I recently decided to make my own ferrofluid toy to play with. I built a sealed enclosure at my local fablab, and the end result was perfect. It worked just like in the videos you see online, and I couldn't have been more pleased.
But then, after a week or so, I noticed that the fluid was moving a bit more sluggishly, and not long after that, it would barely react to the magnet at all. Now when I try to play with it, I have to press a big magnet right against the surface, and then the "fluid" moves completely uniformly -- less like a fluid and more like a big black rock.
The only thing I can think is that maybe the suspension fluid I used had a gradual drying-out effect on the ferrofluid. When I was originally building the thing, I did some googling to find out how best to do it all, and I found an old post on this very forum, suggesting the use of isopropyl alcohol, so that's what I used. I also used the same type of ferrofluid as the OP in that thread.
So would a different type of suspension fluid keep it from hardening like this? Or am I wrong about the cause? The only other thing I can think of is that maybe it's because there's a little air bubble in my enclosure. It can't be any external factors, because it's sealed completely airtight.
Any ideas? Thanks!