Emergent Microrobotic Oscillators via Asymmetry-Induced Order

Abstract

Spontaneous low-frequency oscillations on the order of several hertz are the drivers of many crucial processes in nature [1][2][3][4]. From bacterial swimming [5, 6] to mammal gaits [7, 8], the conversion of static energy inputs into slowly oscillating electrical and mechanical power is key to the autonomy of organisms across scales [9][10][11][12]. However, the fabrication of slow artificial oscillators at micrometre scales remains a major roadblock [13][14][15][16] towards the development of fully-autonomous microrobots [17][18][19][20]. Here, we report the emergence of a lowfrequency relaxation oscillator from a simple collective of active microparticles interacting at the air-liquid interface of a hydrogen peroxide drop. Their collective oscillations form chemomechanical [21] and electrochemical [22] limit cycles that enable the transduction of ambient chemical energy into periodic mechanical motion and on-board electrical currents. Surprisingly, the collective can oscillate robustly even as more particles are introduced, but only when we add a single particle with modified reactivity to intentionally break the system's permutation symmetry [23]. We explain such emergent order through a novel thermodynamic mechanism for asymmetry-induced order [24, 25]. The energy harvested from the stabilized oscillations enables the use of on-board electronic components, which we demonstrate by cyclically and synchronously driving a microrobotic arm [26][27][28]. This work highlights a new strategy for achieving low-frequency oscillations at the microscale that are otherwise difficult to observe outside of natural systems, paving the way for future microrobotic autonomy.