EPJ E Colloquium - Thermophoresis and thermal orientation of Janus nanoparticles in thermal fields

Thermal gradients induce thermodiffusion in aqueous solutions and liquid mixtures and thermophoretic forces that drive the motion of colloids towards hot or cold regions. The Soret coefficient quantifies the strength of the thermophoretic force and varies with temperature, colloid mass and diameter, and colloid-solvent interactions. Janus colloids (JCs ) are nanoparticles with heterogeneous compositions and two contrasting properties, or "two faces" like the Roman god Janus. For example, in spherical JCs, one hemisphere might be hydrophilic and the other hydrophobic. The interest in JCs has grown steadily given their applicability in materials science. While the behaviour of JCs under equilibrium conditions has been explored, their response to thermal gradients is still not fully understood. Explaining the behaviour of JCs in a thermal field might expand their use in materials science and biomedical applications.

Due to their heterogeneous composition JCs feature an internal mass distribution, which may couple to a thermal field, resulting in novel physical effects. In a new Colloquium published in EPJ E, authors from Imperial College London and from University of Cantabria, Santander, demonstrate that such coupling induces the alignment of the JCs in a thermal field and modifies the strength of the thermophoretic force. They also show that the Janus colloid alignment with the thermal field depends very strongly on the thermodynamic conditions and we predict complete alignment using weak thermal fields in the vicinity of a critical point. Overall, this work demonstrates that colloid mass anisotropy plays a crucial role in soft matter thermophoresis providing an innovative approach to manipulating colloidal suspensions using thermal fields.

This Colloquium is part of the Topical Issue Thermal non-equilibrium phenomena in fluid mixtures. Guest editors: Fernando Bresme, Bjørn Hafskjold, Werner Köhler, Francois Montel

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