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Magnetic particles that flock like birds

(Nanowerk News) Understanding how to control the mixing of particles in a liquid—like shaking a bottle from your refrigerator that has separated into two parts with the solids on the bottom, liquid on the top—play an important role in everyday life including pharmaceuticals, oil recovery, and fabrication of electronics.
In this research (Science Advances, "Flocking ferromagnetic colloids"), scientists discovered how to use magnetic fields to control the collective actions of self-assembled particles. The result? A striking organization and reconfiguration behavior reminiscent of bird swarms or fish schools.
Magnetic Particles that Flock Like Birds
Not surprising, the “north” and “south” magnetic poles that we learned about in elementary school provide a way to use alternating magnetic fields to cause magnetic particles in a mixture to roll (inset). Translating this approach to particles whose diameter is about the same as the width of a human hair, and using very well controlled magnetic fields, the “rolling” particles self-assemble into groups with the same rolling direction. In the snapshot on the left, the color indicates the direction that the individual particles are rolling and same-color patches suggest the formation of “flocks,” groups of particles moving coherently, like birds in flight. On the right, the particles have formed a mixing vortex—a funnel shape with the fastest particles in the center—with the color changes indicating differences of particle velocity. (Image: Argonne National Laboratory)
New devices for energy technologies demand new materials. The team’s results offer new insights into the design of out-of-equilibrium materials. These materials exhibit tunable transport at the microscale. Creating such materials requires learning how collective self-organizing behavior is regulated. Such knowledge is vital to control self-assembly at the molecular scale.
The findings shed light on the onset of how flocks form, over space and time. Flocks are both synthetic, such as swarms of robots and biopolymers, and living, such as cell colonies and bird flocks.
Previous investigations of colloids energized by magnetic fields revealed that they are a form of “active matter”—assemblies of self-propelled particles that can convert energy stored in the environment into mechanical motion.
A characteristic of these active matter assemblies is that they demonstrate large-scale dynamic behavior similar to that observed in living systems, such as bacterial swarms, bird flocks, and schools of fish that collectively respond to environmental signals.
Scientists have yet to get a comprehensive grasp of the mechanisms regulating this collective response, but it is critical to predictably synthesize materials with specific designs that generate and maintain this same level of spontaneous reorganization.
Scientists at Argonne National Laboratory conceived a new concept of active assemblies of rolling magnetic colloids whose rotational activity is controlled by an externally applied alternating magnetic field.
A critical element of this design is that each and every particle can be tracked and its trajectory in response to the applied magnetic field can be analyzed, enabling fundamental insights into the behavior of synthetic and living active matter. At low magnetic field energies, the team observed unorganized gas-like motion of individual particles. Increasing the frequency of the magnetic field resulted in large groups of particles that moved coherently and formed well-defined bird-like flocks that spontaneously break up and reassemble in different locations.
Further increases in the field strength resulted in increased flock size and lifetime. Increasing field strength beyond this point produced a vortex, spanning the entire system and continuously rotating in a certain direction that is determined by the initial particle configuration.
By combining experiments and simulations, the team identified the primary physical mechanisms leading to the emergence of large-scale collective motion. The mechanism involves combinations of spontaneous symmetry breaking of the clockwise/counterclockwise particle rotation, collisional alignment of particle velocities, and random particle re-orientations due to shape imperfections. These findings shed light on why the interplay of long-range and short-range interactions triggers the onset of spatial and temporal coherence in a large class of synthetic systems (colloids, swarms of robots, biopolymers).
In addition, the fundamental understanding of active assembly mechanisms could enable precise, deterministic synthesis of materials capable of defect management, self-regulation, reconfiguration of function, and autonomous self-healing.
Source: U.S. Department of Energy, Office of Science
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