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?