Microstructure and thermodynamic properties of systems of particles with different shape

В ходе работы по НИР были проведены исследования систем магнитных эллипсоидов/цилиндров при комнатных температурах. Было показано, что микроструктура системы изменятся от цепочек (образующихся в системе практически изотропных частиц) до практически пространственно-однородной системы (для вытянутых частиц). Было начато исследование основного состояния системы магнитных кубов. Для системы коротких дуплексов ДНК в нематической фазе были получены выражения для вычисления радиальной функции распределения. Было показано, что анизотропия формы, а также энергия взаимодействия дуплексов может способствовать или препятствовать образованию нематической фазы.During the work on this research project systems of magnetic ellipsoids/cylinder in room temperature conditions have been conducted. It was shown that the microstructure of the system varies from the chains (formed in the substantially isotropic particles) to substantially spatially homogeneous system (for elongated particles). The study of the ground state of the magnetic cubes was undertaken. The expressions for the calculation of the radial distribution function have been done for a system of short DNA duplexes in the nematic phase. It has been shown that the shape anisotropy and the interaction energy of the duplexes can promote or inhibit the formation of the nematic phase.Программа развития УрФУ на 2013 год (п.2.1.2.1

Influence of the particle shape on the equilibrium morphologies of supracolloidal magnetic filaments

We investigate the equilibrium morphologies of linear and ring-shaped magnetic filaments made from crosslinked ferromagnetic spherical or ellipsoidal colloidal particles. Using Langevin dynamics simulations, we calculate the radius of gyration and total magnetic moment of a single filament at zero field and different temperatures, analyzing the influence of the particles shape, the strength of their magnetic moment and the filament length. Our results show that, among such parameters, the shape of the particles has the strongest qualitative impact on the equilibrium behavior of the filaments

The structure of clusters formed by Stockmayer supracolloidal magnetic polymers

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: [Figure not available: see fulltext.]. © 2019, The Author(s)

Suspensions of supracolloidal magnetic polymers: self-assembly properties from computer simulations

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 tend to form very large loose aggregates

Behaviour of a Magnetic Nanogel in a Shear Flow

Magnetic nanogels (MNG) – soft colloids made of polymer matrix with embedded in it magnetic nanoparticles (MNPs) – are promising magneto-controllable drug carriers. In order to develop this potential, one needs to clearly understand the relationship between nanogel magnetic properties and its behaviour in a hydrodynamic flow. Considering the size of the MNG and typical time and velocity scales involved in their nanofluidics, experimental characterisation of the system is challenging. In this work, we perform molecular dynamics (MD) simulations combined with the Lattice-Boltzmann (LB) scheme aiming at describing the impact of the shear rate (γ̇) on the shape, magnetic structure and motion of an MNG. We find that in a shear flow the centre of mass of an MNG tends to be in the centre of a channel and to move preserving the distance to both walls. The MNG monomers along with translation are involved in two more types of motion, they rotate around the centre of mass and oscillate with respect to the latter. It results in synchronised tumbling and wobbling of the whole MNG accompanied by its volume oscillates. The fact the an MNG is a highly compressible and permeable for the carrier liquid object makes its behaviour different from that predicted by classical Taylor-type models. We show that the frequency of volume oscillations and rotations are identical and are growing function of the shear rate. We find that the stronger magnetic interactions in the MNG are, the higher is the frequency of this complex oscillatory motion, and the lower is its amplitude. Finally, we show that the oscillations of the volume lead to the periodic changes in MNG magnetic energy. © 2021 Elsevier B.V.This research has been supported by the Russian Science Foundation Grant No.19-12-00209. Computer simulations were performed at the Vienna Scientific Cluster (VSC). I.S.N. and S.S.K. are grateful to Vienna Doctoral School Physics, Doctoral College DCAMF and were partially supported by FWF Project SAM P 33748. The authors thank Pedro S. Sánchez and Dr. Rudolf Weeber for fruitful discussions and useful recommendations

How to calculate structure factors of self-assembling anisotropic particles

We put forward a theoretical approach to analyse the structure factors obtained experimentally for solutions of self-assembling anisotropic particles. This method is applicable for any system of particles forming chains in thermodynamic equilibrium and is based on studying the behaviour of the centre-centre structure factor first peak. In order to calculate the structure factor analytically, we first derive the radial distribution function. For that we use the combination of the density functional theory and density expansion of the pair correlation functions. The first theory allows for the equilibrium chain formation, and the second takes into account particles' shape anisotropy at the level of the Gay-Berne potential. We apply our method to describe centre-centre structure factors of self-assembling short DNA duplexes with various semiaxes ratios. Extensive comparison of the theoretical predictions with the Monte Carlo simulation data shows a very good agreement. We show that the particle shape anisotropy exerts a crucial influence on the behaviour of the structure factors. © 2013 The Royal Society of Chemistry.Seventh Framework Programme, FP7: 226207; Russian Foundation for Basic Research, РФФИ: 12-02-3310

Self-assembly of polymer-like structures of magnetic colloids: Langevin dynamics study of basic topologies

We study the self-assembly of colloidal magnetic particles permanently cross-linked into polymer-like structures with different topologies, that we call supracolloidal magnetic polymers (SMPs). In order to understand the influence of the interparticle permanent links, we investigate SMPs holding the main topologies observed in the self-assembly of non-cross-linked magnetic particles via grand canonical Monte Carlo simulations: chains, rings and simple branched structures. Here, using molecular dynamics simulations, we focus on systems of SMP pairs. Our results evidence that the presence of crosslinkers leads to the formation of new types of aggregates, not previously observed for individual magnetic colloids. © 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.This research has been supported by the Russian Science Foundation [grant number 17-72-10145]. J.J.C. and T.S. 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). T.S. also acknowledges financial support from the Spanish Ministerio de Economía y Competi-tividad and the European Regional Development Fund, [Project number FIS20015-63628-C2-2-R] (AEI/FEDER, UE). P.A.S and S.S.K acknowledge support from the Austrian Research Fund (FWF) [START-Projekt Y 627-N27]. S.S.K. also acknowledges support from the European Commission ETN-COLLDENSE [H2020-MSCA-ITN-2014], [grant number 642774]. The authors would like to thank F. Sciortino for his valuable contribution to the GCMC simulation results

Magnetic properties of clusters of supracolloidal magnetic polymers with central attraction

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. © 2019 Elsevier B.V.Research supported by the Russian Science Foundation Grant No. 19-72-10033 . S.S.K. acknowledges support from the Austrian Research Fund (FWF), START-Projekt Y 627-N27

Adsorption transition of a grafted ferromagnetic filament controlled by external magnetic fields

Extensive Langevin dynamics simulations are used to characterize the adsorption transition of a flexible magnetic filament grafted onto an attractive planar surface. Our results identify different structural transitions at different ratios of the thermal energy to the surface attraction strength: filament straightening, adsorption, and the magnetic flux closure. The adsorption temperature of a magnetic filament is found to be higher in comparison to an equivalent nonmagnetic chain. The adsorption has been also investigated under the application of a static homogeneous external magnetic field. We found that the strength and the orientation of the field can be used to control the adsorption process, providing a precise switching mechanism. Interestingly, we have observed that the characteristic field strength and tilt angle at the adsorption point are related by a simple power law. © 2020 American Physical Society.This research was supported by the Russian Science Foundation, Grant No. 19-12-00209. T.S. acknowledges support by the Spanish AEI/MCI/FEDER(UE), Grant No. RTI2018-095441-B-C22, and The Maria de Maeztu R&D Program (Grant No. MDM-2017-0711). Simulations were carried out at the Vienna Scientific Cluster (VSC)