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I need to make a cloudy white liquid where the particles won't eventually settle out. It will be exhibiting the Tyndall effect for a very long time.

I'd use milk, but I don't want it to become rancid. I've tried white inks and fabric softener, but both eventually tend toward the bottom half of the container. I think it's a matter of particle size.

What substance would hit the sweet spot - small enough to stay dispersed, large enough to scatter light, and not reactive?

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  • $\begingroup$ Bleached wheat flour maybe? See Wikipedia for Tyndall effect. $\endgroup$ – airhuff Jul 3 '17 at 17:09
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    $\begingroup$ Maybe try latex house paint diluted in water. These products are designed to remain usable (colloidal) for years while sitting on the shelf. When dry, house paint is pretty stable to the elements, so perhaps it will not go rancid quickly. If it does, perhaps you could add a mold and bacterial growth inhibitor. $\endgroup$ – iad22agp Jul 3 '17 at 19:08
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Starch in water might work, perhaps with table salt or other bacterial/fungal growth inhibitor. Follow directions in most recipes: mix a bit of starch in cold water and then add gradually to hot water to make a more stable "soup".

BTW, blasting a colloid with ultrasound (e.g. from an ultrasonic humidifier) homogenizes it and may delay separation.

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As a first approximation, there are no stable colloids. I suggest you look for kinetically stable (i.e. metastable) colloids. The academic community is divided between those who insist that no colloid system is stable and those who offer theoretical reasons why certain systems are thermodynamically stable. I used to believe no colloids are stable (thermodynamically) but changed my mind (based on the scientific literature at the time). Unfortunately, that was decades ago, and I'm not able to give you chapter and verse rationale why certain colloids may be stable. To be "stable" the particle needs to both not fall (match density to its media so that thermal motions (and mechanical vibrations, possibly) overwhelm any tendency to fall) and more difficultly have a lower free energy in the highly dispersed state, specifically it must be electrostatically repulsive towards other particles and have virtually no tendency towards Ostwald Ripening. This limits your options to organic (charged) polymers and if you can "structure" the water, that's all to the better. If I recall certain acrylates were able to be synthesized into particles fitting those requirements, but it was a long time ago. Permanent is a long time, I really doubt you need permanent. I'd bet 50, 100, or maybe 200 years would do just fine.

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    $\begingroup$ This is mostly monologue about semantics. Certain acrylates doesn't really answer this. $\endgroup$ – Mithoron Jul 3 '17 at 19:55
  • $\begingroup$ We appreciate your expertise and experience, but providing the actual references to the literature would make this a much stronger answer. Right now, it's kind of a hybrid between an answer and a long comment. $\endgroup$ – jonsca Jul 3 '17 at 23:53

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