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Nucleation is the onset of a phase transition in a small region. The phase transition can be the formation of a bubble or of a crystal from a liquid. Creation of liquid droplets in saturated vapor or the creation of gaseous bubble in a saturated liquid is also characterized by nucleation (see Cloud condensation nuclei).
Nucleation normally occurs at nucleation sites on surfaces containing the liquid or vapor. Suspended particles or minute bubbles also provide nucleation sites. This is called heterogeneous nucleation. Nucleation without preferential nucleation sites is homogeneous nucleation. Homogeneous nucleation occurs spontaneously and randomly, but it requires superheating or supercooling of the medium. Nucleation is involved in such processes as cloud seeding and in instruments such as the bubble chamber and the cloud chamber.
Examples of nucleation
Mechanics of nucleation
Nucleation generally occurs with much more difficulty in the interior of a uniform substance, by a process called homogeneous nucleation. Liquids cooled below the maximum heterogeneous nucleation temperature (melting temperature), but which are above the homogeneous nucleation temperature (pure substance freezing temperature) are said to be supercooled. This is useful for making amorphous solids and other metastable structures, but can delay the progress of industrial chemical processes or produce undesirable effects in the context of casting.
The creation of a nucleus implies the formation of an interface at the boundaries of the new phase. Some energy is consumed to form this interface, based on the surface energy of each phase. If a hypothetical nucleus is too small, the energy that would be released by forming its volume is not enough to create its surface, and nucleation does not proceed. The critical nucleus size can be denoted as by its radius, and it is when r=r* (or r critical) that the nucleation proceeds. As the phase transformation becomes more and more favorable, the formation of a given volume of nucleus frees enough energy to form an increasingly large surface, allowing progressively smaller nuclei to become viable. Eventually, thermal activation will provide enough energy to form stable nuclei. These can then grow until thermodynamic equilibrium is restored.
The spontaneous nucleation rate in, say, water changes very rapidly with temperature, so the spontaneous nucleation temperature can be quite well defined. 'Film boiling' on very hot surfaces and the Leidenfrost effect are both believed to be stabilized by spontaneous nucleation phenomena.
In the case of heterogeneous nucleation, some energy is released by the partial destruction of the previous interface. For example, if a carbon dioxide bubble forms between water and the inside surface of a bottle, the energy inherent in the water-bottle interface is released wherever a layer of gas intervenes, and this energy goes toward the formation of bubble-water and bubble-bottle interfaces. The same effect can cause precipitate particles to form at the grain boundaries of a solid. This can interfere with precipitation strengthening, which relies on homogeneous nucleation to produce a uniform distribution of precipitate particles.
Theory of the spinodal region nucleation
Nucleation processes can also be explained in terms of spinodal decomposition where phase separation is delayed until the system enters the unstable region where a small perturbation in composition leads to a decrease in energy and thus spontaneous growth of the perturbation. This region of a phase diagram is known as the spinodal region and the phase separation process is known as spinodal decomposition and may be governed by the Cahn-Hilliard Equation.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Nucleation". A list of authors is available in Wikipedia.|