Anisotropic Magnetic Colloids: A Strategy to Form Complex Structures Using Nonspherical Building Blocks

Abstract

Microscopic magnetic particles dispersed in a solvent-or dipolar colloidal fluids-commonly assemble into chains due to a directional attractive interparticle potential. These chained structures can impart optical anisotropy (i.e., birefringence) to dipolar fluids, [1] and have been demonstrated as effective matrix materials in the rapid separation of DNA using microfluidic electrophoresis. [2] In the special case of magnetorheological (MR) systems, which require external fields to induce dipoles in polarizable colloids, an abrupt microstructural transition from isolated (unpolarized) particles into oriented chains can produce dramatic changes in the viscous behavior. [3] This responsive property makes MR fluids attractive as field-controllable damping fluids in hydraulic valves, shock absorbers, brakes, and so on. [4] Dipolar fluids also display interesting ordering phenomena beyond the 1D case. For instance, nickel-coated glass microspheres were shown to pack onto square, oblique, triangular, and even quasicrystalline lattices with five-fold symmetry in 2D. [5a] Binary particle mixtures assembled into ''flower''-like aggregates and superlattices in a planar configuration. [5b,c] In 3D, dipolar fluids were predicted to form rich mesophases as a function of the dipole moment strength, particle volume fraction, and the relative ''hard'' or ''soft'' nature of the colloidal interactions. [6] Examples include the body-centered-tetragonal (bct) and body-centered-orthorhombic (bco) crystals, along with a broad fluid-bct coexistence phase. Yethiraj and van Blaaderen [7] demonstrated these phases experimentally using a system with tunable long-range repulsive (isotropic) and dipolar (anisotropic) interactions. Such a model system is useful for the exploration of uncommon colloidal structures and for the elucidation of phase phenomena, including the field-induced solid-solid transformations between face-centered-cubic (fcc) and bct crystals.A recent trend in colloid science suggests an emerging theme of applying complex particles as structural motifs for an ever increasing array of synthetic materials. Significant progress in the fabrication of submicrometer-and micrometer-sized colloids in the past few years has yielded particles with various nonspherical shape, [8] ''patchy'' functionality, [8a,9] and anisotropic properties ranging from amphiphilicity [10] and polarizability [11] to magnetic anisotropy. [12] An important consideration in the pursuit of self-assembly strategies using such building blocks is to ensure that interactions are well-controlled. Thus, selectivity in ''bonds'' and weak interactions are often more desirable for equilibrium structures, as this allows systems to evolve toward the target structure in a reversible fashion. With regards to nonspherical colloids in particular, a major challenge remains in the limited ability to control the orientation, localization, and registry of the particles.Here, we report the synthesis and microstructure of peanut-shaped colloids with a permanent transverse ma...

Engineering of Micro‐ and Nanostructured Surfaces with Anisotropic Geometries and Properties

“…(d) Twisted columns of stacked peanut-shaped particles. Reproduced with permission, [ 223 ] copyright 2009 Wiley. (e) Lock and key assembly of polymer particles.…”

Self-assembly of colloidal hematite cubes: a microradian X-ray diffraction exploration of sedimentary crystals

“…Hematite also possesses a magnetic moment (due to canted antiferromagnetism [21][22][23][24] ) that induces dipole-dipole attractions. To obtain a stable self-assembling system of hematite cubes a balance between the van der Waals and magnetic attraction and the double layer repulsion has to be found.…”

“…Here, we investigate the organization of micron-sized cubelike hematite (a-Fe 2 O 3 ) colloids with a permanent magnetic dipole [21][22][23][24] into three dimensional (3D) structures as a function of inter-cube interactions and external elds. The effect of particle shape, surface charge and alignment in a magnetic eld on the resulting self-assembled structures is investigated.…”

Towards nanoscale composite particles of dual complexity

“…This opens perspectives for magnetic nanofluids with specific rheological properties and as complex building units for the next level in hierarchy of material design. Promising studies in this direction were already reported based on micron-sized particles with complex shapes [56][57][58].…”