NSF-DFG Confine: Chemically-induced phoretic flow, or how to turn a curtain of light into virtual micro-fluidic boundaries

这个奖项是由德国波茨坦大学的一个实验小组和加州理工学院的一个理论小组合作，研究光激活化学诱导的小颗粒在受限几何中的流动。通过应用简单的光学刺激，胶体颗粒，无论是单独的还是散装的，都可以以前所未有的控制和精度进行操作。该研究旨在为“虚拟”微流体装置奠定基础，该装置利用光激活流模式产生的软边界，其强度和位置可调，并允许操纵和控制胶体尺度物体。该研究具有潜在的技术应用，例如，细胞分选，DNA操作和胶体基材料的组装。该奖项的合作性质将有助于拓宽研究学生的视野，因为他们与来自不同国家和背景的其他人一起工作，这在这个全球联系但脆弱的世界中变得越来越重要。该奖项建立并扩展了最近发现的光驱动扩散渗透现象，在这种现象中，化学表面活性剂的疏水性被照明改变，并产生驱动流体和粒子运动的渗透压梯度。本研究旨在对扩散泳流有一个基本的了解；即运动如何取决于化学浓度、离子强度、光强等基本物理性质，以及颗粒之间和与围合底物的水动力相互作用。研究了三种不同的过程来操纵靠近边界的粒子群：(i)光强度的全局空间模式，导致粒子在低（高）溶质浓度区域积聚（从），允许人们用胶体“涂”；（ii）多孔（源）颗粒之间的自生排斥，使其结晶并增强被捕获的被动胶体的运动；（iii）速度和持续时间可动态控制的自推进Janus粒子。理论表明，光诱导的流动剖面尽管是非平衡现象，但可以用类似平衡的化学“溶质势”来表示，这与结晶、相分离等有着有趣的类比。该奖项是关于一个独特的非平衡系统的理论和实验的紧密结合，该系统涉及光化学驱动粒子动力学和流体动力学的许多前沿问题，例如粒子混合物中的偏析动力学和动态波动受限几何中主动自推进粒子的运动。该项目是通过“密闭空间中的化学和传输（NSF-DFG）”机会获得的，这是一项由美国国家科学基金会和德国科学研究协会（DFG）参与的合作征集。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。

This award is a collaboration between an experimental group at the University of Potsdam in Germany and a theoretical group at the California Institute of Technology to study the light-activated chemically induced flow of small particles in confined geometries. By applying simple optical stimuli, colloidal particles, both individually and in bulk, can be manipulated with unprecedented levels of control and precision. The research aims to establish the foundation for ‘virtual’ microfluidic devices that utilize soft boundaries generated from light-activated flow patterns whose strength and location are tunable and allow the manipulation and control of colloidal-scale objects. The research has potential technological applications in, for example, cell sorting, DNA manipulation, and the assembly of colloidal-based materials. The collaborative nature of the award will help broaden the horizon of research students as they work with others from different countries and backgrounds, which is increasingly important in this globally connected but fragile world.This award builds on and extends the recently discovered phenomenon of light-driven diffusion-osmosis in which a chemical surfactant’s hydrophobicity is altered by illumination and generates an osmotic pressure gradient that drives fluid and particle motion. The study is aimed at a fundamental understanding of the diffusiophoretic flow; that is, how the motion depends on the basic physical properties of chemical concentration, ionic strength, light intensity, etc., as well as the hydrodynamic interactions among particles and with the confining substrate. Three different processes to manipulate ensembles of particles adjacent to a boundary are investigated: (i) global spatial patterns of light intensity that cause particles to accumulate in (vacate from) regions of low (high) solute concentration, allowing one to ‘paint’ with colloids; (ii) self-generated repulsion between porous (source) particles that crystalize and enhance motion of trapped passive colloids; and (iii) self-propelled Janus particles whose speed and duration can be dynamically controlled. Theory suggests that the light-induced flow profiles, despite being non-equilibrium phenomena, can be expressed in terms of an equilibrium-like chemical ‘solute potential,’ which suggests intriguing analogies to crystallization, phase separation, etc. The award is a close alignment of theory and experiment regarding a unique non-equilibrium system that touches upon many cutting-edge problems of phoretically-driven particle dynamics and hydrodynamics, such as segregation dynamics in mixtures of particles and the motion of active self-propelled particles in dynamically fluctuating confined geometries.This project was awarded through the “Chemistry and Transport in Confined Spaces (NSF-DFG Confine)" opportunity, a collaborative solicitation that involves the National Science Foundation and Deutsche Forschungsgemeinschaft (DFG).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.