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

该奖项是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的，目的是研究受限几何空间中光激活的化学诱导的小颗粒流动。通过施加简单的光学刺激，胶体颗粒，无论是单独的还是成批的，都可以以前所未有的控制和精度来操纵。这项研究旨在为“虚拟”微流控设备奠定基础，这种设备利用光激活流动模式产生的软边界，其强度和位置是可调的，并允许操纵和控制胶体尺度的对象。这项研究在细胞分选、DNA操纵和胶体材料组装等方面具有潜在的技术应用。该奖项的合作性质将有助于拓宽研究型学生的视野，因为他们与来自不同国家和背景的其他人合作，这在这个全球互联但脆弱的世界中变得越来越重要。该奖项建立在最近发现的光驱动扩散-渗透现象的基础上并扩展了这一现象，在这种现象中，化学表面活性剂的疏水性因光照而改变，并产生渗透压梯度，驱动流体和颗粒运动。这项研究的目的是从根本上了解扩散潜流；即运动如何取决于化学浓度、离子强度、光强度等基本物理性质，以及颗粒之间和约束基质之间的流体动力相互作用。研究了三种不同的操作边界附近粒子系综的过程：(I)导致粒子在低(高)溶质浓度区域积累(腾出)的全球光强模式，使人们能够用胶体‘绘画’；(Ii)多孔(源)粒子之间的自产生排斥，它结晶并增强被捕获的被动胶体的运动；以及(Iii)自推进的Janus粒子，其速度和持续时间可动态控制。理论表明，光诱导的流型，尽管是非平衡现象，但可以用类似平衡的化学物质‘溶质势’来表示，这意味着有趣的类似于结晶、相分离等。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，该系统触及了许多由光驱动的粒子动力学和流体动力学的前沿问题，例如粒子混合物中的分离动力学和活跃的自推进粒子在动态波动的受限几何图形中的运动。该项目通过“受限空间中的化学和运输(NSF-DFG confinine)”机会获奖，国家科学基金会和德国科学基金会(DFG)的合作征集。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.