CAREER: Microscale contactless reconfigurable swarms with active random mutations (MICROSWARMS)

自然界充满了活动组件之间合作互动的例子，这些组件聚集在一起形成的实体比它们各部分的总和更大。例如，数以万亿计的细胞在物理和化学上相互作用，形成高度复杂的生物有机体。这些有机体的行为比单个细胞的行为更加多样化。当许多简单的部分产生复杂的行为时，通常使用的术语是“涌现”。自推进粒子（SPP’s）是能够在没有外界因素影响的情况下移动的实体。这些SPP集合的行为可以导致类似于在迁徙动物群中观察到的复杂形成的紧急现象。一般来说，当唯一可用的信息是SPP如何移动和相互作用时，很难预测会出现什么属性。因此，能够对相互作用的SPP进行控制实验有望帮助弥合这一差距，但目前这种类型的系统很少。该项目致力于开发一个小规模SPP系统，以显示依赖于粒子运动速度的细节的紧急现象，更重要的是，它们如何相互作用；这两个参数都可以调优，甚至可以实时调优。这个新系统的另一个关键特征是粒子不直接与其他粒子接触，模仿自然界中看到的成群结队的行为。这些非接触粒子群允许研究新现象，并使新型纳米和微尺度材料制造成为可能。该项目将对北亚利桑那大学本科生的研究机会产生深远的影响。这也将影响到NAU应用物理学的研究生课程，新开设的课程旨在向学生介绍专业科学家的日常活动。对当地社区的影响将包括研究人员与一所中学密切合作，开展与活性物质有关的项目，并向霍皮高中的美国原住民学生伸出援手。活性物质由驱动的可移动实体组成，这些实体移动和相互作用形成动态结构和模式，其特性在单个自推进粒子（SPP’s）的水平上是看不到的。最近的一些研究关注的是在活性胶体物质中观察到的集体和涌现现象，但本文提出的研究的独特之处在于，粒子形成团簇，不直接接触，克服了该领域目前的许多实验限制，并允许测试当前的理论预测。特别是，该系统结合了使用外部光源的可控活动和粒子之间磁偶极子-偶极子相互作用产生的可调“非接触”远程相互作用。根据几何形状的不同，这些相互作用可以采取不同的形式，可以是吸引的，排斥的，或者是这些的组合。大量非接触SPP之间的相互作用导致集体运动，这在过去没有被证明，并且随着粒子类型的变化而变化。此外，该系统是探索新现象的理想选择，例如当立体相互作用不存在时，复杂3D胶体的形状依赖集体行为。这种独特的系统不仅允许设计控制粒子-粒子相互作用和多粒子集体现象的新形式，而且这些行为可以通过主动自组装来制造新材料。由于该项目的许多实验技术和方法非常适合首次向学生介绍研究，因此不仅本科生将成为这项工作的组成部分，而且参与该项目的研究人员还将与STEM City密切合作，每两年访问Flagstaff的一所中学，学生将有机会参加有关活性物质的研究项目。在大学之外传播这项研究的其他努力包括通过Flagstaff科学节向K-12学生和亚利桑那州当地Flagstaff社区的更大公众推广，以及每年访问霍皮印第安人保留地，与高中生一起实践演示。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。

The natural world is full of examples of cooperative interactions between active components, which come together to form entities greater than the sum of their parts. For example, trillions of cells interact both physically and chemically to form highly complex biological organisms. These organisms are then capable of behaviors infinitely more diverse than those of the individual cells. When many simple parts give rise to complex behaviors, the term often used is "emergent." Self-propelled particles (SPP's) are entities capable of moving without the influence of external factors. The behavior of a collection of these SPP's can lead to emergent phenomena similar to the complex formations observed in herds of migrating animals. In general, it is difficult to predict what properties will emerge when the only information available is how the SPP's move and interact with each other. Thus, being able to perform controlled experiments on interacting SPP's is expected to help bridge this gap, but very few systems of this type are currently available. This project is dedicated to developing a system of small-scale SPP's that show emergent phenomena that are dependent upon the details of how fast the particles move, and more importantly, how they interact; both parameters can be tuned, even in real time. Another key feature of this novel system is the particles do not come into direct contact with others, mimicking behaviors such as swarming and flocking as seen in nature. These contactless particle swarms allow the study of new phenomena and make possible novel nano- and microscale material fabrication. This project will have far reaching impact on undergraduate research opportunities at Northern Arizona University (NAU). It will also impact the graduate program in Applied Physics at NAU, with a new class being developed aimed at introducing the students to the everyday activities of a professional scientist. Impact upon the local community will include the researchers working closely with a middle school on a project related to active matter as well as outreach to Native American students at the Hopi High School.Active matter consists of driven mobile entities that move and interact to form dynamic structures and patterns with properties not seen at the level of the individual self-propelled particles (SPP's). Several recent studies have been concerned with the collective and emergent phenomena observed in active colloidal matter, but the proposed research herein is unique in that the particles form clusters that do not come into direct contact, overcoming numerous current experimental limitations in this field and allowing for testing current theoretical predictions. The system, in particular, combines controllable activity using an external light source and tunable "contactless" long-range interactions arising from magnetic dipole-dipole interactions between the particles. Depending upon the geometry, these interactions can take on different forms that can be attractive, repulsive, or a combination of these. Interactions between large numbers of contactless SPP's results in collective motion that has not been demonstrated in the past and changes with particle type. Moreover, the system is ideal for the exploration of new phenomena such as shape-dependent collective behavior of complex 3D colloids when steric interactions are absent. This unique system not only allows for engineering new forms of controlled particle-particle interactions and multiple-particle collective phenomena, but these behaviors can be harnessed to fabricate new materials by active self-assembly. As many of the experimental techniques and methods in this project are excellent for introducing students to research for the first time, not only will undergraduates be an integral part of this work, but the researchers involved in this project will also work closely with STEM City to make bi-yearly visits to a middle school in Flagstaff, where the students will have the opportunity to take part in a research project on active matter. Additional efforts to disseminate this research beyond the University include outreach to K-12 students, and the larger public, in the local Flagstaff, AZ community via the Flagstaff Festival of Science and beyond by making yearly visits to the Hopi Native American Reservation for hands-on demonstrations with high school students.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.