During a test in colloid and surface chemistry, students were asked to distinguish between two terms: orthokinetic and perikinetic coagulation.

The rate of aggregation is in general determined by the frequency of collisions and the probability of cohesion during collision.

  • If the collisions are caused by Brownian motion, the process is called perikinetic aggregation;
  • [If the collisions are caused] by hydrodynamic motions (e.g. convection or sedimentation), one may speak of orthokinetic aggregation.

from Everett, D. H. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry'. Pure and Applied Chemistry, 2009, 31 (4), 577–638, p 610. doi: 10.1351/pac197231040577

Perhaps unsurprisigly, I got the terms mixed up, and used definitions the wrong way round. The problem was that nothing too obvious popped out when considering the prefixes and the respective processes.

  • Question: To which meaning or association are the prefixes peri- and ortho- referring? The definitions are mostly clear. It is just that I would rather avoid memorisation, and would prefer to be able to always recall which is which based on the prefixes alone.

This means other meanings and etymology of peri- and ortho- are welcome, but will be considered off-topic on their own, unless a clear connection to the definitions of orthokinetic and perikinetic is provided. A short answer with an authoritative reference is very acceptable.

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    $\begingroup$ Haven't found a good source, but my guess is the difference is related to undirected vs directed motion. Peri- means near or around, so perikinetic aggregation is just due to any motion around the cluster. Ortho- can mean straight, which to me would suggest orthokinetic aggregation is caused by processes directed at the cluster. $\endgroup$ – Tyberius Dec 16 '17 at 16:07

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