Hydrodynamic instabilities in ferronematics

Anomalous transport of magnetic colloids in a liquid crystal-magnetic colloid mixture

We report an extensive molecular dynamics study on the translational dynamics of a hybrid system composed of dipolar soft spheres (DSS), representing ferromagnetic particles, suspended in a liquid crystal (LC) matrix. We observe that the LC matrix strongly modifies the dynamics of the DSS. In the isotropic regime, the DSS show a crossover from subdiffusive to normal diffusive behavior at long times, with an increase of the subdiffusive regime as the dipolar coupling strength is increased. In the nematic regime, the LC matrix, due to the collective reorientation of LC particles, imposes a cylindrical confinement on the DSS chains. This leads to a diffusive dynamics of DSS along the nematic director and a subdiffusive dynamics (with an exponent of ∼0.5) in the perpendicular direction. The confinement provided by the LC matrix is also reflected by the oscillatory behavior of the components of the velocity autocorrelation function of the DSS in the nematic phase.

Alternating current magnetic susceptibility of a ferronematic

We report on experimental studies focusing on the dynamic ac magnetic susceptibility of a ferronematic. It has been shown recently, that in the isotropic phase of a ferronematic, a weak dc bias magnetic field of a few oersteds increases the ac magnetic susceptibility. This increment vanishes irreversibly if the substance is cooled down to the nematic phase, but can be reinduced by applying the dc bias field again in the isotropic phase [Tomašovičová, N. et al. Soft Matter2016, 12, 5780-5786]. The effect has no analogue in the neat host liquid crystal. Here, we demonstrate that by doubling the concentration of the magnetic nanoparticles, the range of the dc bias magnetic field to which the ferronematic is sensitive without saturation can be increased by about two orders of magnitude. This finding paves a way to application possibilities, such as low magnetic field sensors, or basic logical elements for information storage.

Influence of a magnetic field on the Soret-effect-dominated thermal convection in ferrofluids

We investigate theoretically the influence of a magnetic field on the growth of convective rolls in a slab of ferrofluid subject to a vertical temperature gradient. Due to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, a dynamic description as a binary mixture is appropriate. We first derive a comprehensive set of magnetic field effects in the statics and dynamics of binary mixtures. Among those, the two prominent ones, the Kelvin force and magnetophoresis, are studied in detail with respect to their influence on the thermal convection behavior. The main difference from the case without an external field rests in the importance of the boundary layers, which influence the bulk problem through the magnetic boundary conditions. We discuss an analytical approximate solution and compare it with a numerical multimode expansion.