What video settings should I use to record ink droplets in water?

I'm trying to film the movement of ink droplets in water as well as swirling colors dropped into milk and distored with dish soap. I'm using a Nikon D5100. I'm new to using my DSLR so i'm not exactly sure how I can get my camera to stay focused on the swirling ink. I had my settings as 'subject tracking AF' for AF-area mode and the focus mode on full-time-servo AF. It had a hard time focusing on the swirling ink as it changed directions. It would keep refocusing but it took a while for it to finally focus (thats if it even could) and i really don't want these blurry transitions in my video.

Can anyone please suggest how I can record this movement with a proper focus?

Use manual focus!