Emergent microrobotic oscillators via asymmetry-induced order

The ability to generate low-frequency oscillations is fundamental to the autonomy of living organisms, underpinning vital processes like heartbeats, neuronal firings, breathing, and locomotion. While advanced electronics operate at gigahertz frequencies, biological oscillations rarely exceed 100 Hz, reflecting the need to balance energy budgets and the timescales of underlying biological processes. These self-oscillations emerge spontaneously from the interplay of competing dynamical processes, a hallmark of living systems. In contrast, producing slow, self-sustaining oscillations in artificial microsystems is challenging. Generating self-sustaining mechanical oscillations typically requires transducing complex chemical oscillators into periodic physical changes, or employing intricate dynamic coupling mechanisms, often demonstrated only in millimeter-scale devices. Generating slow, self-sufficient electrical signals in untethered microscale devices remains especially difficult due to the limited scalability of capacitors and inductors, and the power demands of CMOS oscillators. This research introduces a novel method for producing robust electromechanical oscillations aboard a simple collection of microparticles, circumventing complex chemistries, integrated electronics, and elaborate mechanical structures. By intentionally breaking the permutation symmetry of the particle collective at an air-liquid interface, the researchers reliably control their dynamics to achieve simultaneous chemomechanical and electrochemical periodic energy transduction.

The generation of self-sustaining oscillations at the microscale has been a significant challenge. Existing methods often rely on complex chemical reactions (like the Belousov-Zhabotinsky reaction) translated into periodic physical changes or carefully designed dynamic coupling between materials and stimuli (thermal, chemical, or moisture). These approaches are often demonstrated in larger, millimeter-scale devices, while producing slow, periodic electrical signals in untethered microscale systems remains particularly difficult. This difficulty stems from the limitations in scaling down capacitors and inductors, as well as the power and size constraints of CMOS oscillators and energy modules. Recent advances have shown some promise in generating self-sustaining electrical oscillations by modulating electrical resistance with mechanical feedback loops, but these have mainly been demonstrated in devices larger than 500 µm. The current work aims to address these challenges by exploring a new approach based on collective particle behavior and symmetry-breaking.

The researchers fabricated microparticles consisting of a nanometer-thick platinum (Pt) patch beneath a polymeric microdisc. These particles, when placed at the air-liquid interface of a hydrogen peroxide (H₂O₂) drop, catalyze the decomposition of H₂O₂, generating oxygen bubbles. For a single particle, this reaction is self-limiting as the bubble grows and blocks the catalyst. However, introducing a second identical particle leads to bubble merger, restoring catalytic activity and initiating a cyclical process: bubble merger, growth beyond a threshold, rupture, and impulse-driven particle movement. This is followed by the particles being drawn back together by buoyancy and capillary forces (the "Cheerios effect"), creating a repetitive cycle. The researchers tracked particle motion and the "breathing radius" (average distance from the collective's centroid to each particle) to characterize the oscillations. They also examined the effect of increasing the number of particles, finding that periodicity decreased in homogeneous systems. To maintain periodicity in larger systems, they introduced a "designated leader" (DL) particle with a larger Pt patch, breaking the permutation symmetry. A theoretical model based on Rattling Theory was used to explain the asymmetry-induced order observed with the DL particle. This model connects bubble size to system-level fluctuations and collective order. To harness the oscillations for electrical work, the researchers fabricated particles with a Pt pattern and a second metal patch (Au or Ru), creating an on-board fuel cell. The periodic bubble formation modulates the electrical conductance, generating an oscillatory current. The generated current was used to drive a state-of-the-art microrobotic arm.

The study revealed that the interaction of two identical microparticles at the air-liquid interface of a hydrogen peroxide drop leads to emergent low-frequency (several hertz) chemomechanical oscillations. This self-sustained oscillation results from a cyclical process involving bubble merger, growth, rupture, and particle movement driven by buoyancy and capillary forces. In homogeneous systems with more than two particles, however, the periodicity of these oscillations decays, transitioning towards aperiodicity as the number of particles increases. This loss of periodicity was shown to be due to frequent and unpredictable bubble mergers and collapses among particle subsets. Interestingly, the introduction of a single particle with a larger catalytic patch ("designated leader") dramatically stabilizes the oscillations, maintaining periodicity even in larger ensembles. This effect was explained by the researchers' theoretical model based on Rattling Theory, which shows that breaking permutation symmetry via heterogeneity (in this case, the size of the bubbles) reduces system-level fluctuations and promotes emergent order. The chemomechanical oscillations were harnessed to generate oscillatory electrical currents using a bimetallic (Pt-Ru or Pt-Au) fuel cell design integrated onto the microparticles. The periodic bubble growth and collapse modulate the electrical conductance, resulting in a current that oscillates in phase with the mechanical beating. The generated oscillatory current was successfully used to drive a microrobotic arm, demonstrating the potential for self-powered microrobotics.

The findings demonstrate a novel mechanism for generating low-frequency oscillations in microrobotic systems, leveraging collective behavior and symmetry-breaking. This approach contrasts with traditional methods that rely on complex designs and intricate control mechanisms. The use of a "designated leader" particle to stabilize oscillations in larger systems highlights the importance of heterogeneity in achieving emergent order. The successful integration of a fuel cell to generate and utilize oscillatory electrical currents represents a significant step toward self-powered microrobotics. The observed multifunctionality—simultaneous generation of mechanical oscillation, electrical voltage, and conductance modulation—is reminiscent of embodied energy principles and crucial for developing efficient microsystems. The results suggest a promising path for creating more robust and functional microrobots and active matter systems by designing them to exploit the physics of their environments.

This study introduces a novel strategy for achieving low-frequency oscillations in microrobotic systems by exploiting the self-organized properties of simple microparticles and a thermodynamic mechanism for asymmetry-induced order. The ability to generate and harness oscillatory electrical currents from these oscillations opens exciting avenues for self-powered microrobotics. Future research could explore larger particle collections, investigating general principles for designing active matter systems and enhancing their task capabilities by understanding the interplay between system symmetries and environmental forcing.

While the study successfully demonstrates the generation of self-oscillating microrobotic systems, the current work focuses primarily on relatively small ensembles of particles. The scalability and robustness of this approach in significantly larger systems warrant further investigation. Additionally, the long-term stability and reliability of the fuel cell under continuous operation need to be assessed. Finally, the specific choice of materials and their impact on the efficiency and longevity of the system require more detailed exploration.