Phase transitions in nanoconfined binary mixtures of highly oriented colloidal rods

Abstract

We analyse a binary mixture of colloidal parallel hard cylindrical particles with identical diameters but dissimilar lengths $L_1$ and $L_2$, with $s=L_2/L_1=3$, confined by two parallel hard walls in a planar slit-pore geometry, using a fundamental--measure density functional theory. This model presents nematic (N) and two types of smectic (S) phases, with first- and second-order N-S bulk transitions and S-S demixing, and surface behaviour at a single hard wall which includes complete wetting by the S phase mediated (or not) by an infinite number of surface-induced layering (SIL) transitions. In the present paper the effects of confinement on this model colloidal fluid mixture are studied. Confinement brings about profound changes in the phase diagram, resulting from competition between the three relevant length scales: pore width $h$, smectic period $d$ and length ratio $s$. Four main effects are identified: (i) Second-order bulk N-S transitions are suppressed. (ii) Demixing transitions are weakly affected, with small shifts in the ${\mu }_1-{\mu }_2$ (chemical potentials) plane. (iii) Confinement-induced layering (CIL) transitions occurring in the two confined one-component fluids in some cases merge with the demixing transition. (iv) Surface-induced layering (SIL) transitions occurring at a single surface as coexistence conditions are approached are also shifted in the confined fluid. Trends with pore size are analysed by means of complete ${\mu }_1-{\mu }_2$ and $p-\bar{x}$ (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 restricted geometry.

29 pages, 13 figures.

References in corpus (5)

• Phase behavior and structure of model colloid-polymer mixtures confined between two parallel planar walls
• Capillary ordering and layering transitions in two-dimensional hard-rod fluids
• A fundamental-measure density functional for mixtures of parallel hard cylinders
• Phase behavior of parallel hard cylinders
• Surface and smectic layering transitions in binary mixtures of parallel hard rods

Cited by in corpus (3)

• Confinement of two-dimensional rods in slit pores and square cavities
• Orientational ordering and layering of hard plates in narrow slit-like pores
• Biaxial layering transition of hard rod-like particles in narrow slit-like pores