(Nanowerk Spotlight) The remarkable collective motions exhibited by natural groups like schools of fish or flocks of birds have captivated scientists for generations. Teasing apart the factors enabling such coordinated swarming behaviors remains deeply challenging though, despite extensive observation and modeling efforts. Beyond satisfying curiosity, a deeper grasp of these phenomena could inspire breakthroughs in fields from biology to robotics.
Recently, researchers have turned to synthetically fabricated ‘active particles’ as model systems to systematically study collective dynamics. These microscopic entities propel autonomously in response to stimuli like light, enabling groups to mimic mobile natural organisms. However, limitations in external control technologies have thus far constrained most experiments to studying relatively simple interaction mechanisms. Key capabilities for peering deeper into emergent phenomena like dynamic state transitions remain lacking.
Now, scientists from the University of Texas at Austin have engineered a novel optical manipulation platform that promises unmatched versatility. Their system’s simultaneous and synchronous control over each individual member of customizable synthetic swarms opens new vistas for investigating collective behavior complexity. From replicating classic Vicsek models to quantifying the impacts of environmental uncertainty, this new tool greatly expands the scope for precision studies.
As detailed in a recent paper in Advanced Materials (“Synchronous and Fully Steerable Active Particle Systems for Enhanced Mimicking of Collective Motion in Nature”), this new platform centers around an optical feedback system that can independently steer multiple simple microparticles using computer-calculated holographic laser spots. This technique enables every member of a group of particles to have its own “custom” nudging laser beam, granting researchers synchronous control over each individual. By making real-time adjustments to the group’s complex laser light landscape, intricate choreographies can be programmed into the ensemble.
Collective motion of 25 active particles confined in a circular domain regulated by the Vicsek model. (Video: Zheng Group, University of Texas at Austin)
Principal investigator Yuebing Zheng elaborates on the advance, “Compared to existing ways to manipulate active particles, our platform’s unmatched flexibility in steering multiple particles simultaneously enables the study of a whole new class of models and dynamics closer to what nature can produce. We can now begin to uncover the precise factors that allow the stunning, supple collective motions found in living groups.”
The key to this newfound command lies in the automated optical feedback-control platform integrated with camera, optical microscope and spatial light modulator, which rapidly sculpts a single input laser beam into countless beams dispersing at each target particles. The ability to shift the angles of each beam in real-time via closed-loop feedback correspondingly grants swift control over every constituent’s motion. This technique marks a decisive upgrade from existing methods relying on acousto-optic deflectors to scan just one laser across many particles, producing lag in adjustments and limiting accessible interaction rules.
Another benefit stems from their newly developed particle manipulation technique based on optothermal forces, which operates at low optical power and enables the instantaneous reorientation of a large number of controlled particles once the laser-particle relative positions change. Combined with the swift laser manipulation facilitated by the spatial light modulator, even intricate velocity-alignment models like the famous Vicsek model can now be implemented and studied experimentally.
To showcase the platform’s capabilities, Zheng’s team first delved into an experimental realization of the seminal Vicsek model of collective motion. Initially proposed over 25 years ago, this model focuses on dynamical alignment of velocities between group members as the key coordinator enabling coherent collective motion. Despite its simplicity, the Vicsek model profoundly shaped later understanding of swarming and flocking by proving ordered group motion can spontaneously emerge from just local interactions.
By quantifying various sources of noise and uncertainty and explicitly incorporating long-range interparticle forces, Zheng’s team created a modified experimental setup mirroring real-world conditions. Under closed-loop feedback control of each member’s velocity based on the Vicsek alignment rule, stunning dynamic structures took shape. As if following an unseen choreographer’s directions, the synthetic swarm could transition between distinct collective states – rotating in synchrony one minute only to rearrange into a coherent travelling band the next, due to the uncertainties existing in the real world.
In another first, the effects of density variation could be systematically studied as well thanks to the precise control afforded. Intriguing density-dependent transitions were uncovered, with a critical threshold value marking the shift from disordered motion to ordered collective structures.
By tracking how various precisely known sources of uncertainty like Brownian motion changed group behaviors, Zheng’s experiments further quantified their impacts on state transitions and structure stability. Adding heterogeneity between particles to mimic natural groups increased transition frequencies but maintained coherence. Overall, even with perturbations, the groups proved impressively adept at self-correcting back to ordered collective motions.
Simulated collective motion of 25 active particles regulated by the Vicsek model but without long-range physical interactions. (Video: Zheng Group, University of Texas at Austin)
Beyond studying classical models like Vicsek’s, the new platform promises essentially unlimited flexibility to investigate more obscure or complex variants. Zheng elaborates “We can easily program entirely new interaction rules or environmental effects like time delays, obstacles, external flows, and particle differences using our real-time control software.” The team already demonstrated implementations of topological distance-based velocity matching and the effects of boundary geometry.
Summing up the tool’s potential, he continues “This programmable platform really builds a bridge between abstract modeling and experiments on natural systems. We can now dig systematically into which precise factors enable the magnificent collective behaviors we see in nature. And we can expand the capabilities of groups of simple robots to better emulate living groups.”
With unmatched versatility in constructing and steering customizable synthetic active matter systems, Zheng’s novel optical approach looks set to push the boundaries of collective dynamics research even further. Unravelling the elaborate mechanisms behind the coordinated motions found everywhere in nature may finally be within reach. And mastering simple rules for swarming and flocking could see groups of microbots reach new levels of autonomous coordination for future real-world applications.
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