Transport Processes Driven by Electrochemically-Generated Gradients in Concentration of Electro-Active Surfactants

Nicholas L.威斯康星州-麦迪逊的AbbottU“电活性表面活性剂浓度中由电化学产生的杂质驱动的传输过程。“该项目涉及到大量溶液中传输过程的实验和理论研究，这些溶液由表面活性剂浓度的外部控制梯度驱动。实验方法围绕使用氧化还原活性表面活性剂和电化学方法来产生微流体通道内的表面活性剂浓度梯度。氧化还原活性纳米级表面活性剂的两亲性质可以通过表面活性剂的电化学控制的氧化和还原而显著且可逆地改变。通过用两个电极排列微流体通道-一个是表面活性剂“打开”，另一个是表面活性剂“关闭”，研究将展示一般和简单的方法，产生瞬态和稳态梯度的表面活性剂浓度在一个通道内包含的溶液的主体。胶束的空间浓度梯度也进行了研究。使用微米大小的油滴作为模型系统和表面活性剂的浓度低于临界胶束浓度，浓度梯度的氧化还原活性表面活性剂对液滴的运输的影响进行了研究。由表面活性剂浓度的外部控制梯度引起的液滴表面处的Marangoni应力将驱动液滴穿过梯度的运动。集体运动的合奏液滴驱动的外部控制的表面活性剂浓度梯度也进行了研究。一个互补的分析和数值研究（有限元和边界元法）的液滴的运动中存在的表面活性剂的浓度梯度进行。因为表面活性剂的吸附和解吸的动力学预计对液滴的迁移具有强烈的影响，并且因为它们是模拟所需的输入，所以表征氧化还原活性表面活性剂在油-水界面处的吸附动力学的参数通过实验确定。除了帮助解释实验，模拟将提供一个场所，探索关键参数的基础现象（如吸附动力学的作用），并指导后续实验的设计。更广泛的影响：首先，这项研究有助于社会的界面现象和纳米技术的知识，这是一系列商品产品的基础（例如，食品，药物）和工艺（例如，乳液聚合、油加工）。第二，允许在表面活性剂浓度的电化学控制梯度中控制传输过程的方法的开发具有形成用于进行分子种类（例如药物）分离、用于引导颗粒的介观尺度组装、以及用于对乳液液滴进行分选或将乳液液滴驱动成阵列（介观尺度材料合成）的技术的基础的潜力。虽然所提出的研究本质上是基础性的，但从中产生的原则具有技术前景。例如，专门的微型药物输送系统的潜力也是可能的。第三，发现为导向的实验和详细的基本面分析的融合提供了一个不寻常的环境，为研究生的教育在实验和理论方法在界面科学。第四，通过将本科生研究人员纳入项目（正如PI和co-PI过去所做的那样，包括女性和少数民族本科生参与者），拟议的研究将刺激本科生考虑科学和工程高等教育带来的机会。

Nicholas L. AbbottU of Wisconsin - Madison "Transport Processes Driven by Electrochemically-Generated Gradients in Concentration of Electro-Active Surfactants."This project involves a combined experimental and theoretical study of transport processes in the bulk of solutions that are driven by externally controlled gradients in concentration of surfactant. The experimental approach revolves around the use of redox-active surfactants and electrochemical methods to generate gradients in surfactant concentration within microfluidic channels. The amphiphilic properties of the redox-active nano-scale surfactants can be substantially and reversibly changed by electrochemically-controlled oxidation and reduction of the surfactant. By lining themicrofluidic channels with two electrodes - one at which the surfactant is "turned on" and the other at which the surfactant is "turned off", the research will demonstrate general and facile methods that generate transient and steady-state gradients in surfactant concentration across the bulk of a solution contained within a channel. Spatial concentration gradients of micelles are also investigated. Using micrometer-sized droplets of oil as a model system and concentrations of surfactant below the critical micelle concentration, the influence of the gradients in concentration of the redox-active surfactants on the transport of the droplets is investigated. Marangoni-stresses at the surfaces of the droplets induced by the externally controlled gradient in surfactant concentration will drive the motion of the droplets across the gradient. The collective motion of ensembles of droplets driven by externally controlled gradients in surfactant concentration are also investigated. A complementary analytical and numerical study (finite elements and boundary element methods) of the motion of droplets in the presence of gradients in concentration of surfactant is performed. Because the kinetics of adsorption and desorption of the surfactants is expected to have a strong influence on the migration of the droplet and because they are a required input for the simulation, parameters characterizing the kinetics of adsorption of the redox-active surfactants at oil-aqueous interfaces are experimentally determined. In addition to aiding interpretation of the experiments, the simulations will provide a venue to explore key parameters that underlie the phenomenon (such as the role of the kinetics of adsorption) and guide the design of subsequent experiments.Broader Impacts: First, this research contributes to society's knowledge of interfacial phenomena and nanotechnology which underlies a range of commodity products (e.g., foods, pharmaceuticals) and processes (e.g., emulsion polymerization, oil processing). Second, the development of methods that permit control of transport processes in electrochemically-controlled gradients in concentration of surfactant have the potential to form the basis of technologies for performing separations of molecular species (such as drugs), for directing the meso-scale assembly of particles, and for sorting of emulsion droplets or driving emulsion droplets into arrays (meso-scale materials synthesis). While the research proposed is fundamental in nature, the principles that will emerge from it have technological promise. For example, the potential for specialized micro-drug delivery systems is also possible. Third, the blend of discovery-oriented experimentation and detailed fundamental analysis provides an unusual environment for the education of graduate students in both experimental and theoretical methods in interfacial science. Fourth, by incorporating undergraduate researchers into the project (as the PI and co-PI have done in the past, including female and minority undergraduate participants), the proposed research will stimulate undergraduates to consider the opportunities that follow from higher education in science and engineering.