Crossover from Normal to Anomalous Diffusion in Systems of Field-Aligned Dipolar Particles

Jordanovic, Jelena; Jäger, Sebastian; Klapp, Sabine H. L.

doi:10.1103/physrevlett.106.038301

Cited by 33 publications

(46 citation statements)

References 29 publications

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“…Indeed, a rotating inplane field generates, on averaging over time, an inverted dipolar pair interaction with in-plane attraction and repulsion along the rotation axis. 11,12 Therefore, the structures induced by planar rotating fields are markedly different from those observed in a constant and homogeneous field, which supports the formation of field-aligned chains (low densities) [13][14][15] and bulk crystals.…”

Section: 3mentioning

confidence: 71%

“…In the following, we will investigate in more detail to what extent this assumption is actually fulfilled within the layered region indicated in Fig. 5 on a microscopic 15) where Q is the Heaviside function, Df is the interval length to which we want to resolve the distribution, n is a positive integer or zero that satisfies nDf # f < (n + 1)Df, and, as before, h/i denotes a time-average.…”

Section: Microscopic Rotational Dynamics In the Layered Statementioning

confidence: 99%

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

Jäger

Klapp

2011

Soft Matter

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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.

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Section: 3mentioning

confidence: 71%

Section: Microscopic Rotational Dynamics In the Layered Statementioning

confidence: 99%

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

Jäger

Klapp

2011

Soft Matter

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“…These building blocks, anisotropic in both shape and chemical constituents, enable one to test some of the classical predictions from extensive theory and simulation [11][12][13][14][15][16][17][18][19] , although to do so is not the main point of this study. The microscopic size of our rods allows direct visualization of dynamic pathways of the resulting dipolar rings in response to external fields.…”

Section: Discussionmentioning

confidence: 99%

“…Experiments lag behind extensive theory and simulation of dipolar assembly, even for spheres [11][12][13][14][15][16][17][18][19] . On the experimental side, structures of ferromagnetic nanoparticles were inspected by cryogenic electron microscopy 20,21 , but all dynamics were missing owing to the sample preparation.…”

mentioning

confidence: 99%

Colloidal ribbons and rings from Janus magnetic rods

et al. 2013

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Dipolar particles are fundamental building blocks in nature and technology, yet the effect of particle anisotropy is seldom explored. Here, we fabricate colloidal silica rods coated with a hemicylindrical magnetic layer to satisfy multiple criteria: nearly monodisperse, easily imaged and magnetic interaction that dominates over gravity. We confirm long-predicted features of dipolar assembly and stress the microstructural variety brought about by shape and constituent anisotropy, especially by extrapolating knowledge learned from literal molecules. In this colloidal system, we describe analogies to liquid crystalline deformations with bend, splay and twist; an analogy to cis/trans isomerism in organic molecules, which in our system can be controllably and reversibly switched; and a field-switching methodology to direct single ribbons into not only single but also multiple rings that can subsequently undergo hierarchical self-assembly. We highlight subtle material issues of control and design rules for reconfigurable dipolar materials with building blocks of complex shape.

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“…1(a). Vice versa, the LC stabilizes the mutual alignment of the chains, which is absent in pure DSS fluids at the densities considered here [32,33]. We note, however, that while the chains themselves form ferromagnetic entities, the overal magnetization is zero, as reflected by the negligible values of P 1 plotted in Fig.…”

mentioning

confidence: 84%

Spontaneous ordering of magnetic particles in liquid crystals: From chains to biaxial lamellae

Peroukidis

Klapp

2015

Phys. Rev. E

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Using Monte Carlo (MC) computer simulations we explore the self-assembly and ordering behavior of a hybrid, soft magnetic system consisting of small magnetic nano-spheres in a liquid-crystalline (LC) matrix. Inspired by recent experiments with colloidal rod-matrices we focus on conditions where the sphere and rod diameters are comparable. Already in the absence of a magnetic field, the nematic ordering of the LC can stabilize formation of magnetic chains along the nematic or smectic director, yielding a state with local (yet no macroscopic) magnetic order. The chains, in turn, increase the overall nematic order, reflecting the complex interplay of the structure formation of the two components. Increasing the sphere diameter the spontaneous uniaxial ordering is replaced by biaxial lamellar morphologies characterized by alternating layers of rods and magnetic chains oriented perpendicular to the rod's director. These novel ordering scenarios at zero field suggest complex response of the resulting hybrid to external stimuli such as magnetic fields and shear forces.Mixtures of magnetic nanoparticles (MNP) in liquidcrystalline (LC) matrices attract attention in fundamental physics and material science since about four decades ago: in 1970, Brochard and de Gennes [1] predicted, based on free energy considerations, various new effects including spontaneous magnetization in the liquid state, "compensated" phases and giant field-induced effects. Whereas the field sensitivity has been investigated and verified in many experiments [2][3][4][5][6][7][8], new interest was stimulated by the recent discovery of spontaneous magnetic ordering in a hybrid system of large, micron-sized magnetic plates embedded in a lyotropic nematic LC [9]. A crucial ingredient for the observed behavior are the LCmediated interactions stemming from local distortions of the LC director via the presence of the plates, whose size is much larger than that of the LC molecules. Indeed, LC-mediated elastic interactions and their consequences for colloidal self-assembly and ordering have intensively studied by experiments [10] and simulations [11,12] for many (non-magnetic) colloid/LC mixtures.In the present letter we focus on the opposite case of small MNPs in LC matrices, where the sizes are comparable and distortions of the LC do not play a significant role. An example of such systems are lyotropic suspensions of colloid pigment rods and magnetite MNPs which have been recently studied to investigate field-induced ordering effects [13, 14]. However, although there is strong interest in using such hybrids as new, stimuli-responsive or even adaptive materials [9], the microscopic structure of small MNPs in LC matrices is essentially unexplored; although this is clearly a prerequisite in terms of advancing new functionalities. Experimentally, the challenge is to actually "see" the relatively small MNPs with typical diameters of 10-15 nm; a solution of this problem might be provided by recently developed tomography techniques [15]. Suprisingly, however, the...

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Crossover from Normal to Anomalous Diffusion in Systems of Field-Aligned Dipolar Particles

Cited by 33 publications

References 29 publications

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

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

Colloidal ribbons and rings from Janus magnetic rods

Spontaneous ordering of magnetic particles in liquid crystals: From chains to biaxial lamellae

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