Dynamic behavior of simple magnetic hole systems

Abstract

Experiments on a system of two magnetic holes (nonmagnetic microspheres in ferrofluid) subject to a rotating magnetic field show various types of behavior depending on the driving frequency. For two spheres (holes) mechanically bound together the stable rotation mode at low frequencies is replaced above a critical frequency by a mode with alternating rotation directions. This is described by simple nonlinear equations, and simulations show good quantitative agreement with the experiments. For two free spheres not bound together one observes a transition to a mode in which the spheres undergo both angular and radial motion around the center of mass of the system. Depending on the frequency and the anisotropy of the rotating magnetic field the motion passes through a sequence of states with sphere rotation mode-locked to the driving frequency at diR'erent ratios in good qualitative agreement with simulations. Adding a constant magnetic field normal to the plane causes the minimum sphere separation to increase from contact to a finite value. At high frequencies tlie spheres are influenced by an effective magnetic potential, giving rise to a static particle separation proportional to the normal component of the field.

Magnetic wire-based sensors for the microrheology of complex fluids

“…It was shown that for frequency above the wire undergoes an hydrodynamic instability between two rotation regimes [32,33]. In Regime I, the wire rotates at the same frequency as the field, whereas in Regime II it is animated of asynchronous back-and-forth motions.…”

“…2 illustrate typical temporal evolutions of the wire orientation for viscous (a, b), elastic (c, d) and viscoelastic (e, f) cases. The red lines indicate the average angular velocity Ω = [18,32,33]. For viscous and viscoelastic fluids, Ω is positive whereas it is zero for an elastic solid.…”

Arranging matter by magnetic nanoparticle assemblers

“…The force on each nonmagnetic particle in a magnetic field gradient is proportional to the product of the net fluid magnetization ͗M ជ ͘, with the fluid volume V displaced by the nonmagnetic particle (14,15).…”

“…Magnetic forces on nonmagnetic particles are transmitted through a fluid dispersion of magnetic nanoparticles, such as commercially available ferrofluid. Previously, magnetic forces have been exploited to assemble nonmagnetic particles into somewhat regular structures within the bulk of the fluid, but little control over the assembly process was achieved (14,15). In the present work, we employ magnetization patterns as a template for producing reprogrammable magnetic field maps, which leads to improved control over particle assembly and manipulation.…”

The transition strength from solid to liquid colloidal dipolar clusters in precessing magnetic fields

“…This orientation dependence of the dipolar interactions can be further exploited by using time-dependent magnetic fields with large Mason number [5] that vary on a time scale too fast for the colloids to relax to their instantaneous equilibrium position [3,6,7]. The colloids therefore experience the time-averaged dipolar interactions.…”