Problem with Synthetic Oil in a Science Project

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Hey everyone,

Hopefully this question will be unique since I don't actually care at all about the engine performance of the oil, only the chemistry and material properties.

I am doing experiments for a project where I need to record video of water droplets precipitating through an oil layer and then fall into a water layer. The general goal is to characterize the behavior of the velocity. We have made an acrylic tank (like an aquarium) and about half the tank has water and half the tank has oil on top.

Here's the problem we are facing: after a while, the oil/water interface becomes cloudy and milky and begins swirling all over, making visibility worse than it already was. We are using a couple blends to vary viscosity and density, but for now I'll just keep it to Synthetic Castrol 5W-30. It seems from researching BITOG that maybe it's the hydrolysis of the ester in the oil.

Is that what it looks like when hydrolysis occurs in a synthetic blend like that when water is introduced (milky/mayonnaise/peanutbutter)?

Are there any additives that might be culprits, or are there too many unknown additives?

Is there a type of oil we can switch to with similar viscosity and density properties without the 'problem ingredient,' whether it's ester or otherwise?

I look forward to hearing any answers or responses.
Thanks!
 
If you can, find an API SA rated oil (technically not legal in some states). That'll be mineral and have no additives in it. Or you could get mineral oil from the pharmacy; although color might make the visual more difficult in that case.
 
Maybe try a usp grade mineral oil as old oak mentione,d or, an API SA or other non additised 30 grades.

I got some at autozone to use as ODPE top oil -the bonus in a birn appilicatio is they are minus the modern metallo-organic adds.
 
I think SA rated oil will work. Most WalMarts carry it, or Dollar General. It will be labeled as to not for use in modern engines or something like that.
 
Originally Posted By: Snagglefoot
Olive oil or Canola would probably give you the result you want, if it doesn't need to be synthetic oil.

SF

Vegetable and seed oils are methyl esters and hydrolyze pretty readily.
 
Ester base oils will not hydrolyze at room temperature, although they may dissolve some water and turn milky if the water content exceeds about 2000 ppm, which may happen at the interface. In any case, it is highly unlikely that the Castrol 5W-30 contains any ester base oils, probably all Group III. I suspect you are seeing an effect from the additives, probably the polar detergents or dispersants dissolving or emulsifying some water at the water surface.

I agree an SAE 30 non-detergent SA motor oil would be best - no detergents, dispersants, or esters. I would avoid vegetable oils are they are somewhat polar and may dissolve some water. Silicone fluids may work but they are water white, expensive, and have a higher density that an SAE 30 SA oil.

Tom NJ/VA
 
Wow! I'm blown away with the timely and thoughtful responses! It sounds like there are plenty of viable options to explore as replacements. Thanks everybody!

Grady
 
I'd try pharmaceutical grade liquid paraffin. If you can't colour that, you can perhaps dye the water phase with ink, food dye or potassium permangenate (that's an oxidising agent but used dilute it shouldn't attack the oil and its an antiseptic which might discourage microbial growth) to give you contrast.

Kerosene would be cheaper but more smelly, and is often sold with dye in it, which might not suit.

As suggested above, silicon brake fluid might also work.

Vegetable oil would probably grow microbes at the interface.
 
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