Opto-Thermocapillary Nanomotors on Solid Substrates

Abstract

Motors that can convert different forms of energy into mechanical work are of profound importance to the development of human societies. The evolution of micromotors has stimulated many advances in drug delivery and microrobotics for futuristic applications in biomedical engineering and nanotechnology. However, further miniaturization of motors toward the nanoscale is still challenging because of the strong Brownian motion of nanomotors in liquid environments. Here, we develop light-driven opto-thermocapillary nanomotors (OTNM) operated on solid substrates where the interference of Brownian motion is effectively suppressed. Specifically, by optically controlling particle-substrate interactions and thermocapillary actuation, we demonstrate the robust orbital rotation of 80 nm gold nanoparticles around a laser beam on a solid substrate. With on-chip operation capability in an ambient environment, our OTNM can serve as light-driven engines to power functional devices at the nanoscale.

Keywords: asymmetry; nanoparticles; optical manipulation; optical nanomotors; optical nanorotors.

Figure 1.

General concept of OTNM. (a) Schematic of OTNM on a solid substrate. (b) Time-resolved dark-field optical images showing the orbital rotation of an 80 nm AuNP. Laser power: 6 mW. Scale bar: 1 μm. (c) Centroid tracking and (d) displacement of the rotating AuNP in (b). The origin of the coordinates is at the center of the laser beam. The curved arrows in (a)−(c) indicate the rotation direction of the nanomotor. The solid lines in (d) correspond to the sinusoidal fitting curves.

Figure 2.

Working principle of OTNM. (a) STEM image of 80 nm AuNPs used in experiments. Scale bar: 100 nm. (b) 3D reconstructed asymmetric AuNP based on STEM images, which is used for numerical simulations. (c) In-plane force analysis of OTNM. F_opt is optical force, F_d refers to resistant drag force, and F_TC,t and F_TC,r are the tangential and radial components of thermocapillary forces, respectively. (d) Simulated temperature distribution for an 80 nm AuNP under 660 nm laser illumination. Scale bar: 50 nm. (e) Close view of the temperature distribution at the AuNP surface. The laser power is 10 mW, and the laser–particle distance is 400 nm. (f) Simulated temperature gradient mapping (bottom view) on the surface of the AuNP under 660 nm laser irradiation (10 mW). The arrows indicate the in-plane temperature gradient parallel to the surface. (g) Calculated total in-plane torques as a function of the orientation angle of the AuNP. The red arrows show two equilibrium orientation angles where the torque equals zero. The black and green arrows indicate the orientation of the AuNP and the rotation direction at a certain orientation, respectively. (h) Calculated total forces in the radial direction (F_r), thermocapillary force in the tangential direction (F_TC,t) that is balanced by the resistant drag force, and the potential in the radial direction (P_r) as a function of laser–particle distance.

Figure 3.

Modeling of the motion of OTNM. (a) At the initial time, an asymmetric AuNP with a random orientation was placed at a laser−particle distance of 550 nm. All forces and torques based on numerical simulations were exerted on the AuNP. The green arrow indicates the orientation of the AuNP. r is the laser–particle distance, and α defines the orientation angle of the AuNP with respect to radial direction (gray dashed arrow). (b, c) The AuNP approached the circular orbit with reorientation at (b) t = 60.6 ms and (c) t = 160.6 ms. v is the instant velocity of the AuNP, and the black dot line shows the trajectory of the AuNP. (d) Steady rotation state of the AuNP.

Figure 4.

Power-dependent rotation behaviors of OTNM. (a) Rotation radius and (b) rotation rate of OTNM as a function of laser power. The shaded areas correspond to the ranges obtained via the calculated theoretical values from different asymmetric AuNPs.