Consider a solid yogurt. Can we assign a specific state of matter to it?
I mean, it behaves like solid. However, if we "mix" it with a spoon, it becomes liquid, or at least in the usual sense. It is also quite plastic, which complicates.
My current guess is that it is either a glass or a gel, but I'm quite unsure about it.
The thickness of yoghurt is due to bacteria producing lactic acid and flocculating the casein molecules. There's an excellent article on this here.
We normally reserve the word gel for systems that display a yield stress. There are set yoghurts that would be described as a gel, but the more liquid yoghurts are just a non-Newtonian fluid.
Ice is able to "flow" under stress [as in glaciers], but is classified as a solid. Therefore, it seems safe to think that yogurt is a solid, too, as it flows when moved, but, does not flow on its own unless it is thin.
