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.
