Design considerations for effective thermal management in mobile nanotweezers

Controlled manipulation of nanoscale objects in fluids is relevant to both fundamental studies and advances in nanotechnology. While standard techniques of nanomanipulation, such as optical and plasmonic tweezers have limitations in simultaneous trapping and transport of nanoscale cargo, magnetically driven plasmonic nanorobots under optical illumination provide a promising solution. These so called mobile nanotweezers[1] (MNT) use strongly localized electromagnetic field near plasmonic nanostructures to trap objects with high efficiency and can simultaneously be driven by magnetic fields to selectively trap, transport and release colloidal cargo. Upon illumination, apart from gradient forces due to local electric field enhancement, additional forces arise from heating generated by absorption of light. Here, we present a method to understand and engineer the thermally induced forces in mobile nanotweezers. The temperature enhancement and associated thermofluidic forces are studied as a function of MNT geometry. Illumination slightly detuned from plasmon resonance frequency produces sufficient field enhancement with negligible generation of heat, and therefore much reduced thermophoretic and convective forces. This allowed us to engineer thermoplasmonic forces in MNTs for enhanced trapping performance.