Editing Crystallization (section)

===Growth===
{{Main|Crystal growth}}
Once the first small crystal, the nucleus, forms it acts as a convergence point (if unstable due to supersaturation) for [[molecules]] of solute touching – or adjacent to – the crystal so that it increases its own dimension in successive layers. The pattern of growth resembles the rings of an onion, as shown in the picture, where each colour indicates the same mass of solute; this mass creates increasingly thin layers due to the increasing surface area of the growing crystal. The supersaturated solute mass the original nucleus may ''capture'' in a time unit is called the ''growth rate'' expressed in kg/(m<sup>2</sup>*h), and is a constant specific to the process. Growth rate is influenced by several physical factors, such as [[surface tension]] of solution, [[pressure]], [[temperature]], relative crystal [[velocity]] in the solution, [[Reynolds number]], and so forth.

The main values to control are therefore:
* Supersaturation value, as an index of the quantity of solute available for the growth of the crystal;
* Total crystal surface in unit fluid mass, as an index of the capability of the solute to fix onto the crystal;
* Retention time, as an index of the probability of a molecule of solute to come into contact with an existing crystal;
* Flow pattern, again as an index of the probability of a molecule of solute to come into contact with an existing crystal (higher in [[laminar flow]], lower in [[turbulent flow]], but the reverse applies to the probability of contact).

The first value is a consequence of the physical characteristics of the solution, while the others define a difference between a well- and poorly designed crystallizer.