Viscous is a strange green gel that vaguely resembles toothpaste. Viscous is thicker and flows much slower than normal elements. When mixed with another material it is useful to make them have more friction, for example Tensile + Viscous to make honey, or making Rigid/Wall less slippery. Counterintuitively, it also acts as a decent lubricant because it gets stuck where it is put and makes a layer between objects.
There is a special material called Anti-Viscous. This is created by setting viscosityCoefficient in the Parameters to a negative value. This makes Viscous want to flow more instead of less, and is useful for creating some very strange materials. Anti-Viscous can make rivers that flow uphill and extra-slippery materials.
Viscous with the default settings has an unusual property. Under the right conditions, Viscous explodes into a swarm in a similar way to burning Powder. When a Viscous particle is compressed against a non-Viscous particle with enough speed and force, for example by an explosion or heavy hammer, Viscous "glitches out" and explodes, sending particles flying away at maxSpeed. Exactly how and why this reaction occurs is not currently well known.
My theory is that, within the programming of Viscous, there is a part that was not designed to handle absolute maximum possible pressure. Under certain circumstances, something happens to individual particles to make them do something different. The reaction most commonly seems to happen between Gas and Viscous, but under unusual conditions I have seen Viscous react with Wall. I think maybe there is an Integer Overflow that suddenly snaps the particles into a high-speed state. The reaction is too fast to easily see exactly what happens between two particles, but it seems as if the Viscous snaps onto particles and orbits them for a split second, then catapults off in a random direction. The strange part is that it doesn't just happen once, the Viscous particles stay in this agitated state for an extended period of time, sometimes forever. It's almost as if they hold momentum.