Sommaire:

The dipolar hard-sphere (DHS) model, a dipole embedded in the centre of a hard sphere, is a paradigm for the self
assembly of anisotropic particles. Notwithstanding its simplicity, the phase diagram of the DHS model is still
partially unknown. In this talk I will focus on the structure and thermodynamics of the low-density DHS fluid, in the region
of the phase diagram where a so-called topological phase transition was predicted to occur[1,2]. I will first
show that the system does not phase separate and that the formation of self-assembled, stable rings suppresses
phase separation[3].
In order to confirm this picture, a much simpler patchy toy model, incorporating the main features of the DHS model, will
be used to acquire more insight on the phase behaviour of dipolar hard spheres. Indeed, the model can be tuned to favour
or disfavour the formation of rings. As ring formation is enhanced, the region of thermodynamic instability shrinks
and, eventually, disappears[4]. Given the similarities between the two systems, ring formation can thus be linked with
the lack of a critical point in the DHS fluid.
In addition to the suppression of the gas-liquid-like phase separation, I will also show that the tendency of the DHS
fluid to form magnetically inert structures such as rings provides a natural explanation for the experimentally
observed non-monotonicity of the initial magnetic susceptibility[5,6].

[1] T. Tlusty and S. A. Safran, Science 290, (2000) p. 1328
[2] G. Ganzenmüller, G. N. Patey, and P. J. Camp, Mol. Phys. 107, (2009) p. 403
[3] L. Rovigatti, J. Russo and F. Sciortino, Phys. Rev. Lett., 107, (2011) p. 237801
[4] L. Rovigatti, J. M. Tavares and F. Sciortino, Phys. Rev. Lett., 111, (2013) p. 168302
[5] A. Lebedev, Colloid J. 72, (2010) p. 815
[6] S. Kantorovich, A. O. Ivanov, L. Rovigatti, J. M. Tavares and F. Sciortino, Phys. Rev. Lett., 110, (2013) p. 148306

Pour plus d'informations, merci de contacter Kob W.