Crystal-like instabilities

158°F). Cool the solution and bring to volume. Because NaVOs does not fully dissolve, solutions must be filtered through Whatman no. 2 (or equivalent) filter paper prior to use. 

Collect a portion of wine sample containing the suspect sediment either by filtration or centrifugation. 

Rinse the sediment or crystals with a small volume of distilled water, transfer to a membrane filtration apparatus, and apply vacuum. 

Transfer the membrane to a watch glass. Alternatively, crystals can be collected and placed into a well of a spot plate. 

Place a drop of 1 + 3 H2SO4 on the precipitate and then a drop of metavanadate solution before examining the color of the sediment. 

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Another type of interfacial instability occurred in both systems whenever liquid crystal penetrated the brine to contact the brine-microemulsion interface. At high magnification (40x), rapid convection of liquid crystal particles to the interface was observed at volcano-like instabilities (Figure 11). Reported earlier for the same systems (4), this type of instability forms convection currents in the surrounding brine phase. After times ranging from a few seconds to a few hours, the instabilities choke-off." The mechanism by which this small-scale convection is initiated, maintained, and terminated is as yet unknown. 

In both the cases considered, an optical contrast of the patterns observed in isotropic liquids is very small. Certainly, the anisotropy of Uquid crystals brings new features in. For instance, the anisotropy of (helectric or diamagnetic susceptibility causes the Fredericks transition in nematics and wave like instabilities in cholesterics (see next Section), and the flexoelectric polarizaticm results in the field-controllable domain patterns. In turn, the anisotropy of electric conductivity is responsible for instability in the form of rolls to be discussed below. All these instabilities are not observed in the isotropic liquids and have an electric field threshold controlled by the corresponding parameters of anisotropy. In addition, due to the optical anisotropy, the contrast of the patterns that are driven by isotropic mechanisms , i.e. only indirectly dependent on anisotropy parameters, increases dramatically. Thanks to this, one can easily study specific features and mechanisms of different instability modes, both isotropic and anisotropic. The characteristic pattern formation is a special branch of physics dealing with a nonlinear response of dissipative media to external fields, and liquid crystals are suitable model objects for investigation of the relevant phenomena [39]. 

G. Porte and J.P. Jadot, A phase transition-like instability in static samples of twisted nematic liquid crystal when the surfaces induce tilted alignments, J. de Physique, 39, 213-223 (1978). 

There is no ready explanation for the relative instability of the alkyl-substituted cyclopentadienylthallium(I) compounds. The change in the hydrocarbon substrate, which results from introduction of an alkyl group, is likely to be small and insufficient to account for the dramatic stability differences [in the case of 9-R-substituted fluorenes, for example, where R = H, CHj, C2H5, or terf-butyl, the maximum difference in p. units between any two compounds is only 1.6 (9)]. The change in orbital overlap caused by introduction of an alkyl group is also likely to be small the l,T-dialkylferrocenes, for example, are stable compounds 137). At present, the only plausible explanation appears to be that the relative stabilities of the crystal lattices vary markedly 169). 

Ammonium nitrate (AN) was considered early as an environment-friendly alternative to AP but its multiple crystal phase-transitions at low temperatures and its poor performance precluded its use. The nitramine-based propellants are also likely to emerge as potential eco-friendly propellants as the combustion products are non-toxic and non-smoky, although the present day nitramine propellants do not match the high performance and high burn rates of AP-based composite propellants [62, 63]. At the same time, high pressure exponent and unstable combustion prevent their application in large rockets due to safety considerations [20]. The inclusion of Al powder and other additives increases the burn rate and also eliminates the combustion instability. 

Liquid crystals, as the name implies, are condensed phases in which molecules are neither isotropically oriented with respect to one another nor packed with as high a degree of order as crystals they can be made to flow like liquids but retain some of the intermolecular and intramolecular order of crystals (i.e., they are mesomorphic). Two basic types of liquid crystals are known lyotropic, which are usually formed by surfactants in the presence of a second component, frequently water, and thermotropic, which are formed by organic molecules. The thermotropic liquid-crystalline phases are emphasized here they exist within well-defined ranges of temperature, pressure, and composition. Outside these bounds, the phase may be isotropic (at higher temperatures), crystalline (at lower temperatures), or another type of liquid crystal. Liquid-crystalline phases may be thermodynamically stable (enantiotropic) or unstable (monotropic). Because of their thermodynamic instability, the period during which monotropic phases retain their mesomorphic properties cannot be predicted accurately. For this reason it is advantageous to perform photochemical reactions in enantiotropic liquid crystals. 

First, laboratory testing is conducted to ascertain the stability of the wine. Like tests for protein stability, tests for determining stability and method for correcting instability vary from winery to winery. Berg (34) suggested that a wine stored at — 4° C for four days, without a bitartrate crystalline deposit, may be considered stable. The wines usually are allowed to warm to room temperature before test results are read. Absence of crystals indicates stability. A quantitative method, the concentration product (36), also can be used to evaluate tartrate stability. 

Instabilities in a 1-D system, driven by a strong on-site electron-electron Coulomb repulsion U, lead to a Mott-Hubbard insulator [161], particularly for p = 1 systems this causes charge localization, and the crystal becomes insulating. For a chemist, a Mott-Hubbard insulator is like a NaCl crystal, where the energy barrier to moving a second electron onto the Cl site is prohibitively high, as is the cost of moving an electron off a Na site. 

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