how Brag hex hole forming
 

Hi, forum:

Recently I come up with a interesting phenomenon with viscos flow during heating. The viscos liquid was hold in a container like a pan, and the liquid began to become unstable when the container(pan) was heated from the bottom. Certainly, it is far from boiling. after that amazing hex hole come out.

Here i attach some pic of what i said above. I watch this from a Vedio. Can you tell me more about this phenomenon? what is the mechanisim ?
From which book (or other media) I can get any further imformation about it?
Thanks !