An abundant cheap dense liquid?

[Gigel]

Mercury is rare, expensive, toxic and probably illegal to owe.

Baluncore has a neat idea: some sort of mud. It's not very dense, but it's denser than water. You can go one step further and put iron powder into it. The idea is to mix substances with increasing densities, so they can hold each other. But they will precipitate sooner or later.

 

[Mark Harder]

Mercury is rare, expensive, toxic and probably illegal to owe.

Baluncore has a neat idea: some sort of mud. It's not very dense, but it's denser than water. You can go one step further and put iron powder into it. The idea is to mix substances with increasing densities, so they can hold each other. But they will precipitate sooner or later.

In the oil drilling business, they inject something called drilling mud down the hole to seal it up while drilling. It needs to be dense, so a suspension of barium sulfate (insoluble in water ) is a component of the mud. I wouldn't be surprised if drilling is where most of our barium resources go.

 

[Gigel]

About 80% of BaSO4 is used for drilling, according to Wikipedia.

 

[Baluncore]

The density of drilling mud is increased so that rock chips from the mechanical drilling are buoyant in the flowing mud. As mud is pumped down the centre of the drill string, the rock chips float in the circulating mud up the hole outside the drill string. The pressure of the column of drilling mud in the hole opposes the hydrostatic fluid pressure in the rock and so reduces blow-outs. Bigger is not always better. Drillers despair when they get the mud density too high as the overly expensive mud goes down the hole to be lost in the rock formations below.

Engineering involves solving real problems in an economical way. The problem with this thread was that rather than having a specific problem to solve, the OP was considering the wider possible complexities of buoyancy in differential density fluids. Neither toxic mercury nor expensive drilling mud can be engineered to solve an unspecified problem. Indeed, most engineering effort goes into identifying the problem and the specifications required.