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

威斯康星-麦迪逊分校的Nicholas L.AbbottU：“由电化学产生的电活性表面活性剂浓度梯度驱动的传输过程。”该项目涉及一项由外部控制的表面活性剂浓度梯度驱动的溶液整体传输过程的实验和理论研究相结合的研究。实验方法围绕着使用氧化还原活性表面活性剂和电化学方法在微流控通道内产生表面活性剂浓度的梯度。氧化还原活性纳米表面活性剂的两亲性可以通过电化学控制表面活性剂的氧化和还原来显著和可逆地改变。通过在微流控通道中放置两个电极--一个电极“开启”表面活性剂，另一个电极“关闭”表面活性剂，研究将展示一种通用而简便的方法，这种方法可以在通道内所含溶液的大部分上产生瞬时和稳态的表面活性剂浓度梯度。此外，还研究了胶束的空间浓度梯度。以微米级油滴为模型体系，研究了表面活性剂浓度低于临界胶束浓度时，氧化还原活性表面活性剂浓度梯度对液滴输运的影响。表面活性剂浓度的外部控制梯度在液滴表面产生的Marangoni应力将驱动液滴在梯度上的运动。此外，还研究了表面活性剂浓度的外控梯度驱动下液滴系综的集体运动。对存在表面活性剂浓度梯度的液滴运动进行了补充的解析和数值研究(有限元和边界元方法)。由于表面活性剂的吸附和解吸动力学预计将对液滴的迁移有很大的影响，并且它们是模拟所需的输入，因此通过实验确定了表征氧化还原活性表面活性剂在油-水界面上的吸附动力学的参数。除了帮助解释实验，模拟还将提供一个场所，以探索现象背后的关键参数(如吸附动力学的作用)，并指导后续实验的设计。广泛的影响：首先，这项研究有助于社会对界面现象和纳米技术的了解，这是一系列商品产品(如食品、药品)和工艺(如乳液聚合、石油加工)的基础。第二，开发的方法允许在电化学控制的表面活性剂浓度梯度中控制传输过程，这有可能成为进行分子物种(如药物)分离、指导颗粒的介观尺度组装以及分选乳液滴或将乳液滴驱动成阵列(介观材料合成)的技术的基础。虽然拟议的研究在本质上是基本的，但由此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，以发现为导向的实验和详细的基础分析的结合为研究生提供了一个不同寻常的环境，无论是在界面科学的实验方法还是理论方法方面。第四，通过将本科生研究人员纳入该项目(正如PI和共同PI过去所做的那样，包括女性和少数族裔本科生参与者)，拟议的研究将激励本科生考虑高等教育带来的理工科教育机会。

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.