The base will dissociate readily when it hits the initially acidic aqueous layer (thus invisible), so any whirlpool you can see is between the NP solvent and the base solultion as it's travelling down to the aqueous layer. Above ph=7, the base solution you're adding won't be able to dissociate so quickly, or at all, and what is visible to the swirl-watcher is either precipitate or diffuse interfaces between the two liquids as they mix. I think.
Now, I would advise against feeling your solution until it feels "slimey". Sodium Hydroxide is a strong base (in case you missed any warnings on the package). The whole reason strong bases feel slimey is that they're persistantly dissolving your skin cells (see: Caustic). In contrast, when a strong acid gets on your skin, it denatures the proteins causing a ~'melted' barrier of protein past which the acid cannot go further. Strong bases will keep on digging. Luckily we're composed of water -among other things- which may nuetralize the "slippery" solution before it bores all the way through the appendage in question.
Needless to say: NOT a good experimental technique!
