What Is Nucleation?
Nucleation is a physical reaction which occurs when components in a solution start to precipitate out, forming nuclei which attract more precipitate. In a simple example of nucleation, supersaturated sugar water is used to make rock candy, with the sugar crystals nucleating and growing into crystals. This physical reaction is the basis for a variety of manufacturing processes and interesting natural phenomena.
In homogeneous nucleation, the solution is totally uniform, and the process occurs spontaneously, usually in response to something like a change in pressure or temperature. Heterogeneous nucleation involves a solution which is mixed in nature. Nucleation tends to occur much more quickly in such mixtures. As an example of the difference between heterogeneous and homogeneous nucleation, purified water freezes at a lower temperature than conventional water, because the purified water lacks small particles which can be used as “seeds” to start the nucleation process.
Typically, once nucleation starts to occur, it experiences an exponential growth curve. For example, when crystals form to make rock candy, they start slowly, but as they grow, the surface area increases, attracting more sugar and promoting the growth of crystals at an ever increasing rate, until the solution stabilizes and no more sugar can precipitate out. This also explains why it takes time for a river to freeze over in the winter, but once the ice starts forming in the body of the river, it usually covers the river very rapidly.
There are a wide variety of interesting applications for this principle, and many manufacturers take advantage of it to make an assortment of products, such as electronic components and candies. Nucleation can also be used in activities such as cloud seeding, in which people encourage clouds to form, and it can be used for lab experiments and scientific research on various compounds and solutions.
Some people may be familiar with the rather explosive results of mixing Mentos mints with diet soda, especially Diet Coke®. This is an example of this physical reaction in which the dissolution of mints attracts the dissolved carbon dioxide in the soda, with the process appearing so rapidly that the carbon dioxide precipitates out rather explosively. The reaction cannot be duplicated with unsweetened or sugar-sweetened sodas because the solution does not have the right properties, and the composition of the mints is also important, as specific ingredients in the mints trigger the rapid rate of nucleation which creates a spray of soda.
Written by S.E. Smith
