In short, I am looking for a fluid that meets these criteria:

  • Hydrophobic
  • Denser than water
  • Low viscosity
  • Food safe


We are working on adding a pool and designing a backyard recreational area. One design element that we would like to include in multiple places is waterfalls of varying sizes and styles; I love flowing water. As an extension of that I started wondering if it would be possible to create an underwater waterfall.

Initially I was thinking this could be accomplished by creating a regular waterfall in the pool with normal water, which could then be capped with a clear glass or acrylic watertight cover. This certainly works as an underwater waterfall, but the cover will be visible, and will likely cloud over time, not to mention the condensation issues.

So I'm wandering now if some sort of fluid exists that could actually flow underwater. If it's dense enough it will flow down a rockface even under the water, and if hydrophobic will not mix with the surrounding water. As it will be within a pool that humans will be in for extended periods it must be externally safe, and I'd like something food-safe as kids swallow pool water and it's possible this stuff might get kicked up. And to add to that tall order, the wrong viscosity will ruin the visual effect.

Does such a fluid exist in a form that a normal person can purchase for a reasonable cost? I live in the US (Pennsylvania). Another question, Is there a readily available liquid that's more dense than water and insoluble?, seemed to be looking for something similar but the answer doesn't meet my safety requirements.

  • $\begingroup$ The only thing that comes to mind is brominated vegetable oil, but over the last few years this too has come under scrutiny for possible toxicity. en.wikipedia.org/wiki/Brominated_vegetable_oil $\endgroup$ – Curt F. Jun 21 '15 at 18:08
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    $\begingroup$ Sorry to put a damper on your idea, but even though the fluid might be immiscible with water, natural saponins from plants, detergent residue in the pool (0r even on skin) and even lecithins from insect carcasses or other sources would eventually create an emulsion with some of the fluid and water. How about a gallium fountain (Ga is mildly toxic, though)? It would need to be kept above 30 C, though, which might be OK for a swimming pool. $\endgroup$ – DrMoishe Pippik Jun 21 '15 at 19:14
  • $\begingroup$ What visual effect did you have in mind? Even if there exists a liquid that meets all your criteria, I can't imagine it'll make an interesting waterfall. At best, it might slowly flow down the side of the pool like molasses (EDIT: Not quite as slow as molasses, but I don't know if it would have any striking visual effect) $\endgroup$ – chipbuster Jun 21 '15 at 20:51
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    $\begingroup$ @DrMoishePippik That's a good point; I hadn't considered the effect of contaminants. I wonder if that could be circumvented by replacing the fluid often enough. A gallium fountain is an awesome idea, but is unfortunately economically impractical. :( $\endgroup$ – Nicholas Jun 22 '15 at 2:37
  • $\begingroup$ @chipbuster I do realize the waterfall will be slow. My 'low viscosity' requirement was to try and address this issue. I imagine molasses would make a poor waterfall because of its goopy nature, but a more free-flowing liquid could result in what appears to be a waterfall in slow motion; a very striking effect. Part of the appeal of this idea, of course, is that it's unusual, and will take some creativity and personal effort to bring to life. It will help to personalize the space for me and provide an interesting conversation piece. $\endgroup$ – Nicholas Jun 22 '15 at 2:39

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