Editing Colloid (section)

==Crystals==

{{Main|Colloidal crystal}}

A colloidal crystal is a highly [[Order (crystal lattice)|ordered]] array of particles that can be formed over a very long range (typically on the order of a few millimeters to one centimeter) and that appear [[analogous]] to their atomic or molecular counterparts.<ref>{{cite journal|author =Pieranski, P.|year =1983| title = Colloidal Crystals| journal= Contemporary Physics| volume= 24| pages =25–73|doi =10.1080/00107518308227471|bibcode = 1983ConPh..24...25P }}</ref> One of the finest [[natural]] examples of this ordering phenomenon can be found in precious [[opal]], in which brilliant regions of pure [[wikt:spectrum|spectral]] [[color]] result from [[close-packed]] domains of [[amorphous]] colloidal spheres of [[silicon dioxide]] (or [[silica]], SiO<sub>2</sub>).<ref>{{cite journal|author = Sanders, J.V.|year =1964|title = Structure of Opal|journal = Nature |volume=204|page =1151|doi=10.1038/204990a0|last2 = Sanders|first2 = J. V.|last3 = Segnit|first3 = E. R.|s2cid =4191566|bibcode = 1964Natur.204..990J|issue=4962}}</ref><ref>{{cite journal|author = Darragh, P.J.|year =1976|journal = Scientific American|volume=234|issue =4|pages=84–95|display-authors=etal|doi=10.1038/scientificamerican0476-84|title=Opals|bibcode=1976SciAm.234d..84D}}</ref> These spherical particles [[precipitate]] in highly [[siliceous]] pools in [[Australia]] and elsewhere, and form these highly ordered arrays after years of [[sedimentation]] and [[compression (physical)|compression]] under [[hydrostatic]] and gravitational forces. The periodic arrays of submicrometre spherical particles provide similar arrays of [[interstitial defect|interstitial]] [[wikt:void|voids]], which act as a natural [[diffraction grating]] for [[visible spectrum|visible]] [[light]] [[wave]]s, particularly when the interstitial spacing is of the same [[order of magnitude]] as the [[Optical physics|incident]] lightwave.<ref>{{cite journal |last1=Luck |first1=Werner |last2=Klier |first2=Manfred |last3=Wesslau |first3=Hermann |title=Über Bragg-Reflexe mit sichtbarem Licht an monodispersen Kunststofflatices. II |journal=Berichte der Bunsengesellschaft für Physikalische Chemie |date= 1963 |volume=67 |issue=1 |pages=84–85 |doi=10.1002/bbpc.19630670114}}</ref><ref>{{cite journal|author1=Hiltner, P.A.  |author2=Krieger, I.M.|year =1969|title = Diffraction of light by ordered suspensions|journal=J. Phys. Chem.|volume=73|page=2306|doi = 10.1021/j100727a049|issue = 7}}</ref>

Thus, it has been known for many years that, due to [[Coulomb's Law|repulsive]] [[Coulombic]] interactions, [[electrically charged]] [[macromolecule]]s in an [[aqueous]] environment can exhibit long-range [[crystal]]-like correlations with interparticle separation distances, often being considerably greater than the individual particle diameter. In all of these cases in nature, the same brilliant [[iridescence]] (or play of colors) can be attributed to the diffraction and [[constructive interference]] of visible lightwaves that satisfy [[Bragg’s law]], in a matter analogous to the [[scattering]] of [[X-ray]]s in crystalline solids.

The large number of experiments exploring the [[physics]] and [[chemistry]] of these so-called "colloidal crystals" has emerged as a result of the relatively simple methods that have evolved in the last 20 years for preparing synthetic monodisperse colloids (both polymer and mineral) and, through various mechanisms, implementing and preserving their long-range order formation.<ref>{{Cite journal|last1=Liu|first1=Xuesong|last2=Li|first2=Zejing|last3=Tang|first3=Jianguo|last4=Yu|first4=Bing|last5=Cong|first5=Hailin|date=2013-09-09|title=Current status and future developments in preparation and application of colloidal crystals|journal=Chemical Society Reviews|volume=42|issue=19|pages=7774–7800|doi=10.1039/C3CS60078E|pmid=23836297}}</ref>