skip to main content

Title: Hierarchical assemblies of superparamagnetic colloids in time-varying magnetic fields
Magnetically-guided colloidal assembly has proven to be a versatile method for building hierarchical particle assemblies. This review describes the dipolar interactions that govern superparamagnetic colloids in time-varying magnetic fields, and how such interactions have guided colloidal assembly into materials with increasing complexity that display novel dynamics. The assembly process is driven by magnetic dipole–dipole interactions, whose strength can be tuned to be attractive or repulsive. Generally, these interactions are directional in static external magnetic fields. More recently, time-varying magnetic fields have been utilized to generate dipolar interactions that vary in both time and space, allowing particle interactions to be tuned from anisotropic to isotropic. These interactions guide the dynamics of hierarchical assemblies of 1-D chains, 2-D networks, and 2-D clusters in both static and time-varying fields. Specifically, unlinked and chemically-linked colloidal chains exhibit complex dynamics, such as fragmentation, buckling, coiling, and wagging phenomena. 2-D networks exhibit controlled porosity and interesting coarsening dynamics. Finally, 2-D clusters have shown to be an ideal model system for exploring phenomena related to statistical thermodynamics. This review provides recent advances in this fast-growing field with a focus on its scientific potential.  more » « less
Award ID(s):
Author(s) / Creator(s):
; ; ; ;
Date Published:
Journal Name:
Soft Matter
Page Range / eLocation ID:
1120 to 1155
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Magnetic nanoparticle chains offer the anisotropic magnetic properties that are often desirable for micro‐ and nanoscale systems; however, to date, large‐scale fabrication of these nanochains is limited by the need for an external magnetic field during the synthesis. In this work, the unique self‐assembly of nanoparticles into chains as a result of their intrinsic dipolar interactions only is examined. In particular, it is shown that in a high concentration reaction regime, the dipole–dipole coupling between two neighboring magnetic iron cobalt (FeCo) nanocubes, was significantly strengthened due to small separation between particles and their high magnetic moments. This dipole–dipole interaction enables the independent alignment and synthesis of magnetic FeCo nanochains without the assistance of any templates, surfactants, or even external magnetic field. Furthermore, the precursor concentration ([M] = 0.016, 0.021, 0.032, 0.048, 0.064, and 0.096m) that dictates the degree of dipole interaction is examined—a property dependent on particle size and inter‐particle distance. By varying the spinner speed, it is demonstrated that the balance between magnetic dipole coupling and fluid dynamics can be used to understand the self‐assembly process and control the final structural topology from that of dimers to linear chains (with aspect ratio >10:1) and even to branched networks. Simulations unveil the magnetic and fluid force landscapes that determine the individual nanoparticle interactions and provide a general insight into predicting the resulting nanochain morphology. This work uncovers the enormous potential of an intrinsic magnetic dipole‐induced assembly, which is expected to open new doors for efficient fabrication of 1D magnetic materials, and the potential for more complex assemblies with further studies.

    more » « less
  2. The self-assembly of colloidal magnetic Janus particles with a laterally displaced (or shifted), permanent dipole in a quasi-two-dimensional system is studied using Brownian dynamics simulations. The rate of formation of clusters and their structures are quantified for several values of dipolar shift from the particle center, which is nondimensionalized using the particle's radius so that it takes values ranging from 0 to 1, and examined under different magnetic interaction strengths relative to Brownian motion. For dipolar shifts close to 0, chain-like structures are formed, which grow at long times following a power law, while particles of shift higher than 0.2 generally aggregate in ring-like clusters that experience limited growth. In the case of shifts between 0.4 and 0.5, the particles tend to aggregate in clusters of 3 to 6, while for all shifts higher than 0.6 clusters rarely contain more than 3 particles due to the antiparallel dipole orientations that are most stable at those shifts. The strength of the magnetic interactions hastens the rate at which clusters are formed; however, the effect it has on cluster size is lessened by increases in the shift of the dipoles. These results contribute to better understand the dynamics of magnetic Janus particles and can help the synthesis of functionalized materials for specific applications such as drug delivery. 
    more » « less
  3. Abstract

    Under an applied magnetic field, superparamagnetic Fe3O4nanoparticles with complementary DNA strands assemble into crystalline, pseudo‐1D elongated superlattice structures. The assembly process is driven through a combination of DNA hybridization and particle dipolar coupling, a property dependent on particle composition, size, and interparticle distance. The DNA controls interparticle distance and crystal symmetry, while the magnetic field leads to anisotropic crystal growth. Increasing the dipole interaction between particles by increasing particle size or external field strength leads to a preference for a particular crystal morphology (e.g., rhombic dodecahedra, stacked clusters, and smooth rods). Molecular dynamics simulations show that an understanding of both DNA hybridization energetic and magnetic interactions is required to predict the resulting crystal morphology. Taken together, the data show that applied magnetic fields with magnetic nanoparticles can be deliberately used to access nanostructures beyond what is possible with DNA hybridization alone.

    more » « less
  4. Phase separation processes are widely utilized to assemble complex fluids into novel materials. These separation processes can be thermodynamically driven due to changes in concentration, pressure, or temperature. Phase separation can also be induced with external stimuli, such as magnetic fields, resulting in novel nonequilibrium systems. However, how external stimuli influence the transition pathways between phases has not been explored in detail. Here, we describe the phase separation dynamics of superparamagnetic colloids in time-varying magnetic fields. An initially homogeneous colloidal suspension can transition from a continuous colloidal phase with voids to discrete colloidal clusters, through a bicontinuous phase formed via spinodal decomposition. The type of transition depends on the particle concentration and magnitude of the applied magnetic field. The spatiotemporal evolution of the microstructure during the nucleation and growth period is quantified by analyzing the morphology using Minkowski functionals. The characteristic length of the colloidal systems was determined to correlate with system variables such as magnetic field strength, particle concentration, and time in a power-law scaling relationship. Understanding the interplay between particle concentration and applied magnetic field allows for better control of the phases observed in these magnetically tunable colloidal systems. 
    more » « less
  5. For magnetite spherical nanoparticles, the orientation of the dipole moment in the crystal does not affect the morphology of either zero field or field induced structures. For non-spherical particles however, an interplay between particle shape and direction of the magnetic moment can give rise to unusual behaviors, in particular when the moment is not aligned along a particle symmetry axis. Here we disclose for the first time the unique magnetic properties of hematite cubic particles and show the exact orientation of the cubes' dipole moment. Using a combination of experiments and computer simulations, we show that dipolar hematite cubes self-organize into dipolar chains with morphologies remarkably different from those of spheres, and demonstrate that the emergence of these structures is driven by competing anisotropic interactions caused by the particles' shape anisotropy and their fixed dipole moment. Furthermore, we have analytically identified a specific interplay between energy, and entropy at the microscopic level and found that an unorthodox entropic contribution mediates the organization of particles into the kinked nature of the dipolar chains. 
    more » « less