Non-equilibrium dynamics of magnetically anisotropic particles under oscillating fields

Abstract

In this article, we demonstrate how magnetic anisotropy of colloidal particles can give rise to unusual dynamics and controllable rearrangements under time-dependent fields. As an example, we study spherical particles with a radially off-centered net magnetic moment in an oscillating field. Based on complementary data from a numerical simulation of spheres with shifted dipole and experimental observations from particles with hemispherical ferromagnetic coating, it is explained on a two particle basis how this magnetic anisotropy causes nontrivial rotational motion and magnetic reorientation. We further present the behavior of larger ensembles of coated particles. It illustrates the potential for controlled reconfiguration based on the presented two-particle dynamics.

Non-monotonic response of a sheared magnetic liquid crystal to a continuously increasing external field

“…The anisotropic shape of the MNPs also offers the possibility of aligning the magnetic dipole along different axes of the particles. For further studies, one can consider embedding the magnetic dipoles along the shortest axis, as done in recent experiments 13 , or include an offset from the center as done for spherical particles [99][100][101][102][103][104] . Depending on different embeddings, by increasing the dipole-dipole interaction strength we speculate to find intriguing structures where an external magnetic field can play an important role in stabilizing or destabilizing them.…”

A Two‐Parameter Model for Colloidal Particles with an Extended Magnetic Cap

“…Thus, the magnetization is rotationally symmetric and extends over the whole cap and is ferromagnetically ordered perpendicular to most of the cap surface . Video microscopy on particle dimers indicates that in the far field, the dipole component parallel to the rotation axis prevails, whereas in the near field, the curved magnetization distribution leads to a canted antiparallel arrangement …”

“…Magnetic colloidal particle suspensions have been used to study a wealth of physical phenomena which span from fundamental aspects to application‐oriented topics, e.g., in medical science and imaging, in microfluidic sensing, or in environmental engineering . Recent examples address directed motion in anisotropic media or along chemical, thermal, and magnetic field gradients, the pattern formation under static and dynamic external field conditions with applied shear forces, or in conjunction with magnetic frustration, as well as applications as bit‐patterned magnetic media, ferrofluids, magnetically directed or actuated swimmers, or microscale motors . Both ferromagnetic particles with a permanent magnetic moment and (super)paramagnetic particles have been studied .…”

“…Complex, even chiral, structures have been obtained from simple, structurally isotropic building blocks, especially for 2D assemblies in the presence of external magnetic fields . More elaborate anisotropies of the assembly pattern have been introduced via the particle shape, by an inhomogeneous distribution of the magnetic phase within the particle or on its surface or by both effects combined …”

Ground state of dipolar hard spheres confined in channels

“…Other ground-state structures have been explored for spheres with shifted dipole moments [11][12][13][14], for spheres in an external magnetic field [15] and confined onto the plane [16,17]. Magnetic hard spheres are realized in the macroworld as heavy balls [18] and granulates [19], yet a plethora of possibilities are found in the mesoscopic regime of magnetic colloids [20][21][22][23][24][25][26][27][28][29][30][31], magnetic nanoparticles [32][33][34] (in particular, when the magnetic interaction energy dominates thermal fluctuations), colloidal particles with an induced electric dipole moment [35,36] and dipolar dusty plasmas [37]. As these particles and their clusters constitute the main building blocks of prospective materials such as ferromagnetic filaments [38][39][40] for the creation of microdevices and for magnetorheological fluids and ferrogels with tunable and unusual visco-elastic properties [41][42][43][44], an understanding of their structure is of prime interest.…”