We study the temperature field and the resulting flow pattern in the vicinity of a heated metal-capped Janus particle. If its thickness exceeds about 10 nm, the cap forms an isotherm and the flow pattern comprises a quadrupolar term that decays with the square of the inverse distance $$\sim${}$${r}^{$-${}2}$. For much thinner caps the velocity varies as $$\sim${}$${r}^{$-${}3}$. These findings could be relevant for collective effects in dense suspensions and for the circular tracer motion observed recently in the vicinity of a tethered Janus particle.

We study the Soret effect of charged polystyrene particles as a function of temperature and electrolyte composition. As a main result we find that the Soret coefficient is determined by charge effects, and that non-ionic contributions are small. In view of the well-known electric-double layer interactions, our thermal field-flow fractionation data lead us to the conclusion that the Soret effect originates to a large extent from diffusiophoresis in the salt gradient and from the electrolyte Seebeck effect, both of which show strong specific-ion effects. Moreover, we find that thermophoresis of polystyrene beads is fundamentally different from proteins and aqueous polymer solutions, which show a strong non-ionic contribution.

Thermally driven colloidal transport is, to a large extent, due to the thermoelectric or Seebeck effect of the charged solution. We show that, contrary to the generally adopted single-particle picture, the transport coefficient depends on the colloidal concentration. For solutions that are dilute in the hydrodynamic sense, collective effects may significantly affect the thermophoretic mobility. Our results provide an explanation for recent experimental observations on polyelectrolytes and charged particles and suggest that for charged colloids collective behavior is the rule rather than the exception.

We propose a novel actuation mechanism for colloids, which is based on the Seebeck effect of the electrolyte solution: Laser heating of a nonionic particle accumulates in its vicinity a net charge Q, which is proportional to the excess temperature at the particle surface. The corresponding long-range thermoelectric field E$\propto${}1/r2 provides a tool for controlled interactions with nearby beads or with additional molecular solutes. An external field Eext drags the thermocharged particle at a velocity that depends on its size and absorption properties; the latter point could be particularly relevant for separating carbon nanotubes according to their electronic band structure.

The interaction between two chemically identical charge-regulated surfaces is studied using the classical density functional theory.

We discuss the thermoelectric properties of a locally heated micron-size volume in an electrolyte solution. We find that such a hot spot carries a net charge Q which, for an excess temperature of 10 K, may attain hundreds of elementary charges. The corresponding Seebeck electric field E increases linearly with the radius r inside the heated area, then goes through a maximum, and decays as 1/r² at larger distances. Our results could be relevant to optothermal actuation of electrolytes and colloidal suspensions.