Light-Ultrasound Driven Collective "Firework" Behavior of Nanomotors

Abstract

It is of great interest and big challenge to control the collective behaviors of nanomotors to mimic the aggregation/separation behavior of biological systems. Here, a light-acoustic combined method is proposed to control the aggregation/separation of artificial nanomotors. It is shown that nanomotors aggregate at the pressure node in acoustic field and afterward present a collective "firework" separation behavior induced by light irradiation. The collective behavior is found to be applicable for metallic materials and polymers even different light wavelengths are used. Physical insights on the collective firework behavior resulting from the change of acoustic streaming caused by optical force are provided. It is found that diffusion velocity and diffusion region of cluster can be controlled by adjusting light intensity and acoustic excitation voltage, and irradiation direction, respectively. This harmless, controllable, and widely applicable method provides new possibilities for groups of nanomachines, drug release, and cargo transport in nanomedicine and nanosensors.

Keywords: aggregation; collective behavior; diffusion; firework behavior; nanomotors.

Figure 1

Schematic demonstration of experimental details and “firework” behaviors of light driven acoustic nanomotors. a) Schematic of the light combined acoustic cell for Au nanomotors. b) Schematic of the experimental processes. c) Time‐lapse snapshots of behaviors of Au naomotors. Excitation voltage, frequency, and light intensity are 10 V, 3 MHz, and 17 mW mm⁻², respectively. Scale bar: 30 µm.

Figure 2

Illustration of forces balance. a) Schematic of Au nanomotors in acoustic field (Top view). b) Schematic of Au nanomotors in combined acoustic and light field (Top view). c) The force balance diagram on the cluster and Au nanomotors on the edge or close proximity of the cluster. d) The force balance diagram on the cluster and Au nanomotors with additional optical radiation force, F _o.

Figure 3

Streaming line around the cluster and the trajectory of tracers. a) Field vector for the acoustic streaming around nanomotor cluster without and with additional optical radiation force. b) “Firework” behaviors of tracer particles in acoustic field without and with light irradiation (with excitation voltage, frequency, and light intensity of 10 V, 3 MHz, and 17 mW mm⁻², respectively), scale bar: 15 µm. c) Influence of light intensity on diffusion velocity of Au nanomotors (with excitation voltage of 6 V and frequency of 3 MHz). d) Influence of excitation voltage on diffusion velocity of Au nanomotors (with frequency of 3 MHz and light intensity of 17 mW mm⁻²).

Figure 4

Varied “firework” behaviors of nanomotors prepared with different materials. a,c,e) Aggregation of Ag, PPY, and SiO₂ nanomotors before and after light irradiation in acoustic field. Excitation voltage, frequency, and light intensity are 10 V, 3 MHz, and 17 mW mm⁻², respectively. Scale: bar 20 µm. b,d,f) Illustration of “firework” behavior of Pd, PPY, and SiO₂ nanomotors in combined acoustic and light field. g) Diffusion velocity of different nanomotors with light irradiation in acoustic field. h) Area change percentage of different nanomotors cluster before and after light irradiation in acoustic field. i) Ratio of diffusion velocity and density of nanomotors with different materials.

Figure 5

Lights driven collective “firework” of PPY nanomotors for different lasers. a) Blue laser b) Green laser c) Red laser. All experiments were performed under 10 mW mm⁻² light intensity, 10 V excitation voltage, and 3 MHz frequency. Scale bar: 20 µm.

Figure 6

Controlling diffusion position of PPY nanomotors cluster by altering the light direction. a,d,g) Schematic of light irradiation on PPY nanomotors cluster at different direction (top right, top, top left). b,e,h) Illustration of “firework” behaviors of PPY nanomotors cluster at different position in acoustic field. c,f,i) Experimental results of “firework” behavior of PPY nanomotors cluster at different position in acoustic field. Excitation voltage, frequency and light intensity were 10 V, 3 MHz, and 17 mW mm⁻², respectively. Scale bar: 20 µm.