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So I'm trying to wrap my head around the physics of how a Soxhlet extractor works, not the chemical, solvent and chemical components part, but rather the physical, hydrodynamic, siphoning part : why exactly does the solvent flow back through the siphon tube into the flask ? I imagine Bernoulli's principle, comes into play, but, unlike a usual siphon tube, there's nothing or no one physically initiating the siphoning by sucking air out, or lowering the pressure in the flask. I've tried to find any theoretical study explaining the physics behind a Soxhlet extractor, but to no avail. Any help would be appreciated, thanks.

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Soxhlet extractor, source: https://commons.wikimedia.org/wiki/File:Soxhlet_mechanism.gif

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Vapours of the solvent continuously rise, condense on the Dimroth condenser, and drop as liquid into the thimble. The level of the liquid in the thimble rises, where it yields a hot solution (liquid/solid extraction). The thimble is penetrated by the solution, passing into the outer volume. The level of the liquid in the Soxhlet rises up to the point where the level in the inner is greater than the one in the equally rising arm of the siphon closer to the inner side of the Soxhlet.

By cohesion (of the solution) and gravity (acting on the solution), the potential energy of the solution now is lowered as it passes across the more distant arm, falling (again) from the higher reservoir to the lower one, the still pot. When the solution returns to the lower flask, there is little reason to think there would be a vacuum in the lower flask because vapours still ascend to the Dimroth condenser on top (which, for obvious safety reasons, stays open to atmosphere/is connected to the Schlenk line for pressure equalization).

As for the Bernoulli principle: Indeed it is helpful that the open diameter of the siphon is much smaller than the one containing the thimble. Thus, the local pressure difference generating by the solution returning falling down is large, causing suction next to the thimble during the then swift transfer.

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  • $\begingroup$ +1 I think I doubt that Bernuilli's principle has any significant effect here however ... $\endgroup$
    – Karl
    Commented Dec 21, 2021 at 18:31
  • $\begingroup$ @Karl As long as liquid flows in the siphon, some suction may occur. With fixed length, filling a tube of small section requires less volume of liquid than a tube of large section when length. I agree that a capillary would not be efficient since a small section equally limits flow (as in volume per unit of time) with liquids of fixed density and impractical for cleaning. $\endgroup$
    – Buttonwood
    Commented Dec 22, 2021 at 12:11
  • $\begingroup$ Not sure I follow. Bernuilli says that a flowing substance reduces the static pressure. The pressure difference between the just emptied vessel and the end of the tube is however simply proportional to the filled length of the downward part of the tube minus the filled length of the upward part. $\endgroup$
    – Karl
    Commented Dec 22, 2021 at 16:23
  • $\begingroup$ Not sure I meant this one, intention was «greater velocity for smaller sections if flux is fixed». But ok, Bernoulli dynasty in mathematics, as the Couperin in music. $\endgroup$
    – Buttonwood
    Commented Dec 22, 2021 at 16:35

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