Browsing by Author "Dašić, Miljan"

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    A platform for nanomagnetism – assembled ferromagnetic and antiferromagnetic dipolar tubes
    Stanković, Igor; Dašić, Miljan; Otálora, Jorge A.; García, Carlos
    We report an interesting case where magnetic phenomena can transcend mesoscopic scales. Our system consists of tubes created by the assembly of dipolar spheres. The cylindrical topology results in the breakup of degeneracy observed in planar square and triangular packings. As far as the ground state is concerned, the tubes switch from circular to axial magnetization with increasing tube length. All magnetostatic properties found in magnetic nanotubes, in which the dipolar interaction is comparable to or dominant over the exchange interaction, are reproduced by the dipolar tubes including an intermediary helically magnetized state. Besides, we discuss the antiferromagnetic phase resulting from the square arrangement of the dipolar spheres and its interesting vortex state.
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    Effects of Water Content on the Transport and Thermodynamic Properties of Phosphonium Ionic Liquids
    Stanković, Igor; Dašić, Miljan; Jovanović, Mateja; Martini, Ashlie
    We present a numerical investigation of the influence of water content on the dynamic properties of a family of phosphonium-based room-temperature ionic liquids. The study presents a compelling correlation between structural changes in water-ionic liquid solutions and thermodynamic and transport properties across diverse systems. The results for phosphonium ionic liquids are compared with 1-butyl-3-methylimidazolium hexaphosphate ([bmim]PF6) as a reference. Through this approach, phosphonium cation structure-related characteristics can be identified and placed within the broader context of ionic liquids. These insights are underpinned by observed changes in interaction energy, boiling point, diffusion rate, and viscosity, highlighting the crucial role of water molecules in weakening the strength of interactions between ions within the ionic liquid. The investigation also explains temperature-dependent trends in phosphonium cations, showing that alkyl group length and molecular symmetry are important tuning parameters for the strength of Coulomb interactions. These results contribute to a refined understanding of phosphonium ionic liquid behavior in the presence of water, offering valuable insights for optimizing their use in diverse fields.
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    Influence of confinement on flow and lubrication properties of a salt model ionic liquid investigated with molecular dynamics
    Dašić, Miljan; Stanković, Igor; Gkagkas, Konstantinos
    We present a molecular dynamics study of the effects of confinement on the lubrication and flow properties of ionic liquids. We use a coarse-grained salt model description of ionic liquid as a lubricant confined between finite solid plates and subjected to two dynamic regimes: shear and cyclic loading. The impact of confinement on the ion arrangement and mechanical response of the system has been studied in detail and compared to static and bulk properties. The results have revealed that the wall slip has a profound influence on the force built-up as a response to mechanical deformation and that at the same time in the dynamic regime interaction with the walls represents a principal driving force governing the behaviour of ionic liquid in the gap. We also observe a transition from a dense liquid to an ordered and potentially solidified state of the ionic liquid taking place under variable normal loads and under shear.
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    Molecular dynamics investigation of the influence of the shape of the cation on the structure and lubrication properties of ionic liquids
    Dašić, Miljan; Stanković, Igor; Gkagkas, Konstantinos
    We present a theoretical study of the influence of the molecular geometry of the cation on the response of ionic liquids (ILs) to confinement and mechanical strain. The so-called tailed model includes a large spherical anion and asymmetric cation consisting of a charged head and a neutral tail. Despite its simplicity, this model recovers a wide range of structures seen in ILs: a simple cubic lattice for small tails, a liquid-like state for symmetric cation-tail dimers, and a molecular layer structure for dimers with large tails. A common feature of all investigated model ILs is the formation of a fixed (stable) layer of cations along solid plates. We observe a single anionic layer for small gap widths, a double anionic layer for intermediate ones, and tail-to-tail layer formation for wide gaps. The normal force evolution with gap size can be related to the layer formed inside the gap. The low hysteretic losses during the linear cyclic motion suggest the presence of strong slip inside the gap. In our model the specific friction is low and the friction force decreases with tail size.
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    Tribological properties of vanadium oxides investigated with reactive molecular dynamics
    Dašić, Miljan; Ponomarev, Ilia; Polcar, Tomas; Nicolini, Paolo
    We present a reactive molecular dynamics study on tribological properties of five vanadium oxides (V2O3, V3O5, V8O15, V9O17, VO2) under elevated temperatures and pressures. All considered stoichiometries provide lubrication with a comparatively low coefficient of friction (COF similar to 0.2 at 600 K, COF < 0.2 at 800 and 1000 K) which is a valuable information relevant for the design of coatings containing vanadium as a lubricious agent. An overall tendency of the decrease of friction coefficient with the increase of temperature represents a tribological effect useful for self-adjusting lubrication. We observed the increasing trend of adhesion-related offset of the friction force with the decrease of oxygen content in vanadium oxides.