Pattern formation of dipolar colloids in rotating fields: layering and synchronization

Abstract

We report Brownian dynamics (BD) simulation and theoretical results for a system of spherical colloidal particles with permanent dipole moments in a rotating magnetic field. Performing simulations at a fixed packing fraction and dipole coupling parameter, we construct a full non-equilibrium phase diagram as a function of the driving frequency (u 0 ) and field strength (B 0 ). This diagram contains both synchronized states, where the individual particles follow the field with (on average) constant phase difference, and asynchronous states. The synchronization is accompanied by layer formation, i.e., by spatial symmetry-breaking, similar to systems of induced dipoles in rotating fields. In the permanent dipole case, however, too large u 0 yields a breakdown of layering, supplemented by complex changes of the single-particle rotational dynamics from synchronous to asynchronous behavior. We show that the limit frequencies u c can be well described as a bifurcation in the nonlinear equation of motion of a single-particle rotating in a viscous medium. Finally, we present a simple density functional theory, which describes the emergence of layers in perfectly synchronized states as an equilibrium phase transition.

Chemical reactions induced by oscillating external fields in weak thermal environments

“…For example, field-matter interactions constitute processes in which energy is exchanged with a reacting system. These interactions lead to emergent and controllable behavior in assembly phenomena, [27][28][29][30] organic synthesis, 31 protein folding, 32 the detection of DNA, 33 and photodissociation. 34 Knowledge of the mechanism by which these interactions mediate reactive flow provides a methodological tool in the design of molecular devices with unique functionality.…”

Dynamics of cluster formation in driven magnetic colloids dispersed on a monolayer

“…The interplay of particlefield interactions and the anisotropic (dipole-dipole) magnetic interactions between the particles then leads to a wide variety of structures such as chains [7,8,9], layers [9,8,10,11,12], and intricate honeycomb-like structures [13,14,15].…”

Dynamical states in driven colloidal liquid crystals

“…10 Clearly, in our Langevin dynamics simulation inertia plays a non-negligible role, although we tried to minimize it (see Sec. II), while in Ref.…”

“…For example, spheres with a permanent dipole assemble into layers under a rotating magnetic field. 9,10 In systems of platelets between two flat parallel walls, rotating magnetic fields have been experimentally shown to induce a bend-splay Frederiks transition with transient spatially periodic patterns. 11 For rod-like particles, a linearly oscillating electric field will lead to a number of complex states and phases consisting of isotropic, nematic, and chiral nematic domains.…”