Scattering spatially correlated fluctuations

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. 

Scattering signals in a gelling solution are dominated by interference between different clusters evolving by gelation. This interference conceals structural information on individual clusters. Hence, in order to investigate the spatial correlation of concentration fluctuations, a scattering experiment was carried out on a diluted system. 

Therefore, such a function contains all the information regarding spatial correlations of scattering elements. The scattering is then completely defined in terms of the mean squared fluctuation in the scattering power, and the correlation function yCr) For several systems... 

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