Demixed and ordered phases in hard-rod mixtures

Andras Gabor (1) , Szabolcs Varga (1) , Franz J Vesely (2) , Enrique Velasco (3) , Luis Mederos (4)

1 Institute of Physics
2 Faculty of Physics [Vienna]
3 Departamento de Física Teórica de la Materia Condensada and Instituto de Ciencia
4 ICMM - Instituto de Ciencia de Materiales de Madrid

We analyse demixing and ordering transitions in systems of hard cylindrical particles. The second virial approximation of Onsager and a bifurcation analysis, as introduced by Koda and Kimura, are used to evaluate the free energies, pressures, and density distribution functions in mixtures of equally long but differently wide cylinders. The spatial density distribution along the one relevant coordinate is of particular importance as it provides a more detailed information about the nature of the phase transition than the bare bifurcation diagnosis. Detailed results are given for the nematic-nematic spinodal and the nematic-smectic transitions. Allowing for the absence of an isotropic phase, our results are in good qualitative agreement with those for freely orienting rods reported previously, and indicate a complex sequence of phase diagrams as the diameter dissimilarity of the two components is increased, with upper and lower critical points bounding nematic and smectic demixing regions. However, experimental results on colloidal rods show that nematic demixing occurs at a diameter ratio much smaller than ours or those for freely rotating fluids, indicating that Onsager-type theories may be insufficient to reproduce this phenomenon in a quantitative manner and, consequently, that more sophisticated approaches, presumably incorporating particle flexibility and additional interactions, are required.

Mots clés: Physical Sciences

Andras Gabor, Szabolcs Varga, Franz J Vesely, Enrique Velasco, Luis Mederos. Demixed and ordered phases in hard-rod mixtures. Molecular Physics, 2008, 106 (15), pp.1939-1947. ⟨10.1080/00268970802385253⟩. ⟨hal-00513226⟩