Cellular solid behaviour of liquid crystal colloids 1. Phase separation and morphology

Abstract

Abstract. We study the phase ordering colloids suspended in a thermotropic nematic liquid crystal below the clearing point Tni and the resulting aggregated structure. Small (150 nm) PMMA particles are dispersed in a classical liquid crystal matrix, 5CB or MBBA. With the help of confocal microscopy we show that small colloid particles densely aggregate on thin interfaces surrounding large volumes of clean nematic liquid, thus forming an open cellular structure, with the characteristic size of 10 − 100 µm inversely proportional to the colloid concentration. A simple theoretical model, based on the Landau mean-field treatment, is developed to describe the continuous phase separation and the mechanism of cellular structure formation.

Nematic-Wetted Colloids in the Isotropic Phase: Pairwise Interaction, Biaxiality, and Defects

“…This effect was shown to arise also in other anisotropic fluid hosts, e.g., lyotropic solutions of anisotropic micelles [4], and in different liquid crystal phases, e.g., cholesterics [5]. Lately, such systems were shown to form liquid or solid composite materials with unusual properties [6,7].…”

“…This effect was shown to arise also in other anisotropic fluid hosts, e.g., lyotropic solutions of anisotropic micelles [4], and in different liquid crystal phases, e.g., cholesterics [5]. Lately, such systems were shown to form liquid or solid composite materials with unusual properties [6,7].It is well known that solid surfaces influence not only the orientation of liquid crystals, but also, in general, the degree of order of their constituent molecules. In particular, a surface can induce a wetting nematic layer even above the transition temperature at which the nematic becomes an ordinary isotropic liquid [8][9][10].…”

Surface tension and capillary waves at the nematic-isotropic interface in ternary mixtures of liquid crystal, colloids, and impurities

“…This is what happens in the experiments performed with PMMA colloids where the remnant alkane slowly diffuses in the mixture or is directly added. 12,14, 15 The followed path in this case corresponds to the dashed-double dotted 10͒. In the isotropic phase, the hard-sphere colloid is in the liquid state ͑IL͒ below the coexistence boundary ⌽ 3iso Ͻ 0.52, whereas it is in the isotropic solid state ͑IS͒ at higher concentration 0.52Ͻ⌽ 3iso Ͻ⌽ c = 0.637, where ⌽ c is the random close-packing fraction.…”

“…This second effect could explain qualitatively why the mesh size of the observed cellular superstructure decreases when the concentration of colloids is increased. 14 The elastic modulus GЈ should also depend on the impurity concentration. Such effects were indeed reported recently in experiments 18 but they are not yet well documented.…”

“…This separation is due ͑only in part, as we shall see later͒ to the distortions of the director field cost energy. [12][13][14][15][16][17] It turns out that the first studies have focused on dispersions of Ϸ200 nm size poly͑methyl methacrylate͒ ͑PMMA͒ spheres in 4Ј-pentyl-4-cyanobiphenyl ͑5CB͒ or N-4-methoxybenzylidene-4Ј-butylaniline. Microscopic observations have shown that below the nematic-toisotropic phase transition temperature, the two phases coexist by forming a three-dimensional network which gives to the mixture the consistency of a soft cellular solid.…”