Optofluidic Convection around Au Nanorods and Nanospheres by Photothermal Heating: Effect of Nanoparticle Geometry and Boundary Conditions

Laser mediated heat transfer and optofluidic nano-scale convection can be exploited in the photothermal applications of plamonic nanoparticles in different micro-environment. In this work, heat transfer and fluid flow around Au nanospheres and nanorods in water medium under continuous and pulsed wave laser irradiance was investigated using an FEM based numerical framework. Au nanospheres of a wide range of diameter: 40 nm ≤ Diameter (D) ≤ 180 nm and nanorods with varying aspect ratio (1 ≤ Aspect ratio (A) ≤ 5) and orientation (0o ≤ θ ≤ 90o, ϕ = 0o, 90o) with respect to the incident EM radiation were investigated for continuous wave (CW) and pulsed wave laser. Relatively large Au nanorods dimensions similar to the models were also synthesized. The numerical results were validated by comparison with the experimental optical cross-section of the synthesized nanorods. It was found that although nanorods can attain much higher temperature than nanospheres, orientation of a nanorod is an important factor to be carefully considered in applications. In micro-scale spherical and hemispherical confinements (diameter < 14.4 μm) convective velocity fields around nanoparticles were determined for slip and no-slip condition on the confining walls. It was found that slip boundary condition on the confining walls has a moderate effect on the strength of the flow field. While, the effect of the proximity of the nanoparticle from the walls is considerable.