Magnetic Soret effect: Application of the ferrofluid dynamics theory

The ferrofluid dynamics theory is applied to thermodiffusive problems in magnetic fluids in the presence of magnetic fields. The analytical form for the magnetic part of the chemical potential and the most general expression of the mass flux are given. By employing these results to experiments, global Soret coefficients in agreement with measurements are determined. Also an estimate for a hitherto unknown transport coefficient is made.Comment: 7 pages, 2 figure

Capillary-gravity wave resistance in ordinary and magnetic fluids

Wave resistance is the drag force associated to the emission of waves by a moving disturbance at a fluid free surface. In the case of capillary-gravity waves it undergoes a transition from zero to a finite value as the speed of the disturbance is increased. For the first time an experiment is designed in order to obtain the wave resistance as a function of speed. The effect of viscosity is explored, and a magnetic fluid is used to extend the available range of critical speeds. The threshold values are in good agreement with the proposed theory. Contrary to the theoretical model, however, the measured wave resistance reveals a non monotonic speed dependence after the threshold.Comment: 12 pages, 4 figures, 1 table, submitted to Physical Review Letter

Magnetization of rotating ferrofluids: the effect of polydispersity

The influence of polydispersity on the magnetization is analyzed in a nonequilibrium situation where a cylindrical ferrofluid column is enforced to rotate with constant frequency like a rigid body in a homogeneous magnetic field that is applied perpendicular to the cylinder axis. Then, the magnetization and the internal magnetic field are not longer parallel to each other and their directions differ from that of the applied magnetic field. Experimental results on the transverse magnetization component perpendicular to the applied field are compared and analyzed as functions of rotation frequency and field strength with different polydisperse Debye models that take into account the polydispersity in different ways and to a varying degree.Comment: 11 pages, 7 figures, to be published in Journal of Physics

Onset of Wave Drag due to Generation of Capillary-Gravity Waves by a Moving Object as a Critical Phenomenon

The onset of the {\em wave resistance}, via generation of capillary gravity waves, of a small object moving with velocity $V$, is investigated experimentally. Due to the existence of a minimum phase velocity $V_c$ for surface waves, the problem is similar to the generation of rotons in superfluid helium near their minimum. In both cases waves or rotons are produced at $V>V_c$ due to {\em Cherenkov radiation}. We find that the transition to the wave drag state is continuous: in the vicinity of the bifurcation the wave resistance force is proportional to $\sqrt{V-V_c}$ for various fluids.Comment: 4 pages, 7 figure

Ferrofluids as thermal ratchets

Colloidal suspensions of ferromagnetic nano-particles, so-called ferrofluids, are shown to be suitable systems to demonstrate and investigate thermal ratchet behavior: By rectifying thermal fluctuations, angular momentum is transferred to a resting ferrofluid from an oscillating magnetic field without net rotating component. Via viscous coupling the noise driven rotation of the microscopic ferromagnetic grains is transmitted to the carrier liquid to yield a macroscopic torque. For a simple setup we analyze the rotation of the ferrofluid theoretically and show that the results are compatible with the outcome of a simple demonstration experiment.Comment: 4 pages, 3 figures, corrected version, improved figures, to be published in Phys. Rev. Let

Rolling ferrofluid drop on the surface of a liquid

We report on the controlled transport of drops of magnetic liquid, which are swimming on top of a non-magnetic liquid layer. A magnetic field which is rotating in a vertical plane creates a torque on the drop. Due to surface stresses within the immiscible liquid beneath, the drop is propelled forward. We measure the drop speed for different field amplitudes, field frequencies and drop volumes. Simplifying theoretical models describe the drop either as a solid sphere with a Navier slip boundary condition, or as a liquid half-sphere. An analytical expression for the drop speed is obtained which is free of any fitting parameters and is well in accordance with the experimental measurements. Possible microfluidic applications of the rolling drop are also discussed

Influence of structural and magnetic properties in the heating performance of multicore bioferrofluids

Biomedical applications of superparamagnetic iron oxide particles have been of interest for quite a number of years. Recent developments show that multifunctionality can be efficiently achieved using polymers to coat the particles and to provide anchoring elements to their surface. This leads to the formation of nanobeads with a reduced number of particles trapped by the polymeric structure. While the magnetothermic behavior of isolated nanoparticles has been a subject of interest over the past several years, multicore magnetic nanobeads have thus far not received the same attention. The influence of structural and magnetic properties in the hyperthermia performance of a series of magnetic fluids designed for biomedical purposes is studied here. The fluids are made of maghemite multicore polymeric beads, with variable nanoparticle size and hydrodynamic size, dispersed in a buffer solution. The specific loss power (SLP) was measured from 5 to 100 kHz with a field intensity of 21.8 kA/m. SLP increases with increasing magnetic core size, reaching 32 W/g Fe2O3 at 100 kHz for 16.2 nm. Within the framework of the linear response theory, a graphical construction is proposed to describe the interplay of both size distributions and magnetic properties in the heating performance of such fluids in a given frequency range. Furthermore, a numerical model is developed to calculate the spare contribution of Neel and Brown relaxation mechanisms to SLP, which gives a fair reproduction of the experimental data

Comparative study of cytotoxicity of ferromagnetic nanoparticles and magnetitecontaining polyelectrolyte microcapsules

The work was supported by Ministry of Education and Science of the Russian Federation as part of the State task for National Research Mordovia State University, project No. 2952 and the Government of the Russian Federation (grant №14.Z50.31.0004 to support scientific research projects implemented under the supervision of leading scientists

Self-oscillatory convection caused by the Soret effect

A new kind of finite-amplitude convective oscillations in binary mixtures caused by the Soret effect is predicted. Such oscillations can be observed in colloidal solutions of nanoparticles whose diffusion coefficient is about two orders of magnitude less than that for molecules. Due to the small particle mobility the concentration gradient sets in so slowly, that Soret convection starts before this gradient reaches its stationary value. Soon after the onset, however, the convective motion mixes the fluid, that results in rapid damping of the motion. The concentration gradient is then formed again and the process is repeated. A period of these self-sustained oscillations is estimated for real magnetic colloids in the Bénard configuration