CAREER: High Speed Imaging and Chronocoulometry of Charge Transfer Events in Emulsions

1056138Ristenpart该项目的主要科学目标是回答这个问题：液滴如何获得电荷？ PI最近的工作揭示了临界电荷密度的存在，在该临界电荷密度以上，带相反电荷的液滴不会聚结（Ristenpart等人，Nature 2009）。 的非聚结解释的毛细管压力模型，其中一个短暂的弯月面桥夹断以下的快速电荷交换之间的下降。 该模型有助于阐明已经观察到的聚结效率平台的各种现象，包括大气电荷传导、石油和植物油的去乳化以及芯片实验室设备中液滴的电操纵。然而，几个基本问题仍然没有答案。 首先，也是最重要的：是什么机制控制着液滴获得的电荷量？ 传统的研究焦点一直是带电液滴的产生，其中通过施加足够强的电场（例如，电喷雾电离）。 在许多应用中，包括上面列出的那些，这种不稳定机制是不起作用的。 相反，液滴在接触另一个带电表面时获得电荷，例如另一个液滴或固体电极。 引人注目的是，没有现存的模型可以准确预测与带电表面接触时转移到液滴的电荷量。 智力优点：这里提出的实验框架将允许突破性的，基础研究的电荷转移动力学乳液相结合的同时高速可视化和计时库仑技术。 先前的研究仅间接地探测了液滴电荷，假设液滴上的流体动力学拖曳力由电泳驱动力平衡。建议的实验系统，而不是允许使用高分辨率静电计从液滴转移的电荷的直接测量，并将提供第一个独立的测试与已知的电荷密度的液滴上的电泳力。此外，计时库仑测量将提供重要的信息，用于解释液滴变形，法拉第反应，和瞬态弯月面桥动力学的电荷转移量的影响。 在所有情况下，PI将强调最简单，最直观的系统来阐明关键现象。 更广泛的影响：PI的高速摄像机，以及接受过操作培训的本科生，代表了拟议研究计划的核心能力。 这项职业建议的一个关键目标是在小学阶段利用这些能力。 具体来说，PI将与加州戴维斯的Explorit科学中心合作，开发一个名为“时间扭曲”的互动式实践展览，该展览以高速视频为特色。 小学生将在PI培训的本科生的监督下亲自操作相机软件。 初步的“试运行”已经证实，当学生们拍摄他们自己和他们的同学对物理现象（如爆裂水球或打破木板）进行实践调查时，他们很感兴趣，这些现象说明了关键的科学原理。 学历：PI的研究计划的一个一贯主题是将本科生直接纳入前沿研究，因此PI的本科课程开发侧重于鼓励本科生参与研究。 PI正在开发两个较低级别的研讨会（化学工程和食品科学各一个），这将突出机会，并促进本科生进入研究项目的安置。 在研究生阶段，PI正在开发一门名为“电流体学”的选修课，该课程将调查该领域的最新进展，并结合他自己小组的最新研究活动。

1056138RistenpartThe main scientific goal of this project is to answer the question: how do droplets acquire charge? Recent work by the PI has revealed the existence of a critical electric charge density above which oppositely charged drops do not coalesce (Ristenpart et al., Nature 2009). The non-coalescence was interpreted in terms of a capillary pressure model, in which a short-lived meniscus bridge pinches off following rapid charge exchange between the drops. This model helps shed light on a wide range of phenomena where plateaus in coalescence efficiency have been observed, including atmospheric charge conduction, de-emulsification of petroleum and vegetable oils, and electrical manipulation of drops in lab-on-a-chip devices. Several fundamental questions, however, remain unanswered. First and foremost: what mechanism governs the amount of charge a droplet acquires? The traditional research focus has been on the generation of charged droplets, where a large droplet is destabilized and broken into smaller charged droplets by application of a sufficiently strong electric field (e.g., electrospray ionization). In many applications, including those listed above, this destabilization mechanism is not operative. Instead, droplets acquire charge when they contact another electrified surface, such as another drop or a solid electrode. Strikingly, there are no extant models that accurately predict the amount of charge transferred to a liquid drop upon contact with an electrified surface. Intellectual merit: The experimental framework proposed here will allow groundbreaking, fundamental studies of charge transfer dynamics in emulsions by combining simultaneous high- speed visualization and chronocoulometric techniques. Previous studies have probed the droplet charge only indirectly, by assuming that the hydrodynamic drag force on a drop is balanced by an electrophoretic driving force. The proposed experimental system instead allows direct measurements of the charge transferred from a drop using a high-resolution electrometer, and will provide the first independent tests of the electrophoretic force on drops with a known charge density. Furthermore, the chronocoulometric measurements will provide crucial information for interpreting the effects of droplet deformation, Faradaic reactions, and transient meniscus bridge dynamics on the amount of charge transferred. In all cases, the PI will emphasize the simplest, most intuitive systems to elucidate key phenomena. Broader impacts: The PI's high-speed camera, and the undergraduate students who are trained to operate it, represent core capabilities of the proposed research program. A key goal of this CAREER proposal is to leverage these capabilities at the elementary school level. Specifically, the PI will collaborate with the Explorit Science Center in Davis, CA to develop an interactive, hands-on exhibition named "Time Warp" that features high-speed video. Elementary school students will personally operate the camera software under the supervision of an undergraduate trained by the PI. Preliminary "trial runs" have confirmed that the students are intrigued as they film themselves and their classmates performing hands-on investigations of physical phenomena (such as popping a water balloon or breaking a board) that illustrate key scientific principles. Education: A consistent theme in the PI's research program is incorporation of undergraduate students directly in cutting-edge research, and accordingly the PI's undergraduate curriculum development is focused on encouraging undergraduate participation in research. The PI is developing two lower-division seminars (one each in chemical engineering and food science) that will highlight opportunities and facilitate placement of undergraduates into research programs. At the graduate level, the PI is developing an elective course titled "Electrofluidics" that will survey recent advances in the field and incorporate state of the art research activities from his own group.