Tailored optical propulsion forces for controlled transport of resonant gold nanoparticles and associated thermal convective fluid flows

Optofluidics: Light forces for controlling heat nanosources and fluid flows New opportunities for controlling the motion of laser-heated gold nanoparticles and the associated convection currents in the surrounding fluid are achieved by using tailored light forces. The procedure has been developed by Jose Rodrigo and colleagues at the Complutense University of Madrid. It is based on illuminating nanospheres at wavelengths that create resonant oscillations in surface electrons, a phenomenon known as plasmon resonance. This allows sufficient absorbance of light for the nanospheres to become a heat source. The tailored light forces gave fine control over the direction and speed of the hot nanospheres, both individually and in groups. The refined ability to form, merge and split clusters of nanoparticles could offer new applications in optofluidics-the science of controlling fluids and materials they contain with light. Noble metal nanoparticles illuminated at their plasmonic resonance wavelength turn into heat nanosources. This phenomenon has prompted the development of numerous applications in science and technology. Simultaneous optical manipulation of such resonant nanoparticles could certainly extend the functionality and potential applications of optothermal tools. In this article, we experimentally demonstrate optical transport of single and multiple resonant nanoparticles (colloidal gold spheres of radius 200 nm) directed by tailored transverse phase-gradient forces propelling them around a 2D optical trap. We show how the phase-gradient force can be designed to efficiently change the speed of the nanoparticles. We have found that multiple hot nanoparticles assemble in the form of a quasi-stable group whose motion around the laser trap is also controlled by such optical propulsion forces. This assembly experiences a significant increase in the local temperature, which creates an optothermal convective fluid flow dragging tracer particles into the assembly. Thus, the created assembly is a moving heat source controlled by the propulsion force, enabling indirect control of fluid flows as a micro-optofluidic tool. The existence of these flows, probably caused by the temperature-induced Marangoni effect at the liquid water/superheated water interface, is confirmed by tracking free tracer particles migrating towards the assembly. We propose a straightforward method to control the assembly size, and therefore its temperature, by using a nonuniform optical propelling force that induces the splitting or merging of the group of nanoparticles. We envision further development of microscale optofluidic tools based on these achievements.