Nematic liquid interfaces

We have little information on the way low molecular weight molecules and oligomers adsorb (19). Apparently below DP s of about 100 they lie flat on the surface for concentrations up to a monolayer of segments, then seem to form thicker islands of smectic or nematic structure. Ordered condensed mono, -di, -or multi-layers are primarily the arrangements of smaller, especially amphipa-tic molecules on liquid-liquid interfaces. Polymers are too large to adsorb, in the ordinary sense, on micelles but segments of linear polymers may act as nucleation centers for micelles of small molecules which probably is one of the mechanisms for the lipid-, or detergent-, polymer interaction. 

Moreover, segregation of nanoparticles at liquid crystal/substrate interfaces is also an effective way to promote or alter the alignment of thin nematic liquid crystal films. Depending on their concentration, size, and nature, several types of nanoparticles have been shown to induce homeotropic alignment as well as defects and remarkable defect patterns. 

Tuning of the pre-tilt angle at the interface was also demonstrated by doping commonly used polyimide alignment layers with POSS nanoparticles [339]. In addition, the fabrication of a tunable liquid crystal flat microlens was achieved by placing a drop of a nematic liquid crystal doped with POSS nanoparticles onto a substrate inducing planar alignment (local HAN mode) [340]. Simultaneously, Takatoh and co-workers extended this concept to a series of metal oxide... 

Yokoyama, H., and Van Sprang, H. A. A novel method for determining the anchoring energy function at a nematic liquid crystal-wall interface from director distortions at high fields./. Appl. Phys. 57, 4520 (1985). 

Electron-transfer Reactions - Light-induced electron transfer from a donor to a suitable acceptor has been described for numerous bimolecular systems. The reagents have been dispersed in a polar solvent,at microscopic or macroscopic interfaces, in latex dispersions, in nematic liquid crystals, in reverse micelles, in vesicles, and in lipid bilayer membranes. Additional studies have been concerned with electron transfer... 

In five nematic liquids investigated, the domain structure is similar to that shown in Figure 1. This structure persists indefinitely as long as the field is applied. The behavior of nematic p-methoxycinnamic acid (3) is different and illustrates some interesting features of the interface or wall between domains. 

Optical examination of the effects of applied electric fields provides a sensitive technique for investigating the structure of nematic liquids. Two distinct kinds of interfaces exist in pure nematic liquids. One corresponds to the crystallite boundaries in ordinary polycrystalline solids. The other is the interface between domains which shows effects of interfacial tension in p-methoxycinnamic acid. In other nematic liquids this effect is not found. 

The viscosity coefficients may also be determined by studying the reflexion of ultrasonic shear waves at a solid-nematic interface. The technique was developed by Martinoty and Candau. A thin film of a nematic liquid crystal is taken on the surface of a fused quartz rod with obliquely cut ends (fig. 3.7.1). A quartz crystal bonded to one of the ends generates a transverse wave. At the solid-nematic interface there is a transmitted wave, which is rapidly attenuated, and a reflected wave which is received at the other end by a second quartz crystal. The reflexion coefficient, obtained by measuring the amplitudes of reflexion with and without the nematic sample, directly yields the effective coefficient of viscosity. 

With a DC field, there may be injection of charge carriers at the solid-liquid interface but its role in the electrohydrodynamics of the nematic phase is not yet fully understood. However, as remarked earlier, a frequency of about 10 Hz is enough to suppress charge injection. We shall therefore neglect it in the present discussion. 

Tjipto E, CadweU KD, Quinn IF et al (2006) Tailoring the interfaces between nematic liquid crystal emulsions and aqueous phases via layer-by-layer assembly. Nano Lett 6(10) 2243-2248... 

This question can be answered by considering the experiments, where liquid crystals have been spread across solid crystal surfaces [15]. Macroscopic observations under a polarizing microscope reveal that crystal surfaces do orient nematic liquid crystals, which leads to the conjecture that there may be microscopic positional and orientational order at a crystal-liquid crystal interface. Indeed, this local interfacial order can be revealed in experiments with a Scanning Tunneling Microscope (STM), first perfoi med by Foster and Frommer [16]. An example of an STM image of a crystal-nematic liquid crystal interface is shown in Fig. 1.4 for the case of 8CB on graphite. One can... 

Ellipsometry of the Glass-Isotropic Nematic Liquid Crystal Interface... 

As discussed abfeve, ellipsometry is directly sensitive only to the interfacial variations of the nematic order parameter, which is connected to the optical refraction indices. The interfacial smectic order, which has no direct influence on the optical properties, can only be observed due to its coupling to the nematic (orientational) order. The same experimental setup as described in Sect. 4.1.3 has been used to study the interface between smectic liquid crystal dodecylcyanobiphenyl (12CB) in the isotropic phase and the silanated glass. Although only orientational order is observed, the temperature dependence of pb is in this case quite different from the case with the nematic liquid crystal, as evident from Fig. 4.4. 

S. Murakami, H. Naito, Electrode and interface polarizations in nematic liquid crystal cells. 

Traditionally, the term electroconvection is used in at least four different physical contexts, of which three pertain to flows of liquid dielectrics. Thus, it is used to describe the electric field-induced flow of nematic liquid crystals, the flow of liquid dielectrics caused by the action of electric field on the space charge of ions of the appropriate sign injected in a low quantity into a fluid, or the effects of an electric field acting on the surface charge accumulated at the interface between two weakly conducting fluids. The latter process was studied by G. I. Taylor, who in the mid-1960s introduced the leaky dielectric model... 

Figure 6.2 Examples of static self-assembly, (a) Crystal structure of a ribosome, (b) Self-assembled peptide-amphiphile nanofibers, (c) An array of millimeter-sized polymeric plates assembled at a water/perfluorodecalin interface by capillary interactions, (d) Thin film of a nematic liquid crystal on an isotropic substrate, (e) Micrometer-sized metallic polyhedral folded from planar substrates, (f) A 3-D aggregate of micrometer plates assembled by capillary forces.

Some solid surfaces induce disorder in nematic liquid crystals. It means that the order parameter at the interface is lower than the bulk value. For instance, evaporated SiO layers of a certain thickness due to their roughness decrease the order parameter of MBBA from the bulk value Sb 0.6 down to So 0.1-0.2. In some cases, the surface order parameter may be equal to zero (surface melting). 

In the X-ray experiments on nematic 8CB, the smectic ordering was observed at the free surface (air-nematic interface). The same phenomenon has also been observed at the solid-nematic interface by the X-ray, an electrooptical technique and molecular force measurements. The principle of the latter is shown in Fig. 10.7. For two mica cylinders submerged in nematic liquid crystal, their interaction force measured with a balance oscillates with a distance between the cylinders and the period of oscillations was found to be equal to molecular length 1. This clearly shows the periodicity in density characteristic of a smectic phase [8]. 

Fig. 10.9 Optical second harmonic generation by a polar layer at the interface between nematic liquid crystal and glass due to non-linear interaction with surface layer the incident beam of frequency ...

The total free energy increases with the radius of the cylinder. Note that we neglect the surface energy of the interface between the nematic liquid crystal and the isotropic core. 

This smectic ordering is experimentally observed at the nematic-solid and nematic-air interfaces. X-ray reflectivity measurements showed the existence of the smectic order at the free surface of 80CB (l.xv) both in the nematic and the isotropic phase [14]. Other nematic compounds also exhibit smectic ordering at the free surface [15]. However, the free surface seems not to be favorable for the formation of polar layers. As usual, in polar compounds like 8CB (l.iii), bilayers are formed at the surface, probably due to the favorable orientation of the alkyl chains outward from the liquid crystal. In such a case, polar heads form dimers much easier [16]. 

M. Nobili and G. Durand, Disorientation-induced disordering at a nematic-liquid-crystal-solid interface, Phys. Rev. A 46, R6174 (1992). 

Figure 5.2. Monolayers of the amphiphile 1-monopalmitoyl-( )-glycerol at the air-water interface assemble in domains in which the molecular tilt azimuth is organized in star-shaped patterns. It is possible to preserve this order during the transfer on to a solid support. LB monolayers of this material have been utilized for the anchoring of nematic liquid crystals. The order within the monolayer determines the order within the bulk phase of the nematic liquid crystal (LC). The image here shows the LC cell between crossed polarizers. (From J. Fang, U. Gehlert, R. Shashidar and C. Knobler, Langmuir (1999), 15, 297)...

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