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, 2015 at 18:08
  • 2
    $\begingroup$ 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. $\endgroup$ Jun 21, 2015 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, 2015 at 20:51
  • 1
    $\begingroup$ @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, 2015 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, 2015 at 2:39

1 Answer 1


Hello I know I went late to the party, but I have a solution to this. You all already have the right idea, but you need to put your ideas together.

Double layer your pool

Whether it's gallium, water, oil, or some other liquid it will all work and be safe under glass.

Their are hydrophobic resins that you can treat the glass with, and if you double paned the area that you're waterfall would be under, it would not cause a health problem because there would be no contact.

For example if you had made window panes in your pool (let's just say in a t-shaped stretching halfway under your pool), and you coated the window panes with a hydrophobic resin, and you kept it under let's just say a temperature controlled environment, that would one solve your pool problem of being boring and two, keep your waterfall from fogging up.

So in short 1 Removable hydrophobic panels for cleaning 2 Heated/ chilled pool or temperature controlled environment 3 Heated / cooling pump for waterfall 4 Well designed lighting ( you'd be surprised what a little bit of light in the right spot can do)

I do not know if it's available to the public yet but there is a coolant used for computers that is one consumable and two low maintenance called Coriant. It has a lighter density than water, if you're panes are hydrophobate this would not present a problem.

Colored mineral oil or antifreeze would also work here.

If you use antifreeze and we're creative with its use, it may assist in some other mechanical uses as well as being a display piece.

Please note that any hydrophobic material or substance is going to be toxic to breathe or have extended contact with.

This should not be a problem if it is under a air / water tight encasing.

I apologize that I'm so late in responding to this but I hope someone reads this and finds it useful thank you for your time.


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