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- Low temperature structural transitions in dipolar hard spheres: the influence on magnetic propertiesPublication . Ivanov, A.O.; Kantorovich, Sofia S.; Rovigatti, Lorenzo; Tavares, Jose; SCIORTINO, FrancescoWe investigate the structural chain-to-ring transition at low temperature in a gas of dipolar hard spheres (DRS). Due to the weakening of entropic contribution, ring formation becomes noticeable when the effective dipole-dipole magnetic interaction increases, It results in the redistribution of particles from usually observed flexible chains into flexible rings. The concentration (rho) of DI-IS plays a crucial part in this transition: at a very low rho only chains and rings are observed, whereas even a slight increase of the volume fraction leads to the formation of branched or defect structures. As a result, the fraction of DHS aggregated in defect-free rings turns out to be a non-monotonic function of rho. The average ring size is found to be a slower increasing function of rho when compared Lo that of chains. Both theory and computer simulations confirm the dramatic influence of the ring formation on the rho-dependence of the initial magnetic susceptibility (chi) when the temperature decreases. The rings clue to their zero total dipole moment are irresponsive to a weak magnetic field and drive to the strong decrease of the initial magnetic susceptibility. (C) 2014 Elsevier B.V. All rights reserved.
- Branching points in the low-temperature dipolar hard sphere fluidPublication . Rovigatti, Lorenzo; Kantorovich, Sofia S.; Ivanov, Alexey O.; Tavares, Jose; SCIORTINO, FrancescoIn this contribution, we investigate the low-temperature, low-density behaviour of dipolar hard-sphere (DHS) particles, i.e., hard spheres with dipoles embedded in their centre. We aim at describing the DHS fluid in terms of a network of chains and rings (the fundamental clusters) held together by branching points (defects) of different nature. We first introduce a systematic way of classifying inter-cluster connections according to their topology, and then employ this classification to analyse the geometric and thermodynamic properties of each class of defects, as extracted from state-of-the-art equilibrium Monte Carlo simulations. By computing the average density and energetic cost of each defect class, we find that the relevant contribution to inter-cluster interactions is indeed provided by (rare) three-way junctions and by four-way junctions arising from parallel or anti-parallel locally linear aggregates. All other (numerous) defects are either intra-cluster or associated to low cluster-cluster interaction energies, suggesting that these defects do not play a significant part in the thermodynamic description of the self-assembly processes of dipolar hard spheres. (C) 2013 AIP Publishing LLC.
- Free energy calculations for rings and chains formed by dipolar hard spheresPublication . Ronti, Michela; Rovigatti, Lorenzo; Tavares, Jose; Ivanov, Alexey O.; Kantorovichaf, Sofia S.; SCIORTINO, FrancescoWe employ a method based on Monte Carlo grand-canonical simulations to precisely calculate partition functions of non-interacting chains and rings formed by dipolar hard spheres (DHS) at low temperature. The extended low temperature region offered by such cluster calculations, compared to what had been previously achieved with standard simulations, opens up the possibility of exploring a part of the DHS phase diagram which was inaccessible before. The reported results offer the unique opportunity of verifying well-established theoretical models based on the ideal gas of cluster approximation in order to clarify their range of validity. They also provide the basis for future studies in which cluster–cluster interactions will be included.
- Computing the phase diagram of binary mixtures: A patchy particle case studyPublication . Rovigatti, Lorenzo; de las Heras, Daniel; Tavares, Jose; Gama, Margarida; SCIORTINO, FrancescoWe investigate the phase behaviour of 2D mixtures of bi-functional and three-functional patchy particles and 3D mixtures of bi-functional and tetra-functional patchy particles by means of Monte Carlo simulations and Wertheim theory. We start by computing the critical points of the pure systems and then we investigate how the critical parameters change upon lowering the temperature. We extend the successive umbrella sampling method to mixtures to make it possible to extract information about the phase behaviour of the system at a fixed temperature for the whole range of densities and compositions of interest. (C) 2013 AIP Publishing LLC.
- Quantitative description of the self-assembly of patchy particles into chains and ringsPublication . Tavares, Jose; Rovigatti, Lorenzo; SCIORTINO, FrancescoWe numerically study a simple fluid composed of particles having a hard-core repulsion complemented by two patchy attractive sites on the particle poles. An appropriate choice of the patch angular width allows for the formation of ring structures which, at low temperatures and low densities, compete with the growth of linear aggregates. The simplicity of the model makes it possible to compare simulation results and theoretical predictions based on the Wertheim perturbation theory, specialized to the case in which ring formation is allowed. Such a comparison offers a unique framework for establishing the quality of the analytic predictions. We find that the Wertheim theory describes remarkably well the simulation results.
- Communication: Re-entrant limits of stability of the liquid phase and the Speedy scenario in colloidal model systemsPublication . Rovigatti, Lorenzo; Bianco, Valentino; Tavares, Jose; SCIORTINO, FrancescoA re-entrant gas-liquid spinodal was proposed as a possible explanation of the apparent divergence of the compressibility and specific heat off supercooling water. Such a counter-intuitive possibility, e.g., a liquid that becomes unstable to gas-like fluctuations on cooling at positive pressure, has never been observed, neither in real substances nor in off-lattice simulations. More recently, such a re-entrant scenario has been dismissed on the premise that the re-entrant spinodal would collide with the gas-liquid coexisting curve (binodal) in the pressure-temperature plane. Here we study, numerically and analytically, two previously introduced one-component patchy particle models that both show (i) a re-entrant limit of stability of the liquid phase and (ii) a re-entrant binodal, providing a neat in silico (and in charta) realization of such unconventional thermodynamic scenario.
- Temperature-induced structural transitions in self-assembling magnetic nanocolloidsPublication . Kantorovich, Sofia S.; Ivanov, Alexey O.; Rovigatti, Lorenzo; Tavares, Jose; SCIORTINO, FrancescoWith the help of a unique combination of density functional theory and computer simulations, we discover two possible scenarios, depending on concentration, for the hierarchical self-assembly of magnetic nanoparticles on cooling. We show that typically considered low temperature clusters, i.e. defect-free chains and rings, merge into more complex branched structures through only three types of defects: four-way X junctions, three-way Y junctions and two-way Z junctions. Our accurate calculations reveal the predominance of weakly magnetically responsive rings cross-linked by X defects at the lowest temperatures. We thus provide a strategy to fine-tune magnetic and thermodynamic responses of magnetic nanocolloids to be used in medical and microfluidics applications.
- Nonmonotonic Magnetic Susceptibility of Dipolar Hard-Spheres at Low Temperature and DensityPublication . Kantorovich, Sofia S.; Ivanov, Alexey O.; Rovigatti, Lorenzo; Tavares, Jose; SCIORTINO, FrancescoWe investigate, via numerical simulations, mean field, and density functional theories, the magnetic response of a dipolar hard sphere fluid at low temperatures and densities, in the region of strong association. The proposed parameter-free theory is able to capture both the density and temperature dependence of the ring-chain equilibrium and the contribution to the susceptibility of a chain of generic length. The theory predicts a nonmonotonic temperature dependence of the initial (zero field) magnetic susceptibility, arising from the competition between magnetically inert particle rings and magnetically active chains. Monte Carlo simulation results closely agree with the theoretical findings. DOI: 10.1103/PhysRevLett.110.148306
- Self-assembly in chains, rings, and branches: a single component system with two critical pointsPublication . Rovigatti, Lorenzo; Tavares, Jose; SCIORTINO, FrancescoWe study the interplay between phase separation and self-assembly in chains, rings, and branched structures in a model of particles with dissimilar patches. We extend Wertheim's first order perturbation theory to include the effects of ring formation and to theoretically investigate the thermodynamics of the model. We find a peculiar shape for the vapor-liquid coexistence, featuring reentrant behavior in both phases and two critical points, despite the single-component nature of the system. The emergence of the lower critical point is caused by the self-assembly of rings taking place in the vapor, generating a phase with lower energy and lower entropy than the liquid. Monte Carlo simulations of the same model fully support these unconventional theoretical predictions.