Critical Phenomena
Everything in the universe that occupies space is considered matter. All matter is classified as being a solid, liquid, gas or plasma (very hot ionized gases) depending on its physical state. Each of these states of matter is characterized by different physical properties. Critical phenomena relates to the physical properties of matter that occur at the phase transition points and specifically at the critical points.
While the first theories of matter were proposed by the ancient Greeks as early as 500 B.C., it was not until the nineteenth century that the modern understanding of matter was proposed. At this time, English chemist John Dalton (1766- 1844) introduced the atomic theory that postulated that all matter is composed of individual particles called atoms. He also suggested that various atoms exist with different masses and sizes to make up all of the known elements. Today, it is known that these atoms can bond with each other to produce molecules.
Building on Dalton's theory, scientists deduced that the different states of matter are a result of the physical arrangement of atoms in space. For example, a solid substance like ice is composed of water molecules that are bound relatively close together and neatly ordered. When the ice is exposed to heat under atmospheric pressure, the molecules move farther apart and become less ordered. When the temperature is raised above the first transition temperature (the melting point) the solid ice abruptly becomes liquid water. Similarly, when the temperature is increased above the second transition temperature (boiling point) the molecules move so far apart that the liquid water quickly becomes water vapor, or steam. When the temperature is raised so high that the electrons are separated from their atoms (the atoms are 100% ionized), then plasma is formed. The transitions from solid to liquid and liquid to gas represent first-order phase transitions.
Critical phenomena relate to the changes that occur at the second-order or continuous phase transition. This is a physical transition that occurs above the critical point. The critical temperature is the temperature above which vapor cannot be liquefied no matter how much pressure is applied to the system. The critical pressure is the amount of pressure required to liquefy water at the critical temperature. Together, these values define the critical point. For water, the critical point occurs at a temperature of 705.2°F (374°C) and a pressure of 2.21 x 107 pascals.
Different materials have different critical points. Unlike first-order phase transitions, second-order transitions are gradual. In a pressurized system of water vapor, as the temperature is decreased toward the critical point, the vapor goes through an intermediate "fluid" critical phase transition that is neither liquid nor a gas. It is milky and turbid and represents a phenomenon known as critical opalescence. Another example of a critical phenomenon occurs in solid, ferromagnetic materials such as iron and nickel. At the critical point, also called the Curie point, these materials lose their natural magnetic properties and can only be magnetized by applying a magnetic field.Superconductivity is another example of a critical phenomenon.
One of the primary goals of scientists who study critical phenomena is to develop a common theory to explain and predict the behavior of matter above the critical point. A common feature of all systems experiencing critical phase transitions is order parameters. These are quantities that are zero on one side of the critical point and nonzero on the other. Net magnetism is an order parameter in ferromagnetic systems because above the critical point magnetism is zero. Below this point, it gradually increases.Density and concentration are other properties that can be order parameters. In addition to order parameters, other thermodynamic quantities, such as heat capacity, fluctuate significantly at the critical point.
The mean field theories represent the earliest attempts at describing the behavior of systems experiencing critical transitions. These include the van der Waals model for fluids, proposed in 1873, and the Weiss model of ferromagnets, suggested in 1907. Characteristic of these theories is the assumption that every particle in a system can be described by the average properties of the whole system. While these theories were successful in predicting numerous aspects of critical phenomena, they did not provide adequate quantitative results. This desire for quantitative results fueled the development of the modern theory of critical phenomena.
The modern theory began with the scaling hypothesis suggested by Ben Widom in 1965 and the universality hypothesis proposed by Leo Kadanoff in 1967. These theories were based on the assumption that the values of critical exponents of a system depend only on the general features such as dimensionality and symmetry and not on particle- particle interactions. Using a mathematical procedure originally developed for quantum mechanics, Ken Wilson translated the ideas of the scaling hypothesis and the universality hypothesis into the renormalization group method. This method resulted in more solvable equations.
One of the difficulties in verifying these critical phenomena hypotheses is encountered when measuring the critical parameters experimentally. This is because the experiments must be extremely precise and carefully done. In 1996, Robert Gammon and his colleagues overcame some of these problems by performing a critical phenomena experiment on the space shuttle Columbia. High above Earth, they were able to maintain the critical temperature to millionths of a degree without the interference of gravity. This allowed them to take readings of various properties at the critical point. It is thought that through more experiments like these researchers will gain a fundamental understanding for what actually occurs during critical phase transitions.
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