Opalescence phenomenon

Let us consider now behaviour of the gas-liquid system near the critical point. It reveals rather interesting effect called the critical opalescence, that is strong increase of the light scattering. Its analogs are known also in other physical systems in the vicinity of phase transitions. In the beginning of our century Einstein and Smoluchowski expressed an idea, that the opalescence phenomenon is related to the density (order parameter) fluctuations in the system. More consistent theory was presented later by Omstein and Zemike [23], who for the first time introduced a concept of the intermediate order as the spatial correlation in the density fluctuations. Later Zemike [24] has applied this idea to the lattice systems. 

On the other hand, we note that a demixtion curve corresponds to each polymer length and that by changing the lengths, it is possible to study the variation of parameters associated with the opalescence phenomenon. Thus, in order to reveal the universal properties of the results, we must use scaling variables. 

Point c is a critical point known as the upper critical end point (UCEP).y The temperature, Tc, where this occurs is known as the upper critical solution temperature (UCST) and the composition as the critical solution mole fraction, JC2,C- The phenomenon that occurs at the UCEP is in many ways similar to that which happens at the (liquid + vapor) critical point of a pure substance. For example, at a temperature just above Tc. critical opalescence occurs, and at point c, the coefficient of expansion, compressibility, and heat capacity become infinite. 

In his first paper3 , Stokes called the observed phenomenon dispersive reflexion, but in a footnote, he wrote I confess I do not like this term. I am almost inclined to coin a word, and call the appearance fluorescence, from fluorspar, as the analogous term opalescence is derived from the name of a mineral. Most of the varieties of fluorspar or fluorspath (minerals containing calcium fluoride (fluorite)) indeed exhibit the property described above. In his second paper7, Stokes definitely resolved to use the word fluorescence (Scheme 1.2). 

A combination of SLS and DLS methods was used to investigate the behavior of nonionic micellar solutions in the vicinity of their cloud point. It had been known for many years that at a high temperature the micellar solutions of polyoxyethylene-alkyl ether surfactants (QEOm) separate into two isotropic phases. The solutions become opalescent with the approach of the cloud point, and several different explanations of this phenomenon were proposed. Corti and Degiorgio measured the temperature dependence of D pp and (Ig), and found that they can be described as a result of critical phase separation, connected with intermicellar attraction and long-range fluctuations in the local micellar concentration. Far from the cloud point, the micelles of nonionic surfactants with a large number of ethoxy-groups (m 30) may behave as hard spheres. ... 

The intensity of light scattered from a fluid system increases enormously, and the fluid takes on a cloudy or opalescent appearance as the gas-liquid critical point is approached. In binary solutions the same phenomenon is observed as the critical consolute point is approached. This phenomenon is called critical opalescence.31 It is due to the long-range spatial correlations that exist between molecules in the vicinity of critical points. In this section we explore the underlying physical mechanism for this phenomenon in one-component fluids. The extension to binary or ternary solutions is not presented but some references are given. 

In fluids Xt s generally a well-behaved function of the thermodynamic state. Near the critical point, however, Xt becomes divergent (arbitrarily large). It follows that the intensity of scattered light increases very strongly as the critical point is approached. In fact there is so much scattering that the critical fluid appears cloudy or opalescent. This phenomenon, as mentioned above, is called critical opalescence. 

This suggests that near the critical point a fluid displays unusual behavior. The behavior is unusual because natural fluctuations are not completely suppressed, as they are when Kt- is bounded and positive, but neither are fluctuations able to grow so as to force a phase change, as they can when Kj is negative. Such fluctuations cause the observable phenomenon known as critical opalescence moreover, critical fluctuations are independent of molecular constitution, so that near their critical points all fluids have certain traits in common. Descriptions of critical phenomena are beyond the scope of this book see instead [6]. 

FIGURE 10.16. In many colloidal systems, the interaction energy curve will have a small minimum, the secondary minimum, M , that allows the particles to undergo a lose, reversible flocculation. In some systems of relatively large, monodisperse particles, the secondary minimmn may lead to an optical phenomenon called opalescence in which a very regular structure is developed (similar to a crystal structure) that produces beautiful and interesting patterns with incident light. 

When the second drum is heated, the liquid molar volume increases again, and at the same time the vapor molar volume decreases together with the pressure increase. Closely below the critical point, the phenomenon of the critical opalescence... 

As the diffusion coefficient is not a purely thermodynamic quantity, but also specifics the transport (kinetic) properties of a system, this leads to a most important phenomenon in the critical region, namely, critical retardation, discussed in the literature more seldom than critical opalescence. 

Marian von Smoluchowski (1872-1917). .. was a Polish physicist whose research on discrete state matter is still highly valued in modem science. He is particularly acknowledged for his theory on Brownian motion, which he developed independently of Einstein and which laid the foundation for the theory of stochastic processes. A similar rank is deserved by his discovery of density fluctuations in liquids and gases and their relevance for macroscopic scattering— most prominently explained by the phenomenon of critical opalescence. Both works proved veiy influential for the understanding of colloidal suspensions. Furthermore, he did pioneering work on the quantification of particle aggregation as well as in the field of electrokinetic phenomena. 

Opals and Photonic-Band-Gap Materials 3-D ordered arrays of nanostractures with periods of the order of a fraction of the optical wavelength may show intense Bragg diffraction for specific wavelengths and diffraction angles. This phenomenon is the origin of the iridescent colors (opalescence) of opals. Opals consist of an fcc-like array of silica nanoparticles with sizes in the range 150-900 nm [3.82], with a size dispersion below 5% [3.83]. 

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