FisMat2017 - Submission - View

Abstract's title: Low-Temperature Branched Structures of Dipolar Hard Spheres
Submitting author: Michela Ronti
Affiliation: University of Vienna
Affiliation Address: Computational Physics, Sensengasse 8 1090 Vienna (Austria)
Country: Austria
Oral presentation/Poster (Author's request): Oral presentation
Other authors and affiliations: Lorenzo Rovigatti (University of Rome Sapienza), Josè M. Tavares (University of Lisbon), Alexey O. Ivanov (Ural Federal University), Sofia S. Kantorovich (University of Vienna), Francesco Sciortino (University of Rome Sapienza)

The self-assembly scenario of the dipolar hard sphere (DHS) particles is not limited to linear chains: in our work 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).
We put forward a new method based on Monte Carlo grand-canonical simulations to precisely calculate partition functions at low densities and low temperature. Our approach is much faster than previously used canonical Monte Carlo simulations with large particle ensembles[1]: this advantage in efficiency allows us reaching the part of the DHS phase diagram inaccessible before and observe the precursor of percolation. We present a detailed study of the structure of defects in dipolar hard sphere systems, introducing a systematic way of classifying inter-cluster connections according the their topology.

We confirm that for low concentrations and low temperatures, the majority of magnetic nanoparticles is aggregated in rings; for higher concentrations, low temperature clusters merge together into more complex branched structures. We find that, decreasing the value of the temperature, the relevant configuration is provided by four-way junctions arising from parallel or anti-parallel locally linear aggregate: a structure different from the one predicted by Tlusty and Safran as the responsible of a possible topological phase transition[2].

Our results allow us to describe the next hierarchical level of self-assembly in magnetic nano colloids: the aggregation of basic branched clusters into complex networks.

[1] S. S. Kantorovich, A.O. Ivanov, L. Rovigatti, J.M. Tavares and F. Sciortino, Phys. Chem. Chem. Phys. 17, 16601-16608 (2015)
[2] T. Tlusty and S. A. Safran,
Science 290, 1328 (2000).