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SUMMARY:The phase behaviour of shape-changing spheroids - Teixeira\, P (IS
 EL and Universidade de Lisboa)
DTSTART:20130322T154000Z
DTEND:20130322T160000Z
UID:TALK44079@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:Low-molecular-weight liquid crystals are typically modelled as
  collections of either hard rods or hard discs. However\, small\,flexible 
 molecules known as tetrapodes also exhibit liquid crystalline phases\, inc
 luding the elusive biaxial nematic phase [1\,2]. This is a consequence of 
 the interplay between conformational and packing entropies: the molecules 
 are able to adopt an anisometric stable conformation that allows then to p
 ack more efficiently into orientationally ordered mesophases. Previous the
 oretical studies of such systems have been presented [3]\, but in order to
  capture the essential physics of the process\, we introduce a minimal mod
 el which permits a clear detailed analysis. In our model a particle can ex
 ist in one of two states\, corresponding to a prolate and an oblate sphero
 id. The energies of these two states differ by a prescriamount ε\, and th
 e two conformers are in chemical equilibrium. The interactions between the
  particles are described by the Gaussian Overlap Model [4] and we investig
 ate the phase behaviour using a second-virial (Onsager) approach\, which h
 as been successfully applied to binary mixtures of plate-like and rod-like
  particles [5]. Depending on conditions these mixtures may exhibit biaxial
  nematic phases and N+--N co-existence. We use both bifurcation analysis a
 nd a numerical minimisation of the free energy to show that\, in the L2 ap
 proximation: (u) there is no stable biaxial phase even for ε=0 (although 
 there is a metastable biaxial phase in the same density range as the stabl
 e uniaxial phase)\; (ii) the isotropic-to-nematic transition is into eithe
 r one of two degenerate uniaxial phases\, rod-rich or disc-rich. \n\nRefer
 ences: [1] K. Merkel et al.\, Phys. Rev. Lett. 93\, 237801 (2004). [2] J. 
 L. Figueirinhas et al.\, Phys. Rev. Lett. 94\, 107802 (2005). [3] A. G. Va
 nakaras et al.\, Mol. Cryst. Liq. Cryst. 362\, 67 (2001). [4] B. J. Berne 
 and P. Pechukas\, J. Chem. Phys. 56\, 4213 (1972). [5] P. J. Camp et al.\,
  J.\n\n
LOCATION:Seminar Room 1\, Newton Institute
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