Definition
The ability of an oil or other lubricant to separate cleanly from water after the two have been mixed or shaken together. A fluid with good demulsibility releases water quickly so the water can settle out and be drained, leaving the oil intact and able to lubricate properly.
Plain English
How well an oil pushes water back out of itself after the two get mixed. Good demulsibility means the water separates and sinks to the bottom instead of staying blended with the oil.
Context Anchor
Seen in aircraft maintenance discussions about engine oil, lubricants, contamination, and oil specifications.
Derivation
From 'de-' (to undo or reverse) plus 'emulsify' (to blend two liquids that don't normally mix, like oil and water). Demulsibility is literally the ability to un-emulsify -- to undo the mixing.
Why Pilots Care
Good demulsibility allows water contamination to settle and be drained from sumps, reducing the risk of engine power loss or corrosion.
Analogy
Think of oil-and-vinegar salad dressing. After you shake it, the liquids mix for a while, then separate again. A fluid with good demulsibility separates from water in a similar way.
Intuition Check
Demulsibility is not the ability of oil and water to mix well. It is the ability of oil and water to separate again after being mixed.
Example Sentence 1
Engine oils used in piston aircraft are tested for demulsibility to confirm that any water entering the system will separate out rather than emulsify with the oil.
Example Sentence 2
The mechanic tested the oil sample for demulsibility to confirm it would not hold water in emulsion during engine operation.