Isotropic droplet

This is also observed to be the case for free droplets [116]. Indeed, simulations started from isotropic droplets below the smectic B-isotropic transition form cylindrical rather than spherical droplets these are apparent in Fig. 22. In this way, the molecules can align in parallel layers with the... 

Under these circumstances, it was difficult to distinguish between true high strength singularities and apparent singularities consisting of several points bunched together around an immiscible isotropic droplet. 

Small isotropic droplets, c, 1 im or smaller in size, are not easily resolved by microscopy. A minor isotropic component is often found to trail at the lower temperature range within the biphase and its presence can be overlooked [25]. Yet a finely dispersed I phase can be expected to influence rheological behavior, order and orientation dynamics in the mesophase, as well as the mechanical properties of the resulting solid phase. In contrast to microscopy or DSC which detect macroscopic behavior, NMR provides a molecular or segment level view of morphology and cannot distinguish between phase and microphase separation. Thus accurate biphase delineation may be delicate to accomplish. We should further note that the customary dynamic scans do not provide the equilibrium value of biphase width but rather an apparent value as determined by thermal history (section 6.5). 

Thermal phase transitions have also been revealed in liquid crystal obtained from various tissues (Table 1). Not surprisingly, with thermal stage temperature increase, the birefringent liquid crystal droplets transit to non-refracting isotropic droplets. With temperature decrease, the liquid crystal droplets transit into crystal. However, when the isotropic droplets cool, two different results, controllable by rate of temperature decrease, were possible. If the rate of temperature drop is fast (the slide is placed on a 4°C plate), then the isotropic droplets will transition into liquid-crystals. However, if the rate of temperature decrease is slow (temperature is allowed to drop in step with the slowly cooling cop>per thermo-controller) then the isotropic droplets will transit to crystal (Figure 3). This finding... 

Fig. 3. Thermal phase transitions of liquid crystal droplet, crystal and isotropic droplet.

The phase transition from isotropic droplet to crystal or liquid crystal depends on the rate of temperature decrease (A). B and C exhibit the hepatic liquid crystal droplets in crossed nicols with 90 degree of angle (B) and 45 degree (C). D and E show the hepatic crystal in crossed nicols at 90 degree of angle (D) and 45 degree (E), which transited from liquid crystals. Anisotropic liquid crystals locate in hepatocytes in the cords and are absent in the blood sinus (bs). Bars, 60 pm. 

The perpendicular arrangement of the long axes of the 5CB molecules on the droplet walls resulted in a homeotropic alignment, which was visualized under POM as radial droplets. In addition, when the PDLC samples were heated to the upper isotropization temperature of the liquid crystal, the isotropic droplets remained surrounded by a birefringent shadow—the signature of a polymer matrix ordering... 

The herring-bone moleeular arrangement in the SmO phase of MHTAC was first confirmed by an interferential mieroscope observation of a droplet of racemic MHTAC on a glass surface [13]. The temperature of the droplet was controlled so that a few smectic layers were formed on the isotropic droplet. By changing the temperature under the application of an electric field parallel to the glass surface, the number of layers was changed and the structure was examined. In this way, it was confirmed that the molecular tilt sense alters from layer to layer. It has been confirmed that the SmO and SmCA phases are identical [14]-[16j. 

The distribution of defects in mesophases is often regular, owing to their fluidity, and this introduces pattern repeats. For instance, square polygonal fields are frequent in smectics and cholesteric liquids. Such repeats occur on different scales - at the level of structural units or even at the molecular level. Several types of amphiphilic mesophase can be considered as made of defects . In many examples the defect enters the architecture of a unit cell in a three-dimensional array and the mesophase forms a crystal of defects [119]. Such a situation is found in certain cubic phases in water-lipid systems [120] and in blue phases [121] (see Chap. XII of Vol. 2 of this Handbook). Several blue phases have been modeled as being cubic centred lattices of disclinations in a cholesteric matrix . Mobius disclinations are assumed to join in groups of 4x4 or 8x8, but in nematics or in large-pitch cholesterics such junctions between thin threads are unstable and correspond to brief steps in recombinations. An isotropic droplet or a Ginsburg decrease to zero of the order parameter probably stabilizes these junctions in blue phases. 

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian Hquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin Hquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a large number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the lUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

To determine whether the 8CB droplets condensed above 41°C (trapped in the isotropic phase) sit on a trilayer or on bare silicon, we used the ATM tip to mechanically spread the droplets and thus accelerate their conversion to a stable configuration. The SPFM images shown in Fignre 15 were obtained after such tip-induced spreading. A layered structure with 32-A-high steps typical of the smectic phase is obtained. The first, or bottom, layer is 41 A thick, while the layers above it are all 32 A thick. This indicates that the bottom layer of the film is a trilayer and that the remaining snbstrate is dry silicon, i.e.. 

Many other interesting examples of spontaneous reflection symmetry breaking in macroscopic domains, driven by boundary conditions, have been described in LC systems. For example, it is well known that in polymer disperse LCs, where the LC sample is confined in small spherical droplets, chiral director structures are often observed, driven by minimization of surface and bulk elastic free energies.24 We have reported chiral domain structures, and indeed chiral electro-optic behavior, in cylindrical nematic domains surrounded by isotropic liquid (the molecules were achiral).25... 

In some practical processes, a high relative velocity may not exist and effects of turbulence on droplet breakup may become dominant. In such situations Kolmogorov, 280 and Hinze[27°l hypothesized that the turbulent fluctuations are responsible for droplet breakup, and the dynamic pressure forces of the turbulent motion determine the maximum stable droplet size. Using Clay s data, 2811 and assuming isotropic turbulence, an expression was derived for the critical Weber number 270 ... 

In that case the self diffusion coefficient - concentration curve shows a behaviour distinctly different from the cosurfactant microemulsions. has a quite low value throughout the extension of the isotropic solution phase up to the highest water content. This implies that a model with closed droplets surrounded by surfactant emions in a hydrocarbon medium gives an adequate description of these solutions, found to be significantly higher them D, the conclusion that a non-negligible eimount of water must exist between the emulsion droplets. 

The term microemulsion is applied in a wide sense to different types of liquid liquid systems. In this chapter, it refers to a liquid-liquid dispersion of droplets in the size range of about 10-200 nm that is both thermodynamically stable and optically isotropic. Thus, despite being two phase systems, microemulsions look like single phases to the naked eye. There are two types of microemulsions oil in water (O/W) and water in oil (W/O). The simplest system consists of oil, water, and an amphiphilic component that aggregates in either phase, or in both, entrapping the other phase to form... 

Mashayek, F., F. A. Jaberi, R. S. Miller, and R Givi. 1997. Dispersion and poly-dispersity of droplets in stationary isotropic tnrbnlence. Int. J. Adultiphase Flow 23(2) 337-55. 

Mashayek, F. 1999. Simnlations of reacting droplets dispersed in isotropic turbulence. AMA J. 37(ll) 1420-25. 

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