It's interesting to see that you're physically moving the magnets towards and away from the ferrofluid. Did you investigate electrically switchable permanent magnets?

e.g.: http://www.eclipsemagnetics.com/media/wysiwyg/datasheets/lif...

I know very little about them, other than that they're cool. The ones in that datasheet start about at about 2kg, so I don't know if you can get them small enough for your purposes.