zero viscosity
/ ZEER-oh vis-KOSS-ih-tee /
Stir honey and stir water, and the difference you feel is viscosity — the inner stickiness of a liquid, the way its parts drag against one another. Honey is very viscous and oozes; water is much less so and splashes. Viscosity is also why any spinning drink eventually comes to rest: the friction inside it quietly eats up the motion.
Zero viscosity means that internal drag has vanished entirely. A fluid with zero viscosity can flow through the tiniest gap with no resistance, and a current set spinning in it never winds down because nothing is rubbing it to a stop. In a superfluid this happens because the flowing atoms all occupy one shared quantum state, and there is simply no gentle way to scatter a single atom out of that perfectly coordinated motion.
The idea matters because it is the defining signature of superfluidity, the thing experimenters first measured by watching helium pour through a fine plug that should have blocked it. The honest caveat is that 'zero' only holds below a certain flow speed, the critical velocity; push the liquid faster than that and it starts shedding tiny whirlpools, friction reappears, and the magic stops.
Ordinary liquid helium dribbles through a tightly packed plug of fine powder only very slowly; cool it past the superfluid point and the same liquid gushes straight through, as if the plug were not there at all.
Through a plug that stops normal liquid, a superfluid pours freely.
Zero viscosity is not the same as zero resistance to all motion — a superfluid still resists being pushed faster than its critical velocity, and any normal component mixed in still carries ordinary friction.