Coupled oscillation and spinning of photothermal particles in Marangoni optical traps

Cyclic actuation is critical for driving motion and transport in living systems, ranging from oscillatory motion of bacterial flagella to the rhythmic gait of terrestrial animals. These processes often rely on dynamic and responsive networks of oscillators—a regulatory control system that is challenging to replicate in synthetic active matter. Here, we describe a versatile platform of light-driven active particles with interaction geometries that can be reconfigured on demand, enabling the construction of oscillator and spinner networks. We employ optically induced Marangoni trapping of particles confined to an air–water interface and subjected to patterned illumination. Thermal interactions among multiple particles give rise to complex coupled oscillatory and rotational motions, thus opening frontiers in the design of reconfigurable, multiparticle networks exhibiting collective behavior.

Motion in living systems often relies on coupled dynamics of oscillatory and active networks (1–4). Examples spanning a wide range of size scales include the swimming of lamprey coordinated by oscillatory neuronal networks and the synchronized rotation of flagella coupled by hydrodynamic interactions (5, 6). Synthetic platforms that mimic key elements of these natural systems promise new fundamental understanding and next-generation active material systems: recent advances include chemomechanical oscillators built upon the Belousov–Zhabotinsky reaction (7, 8), synthetic genetic circuits (9), self-regulatory microfluidic devices (10–12), and organic chemical reactions (13). However, in most synthetic systems that rely on complex chemical or biochemical reactions, the nature of interactions between different elements is typically hardwired with limited ability to dynamically adjust coupling parameters. Similar limitations on geometric reconfigurability and tunability of coupling are found in physical particle oscillator networks, such as in acoustically levitated oscillators (14). In contrast, the effectiveness of natural control systems stems from oscillatory elements that can reconfigure their interactions rapidly, as observed in transitions of insect flight modes or animal gait patterns (2, 15). Therefore, developing simple yet versatile experimental platforms that mimic adaptive and active living systems with synthetic components remains a critical challenge.As an important step in this direction, we describe particles at fluid interfaces, driven by two-dimensional (2D) patterns of light, as a powerful approach to coupled motion across easily tunable geometries. In nature, insects such as Microvelia secrete chemicals to generate gradients in surface tension and propel themselves across water surfaces (i.e., Marangoni propulsion) (16). This concept has been exploited to drive translation and rotation of objects on fluid surfaces using both chemical (17, 18) and light-induced photothermal (19–21) gradients. Some of us previously demonstrated how photothermal Marangoni forces incite oscillatory motion of interfacially adsorbed particles, but as this approach relied on the use of curved droplets and permitted only very coarse “patterns” of light defined by the field of view of a microscope objective, it was not amenable to the study of multiparticle systems with well-controlled geometries (22). Indeed, the rational design of systems exhibiting reconfigurable Marangoni interactions between multiple particles remains elusive. In contrast, optical tweezers operating by momentum transfer offer spatial control over trapped objects in almost limitless arrangements (23–25), enabling oscillatory motion of colloids via continuous repositioning of traps (26) and sustained rotation of particles by transferring spin angular momentum (27). However, scaling this platform to larger collections of particles is challenging, since the high-intensity light (∼10⁶ W/cm²) required (28) becomes problematic over large areas and can drive large temperature increases. Here, we show that the Marangoni forces developed during illumination of hydrogel nanocomposite disks (HNDs), consisting of polymer gels with patterned gold nanoparticles (Au NPs), can incite a gamut of coordinated and reconfigurable multiparticle behavior at very modest light intensities (∼1 W/cm²). Arbitrary gray-scale light patterns provide optical boundary conditions to define coupled systems of HNDs, culminating in a versatile materials toolbox of active matter exhibiting complex rhythmic motion.