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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

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    $\begingroup$ 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. $\endgroup$
    – Russell Borogove
    Commented Mar 5, 2018 at 18:59
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    $\begingroup$ Oh, I don't know if the exhaust would be important, but it would be colloid. $\endgroup$
    – Russell Borogove
    Commented Mar 6, 2018 at 2:58
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    $\begingroup$ @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." $\endgroup$
    – uhoh
    Commented Mar 6, 2018 at 3:47
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    $\begingroup$ I've got some bad news for you about how things get named. $\endgroup$
    – Russell Borogove
    Commented Mar 6, 2018 at 4:34
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    $\begingroup$ I like the steam engines idea. I've called them water vapor generators. $\endgroup$
    – Organic Marble
    Commented May 19, 2018 at 13:58

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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.

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  • $\begingroup$ 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. $\endgroup$
    – Tom Spilker
    Commented May 20, 2018 at 1:06
  • $\begingroup$ 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. $\endgroup$
    – Tom Spilker
    Commented May 21, 2018 at 3:47
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    $\begingroup$ 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. $\endgroup$
    – Tom Spilker
    Commented May 21, 2018 at 4:16
  • $\begingroup$ Comment cleanup! $\endgroup$
    – uhoh
    Commented May 21, 2018 at 4:16
  • $\begingroup$ 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. $\endgroup$
    – uhoh
    Commented May 22, 2018 at 1:48

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