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We study via molecular dynamics simulations the self-assembled structures formed by supracolloidal magnetic polymers (SMPs). These are small aggregates of magnetic colloidal particles with polymer-like structures that have been permanently stabilized by means of polymer crosslinking. Here we focus on the fundamental structural and magnetic properties of the self-assembled motifs formed by pairs of identical SMPs under different conditions. We observe that the minimum characteristic density at which the self-assembly of both SMPs into a single aggregate becomes dominant is signaled by a maximum in their net magnetization and radius of gyration.

In the present work we study via molecular dynamics simulations filaments of ring and linear shape. Filaments are made of magnetic nanoparticles, possessing a point dipole in their centres. Particles in filaments are crosslinked in a particular way, so that the deviation of the neighbouring dipoles from the head-to-tail orientation is penalised by the bond. We show how the conformation of a single chain and ring filament changes on cooling for different lengths. We also study filament pairs, by fixing filaments at a certain distance and analysing the impact of inter-filament interaction on the equilibrium configurations. Our study opens a perspective to investigate the dispersions of filaments, both theoretically and numerically, by using effective potentials.

In our earlier work (Steinbach et al., 2016 [1] ) we investigated a homogeneous system of magnetically capped colloidal particles that self-assembled via two structural patterns of different symmetry. The particles could form a compact, equilateral triangle with a three-fold rotational symmetry and zero dipole moment and a staggered chain with mirror symmetry with a net magnetisation perpendicular to the chain. The system exhibited a bistability already in clusters of three particles. Based on observations of a real magnetic particles system, analytical calculations and molecular dynamics simulations, it has been shown that the bistability is a result of an anisotropic magnetisation distribution with rotational symmetry inside the particles. The present study is a logical extension of the above research and forms a preparatory stage for the study of a self-assembly of such magnetic particles under the influence of an external magnetic field. Since the magnetic field is only an additive contribution to the total ground state energy, we can study the interparticle interaction energies of candidate ground state structures based on the field-free terms.

Abstract. Unlike Stockmayer fluids, that prove to undergo gas-liquid transition on cooling, the system of dipolar hard or soft spheres without any additional central attraction so far has not been shown to have a critical point. Instead, in the latter, one observes diverse self-assembly scenarios. Crosslinking dipolar soft spheres into supracolloidal magnetic polymer-like structures (SMPs) changes the self-assembly behaviour. Moreover, aggregation in systems of SMPs strongly depends on the constituent topology. For Y- and X-shaped SMPs, under the same conditions in which dipolar hard spheres would form chains, the formation of very large loose gel-like clusters was observed (E. Novak et al., J. Mol. Liq. 271, 631 (2018)). In this work, using molecular dynamics simulations, we investigate the self-assembly in suspensions of four topologically different SMPs --chains, rings, X and Y-- whose monomers interact via Stockmayer potential. As expected, compact drop-like clusters are formed by SMPs in all cases if the central isotropic attraction is introduced, however, their shape and internal structure turn out to depend on the SMPs topology. Graphical abstract

In the present work magnetic brushes under flow conditions and confined inside narrow slits have been studied using Langevin dynamics simulations. It has been observed that the structural properties of these confined magnetic brushes can be tuned via the application of an external magnetic field, and this control can be exerted with a relatively low content of magnetic colloidal particles in the filaments that form the brushes (20% in the present study). The potential of these brushes to perform a separation process of a size-bidispersed mixture of free non-magnetic colloidal particles flowing through the slit has also been explored. Numerical results show that it is possible to induce a two-fold effect on the bidispersed particle flow: A lateral separation of the two types of flowing colloidal particles and an enhancement of the differences in their velocities. These two features are key elements sought in separation processes and could be very relevant in the design of new chromatographic columns and microfluid separation devices., J. J. Cerdà, C. Bona, and A. Cerrato acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness (MINECO/AEI/FEDER,UE) through the project Proyecto de I + D (excelencia) DPI2017-86610-P. E. N., E. P., P. S., and S. K. acknowledge the support of the Russian Science Foundation (Grant No. 19-12-00209). Simulations were performed at the IFISC's Nuredduna high-throughput computing clusters, supported by the projects GRID-CSIC.69 T. Sintes also acknowledges support via RTI2018-095441-B-C22, funded by MINECO/AEI/FEDER,UE.

Using Molecular Dynamics computer simulations, we study how an applied magnetic field influences the shape, integrity and internal structure of clusters formed by Stockmayer supracolloidal magnetic polymers (SMPs). We consider SMPs of four different topologies: chain-, Y-, X- and ring-like ones. We find that the highest macroscopic deformation and the strongest monomer rearrangements from a liquid to a local hexatic order is observed for clusters formed by chain-like SMPs. The highest magnetic susceptibility, however, is exhibited by the clusters composed of Y- and X-like SMPs. Clusters formed by ring-like SMPs are basically not affected by the field strengths considered here. It is worth mentioning that those fields are sufficient to cause the dissociation of the clusters formed by other topologies of SMPs.

This article is part of the themed collection: Nanoparticle Assembly: From Fundamentals to Applications, We present a theoretical study on the design of a supramolecular magnetoresponsive coating. The coating is formed by a relatively dense array of supracolloidal magnetic filaments grafted to a surface in a polymer brush-like arrangement. In order to determine and optimise the properties of the magnetic filament brush, we perform extensive computer simulations with a coarse-grained model that takes into account the correlations between the magnetic moments of the particles and the backbone crosslinks. We show that the self-assembly of magnetic beads from neighbouring filaments defines the equilibrium structural properties of the complete brush. In order to control this self-assembly, we highlight two external stimuli that can lead to significant effects: temperature of the system and an externally applied magnetic field. Our study reveals self-assembly scenarios inherently driven by the crosslinking and grafting constraints. Finally, we explain the mechanisms of structural changeovers in the magnetic filament brushes and confirm the possibility of controlling them by changing the temperature or the intensity of an external magnetic field., This research has been partially supported by the Austrian Research Fund (FWF): START-Projekt Y 627-N27. Authors are grateful to the Ural Federal University stimulating programme. S. S. K is supported by RFBR mol-a-ved 15-32-20549. E. V. N. acknowledges the support of President RF Grant No. MK-5216.2015.2. The authors are grateful to the Ministry of Education and Science of the Russian Federation (Project 3.12.2014/K) and EU-Project 642774 ETN-Colldense. J. J. C. and T. S. also acknowledge funding from a grant awarded by the Conselleria d'Innovació, Recerca i Turisme del Govern de les Illes Balears and the European Social Fund (ESF).

We investigate the properties of magnetic supracolliodal polymers – magnetic filaments (MFs) – with super-paramagnetic monomers, with and without Van der Waals (VdW) attraction between them. We employ molecular dynamics (MD) simulations to elucidate the impact of crosslinking mechanism on the structural and magnetic response of MFs to an applied external homogeneous magnetic field. We consider two models: plain crosslinking, which results in a flexible backbone; and constrained crosslinking, which provides significant stiffens against bending. We find that for plain crosslinking, even a slight increase of the central attraction leads to collapsed MF conformations. Structures that initially exhibit spherical symmetry evolve into cylindrically symmetric ones, with growing magnetic field strength. Plain crosslinking also allows for conformational bistability. MFs with constrained crosslinking tend to, instead, unravel in field. In both crosslinking scenarios, central attraction is able to hinder low-field magnetic response of MFs, albeit the bistability of plainly crosslinked MFs manifests itself also in the high-field region.

Supracolloidal magnetic polymers (SMPs) are structures made by crosslinking magnetic particles. In this work, using Langevin dynamics simulations, we study the zero-field magnetic properties of clusters formed in suspensions of SMPs with different topologies -- chains, rings, X and Y -- that interact via Stockmayer potential. We find that the presence of central attraction, resulting in the formation of large compact clusters, leads to a dramatic decrease of the suspension initial susceptibility, independently from SMP topology. However, the largest decrease corresponds to chain-like SMPs with strongly interacting particles. This is due to the higher rotational degrees of freedom of SMPs with such topology, which allows the particles to reorganise themselves inside the clusters in such a way that their magnetic moments form energetically advantageous vortex structures with negligible net magnetic moments., Comment: International Conference on Magnetic Fluids - ICMF 2019

We study self-assembly in suspensions of supracolloidal polymer-like structures made of crosslinked magnetic particles. Inspired by self-assembly motifs observed for dipolar hard spheres, we focus on four different topologies of the polymer-like structures: linear chains, rings, Y-shaped and X-shaped polymers. We show how the presence of the crosslinkers, the number of beads in the polymer and the magnetic interparticle interaction affect the structure of the suspension. It turns out that for the same set of parameters, the rings are the least active in assembling larger structures, whereas the system of Y- and especially X-like magnetic polymers tends to form very large loose aggregates. © 2018 Elsevier B.V.