The theory of Brownian motion of a particle in a viscoelastic Jeffreys fluid is extended for the case of rotational motion. The employed rheological model combines two viscous mechanisms (instantaneous and retarded) and, in contrast to the Maxwell model, does not produce artifacts and works robustly when applied to the diffusion of tracer particles in real complex fluids. With the aid of this model, specific features of the dynamic susceptibility of a magnetic Jeffreys suspension and the viscous power losses induced by an ac field are analyzed and conclusions are derived that are valid for active microrheology and magnetic hyperthermia. In general, it is shown that the developed phenomenology provides an archetypal “frame” for a number of mesoscopic models used to describe confined random walk transport processes in a variety of systems of both biological and inorganic origin.

Key parameters were identified to obtain long-term stable ionic liquid-based colloids.

Magnetic droplets obtained by induced phase separation in a magnetic colloid show a large variety of shapes when exposed to an external field. However, the description of the shapes is often limited. Here, we formulate an algorithm based on three-dimensional boundary-integral equations for strongly magnetic droplets in a high-frequency rotating magnetic field, allowing us to find their figures of equilibrium in three dimensions. The algorithm is justified by a series of comparisons with known analytical results. We compare the calculated equilibrium shapes with experimental observations and find a good agreement. The main features of these observations are the oblate–prolate transition, the flattening of prolate shapes with the increase of magnetic field strength and the formation of starfish-like equilibrium shapes. We show both numerically and in experiments that the magnetic droplet behaviour may be described with a triaxial ellipsoid approximation. Directions for further research are mentioned, including the dipolar interaction contribution to the surface tension of the magnetic droplets, accounting for the large viscosity contrast between the magnetic droplet and the surrounding fluid.

Thermodiffusion of different ferrite nanoparticles (NPs), ∼10 nm in diameter, is explored in tailor-made aqueous dispersions stabilized by electrostatic interparticle interactions. In the dispersions, electrosteric repulsion is the dominant force, which is tuned by an osmotic-stress technique, i.e. controlling of osmotic pressure Π, pH and ionic strength. It is then possible to map Π and the NPs’ osmotic compressibility χ in the dispersion with a Carnahan–Starling formalism of effective hard spheres (larger than the NPs’ core). The NPs are here dispersed with two different surface ionic species, either at pH ∼ 2 or 7, leading to a surface charge, either positive or negative. Their Ludwig–Soret S_T coefficient together with their mass diffusion D_m coefficient are determined experimentally by forced Rayleigh scattering. All probed NPs display a thermophilic behavior (S_T < 0) regardless of the ionic species used to cover the surface. We determine the NPs’ Eastman entropy of transfer and the Seebeck (thermoelectric) contribution to the measured Ludwig–Soret coefficient in these ionic dispersions. The NPs’ Eastman entropy of transfer Ŝ_NP is interpreted through the electrostatic and hydration contributions of the ionic shell surrounding the NPs.

An analytical model describing the thermoelectric potential production in magnetic nanofluids (dispersions of magnetic and charged colloidal particles in liquid media) is presented. The two major entropy sources, the thermogalvanic and thermodiffusion processes are considered. The thermodiffusion term is described in terms of three physical parameters; the diffusion coefficient, the Eastman entropy of transfer and the electrophoretic charge number of colloidal particles, which all depend on the particle concentration and the applied magnetic field strength and direction. The results are combined with well-known formulation of thermoelectric potential in thermogalvanic cells and compared to the recent observation of Seebeck coefficient enhancement/diminution in magnetic nanofluids in polar media.