Nucleation

Nucleation is the onset of a phase transition in a small but stable region. The phase transition can be the formation of a gas or of a crystal from a liquid.

Examples of nucleation

Pure water freezes at −42°C rather than at its melting temperature of 0°C if no nucleators are present to form an ice nucleus. Nucleators are therefore important in meteorology because there are often few nucleators present in the upper atmosphere (see cloud seeding).

Bubbles of carbon dioxide nucleate shortly after the pressure is released from a container of carbonated liquid. Nucleation often occurs more easily at a pre-existing interface (heterogeneous nucleation), as happens on boiling chips and string used to make rock candy. A demonstration of nucleation using soda and a candy with many nucleation sites can be observed here.

Nucleation is key to understanding the thermal processing of polymers, alloys, and some ceramics.

In chemistry and biophysics, nucleation can also refer to the phaseless formation of multimers which are intermediates in polymerization processes. This sort of process is believed to be the best model for processes such as crystallisation and amyloidogenesis.

In molecular biology, nucleation is used to term the critical stage in the assembly of a polymeric structure, such as a microtubule, at which a small cluster of monomers aggregates in the correct arrangement to initiate rapid polymerization. For instance, two actin molecules bind weakly, but addition of a third stabilizes the complex. This trimer then adds additional molecules and forms a nucleation site. The nucleation site serves the slow, or lag phase of the polymerization process.

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 heterogenous nucleation temperature (melting temperature), but which are above the homogenous 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 expended 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.

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.

External links

The Extreme Diet Coke & Mentos Experiments - fun with nucleation

polymers | industrial processes | Phase changes

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Examples of nucleation

Mechanics of nucleation

See also

External links