Liquid Crystal - Solid Interface

In spite of the obvious technological importance, surface charging and surface electric field have only recently been directly observed in liquid crystals [52] using AFM in the force spectroscopy mode. In this section, we briefly describe the experiments, that enabled a direct measurement of the surface charge and the Debye screening length of the liquid crystal-solid interface. 

Fig. 10.10 A schematic picture of the charge distribution as a function of the distance from the liquid crystal-solid interface for ionic (a), dipolar (b) and quadrupolar (c) mechanisms of surface polarization Psurf...

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

The state of polarization of light, reflected from an interface, depends strongly on the profile of the dielectric constant across that interface. This simple principle is used in the Brewster angle reflection ellipsometry (BAE), where one measures the ellipticity coefficient of light, reflected from an interface. The method is sensitive enough to detect extremely small changes in the structure of liquid crystalline-solid interfaces. Subnanometer resolution of the adsorption parameter is routinely achieved. The method is therefore very useful for the study of liquid crystal interfaces, where the surface-induced variation of the order can be observed [5,25,33-41]. 

Second, the molecular orientation of the fiber and the prepolymer matrix is important. The rate of crystal nucleation at the fiber-matrix interface depends on the orientation of matrix molecules just prior to their change of phase from liquid to solid. Thus, surface-nucleated morphologies are likely to dominate the matrix stmcture. 

Stelzer et al. [109] have studied the case of a nematic phase in the vicinity of a smooth solid wall. A distance-dependent potential was applied to favour alignment along the surface normal near the interface that is, a homeotropic anchoring force was applied. The liquid crystal was modelled with the GB(3.0, 5.0, 2, 1) potential and the simulations were run at temperatures and densities corresponding to the nematic phase. Away from the walls the molecules behave just as in the bulk. However, as the wall is approached, oscillations appear in the density profile indicating that a layered structure is induced by the interface, as we can see from the snapshot in Fig. 19. These layers are... 

Liquid crystals, commonly referred to as the fourth state of matter, are materials which are intermediate in character between the solid and liquid states. Unlike normal isotropic liquids, they show some time-averaged positional orientation of the molecules, but they retain many of the properties of liquids, such as the ability to flow. In recent decades, liquid crystals have played an increasingly important part in our lives. Probably their most familiar application is in the information displays which provide the visual interface with microprocessor-controlled instrumentation. Liquid crystal displays have superseded more traditional display technology, such as light-emitting diodes and cathode ray tubes, for many appliances principally because of the advantages of visual appeal, low power consumption, and their ability to facilitate the miniaturisation of devices into which they are incorporated. They are encoun-... 

The Laplace equation (eq. 6.27) was derived for the interface between two isotropic phases. A corresponding Laplace equation for a solid-liquid or solid-gas interface can also be derived [3], Here the pressure difference over the interface is given in terms of the factor that determines the equilibrium shape of the crystal ... 

The combustion wave of HMX is divided into three zones crystallized solid phase (zone 1), solid and/or liquid condensed phase (zone 11), and gas phase (zone 111). A schematic representation of the heat transfer process in the combustion wave is shown in Fig. 5.5. In zone 1, the temperature increases from the initial value Tq to the decomposition temperature T without reaction. In zone 11, the temperature increases from T to the burning surface temperature Tj (interface of the condensed phase and the gas phase). In zone 111, the temperature increases rapidly from to the luminous flame temperature (that of the flame sheet shown in Fig. 5.4). Since the condensed-phase reaction zone is very thin (-0.1 mm), is approximately equal to T . 

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