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

在许多化学过程中，分离液体中的微米级颗粒是一项重要的操作。基于成分或表面性质差异的颗粒分离尤其具有挑战性，因为传统的基于过滤器的方法无法实现。然而，这些分离在许多环境和工业应用中是重要的，包括从废水中去除颗粒，评估水道中的微塑料水平，以及使用全血进行细胞分析。电场代表了一种很有前途的分离胶体的解决方案，因为它们允许在大型和小型设备中对颗粒进行空间控制，并且节能。本研究项目探讨了电场和pH梯度的同时使用，以操纵微米尺度的胶体颗粒，并控制它们的组装和分离。胶体表面化学、pH梯度和电场之间的相互作用导致胶体在pH梯度中移动时产生动态表面电荷。这种动态特性导致了广泛的新现象，如颗粒悬浮，基于大小、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，以确定颗粒组成、大小、形状和表面化学成分如何影响电场和pH梯度下胶体颗粒的行为。研究小组将邀请高中生进行为期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STEM夏令营的中学生展示胶体和界面科学现象。研究项目的各个方面将被整合到胶体和界面科学的研究生课程中。本研究项目旨在测量微米尺度胶体在pH梯度和低频（1khz）振荡电场中的电泳平流运动，并建立潜在的电动势。通过电活性醌分子的氧化还原反应，在电极-电解质界面消耗或产生质子，在电极表面附近产生微观的pH梯度。用光学和共聚焦显微镜观察平行板电化学电池中胶体的行为。胶体将经历各种竞争和协同的电泳和平流力，包括电泳、电渗透、电流体动力学流体流动和沉积，这些共同决定了胶体在电极表面上的组装状态和悬浮高度。作用在胶体上的电动力的缩放模型将揭示，动态胶体表面电荷和偶极子场协同作用，控制粒子的组装状态，并使胶体悬浮到电极上方的独特位置，这取决于粒子和电场的性质。这些现象将被用来证明具有不同形状和不同表面化学性质的大小相似的颗粒混合物可以被分离并组装成双层胶体晶体。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.