While the outside of the drop quickly cools, the inside remains hot for a longer time.
I would like to make them as gifts, but is there a reason I cant get them to shatter? Source data are provided as a file.
At some point the glass entering the water will be so thin that it's no longer possible for liquid glass to flow through the center, but by the time that happens the larger portions of the glass will have contracted almost as much as they're going to, so the amount of liquid glass that would still need to be displaced to avoid creating tension will be pretty small, and so the amount of tension created by an inability to displace any more liquid glass from the interior will be likewise small.
In order to break a droplet, it's necessary to create a crack that enters the interior tension zone in the drop.
Prince Rupert's Drops are one of the most spectacular physics demonstrations I have witnessed.