Thermoelectric effects at the nanoscale

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In recent years it has become clear that the thermal forces on charged colloids are to a large extent determined by thermoelectricity. In the bulk, the thermoelectric or Seebeck field is proportional to the applied temperature gradient. Both sign and magnitude of the Seebeck coefficient depend on the electrolyte composition and the connected effects explain a wealth of current experiments on colloidal suspensions. The perspectives of the thermoelectric effects in solution are, however, much wider than currently explored.Thermoelectric effects are, for example, highly relevant for biotechnological and microfluidic applications, where selective colloidal transport, size separation, molecular trapping and confinement are required. Such applications become even more appealing when considering that the required heating for such thermoelectric processes can be supplied by taking advantage of the strong plasmonic interaction of noble metal structures with light. This leads to very strong local temperature gradients, which will allow for a new type of optically controlled micro- and nanofluidics in future applications. This project thus proposes to explore in a unique combined theoretical and experimental effort, the thermoelectric properties at the nano- and micro-scale in an electrolyte solution. There are two main objectives: The first one is to better understand the forces operating in the self-propulsion of hot Janus particles. The second one aims at the design and realization of thermally generated electric fields in confined geometries and nanostructures. On the theoretical side we have to solve the coupled thermo-electro-osmotic equations relating the salt-ion currents and the Seebeck field. Then the particle motility is obtained from plugging the resulting thermodynamic forces in the Stokes equation. As main results we expect to determine the charge distribution in the vicinity of a hot particle, in particular the net thermo-charge and the dipole moment, and the resulting translational and rotational motion. We intend to work out possible microfluidic applications for colloidal transport and separation by size.The experiments proposed in this project are directly related to the theoretical tasks. They focus on the study of the influence of thermoelectric effects on the motion of noble metal and noble metal capped Janus particles, which are heated by optical means. The experiments involve advanced particle tracking techniques, which are combined with active particle manipulation, such as the recently developed photon nudging. The experimental studies will be completed by an investigation of the electric field distribution around mobile and immobile heated metal nanostructures in electrolyte solution, which will provide the fundamental means to develop new structures for the generation of freely configurable thermoelectric fields for micro- and nano-manipulation.