Critical opalescence is a phenomenon which arises in the region of a continuous, or second-order,

phase transition In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states of ...

. Originally reported by Charles Cagniard de la Tour in 1823 in mixtures of alcohol and water, its importance was recognised by

Thomas Andrews Thomas Andrews Jr. (7 February 1873 – 15 April 1912) was a British businessman and shipbuilder. He was managing director and head of the drafting department of the shipbuilding company Harland and Wolff in Belfast, Ireland. He was the nava ...

in 1869 following his experiments on the liquid-gas transition in

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is tr ...

; many other examples have been discovered since. The phenomenon is most commonly demonstrated in binary fluid mixtures, such as

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is ...

and

cyclohexane Cyclohexane is a cycloalkane with the molecular formula . Cyclohexane is non-polar. Cyclohexane is a colorless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cycloh ...

. As the critical point is approached, the sizes of the gas and liquid region begin to fluctuate over increasingly large length scales (the

correlation length A correlation function is a function that gives the statistical correlation between random variables, contingent on the spatial or temporal distance between those variables. If one considers the correlation function between random variables re ...

of the liquid diverges). As the density fluctuations become of a size comparable to the wavelength of light, the light is scattered and causes the normally transparent liquid to appear cloudy. Tellingly, the

opalescence Opalescence refers to the optical phenomena displayed by the mineraloid gemstone opalopalescent. 2019. In Noah Webster's 1828 American Dictionary of the English Language. Retrieved January 7, 2019, from https://1828.mshaffer.com/d/word/opalesc ...

does not diminish as one gets closer to the critical point, where the largest fluctuations can reach even centimetre proportions, confirming the physical relevance of smaller fluctuations. In 1908 the Polish physicist Marian Smoluchowski became the first to ascribe the phenomenon of critical opalescence to large density fluctuations. In 1910

Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...

showed that the link between critical opalescence and

Rayleigh scattering Rayleigh scattering ( ), named after the 19th-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the ...

is quantitativ

External links

A time-lapse video of critical opalescence in a binary mixture
More-detailed experimental demonstrations of critical opalescence may be found at *https://web.archive.org/web/20071121073241/http://www.physicsofmatter.com/NotTheBook/CriticalOpal/Explanation.html *https://web.archive.org/web/20041119135015/http://www.msm.cam.ac.uk/doitpoms/tlplib/solid-solutions/demo.php Critical phenomena Phase transitions