaling model for the electrokinetic forces acting on the colloids will reveal that dynamic colloid surface charge and dipole field interact synergistically to control particle assembly state and to levitate colloids to a unique position above the electrode that depends on the particle and electric field properties. These phenomena will be exploited to demonstrate that mixtures of similarly sized particles with different shapes and different surface chemistries can be separated and assembled into bilayer colloidal crystals.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.