Thermoelectric effects at the nanoscale – Hotspot

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 E=S?T is proportional to the applied temperature gradient. Both its sign and magnitude 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.

If the bulk Seebeck effect is well understood, little is known, however, on the thermoelectric properties in complex geometries, for example in the vicinity of a hot Janus particle, and on the nano-scale phenomena that occur close to the heat source. These issues require a detailed knowledge of the thermo-charge that accumulates at the boundary of the heated object.

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 thermoelectric 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. On one side they focus on the study of the influence of thermoelectric effects on the motion of noble metal capes Janus particles, which are heated by optical means. The experiments involve advanced particle tracking strategies, which are combined with active particle manipulation, such as the recently developed photon nudging. Within these experiment we will focus on the detailed contribution of the thermoelectric effects on the motion of single noble metal capped Janus particles. The results of this study will be compared to more complex phenomena, which occur when either two Janus particles are rigidly linked together or interact by the thermal, hydrodynamic and electrostatic fields. This will provide a glimpse on how even more complex structures emerge collective effects. Finally, the experimental studies will be completed by an investigation of the electric field distribution around 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.