| We analyse a binary mixture of colloidal parallel hard cylindrical particles with identical diameters but dissimilar lengths, confined by two parallel hard walls in a planar slit-pore geometry, using a fundamental--measure density functional theory. Confinement brings about profound changes in the phase diagram, resulting from competition between the three relevant scales: pore width, smectic period and length ratio. Four main effects are identified: (i) Second-order bulk nematic-smectic transitions are suppressed. ii) Demixing transitions are weakly affected, with small shifts in the phase diagram spanned by the chemical potentials. (iii) Confinement-induced layering transitions occurring in the two confined one-component fluids in some cases merge with the demixing transition. (iv) Surface-induced layering transitions occurring at a single surface as coexistence conditions are approached are also also shifted in the confined fluid. Trends with pore size are analysed by means of complete chemical potentials and pressure-mean pore composition phase diagrams for particular values of pore size. This work, which is the first one to address the behaviour of liquid-crystalline mixtures under confinement, could be relevant as a first step to understand self-assembling properties of mixtures of metallic nanoparticles under external fields in the restricted highly confined geometry. |
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