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From this answer to the question "Difference between colloid thrusters and electro spray thrusters" I've extracted the following:

According to the Wikipedia article Colloid thruster they are the same thing:

A colloid thruster (or "electrospray thruster") is a type of low thrust electric propulsion rocket engine that uses electrostatic acceleration of charged liquid droplets for propulsion. In a colloid thruster, charged liquid droplets are produced by an electrospray process and then accelerated by a static electric field. The liquid used for this application tends to be a low-volatility ionic liquid.

The Wikipedia article includes a link to some MIT lecture notes for Aeronautics and Astronautics; On the Lecture notes page for 16-522-space-propulsion-spring-2015 one can find MIT16_522S15_Lecture22-23.pdf, the title of which is:

Session : Cone-jet Electrosprays, or Colloid Thrusters

These two terms indeed seem to be use interchangeably.

But neither of those links explain why the liquid must or may be a colloid.

When I (and Wikipedia) think of "colloid" we think of the most commonly used example of which is milk. Wikipedia goes on to list mayonnaise, hand cream, and latex as examples of a liquid-liquid colloid. A dilute solution of milk will demonstrate opalescence due to the scattering of light from the colloidal suspension of fat particles. You see this by the blue/red color shift depending on angle from white light, or the strong scattering of a laser pointer's light shining in from the side. Opal itself is a colloid crystal.

Question: But do electrospray thrusters really use colloids? And are there colloidal suspensions that are stable in space? Or do they just use ionic or metallic liquids and the use of the term "colloid" is just historical?

below: Got milk? Colloid example; Glass of milk on tablecloth. Click for full size.

enter image description here

uhoh
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    I'm guessing that the term colloid comes from the use of such a thruster in atmosphere, where it would produce exhaust in the form of a colloid aerosol in air. In space it would be more of a nullosol. – Russell Borogove Mar 05 '18 at 18:59
  • @RussellBorogove Oh, I see what you mean. In an atmospheric application the resulting aerosol ("spray") might be the important product, rather than the thrust. – uhoh Mar 06 '18 at 02:19
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    Oh, I don't know if the exhaust would be important, but it would be colloid. – Russell Borogove Mar 06 '18 at 02:58
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    @RussellBorogove well we don't call LOX/RP-1 engines "Soot Engines" nor LOX/LH2 "Steam Engines", so I don't think we would call a space engine that makes colloidal particles "Colloid Engines" unless the colloid had some significance beyond "that's what's in the exhaust." – uhoh Mar 06 '18 at 03:47
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    I've got some bad news for you about how things get named. – Russell Borogove Mar 06 '18 at 04:34
  • @RussellBorogove in the pit of my stomach I have a feeling you might be right. But at least I got to use my "Steam Engines" quip in the process. – uhoh Mar 06 '18 at 05:16
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    I like the steam engines idea. I've called them water vapor generators. – Organic Marble May 19 '18 at 13:58
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    Good question! I've looked at multiple papers on the subject, and the ones discussing "colloid thrusters" immediately start talking about ionic liquids as the propellants, and ionic liquids are not colloids. Russell Borogove might be right about the rather inappropriate naming history. – Tom Spilker May 19 '18 at 17:17
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    @TomSpilker thanks! The earliest publications or reports may have the best clues. – uhoh May 19 '18 at 17:46
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    @uhoh By the way, I didn't have to click on the image of the glass of milk to get a full-size image: I have a "laboratory example" of one on my kitchen table! ;-) – Tom Spilker May 19 '18 at 18:06
  • I suspect that the fact that the droplets are all like-charged means that they don't agglomerate in vacuum. – ikrase Feb 25 '20 at 02:41
  • @ikrase I don't know where or how the electrons are offloaded but to remain charge-neutral the spacecraft has to get rid of them somehow, but probably it's with some electron gun off to one side. But I'm not sure of your point about agglomeration; does it matter some how in terms of the use of the term colloid or is it just a general comment? – uhoh Feb 25 '20 at 03:23

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If this source is right, they're using the term "colloid" to refer to charged microparticles:

At sufficiently high voltages, the micro-jet becomes unstable very near the apex, and breaks up into charged droplets (hence the name “colloid”)....

which does not conform to the formal definition of a colloid:

A homogeneous, noncrystalline substance consisting of large molecules or ultramicroscopic particles of one substance dispersed through a second substance. Colloids include gels, sols, and emulsions; the particles do not settle and cannot be separated out by ordinary filtering or centrifuging like those in a suspension. (emphasis mine)

but I guess they liked the term.

uhoh
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Tom Spilker
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  • Yes, but the Dictionary specifies that it's dispersed within a second substance, like a liquid. A vacuum, even a partial vacuum like the LEO environment, doesn't have enough charge density to qualify. I would have preferred that they invent a new term for a new phenomenon, rather than hijack a pre-existing, and in my view inappropriate, term. – Tom Spilker May 20 '18 at 01:06
  • I had multiple undergraduate chemistry courses, and the first reference I found online agreed with what I'd been taught in those chemistry courses, so I stopped there. It didn't seem there was any controversy. – Tom Spilker May 21 '18 at 03:47
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    No red herring intended. It's just that from the text preceding it, "...the one quoted passage..." it appeared to be pointed in the other direction. – Tom Spilker May 21 '18 at 04:16
  • Comment cleanup! – uhoh May 21 '18 at 04:16
  • The four word, parenthetical sentence fragment isn't really a sufficient explanation of why the term has been used by so many scientists and engineers at places like MIT, NASA... They seem to be convinced that the use of this term is important. – uhoh May 22 '18 at 01:48