Workshop description

The aim of this workshop is to bring together scientists who work on optically heated colloidal nanoparticles for different purpose, and who belong to different communities, thermal sciences, soft matter/statistical physics and nano-optics. These researchers do no not have the opportunity to meet, as there is no conference organized on the subject. The field is clearly interdisciplinary, and we want to break the frontiers between the above mentionned communities. We think that only breaking the frontiers will help in identifying possible advances in the field. A three days workshop is well designed for this purpose.

Topics addressed during this workshop include:

-thermophoresis, thermoosmosis, Kapitza resistance

-hot microswimmers, collective effects

-solar energy conversion by nanofluids

-laser heated nanoparticles for biomedical applications

We will invite experimentalists so as to outline open questions that need to be addressed by computationalists and theoreticians. Exemples of open questions include : competition between heat transfer and external fields, collective effects, heat transfer phase change. Specifically, we will stimulate discussion among the participants to address the following questions :

-How to design hot microswimmers with high propelling speed ?
-Can we transfer heat faster than normal diffusion at the nanoscale ?
-How to optimize the efficiency of solar receivers ?
-Can we use laser heated nanoparticles to destruct cancer cells?

The workshop will be considered a success if partial answers to the listed questions are foreseen. In particular, the interaction with experimentalists will permit to identify clear opportunities for computational modelling. Another issue that will be addressed by the workshop is the necessary coupling between different length and time scales, from picoseconds to seconds.

Obviously, the field has strong societal and economic impacts. There is a tremendous need to define solutions for thermal management at the nanoscale, and in particular for the design of liquid coolers. Light irradiated nanoparticles may be used to sterilize and treat water in remote areas. Metallic particles have also been shown to selectively destroy malignant cells when iluminated with high energy.

Nanoscale heat transfer has attracted the attention of physicists, as it raises a number of new questions regarding thermal transport : violation of Fourier's law, balistic transport, phoretic motion, enhanced role of the interfaces. These questions arose essentially after the development of nanotechnologies during the nineties. In particular, heat generation by metallic nanoparticles in solution have many applications in a broad range of disciplines : biomedecine (thermal therapy treatments), physics (phoretic swimmers), chemistry (enhanced reactions) or engineering (nanofluids, energy conversion devices). Most of these applications rely on the properties of nanoparticles to strongly absorb light leading to controlled local heating. In turn, temperature gradients may be brought to very high levels 10^9 K/m, and possibly drive local phase change. Experimentally, local mapping of the temperature field is often difficult, given the small time and length scales involved.

Computational studies conversely may reach nanoscale-picosecond timescales, and certainly help in understanding the physics of interfacial thermal transport. In order to match the experimental scales a multiscale approach combining molecular models and continuum approaches is highly suitable.
The broad range of applications makes the situation even more complex since each community develops its own models in a rapidly extending field.

Different physical mechanisms compete in the description of thermal transport in colloidal suspensions. These include thermophoresis [1], ie the drift diffusion motion of colloids in an external temperature gradient, but also the Kapitza resistance [2,3]. Recently, there has been a growing interest in the thermal transport properties of metallic colloidal suspensions. Nanofluids have caught the attention of physicists, as they display outstanding thermal conductivity which makes them interesting candidates for liquid coolers [4].
Metallic nanoparticles have also the unique property to strongly absorb the energy from an electromagnetic radiation. This opens the way to create nanosources of heat in a liquid environment [5].
Optically heated nanoparticles represent a simple system driven out of equilibrium, that may serve to probe thermal non-equilibrium statistical physics, including tests of the fluctuation theorems, or the generalization of the theory of Brownian motion to colloids driven out of equilibrium [6]. Metallic nanoparticles could be used as well as local probes of temperatures.
If the nanoparticles are made asymmmetrical, the interaction with light may be exploited to design microswimmers [7,8].
An even richer phenomenology may be revealed by the coupling of thermal fields with other fields, including electric fields [9]. New collective effects also emerge from the mutual interaction of thermally active particles [10].
Lastly, ultrafast boiling has been recently observed experimentally [11] and numerically [12] around intensely heated gold colloidal particles. The generation of fast expanding vapor nanobubbles has important applications in cancer cell therapy.