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In particular, an isopycnic surface is a surface of constant density. It is common in conversational use to hear isopycnic surfaces referred to simply as "iso-density" surfaces, which while strictly incorrect, is nonetheless abundantly more clear. — “Isopycnic - Wikipedia, the free encyclopedia”, en.wikipedia.org
( The word "isopycnic" means "equal density".) Typically, a "self-generating" density gradient is established via equilibrium sedimentation, and then analyte molecules concentrate as bands where the molecule density matches that of the surrounding solution. — “Isopycnic centrifugation - Wikipedia, the free encyclopedia”, en.wikipedia.org
The parts about isopycnic- and ultracentrifugation should be moved to their appropriate articles (see isopycnic centrifugation and ultracentrifuge), and a separate article should be created for rate zonal sedimentation (and merged with Sucrose. — “Talk:Differential centrifugation - Wikipedia, the free”, en.wikipedia.org