Optical manipulation of micro- and nano-objects is a significant field with broad applications. Laser tweezers, using intensity-gradient forces, provide positional control. However, the addition of phase-gradient propulsion forces (scattering forces) allows for more sophisticated control of particle motion and collective behavior. Previous research has explored assembling and disassembling nanoparticle lattices, creating optical matter, and using optical vortex traps for rotation. Freestyle laser traps enable programmable light-driven transport along arbitrary 3D trajectories. Metal nanoparticles, particularly when illuminated near their plasmon resonance, become efficient heat sources due to enhanced light absorption. This optothermal property has applications in photothermal therapy, drug delivery, and thermal optofluidics. Thermal optofluidics uses optothermal control of fluid motion, often employing resonant plasmonic structures. Fluid flow generation involves complex thermal mechanisms; understanding these would allow for precise control of flow direction, velocity, and extent. Previous studies have shown that increased local temperature of a metal nanoparticle (or assembly) causes convective fluid flow, its configuration influenced by nanoparticle position and chamber width. Optically heating periodic plasmonic nanostructure arrays produces short-range convection. The temperature-induced Marangoni effect, occurring at the liquid water/superheated water interface due to nanobubble formation, can result in significant fluid flow speeds. This work aims to demonstrate stable optical transport of single and multiple gold nanoparticles with controllable speed via tailored phase-gradient forces, using the resonant laser wavelength for programmable motion. It also aims to study the formation and manipulation of a moving heat source that induces significant convective fluid flows, building on previous work focusing on fixed plasmonic structures. This combines optical nanoparticle heating with programmable transport, paving the way for more versatile optofluidics tools.

The introduction extensively reviews relevant literature on optical manipulation techniques, including laser tweezers and their advancements using structured beams to incorporate phase-gradient forces. The review covers applications of these techniques in manipulating various types of particles, assembling nanoparticle structures, and creating optical matter. It also delves into the optothermal properties of metal nanoparticles, focusing on their use as heat sources in various applications such as photothermal therapy and thermal optofluidics. The literature review highlights previous research on photothermal-induced fluid flows, including mechanisms such as the Marangoni effect, and discusses the complexity of controlling these flows at the microscale. The review sets the stage for this study by emphasizing the gap in knowledge regarding the simultaneous control of nanoparticle transport and the resulting fluid flows using tailored optical forces.

The study employed a freestyle trap created by a circularly polarized polymorphic beam (λ₀ = 532 nm) focused into a ring trap (radius R = 4 µm) with uniform intensity. The phase gradient distribution was the primary source of the optical propulsion force. The optical propulsion force Fᵢ(R,φ) is proportional to the product of intensity I(R) and phase gradient (∂φ/∂φ). Three different phase-gradient profiles ψ(φ) were used: a uniform ξ-trap, a 2-sector ξ₁,₂-trap with different constant propulsion forces in two sectors, and a ξ(φ)-trap with a linearly increasing phase gradient. The laser trap was positioned near a glass coverslip for stable axial confinement of the nanoparticles. The motion of single and multiple 200 nm radius gold nanoparticles was tracked. The light power was 40 mW (uniform ξ-trap) and 0.73 mW/µm² (2-sector and ξ(φ)-traps). Nanoparticle dynamics were analyzed experimentally and compared to numerical simulations that incorporated Brownian thermal noise. The simulations predicted nanoparticle speed and trajectory, taking into account the temperature increase of the nanoparticles and the resulting viscosity reduction of the surrounding water. The temperature increase was estimated to be ~31 K (uniform ξ-trap) and ~40 K (other traps). Kinetic diagrams and speed profiles were generated to analyze the temporal evolution of nanoparticle motion under different phase-gradient profiles.

The experimental results showed that the nanoparticle speed was controllable by adjusting the phase gradient strength. In the uniform ξ-trap, the nanoparticle moved around the ring with a mean speed of 7 µm/s, consistent with numerical simulation predictions (9.4 µm/s). The 2-sector ξ₁,₂-trap demonstrated that the nanoparticle speed varied in the two sectors according to the designed phase gradient, with the nanoparticle being approximately 2.86 times faster in the higher phase gradient sector. Numerical simulations predicted a 2.47-fold speed increase. The ξ(φ)-trap exhibited an almost linear increase in nanoparticle speed as a function of the polar angle, as predicted by simulations. When multiple nanoparticles were present, they self-organized into a stable group acting as a moving heat source that induced significant convective fluid flow, confirmed by the movement of tracer particles. This group's speed and trajectory were also controlled by the optical propulsion force. The size (and therefore temperature) of the nanoparticle group could be controlled by manipulating the optical propulsion force, causing the group to split or merge.

The findings demonstrate precise control over nanoparticle transport and the associated fluid flow using tailored phase-gradient optical forces. The agreement between experimental results and numerical simulations validates the theoretical understanding of the optical propulsion forces and their effect on nanoparticle motion. The ability to control the speed of individual nanoparticles and the size and motion of nanoparticle assemblies offers significant potential for advanced micro-optofluidic applications. The generation of a controllable moving heat source enables indirect manipulation of fluid flows, opening possibilities for diverse applications in microfluidics and other fields. This study extends beyond previous work that mainly focused on optofluidic effects from fixed structures.

This study successfully demonstrated controlled optical transport of single and multiple resonant gold nanoparticles using tailored phase-gradient forces. The ability to control the speed of individual nanoparticles and to create and manipulate a moving heat source that induces convective fluid flow opens promising avenues for developing sophisticated micro-optofluidic devices. Future research could explore expanding this approach to three dimensions and integrating it with other microfluidic techniques.

The study focused on a 2D ring trap geometry. Extending the methodology to 3D geometries and more complex trajectories would enhance the versatility of the approach. Further investigations could explore the precise mechanisms governing the temperature-induced Marangoni effect in the system and its dependence on various experimental parameters.