Self-assembly of rigid magnetic rods consisting of single dipolar beads in two dimensions

Magnetic field-induced transitions and phase diagram of aggregate structures in a suspension of polydisperse cubic haematite particles

We investigated a polydisperse cubic haematite particle suspension in an external magnetic field and examined the dependence of magnetic field-induced transitions on the standard deviation of the particle size distribution using quasi-two dimensional Monte Carlo simulations. In the case of smaller polydispersity, stable clusters tend to form owing to stable face-to-face contact. In this case, however, larger magnetic particle-particle interaction strengths are necessary. Since the applied magnetic field enables the magnetic moment of each particle to incline in the field direction, it enhances the formation of chain-like clusters. In the case of larger polydispersity, compared to the smaller polydispersity cases, particle aggregates are formed even in the region of smaller magnetic particle-particle interactions. In this case, small particles combine with a growing cluster composed of large particles to form larger clusters. However, these small particles tend to disturb the internal structure of the particle aggregates, leading to chain-like clusters with narrower widths than those in the case of smaller polydispersity. These characteristics of the particle aggregates confirm that the broadness of polydispersity in a magnetic cubic particle suspension is applicable for controlling the internal structure and regime transition in the internal structure of particle aggregates. This may be an important feature in the development of surface modification techniques using magnetic cubic particle suspensions.

Hierarchical Self-Assembly of Magnetic Handshake Materials

Through programmable self-assembly, simple building blocks can be made to form highly complex structures following local rules of interaction. However, materials systems that are most commonly utilized for programmable assembly often lack interactions that exhibit the strength, specificity, and long ranges, which would, as a result, allow for robust and rapid hierarchical self-assembly processes. "Magnetic handshake" building blocks resolve many of these challenges at once, incorporating strong, long-range, and specific magnetic interactions through patterning of magnetic dipoles onto rigid panels. When appropriately designed, the panels organize hierarchically: first into chains, and subsequently those chains combine to form dense stacks. Here, we examine differences in phase behavior and morphology for four panel types. We delineate how perpendicular chaining and stacking interactions between panels compete and how they can be manipulated to reverse the sequence of the hierarchical assembly pathway. Collectively, our work shows the enormous potential for using magnetic handshake materials for self-assembly of hierarchically organized complex structures.

Emergence of synchronization-induced patterns in two-dimensional magnetic rod systems under rotating magnetic fields

We investigate the dynamics of two-dimensional assemblies of rod-shaped magnetic colloids under the influence of an external rotating magnetic field. Using molecular dynamics, we simulate the formation of patterns that emerge based on the synchronization degree between the magnetic rods and the rotating field. We then explore the structural and dynamic characteristics of the resulting steady states, examining their evolution as a function of changes in the rods' aspect ratio, the strength of the external magnetic field, and its rotation frequency. Three distinct synchronization regimes of the rods with the magnetic field are clearly observed. A detailed set of phase diagrams illustrates the complex relationship between the magnitude of the external magnetic field and its rotation frequency and how these parameters govern the formation of unique self-organized structures.

Growth transitions and critical behavior in the non-equilibrium aggregation of short, patchy nanorods

We have carried out Monte Carlo simulations to study the non-equilibrium aggregation of short patchy nanorods in two dimensions. Below a critical value of patch size ([Formula: see text]), the aggregates have finite sizes with small radii of gyration, [Formula: see text]. At [Formula: see text], the average radius of gyration shows a power law increase with time such that [Formula: see text], where [Formula: see text]. Above, [Formula: see text], the aggregates are fractal in nature and their fractal dimension depends on the value of patch size. These morphological differences are due to the fact that below the critical value of patch size ([Formula: see text]), the growth of the clusters is suppressed and the system reaches an 'absorbed state.' Above [Formula: see text], the system reaches an 'active state,' in which the cluster size keeps growing with a fixed rate at long times. Thus, the system encounters a non-equilibrium phase transition. Close to the transition, the growth rate scales as [Formula: see text], where [Formula: see text]. The long-time growth rate varies as [Formula: see text] where [Formula: see text]. These scaling exponents indicate that the transition belongs to the directed percolation universality class. The patchy nanorods also display a threshold patch size ([Formula: see text]), beyond which the long-time growth rate remains constant. We present geometric arguments for the existence of [Formula: see text]. The fractal dimension of the aggregates increases from 1.75, at [Formula: see text], to 1.81, at [Formula: see text]. It remains constant beyond [Formula: see text].

Steady states of non-axial dipolar rods driven by rotating fields

We investigate a two-dimensional system of magnetic colloids with anisotropic geometry (rods) subjected to an oscillating external magnetic field. The structural and dynamical properties of the steady states are analyzed, by means of Langevin dynamics simulations, as a function of the misalignment of the intrinsic magnetic dipole moment of the rods with respect to their axial direction, and also in terms of the strength and rotation frequency of an external magnetic field. The misalignment of the dipole relative to their axial direction is inspired by recent studies, and this is extremely relevant in the microscopic aggregation states of the system. The dynamical response of the magnetic rods to the external magnetic field is strongly affected by such a misalignment. Concerning the synchronization between the magnetic rods and the direction of the external magnetic field, we define three distinct regimes of synchronization. A set of steady states diagrams are presented, showing the magnitude and rotation frequency intervals in which the distinct self-organized structures are observed.

Self-assembly and clustering of magnetic peapod-like rods with tunable directional interaction

Based on extensive Langevin Dynamics simulations we investigate the structural properties of a two-dimensional ensemble of magnetic rods with a peapod-like morphology, i.e, rods consisting of aligned single dipolar beads. Self-assembled configurations are studied for different directions of the dipole with respect to the rod axis. We found that with increasing misalignment of the dipole from the rod axis, the smaller the packing fraction at which the percolation transition is found. For the same density, the system exhibits different aggregation states for different misalignment. We also study the stability of the percolated structures with respect to temperature, which is found to be affected by the microstructure of the assembly of rods.