Measurements and simulations of thermoplasmonically induced Marangoni flows

Licentiate thesis, 2023

Particle transport in microfluidic environments is often dominated by slow diffusion near interfaces. However, by inducing localized fluid flow, it is possible to actively transport suspended nano-objects in confined spaces. One promising method for achieving precise and dynamic control over fluid flow on the microscale is to use photothermal effects based on the illumination of plasmonic metal nanoparticles, which exhibit very high optical absorption for light wavelengths near resonance. The particles can thus be used as nanoscale heat sources that locally increase the temperature of the surrounding fluid, resulting in processes such as thermophoresis, convection, and vapor bubble generation. This thesis focuses on the latter effect and the associated bubble nucleation and thermal Marangoni convection processes.
Marangoni flows result from the surface tension gradient that establishes on a thermoplasmonically induced vapor bubble at equilibrium. However, in addition to this, strong flow transients appear as a bubble is created and expands. Both phenomena lead to similar flow profiles. Here it is shown that the direction of these flows can be controlled by manipulating the temperature gradient on the surface of the bubble. Specifically, it is demonstrated that the direction of the strong transient flows around a nanobubble can be reverted by breaking the photothermal symmetry using two unequal nearby arrays of plasmonic nanoparticles. Furthermore, we investigate the possibility of remotely controlling the flow direction by turning the incident light polarization. The results are based on vectorial flow measurements using optical force microscopy supported by extensive flow profile simulations.